ENGINEERING MATERIALS

Special properties steels

Ferrous Metals

Introduction
The ferrous metals are based on iron, one of the oldest metals known to
humans.
The ferrous metals of engineering importance are alloys of iron and
carbon. These alloys divide into two major groups: steel and cast iron.
Together, they constitute approximately 85% of the metal tonnage in the
United States.

The carbon is added into iron in varying amounts to produce a number of

useful alloys such as mild steel, stainless steel, white cast iron, etc. In
order to understand the microstructure of these alloys, we shall first
discuss the phase transformation occurring at different temperatures in the iron-
carbon system. This discussion of the ferrous metals begins with the iron–carbon
phase diagram.

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Contents:
- Phase diagram of Iron-carbon system.
- Plain Carbon Steel
- Alloy Steel
- Stainless steel
- Cast Iron
- Super Alloys
THE IRON–CARBON PHASE DIAGRAM
The iron–carbon phase diagram is shown in Figure .1. Pure iron melts at
1539°C.
During the rise in temperature from ambient, it undergoes several solid
phase transformations, as indicated in the diagram. Starting at room
temperature the phase is alpha (α), also called ferrite. At 912°C, ferrite
transforms to gamma (γ), called austenite. This, in turn, transforms at
1394°C to delta (δ), which remains until melting occurs. The three phases
are distinct; alpha and delta have BCC lattice structures, and between
them, gamma is FCC.

Figure.1: The iron-iron carbide (Fe-Fe3C) phase diagram.

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From the figure above, the low- carbon – region found 1400°C is not of
any practical importance, however, the region lying in the 700-900 °C
temperature range and (0-1%) carbon range is the most important region in
the phase diagram.
In this region, an engineer can develop within steel, those microstructures
which are required for desired properties.

Solid Phases in Iron- Iron Carbide Phase Diagram:

The iron-Iron carbide phase diagram shown in figure.1 contains four solid
phases:
α – Ferrite, γ- Austenite, Cementite (Fe3 C), and δ- Ferrite.
1. α – Ferrite: Is a solid solution has a (BCC) structure. The
maximum solid solubility of the carbon in α – Ferrite decreases
with the decrease in temperature, until about (0.008%) at 0°C.
The α – Ferrite is soft, ductile , and highly magnetic.

2. γ- Austenite: The solid solution of carbon in γ-Iron is called

austenite, it has a (FCC) structure. The solubility of carbon in
austenite reaches a maximum of 2.11% at 1148°C, and then decrease
to 0.8% at 723°C.
The austenite is soft and ductile, it is not ferromagnetic at any
temperature.

3. Cementite (Fe3C): Is an intermediate phase. It is a metallic

compound of iron and carbon, is called Iron Carbide or Cementite, it
contains 6.7% carbon. It extremely hard and brittle. It is magnetic
below 210°C.

4. δ- Ferrite: It has (BCC) structure. The maximum solubility of

carbon in δ- Ferrite is 0.09% at 1495°C.

** Pearlite(P): Pearlite is the product of the decomposition of austenite by

a eutectoid reaction and comprises a lamellar arrangement of ferrite and
cementite. Has good mechanical properties because it consider as
composite material of ferrite and cementite. See figure. 2.

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Figure.2: Microstructure of Pearlite steel

Eutectoid, Hypo-eutectoid, and Hyper- eutectoid steel:

Steel is an alloy of iron that contains carbon ranging by weight between

0.02% and 2.11% (most steels range between 0.05% and 1.1%C). These
steels are referred as plain carbon steel when they do not contain any
alloying element.
A plain carbon steel containing 0.8% carbon is known as eutectoid steel,
if carbon content of the steel is less than 0.8% it is called hypo-eutectoid
steel. Most of these steels produced, commercially are hypo-eutectoid
steels.
The steel, which contain more than 0.8% of carbon are called hyper-
eutectoid steels.
Hyper-eutectoid steels with carbon content up to (1.4%) are produced
commercially, when the carbon content is more than 1.4% it becomes very
brittle, thus very few steels are made with carbon content more than
(1.4%).
In order to increase the strength of steel. Other alloying elements are
added, these elements increase strength as well as maintain ductility and
toughness.

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Plain Carbon Steel

These steels contain carbon as the principal alloying element, with only
small amounts of other elements. Most of steel produced now-a days is
plain carbon steel.
A plain carbon steel is defined as a steel which has it properties mainly
due to its carbon content and does not contain more than 0.5% of silicon
and 1.5% of manganese.
The plain carbon steel varying from o.o6% to 1.4% carbon are divided into
the following types depending upon the carbon content:

1. Low carbon steel (Mild steel)= 0.06%- 0.19% carbon.

2. Medium carbon steel = 0.20% - 0.55% carbon.
3. High carbon steel = 0.55% - 1.4% carbon.

These steel are (strong , tough, ductile) and has poor atmospheric
corrosion resistance.
The properties of plain carbon steel depends on the presence of carbon
content.
The hardness and strength increase with an increase in carbon content.
These properties increase due to presence of hard and brittle cementite.
The ductility and toughness decrease with an increase in carbon content.
Figure.3 shows the effect of carbon content on steel properties.

Figure.3: Effect of carbon content on mechanical properties

of carbon steel

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Figure.4: Mechanical properties for different plain carbon steels

1. Low Carbon Steel:

These generally contain less than about 0.20 wt%

of carbon. Microstructure consist of ferrite and
pearlite constitutes. These alloys are relatively soft
and weak but have ductility and toughness, in
addition, they are machinable and weldable.

Typical applications include: automobile body

components, structural shapes (I-beam, channel
and angle iron), and sheets that are used in
pipelines, buildings, bridges, and railroad rails.
These steels are relatively easy to form, which accounts for their
popularity where high strength is not required. Steel castings usually fall
into this carbon range, also.

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2. Medium Carbon Steel:

These have carbon concentrations between

0.25and 0.55% carbon. These alloys may heat –
treated by austenitizing, quenching, and then
tempering to improve their mechanical properties.
The plain medium carbon steel has low
hardenability. Theses heat treated alloys are
stronger than the low carbon steel, and are
specified for applications requiring higher strength
than the low-C steels.
Applications of medium carbon steel include
railway wheels and tracks, machinery components and engine parts such
as crankshafts, gears and connecting rods.

3. High Carbon Steel:

These normally have carbon contents between

0.55% and 1.4%. Are hardest, strongest, and yet
least ductile of carbon steel.
These used in a hardened and tempered condition
and as such are especially wear resistant.
Applications of high carbon steel include Springs,
cutting tools and blades, punches, dies, and wear-
resistant parts are examples.

Table.1: The important applications of plain carbon steel.

Types of steels Uses

1. Low- carbon steel or mild steel Chain links, nails, rivets, ship hulls, car
bodies, bridges, cams, light duty gears
,etc.
2. Medium carbon steel Axles, connecting rods, gears ,wheels for
trains and rails, etc.
3. High carbon steel Clutch plate, razor blades scissors, knives,
punches, dies, etc.

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Effect of Impurities on Steel:

The following are important effects of impurities like: silicon (Si), Sulphur
(S), manganese (Mn), and phosphor (P) on steel.

1. Silicon: Range between 0.05% to 0.30% . Is added in low carbon

steel to: -
- Prevent them from becoming porous.
- Remove the gasses and oxides.
- Make the steel tougher and harder.

2. Sulphur: It occurs in steel either as iron sulphide or manganese

sulphide. Iron sulphide because of its melting point produces red
shortness, whereas manganese sulphide does not affect so much.
Sulphur is added to improve machinability only.

3. Manganese: Is added in low carbon steel to:

- It serves as a deoxidizing and purifying agent in steel.
- Manganese combines with sulphur and thereby decreases the harmful
effect of this element in the steel.
- Improve yield strength, toughness, and hardenability.

4. Phosphorus: Is added to steel to:

- It make the steel brittle.
- Raises the yield point.
- Improve the resistance to atmospheric corrosion.

Classification of Metal Alloys

The metal alloys include steel alloys and other metal alloys are classified
according to many designation codes:

The Society of Automotive Engineers (SAE),

The American Iron and Steel Institute (AISI), and

The American Society for Testing and Materials (ASTM) are responsible for the

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classification for these steels as well as other alloys.

- Steel series.
According to the AISI/ SAE designation for these steels can be divided into
different series.

four-digit number + any prefix: the first two digits indicate the alloy content, the
last two digit give the carbon content.

For Plain Carbon Steel (1000 series):

First digit: 1,
Second digit: 0 (carbon steel),
1 (resulphurized carbon steels,
2 (resulphurized and rephosphorized carbon steels)
Third and fourth digits: carbon content *100.
For Alloy Steel : The first two digit indicate the alloy content as in table
below:

SAE designation Type

1xxx Carbon steels
2xxx Nickel steels
3xxx Nickel-chromium steels
4xxx Molybdenum steels
5xxx Chromium steels
6xxx Chromium-vanadium steels
7xxx Tungsten steels
8xxx Nickel-chromium-vanadium steels
9xxx Silicon-manganese steels

The first digit of AISI/SAE Steel Designation represents a general

category grouping of steels. This means that 1xxx groups within the SAE-

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AISI system represent carbon steels. Thus the plain carbon steels are
represented within the 10xx series, resulfurized carbon steels are
represented within the 11xx series, resulfurized and rephosphorized carbon
steels are represented within the 12xx series. The second digit of the series
indicates the presence of major elements, which may affect the properties
of the steel. For example in 1018 steel, indicates non-modified carbon steel
containing 0.18% of carbon.

SAE 5130 indicates a chromium alloy steel containing 1% of chromium

and 0.30% of carbon.

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Alloy Steel / ‫الفوالذ السبائكي‬

Definition: A steel in which elements other than carbon are added in

sufficient quantity in order to obtain special properties, is known as alloy
steel.

• The alloying of steel is generally done to increase its strength,

hardness, toughness, resistance to abrasion and wear, and to improve
electrical and magnetic properties.
• The various alloying elements are: Nickel, Chromium, Molybdenum,
Cobalt, Vanadium, Manganese, Silicon, and Tungsten.
• Low alloy: Added in small percent’s (<5%) to increase strength and
hardenability.
• High alloy: Added in large percent’s (>20%) – i.e. > 10.5% Cr =
stainless steel where Cr improves corrosion resistance and stability at
high or low temps
• The effect of these alloying elements are discussed below:

1. Nickel: It is one of the most important alloying elements:

- Steel sheets contain (2% to 5% nickel and 0.1% to 0.5% carbon).
In this range, nickel improves tensile strength, raises elastic limit,
imparts hardness, toughness, and reduce rust formation. It is largely
used for boiler plates automobile engine parts, crank shafts,
connecting rods.

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- When nickel is added to steel (about 25%) it results in higher
strength steels with improved shock and fatigue resistance. It makes
the steel resistant to corrosion and heat. It is used in the boiler tubes,
valves for gas engine, sparking blogs for petrol engines.
- A nickel steel alloy containing about (36% Ni and 0.5% Carbon) is
known as (Invar). It can be rolled, forged, turned, and drawn, it is
widely used for making pendulums of clocks, precision measuring
instruments.
- 2xxx is Nickel steel alloy.

2. Chromium: Addition of chromium to steel increases its

strength, hardness, and corrosion resistance:
- A chrome steel containing (0.5% to 2% Cr) is used for balls, rollers,
and races for bearings, die, permanent magnets, etc.
- 5xxx is chrome steel alloy
- A steel containing (3.25% Ni, 1.5% Cr, and 0.25% C) is known as
(Nickel- chrome steel 3xxx). The combination of toughening effect
of nickel and the hardening effect of chromium produces a steel of
high tensile strength with great resistance to shock. It is used for
motor car crank shafts, axles and gears requiring strength and
hardness.

3. Vanadium: It is added in low and medium carbon steels in order

to increase their yield and tensile strength properties. It is added to
steel usually about 0.03% to 0.25 % to increase strength without
loss of ductility
- In combination with chromium about (0.5% to 1.5% Cr, 0.15% to
0.25% V, and 0.13% to 1.1% C), it produces a marked effect on
properties of steel and makes the steel tough and strong. These steel
is used for spring steel, high speed tools, and other parts of
automobiles. Code of this alloy is 6xxx.

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4. Tungsten: the addition of tungsten raises the critical temperature
of steel and hence it is used for increasing strength of alloyed steel
at high temperatures. It helps to form stable carbides and
increases hot hardness
- When added to the extent of 5% to 6%, it gives steel good magnetic
properties, thus it is commonly used for magnets in electrical
instruments.
- The tungsten is usually used with other elements, steel containing
(18% tungsten, 4% Cr, 1% V, and 0.7% C) is called tool steel or
high speed steel.
Since the tool made with this steel the ability to maintain its sharp
cutting edge even at elevated temperature, therefore it is used for
making high speed cutting tools such as cutters, drills, dies…. etc.
- Code of this steel is 7xxx.

5. Manganese: It is added to steel to reduce the formation of iron

sulphide by combining with sulphur. It make the steel hard, tough,
and wear resisting.
- The manganese alloy steel containing over (1.5% Mn with carbon
of 0.4% to 0.55%) are widely used for gears, axles, shafts, and other
parts where high strength combined with fair ductility is required.

- A steel containing manganese from (10% to 14% and carbon

1% to 1.3%) form an alloy steel, which is extensively hard and tough
and has high resistance to abrasion. It is Ideal for impact resisting
tools. is largely used for mining, rock crushing and railways
equipment.

6. Cobalt: It is added to high speed steel from (1% to 12%), to give

red hardness by retention of hard carbides at high temperatures.

It increases hardness and strength. But too much of cobalt it

decreases impact resistance of steel.

7. Molybdenum: A small quantity (0.15% to 0.3%) of

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molybdenum is generally used with chromium and manganese
(0.5% to 0.8%) to make molybdenum steel.
- Mo increase hardenability and strength
- Mo-carbides help increase creep resistance at elevated temps
- typical application is hot working tools
- It can replace tungsten in high speed steel.

8. Silicon: It increases the strength and hardness of steel without

lowering its ductility.
- Silicon steel containing from (1% to 2% silicon and 0.1% to 0.4%
carbon) have good magnetic properties and high electrical resistance.
- It can withstand impact and fatigue even at elevated temperature.
- These steel are used for generator and transformers in the form of
laminated cores.

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Tool Steel ‫فوالذ العدد‬

Introduction: These are the steel used in making tools and dies which are
required for cutting, shaping, forming, and blanking of materials.

These steels should have high hardness, greater abrasion or wear

resistance, greater toughness, high impact strength, high thermal
conductivity, low coefficient of friction.

Types of tool steel: The tool and dies steels are of the following types:-

1. Plain carbon steels:

These steels contain carbon from (0.60% to 1.4%) and are hardened
either by oil or water quenching. The important advantage of these
steel is that they low cost, good machinability, and high impact
resistance.

The main disadvantage of carbon tool steel is poor hardenability. It

needs quenching with water, brine or caustic water. Distortion and
cracking tend to be large, and wear resistance and thermal strength
are very low.
Therefore, carbon tool steel can only be used to make small
handmade tools or woodworking tools, as well as small cold working
dies with low precision, simple shape, small size and light load.
These steels are used for: keys, stamping dies, twist drills, general

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wood and leather cutting tools.

Plain carbon tool steel

2. Low alloy tool steel:

These steels containing alloying elements like: vanadium,
chromium, tungsten, and silicon. The presence of alloying elements
refine the structure and increases the toughness and impact
resistance. Compare with carbon tool steel, its hardness, toughness
and wear-resistance are raise. And its hardenability and hot hardness
are raised dramatically.

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The low – alloy tool steel are used for heavy duty pneumatic tools,
pavement breakers. it was used to make measuring tools, molds and
blade tools with big size, complex shape and high requirement of
performance.

Different levels for alloy steel because of the different total amount of
alloy elements. And if the alloy elements amount is less than 5%, then
it is low alloy tool steel. The medium alloy tool steel is at the range of 5%-
10%. And the high alloy tool steels are higher than 10%. At the present
time, most of the alloy tool steel is low alloy tool steel.

3. High speed steels:

These steels are used for cutting metals at a much higher speed than
ordinary carbon tool steel. The carbon steel cutting tools do not
retain sharp cutting edges under heavier loads and higher speeds.
This is due to the fact that at high speed, sufficient heat may be
developed during the cutting operation and causes the temperature of
the cutting edge of the tool to reach a red hot. This temperature
would soften the carbon tool steel and thus the tool will not work
efficiently for a longer period.
The high speed steels have the valuable properly of retaining their
hardness even when heated to red hot.
Following are the different types of high speed steels:-

a. 18-4-1 high speed steel (tungsten series HSS):

This steel contains (18% tungsten, 4% chromium, and 1%
vanadium). It is considered to be one of the best of all purpose
tool steel.
It is widely used for drills, lathes, planer and shaper tool, milling
cutters, etc..
b. Molybdenum series HSS:
This steel contains (6% tungsten, 6% molybdenum, 4%
chromium, and 2% vanadium). It has excellent toughness and
cutting ability. Molybdenum high speed steels are better and

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cheaper than other types of steels.
It is used for drilling and tapping operations.
c. Super high speed steel:
This steel is called cobalt high speed steel, because cobalt is
added from (2% to 4%) in order to increase the cutting efficiency
especially at high temperatures.
This steel contains (20% tungsten, 4% chromium, 2%vandium,
and 12% cobalt). Since the cost of this steel is more, is used for
heavy cutting operation, which impose high pressure and
temperature on the tool.

4. High carbon high chromium steels:

These steels are much cheaper than high speed steel (HSS), but have
greater importance than HSS.
These are widely used for various types of dies like those used for
drawing, coining, blanking, forming, and thread rolling.

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Stainless steel
Introduction
The word STAINLESS could mean a steel that STAINS LESS. They are
highly resistant to corrosion (rusting) in a variety of environments,
especially the ambient atmosphere. Their important alloying element is
chromium , a concentration of at least 11 wt % is required. Corrosion
resistance may also be enhanced by nickel and molybdenum addition.
The family of stainless steel can be split into four main groups:
Martensitic, Ferritic, Austenitic and Duplex. Nearly all end use
applications use either ferritic or conventional
austenitic steel.

Fig. 1: The family tree of

Stainless steel

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Austenitic stainless steel :
Austenitic grades are those alloys which are commonly in use for stainless
applications. The austenitic grades are not magnetic. The most common
austenitic alloys are iron-chromium-nickel steels and are widely known as
the 300 series which contain chromium normally in the range 17-25% and
nickel in range 8-20% with various additional elements to achieve the
desired properties.(commonly referred to as 18/8 steel).

The austenitic stainless steels, because of their high chromium and nickel
content, are the most corrosion resistant of the stainless group providing
unusually fine mechanical properties. They cannot be hardened by heat
treatment, but can be hardened significantly by cold-working.

Table (1) shows some types of austenitic stainless steel(300 serirs).

Type 304 The most common of austenitic grades, containing approximately 18%
chromium and 8% nickel.
It is used for chemical processing equipment, for food, dairy, and beverage
industries, for heat exchangers, and for the milder chemicals.
Type 316 Contains 16% to 18% chromium and 11% to 14% nickel. It also molybdenum
added to the nickel and chrome of the 304. The molybdenum is used to control
pit type attack. Type 316 is used in chemical processing, the pulp and paper
industry, for food and beverage processing and dispensing and in the more
corrosive environments. The molybdenum must be a minimum of 2%
Type 317 Contains a higher percentage of molybdenum than 316 for highly corrosive
environments. It must have a minimum of 3% “moly”. It is often used in stacks
which contain scrubbers
Type 317L Restricts maximum carbon content to 0.030% max. and silicon to 0.75% max. for
extra corrosion resistance

Type 317LM Requires molybdenum content of 4.00% min

Type Requires molybdenum content of 4.00% min. and nitrogen of .15% min.
317LMN
Type 321 These types have been developed for corrosive resistance for repeated
Type 347 intermittent exposure to temperature above 800 degrees F. Type 321 is made
by the addition of titanium and Type 347 is made by the addition of
tantalum/columbium. These grades are primarily used in the aircraft industry.

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Martensitic Stainless steel

Martensitic grades were developed in order to provide a group of stainless
alloys that would be corrosion resistant and hardenable by heat treating.
The martensitic grades are straight chromium steels containing no nickel
which contains a minimum of 12% chrome and usually a maximum of 14%
with carbon in the range of 0.08%-2.00%. They are magnetic and can be
hardened by heat treating. The martensitic grades are mainly used where
hardness, strength,and wear resistance are required.

Table (2) shows some types of martensitic stainless steel (400 series)

Type 410 Basic martensitic grade, containing the lowest alloy content of the three
basic stainless steels (304, 430, and 410). Low cost, general purpose, heat
treatable stainless steel. Used widely where corrosion is not severe (air,
water, some chemicals, and food acids. Typical applications include highly
stressed parts needing the combination of strength and corrosion resistance
such as fasteners.
Type 410S Contains lower carbon than Type 410, offers improved weldability but
lower hardenability. Type 410S is a general purpose corrosion and heat
resisting chromium steel recommended for corrosion resisting applications.

Type 414 Has nickel added (2%) for improved corrosion resistance. Typical
applications include springs and cuttlery.
Type 416 Contains added phosphorus and sulfer for improved machinability. Typical
applications include screw machine parts.

Type 420 Contains increased carbon to improve mechanical properties. Typical

applications include surgical instruments.
Type 431 Contains increased chromium for greater corrosion resistance and good
mechanical properties.Typical applications include high strength parts such
as valves and pumps.

Type 440 Further increases chromium and carbon to improve toughness and
corrosion resistance. Typical applications include instruments.

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Ferritic Stainless steel
Ferritic grades have been developed to provide a group of stainless steel to
resist corrosion and oxidation, while being highly resistant to stress
corrosion cracking. These steels are magnetic but cannot be hardened or
strengthened by heat treatment. They can be cold worked and softened by
annealing. This groub contains a minimum of 17% chrome and carbon in
the range of 0.08%- 2.00%. As a group, they are more corrosive resistant
than the martensitic grades, but generally inferior to the austenitic grades.
Like martensitic grades, these are straight chromium steels with no nickel.
They are used for decorative trim, sinks, and automotive applications,
particularly exhaust systems.

Table (3) shows some types of ferritic stainless steel (400 series)

Type 430 The basic ferritic grade, with a little less corrosion resistance than Type 304.
This type combines high resistance to such corrosives as nitric acid, sulfur
gases, and many organic and food acids.
Type 405 Has lower chromium and added aluminum to prevent hardening when cooled
from high temperatures. Typical applications include heat exchangers.

Type 409 Contains the lowest chromium content of all stainless steels and is also the
least expensive. Originally designed for muffler stock and also used for
exterior parts in non-critical corrosive enviornments.

Type 434 Has molybdenum added for improved corrosion resistance. Typical
applications include automotive trim and fasteners.
Type 436 Type 436 has columbium added for corrosion and heat resistance. Typical
applications include deepdrawn parts.
Type 442 Has increased chromium to improve scaling resistance. Typical applications
include furnace and heater parts.

Type 446 Containes even more chromium added to further improve corrosion and
scaling resistance at high temperatures. Especially good for oxidation
resistance in sulfuric atmospheres.

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Duplex Grades(Austenitic, Ferritic Stainless Steels)

Duplex grades are the newest of the stainless steels. This material is a
combination of austenitic and ferritic material. This material has higher
strength and superior resistance to stress corrosion cracking. An example of
this material is type 2205. It is available on order from the mills.

This class of stainless steel has been available for about ten years. The very
high proof strength is due to the smaller grain size owing to the two phase
micro structure. The very small grain size prevents grain growth and
increases strength and toughness. The duplex alloy contains more
chromium, molybdenum, nickel and nitrogen than either 316 or 304. It has
improved corrosion resistance to the chlorine ion (salt water). It’s yield
strength is 2 - 3 times greater than austentic alloys.

Fig. 2 : Comparison of Tensile Strength Duplex and Austenitic Grades

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CAST IRON
Introduction
Cast iron is a cheap alloy. Ordinary cast iron is an alloy containing a total
of up to 10% of the elements carbon, silicon, manganese, sulphur and
phosphorus; the balance being iron. Alloy cast irons, contain also varying
amounts of nickel, chromium, molybdenum, vanadium and copper.

Graphitization:
Cementite (Fe3C) is a metastable compound, and under some circumstances it can
be made to dissociate or decompose.

1- Composition: Graphite formation is promoted by the presence of silicon

and to less degree phosphorus, nickel and copper. If silicon content is
lower than 1 wt% graphitization may not takes place.

2- Cooling rate: Slower cooling rates during solidification favor

graphitization. While higher rate of cooling during solidification tends to
favor the formation of cementite. This effect is illustrated by casting a
'stepped bar' of cast iron of a suitable composition Here, the thin sections
have cooled so quickly that solidification of cementite has occurred, as
indicated by the white fracture and high Brinell values. The thicker
sections, having cooled more slowly, are graphitic and consequently
softer.

3- Heat treatment

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Types of Cast Iron

1- White Cast Iron
If silicon content is lower and the cooling rates during solidification are
higher, the resulting structure will contain cementite, and the fracture
surface will appears bright .Since white cast iron is extremely hard and
brittle is not used as such but they are made as the first step for conversion
into the malleable iron. It is possible to form white cast iron structure on
the surface layers of grey cat iron by chilling the surface. This is called
chilled iron and is used for making wear resistance surfaces for iron rolls
and ploughs

2- Grey Cast Iron

If silicon content is higher and the cooling rates during solidification are
lower, complete graphitization takes place and the resulting structure will
contain graphite flakes only. Then it is called grey cast iron, the fracture
surface may be dull and grey.

The important engineering properties of grey cat iron are;

1. High compressive strength.

2. Moderate tensile strength

3. Good wear resistance

4. High damping capacity

The shortcoming of grey iron is the brittleness du to the flake form of

graphite which introduces sharp notches at the edges. The most important
applications of cast iron are machine beds, ingot moulds, lamp spots and
others.

3- Spheroidal-graphite (SG) Cast Iron

University of Baghdad / Mechanical Engineering Department Dr. Suhair G. Hussein25

 ENGINEERING MATERIALS
Also known as nodular cast iron or ductile cast iron. In SG cast iron the
graphite flakes are replaced by spherical particles of graphite so that the
sharp stress raisers are eliminated. The formation of this spheroidal
graphite is effected by adding small amounts of cerium or magnesium to
the molten iron just before casting.

4- Compacted-graphite (CG) Iron

The mechanical properties of this type is intermediate between those of
ordinary grey flake-graphite irons and those of SG iron. The graphite
flakes produced are short and stubby and have rounded edges. CG iron is
produced when molten iron of near-eutectic composition is treated with a
single alloy containing appropriate amounts of magnesium, titanium and
cerium. CG has good resistance to scaling and 'growth' at high
temperatures, so CG iron attractive as a heat-resisting material and it was
developed originally for the manufacture of ingot moulds and vehicle
brake components.

5- Malleable Cast Irons

The names of the two original malleabilising processes, the Blackheart and
the White heart, refer to the color of a fractured section after heat
treatment has been completed. Another process used for manufacturing
pearlitic malleable iron. In all three processes the original casting is of
white iron, which will be very brittle before heat-treatment.

University of Baghdad / Mechanical Engineering Department Dr. Suhair G. Hussein26

 ENGINEERING MATERIALS

University of Baghdad / Mechanical Engineering Department Dr. Suhair G. Hussein27

 ENGINEERING MATERIALS
Alloy Cast Irons
The microstructural effects which alloying elements have on a cast iron
are, in most cases, similar to the effects these elements have on the
structure of a steel.

1- Martensitic irons
Martesitic irons, which are very useful for resisting abrasion, usually
contain 4-6% nickel and about 1% chromium. Such an alloy is Ni-hard, is
martensitic in the cast state, whereas alloys containing rather less nickel
and chromium would need to be oil-quenched in order to obtain a
martensitic structure .

2- Austenitic irons
Austenitic irons usually contain between 10 and 30% nickel and up to 5%
chromium. These are corrosion-resistant, heat-resistant, non-magnetic
alloys. Some of them are treated to produce structures containing
spheroidal instead of flake graphite .

University of Baghdad / Mechanical Engineering Department Dr. Suhair G. Hussein28

 Classification of steel
STEEL is an alloy of iron with other elements.

I. Plain carbon (or) non alloy steels

a) Low carbon steels
b) Medium carbon steels
c) High carbon steels
II. Alloy steels
a) Low alloy steels
b) High alloy steels

(Explain the classification of plain carbon steel with its properties and application.)
I. Plain Carbon steel
Carbon is the alloying element that defines the properties of the alloy.
Composition of plain carbon steels:
up to 1.5% Carbon
up to 1.65% Manganese
up to 0.6% Copper
up to 0.6%Silicon
Characteristics of plain carbon steels:
➢ moderately priced steels due to the absence of large amount of alloying elements.
➢ sufficiently ductile
Applications of plain carbon steels:
➢ mass production products such as automobiles and appliances.
➢ production of ball bearings base plates housing, chutes, structural member etc.

a) Low carbon steels:

➢ They are known as mild steels.
➢ contain less than about 0.25% carbon.
Characteristics of low carbon steels:
➢ relatively soft and week.
➢ cannot by hardened by heat treatment.
➢ Strengthened by cold working.
➢ have outstanding ductility and toughness.
➢ micro structure of low carbon steels consists of ferrite and pearlite constituents.
➢ least expensive to produce.
Applications of Low carbon steel
➢ Building frames in construction projects
➢ Machinery parts
➢ Cookware
➢ Pipelines
 ➢ Metal gates and fencing
b) Medium carbon steels
➢ Contains between 0.25 and 0.60% of carbon.
➢ may be heat treated, quenched and then tempered to improve their mechanical
properties.
Characteristics of medium carbon steels:
➢ low hardenabilities.
➢ good formability and weldability
➢ Application of medium carbon steels:
➢ railway wheels, railway tracks, gears, crank shafts, and other machine parts.
Application of medium carbon steels:
➢ railway wheels, railway tracks, gears, crank shafts, and other machine parts.
c) High-Carbon Steels:
➢ have more than 0.6% carbon
Characteristics of high carbon steels:
➢ hardest, and strongest of the carbon steels.
➢ least ductile of the carbon steel
➢ more wear resistant.
➢ They are capable of holding a sharp cutting
Application of high carbon steels:
➢ cutting tools and dies
➢ knives, razors,
➢ hack saw blades,
➢ springs and high strength wire.

II. Alloy steels:

Alloy steels are any steels other than carbon steels.

Composition:
Manganese -1.65, silicon – 0.6%, copper – 0.6%

Alloying elements:
chromium, nickel, molybdenum, vanadium, tungsten, cobalt, boron, copper etc.,
Purpose of Alloying:
➢ To increase its strength
➢ To improve hardness
➢ To improve toughness
➢ To improve resistance to abrasion and wear
➢ To improve machinability
➢ To improve ductility
➢ To achieve better electrical ad magnetic properties
Classification of Alloy steels:

Alloy steels can be divided into two main groups: Low alloy steels and high alloy steels

a) Low alloy steels:

➢ contain up to 3 to 4% of alloying elements.
➢ Have better mechanical properties that prevent corrosion,
➢ Have high temperature performance.
Applications:
➢ Pipes
➢ Automotive & aerospace bodies
➢ Railway lines, off shore & onshore structured engineering plates.

b) High Alloy steels:

➢ more than 5% of one or more alloying elements.
➢ The room temperature structures after normalizing may be austenitic, martensitic or
contain precipitated carbides.
Applications
➢ Automobiles
➢ Ship building
➢ Railways, aircrafts
➢ Finds use in low temperature applications due to its high toughness.
(Explain the cast iron micro structure, properties and application in detail.)
Micro structure of cast iron
Cast iron, an alloy of iron that contains 2 to 4 percent carbon, along with varying amounts
of silicon and manganese and traces of impurities such as sulfur and phosphorus.
Types of cast iron
1. Grey cast iron
2. White cast iron
3. Malleable cast iron
4. Spheroidal graphite cast iron or nodular cast iron
5. Alloy cast iron

1. Gray cast iron

➢ It is the most common type of cast iron.
➢ It has graphite microstructure consisting of many small fractures giving gray
colour.
➢ When gray cast iron is produced, the fractures open up to reveal the gray-colored
graphite underneath the surface.
➢ Gray cast iron is not as strong as steel, nor is it able to absorb the same shock as
steel.
➢ gray cast iron offers similar compressive strength as steel.
 EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

CHAPTER 8

METALS AND ALLOYS

8.1 Types of Alloying

8.2 Hume-Rothery Rules for Alloying
8.3 Material Systems
8.4 One-component material systems
8.5 Two component material systems
8.6 Derivation of Lever Rule
8.7 Metal Processing
8.8 Work Hardening and Annealing
8.9 Precipitation Hardening of Alloys
8.10 Iron-Carbon Systems
8.11 Eutectoid Point for Iron-Carbon Systems
8.12 Continuous-Cooling Transformation Diagram for Iron-Carbon Systems
8.13 Heat Treatment of Steel
8.14 Specifics of Quenching Process for Iron-Carbon Systems
8.15 Case Hardening of Steel

Alloys consist of a combination of two or more metal elements in the solid state. In engineering
work, alloys, rather than pure metals, are generally used because of their superior properties.
Ordinary steels, for example, are alloys of iron and carbon and are preferred to pure iron because
of strength and other considerations. Important mechanical properties of alloys are related to the
solid phases, or homogenous parts, that form from the combination of elements. Such phases are
observable on the microscopic scale, and their study is important in understanding alloy properties.

8.1 Types of Alloying

Interstitial is where the alloy elements are located in spaces between atoms in the unit cell. For
instance, when carbon (Atomic radius 0.129 nm) is added to iron (0.075nm), the carbon fits into
the gap between the Fe atoms (interstitial site). The solubility depends on the size of these gaps
and the crystal structure.

Substitutional is where the alloying elements are located in vacancies in the unit cell. An atom of
one element substitutes for another in the crystal structure.

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Fig. 1

8.2 Hume-Rothery Rules for Alloying

It is not possible for every combination of materials to form an alloy. The following rules are for
maximum solubility in substitutional alloying:

1. Same Crystal Structure (BCC, FCC, HCP for instance)

2. Same Valency Factor
3. Similar Electronegativity – the tendancy for the atom to attract a shared pair of electrons.
This depends on the atomic number and the distance at which the valence electrons are
from the nucleus.
4. Difference in atomic radius <= 15%
In interstitial alloying, the solute atoms should be <= 59% of the atomic radius of the solvent
atoms.

8.3 Material Systems

A material system refers to a definite amount of material or materials.

The components in a system refer to the smallest number or individual substances that must be
listed to describe the chemical composition of the system. These may be elements (Pb, Sn, Fe, C,
etc.) or compounds (H2O, NaCl, etc.)

The phases in a system denote the homogenous parts of the system that are made up of its
components.

Phase diagrams are graphs showing the phases present under selected conditions of temperature,
pressure, and composition when the system is in thermodynamic equilibrium.

A material system may be specified as in the following example:

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Example: Suppose we have 8 gms of copper and 2 gms of nickel. This yields a system having 80
w/o copper and 20 w/o nickel (w/o= weight percent). This can also be expressed as mole percent
(m/o) or atomic percent (a/o).

8 gm (Cu): (8 gm)/(63.54 gm/mole) = 0.1259 moles

2 gm (Ni): (2 gm)/(58.71 gm/mole) = 0.0341 moles

Since the number of atoms in a mole is constant, the atomic percent is:

8 gm (Cu): 0.1259/(0.1259 + 0.0341) = 78.7 m/o = 78.7 a/o

2 gm (Ni): 0.0341/(0.1259 + 0.0341) = 21.3 m/o = 21.3 a/o

8.4 One-component material systems

The phases possible in a one component system are limited to liquid, solid, and gas states (Fig 2).
A phase diagram consists simply of a graph of pressure vs. temperature, with the phase regions
indicated.

Fig. 2 Phase diagram for a one-component system (water)

8.5 Two component material systems

Materials with two components are called binary systems. They are commonly encountered in
engineering materials (brass consisting of copper and zinc, carbon steel consisting of iron and
carbon, etc.) The phase diagram of a binary material is customarily displayed in a temperature vs.
composition format, with the pressure held fixed at atmospheric.

In the phase diagrams,

L denotes liquid
α, β, γ are various solids
α+ β or L+ γ indicate states where both are present

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Two-Components with Complete Solubility: For a system having components completely soluble
in the solid state, the phase diagram is especially simple, since only one phase exists in the solid
state. See Figure 3 for the phase diagram of the material.

Two-Components with Partial Solubility (Eutectic): For a binary system having components only
partially soluble in the solid state, two solid phases will exist, and the phase diagram is much more
complex than Figure 3. A special case is an eutectic diagram shown in Figure 4. “Eutectic” means
that when the two components are mixed, the melting point is lower than the melting temperatures
of the two components separately. At point d, the eutectic reaction Liquid, L → α + β occurs.

Two-Components with Partial Solubility (Eutectoid): When the liquid region is replaced by a third
solid phase γ, we have the eutectoid reaction γ → α + β and associated phase diagram. Figure 5
shows such as phase diagram with both a Eutectic and Eutectoid point.

Two-Components with Partial Solubility (Peritectic): For a binary system having components only
partially soluble in the solid state, we may also have a phase diagram having a peritectic point P,
where the reaction L + β → α occurs (Figure 6)

Two-Components with Partial Solubility (Peritectoid): If the liquid in the peritectic reaction is
replaced by a third solid phase, γ, we have a peritectoid reaction α + γ → β. This is illustrated by
Figure 7.

Figure 3: Components with Complete Solubility (Copper-Nickel System)

Melting temperature depends on percentage of nickel. There is a solid phase, then a mixture of
liquid and solid (L → α) and then a complete liquid phase L

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 EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

Figure 4: Two Components with Partial Solubility (Silver Alloy Copper System)
At the Eutectic point (d) the liquid changes to a solid L → α + β
(This is at a lower temperature than when the separate components become solid)

Figure 5: Case 2 - Two Components with Partial Solubility (Iron Carbon System)
At Eutectic point, the liquid changes to a solid L → α + β
At the Eutectiod point γ → α + β
(This is at a lower temperature than when the change occurs for the individual components)

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 EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

Figure 6: Two Components with Partial Solubility - Peritectic

At the Peritectic point, the reaction L + β → α occurs

Figure 7: Two components with Partial Solubility- Peritectoid Reaction -

peritectoid reaction α + γ → β

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 EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

8.6 Derivation of Lever Rule

Consider a temperature-composition point in a phase diagram where two phases exist, as shown
in Figure 8 for temperature T1 and composition X2. The chemical composition of the two phases
are:
α - phase: X1 w/o B, 100 - X1 w/o A:
β - phase: X3 w/o B, 100 - X3 w/o A.

Figure 8: Phase Diagram for Derivation of Lever Rule

This may be seen by imagining that we start with pure A material and add increasing amounts of
B. When we reach the composition X1 w/o B, the α phase has become saturated and can accept
no additional amount of B. As we add still more B, the second β phase must appear. The β phase
will be saturated, and its composition will be X3 w/o B, as read from the right side of the diagram.

Now we would like to determine how much α phase and β phase material will exist at a temperature
T1, and the composition X2 w/o B. To arrive at this, we use the lever rule.

Let A1 be the amount of α phase present

A3 be the amount of β phase present
A2 be the total amount of material

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 EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

The balance of material requires that

A1 + A3 = A2

And the balance of the B material requires that

X1A1 + X3A3 = X2A2

Combining these two equations yields

A1 (X3 - X2)
A2 = (X3 - X1)

which is the fractional weight of the α phase, and

A3 (X2 _ X1)
A2 = (X3 _ X1)
which is the fractional weight of the β phase.

Example: A Cu-Ni alloy (Fig 9) contains 73% cu and 27% Ni at 1200C. Find (1) Weight % Cu
in solid and liquid phases and (2) % of alloy in solid % liquid phases.

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 EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

Solution:
Step 1: Draw horizontal tie line at 1200C
Step 2: Project down for Cu

Liquid, 𝑊L = 80%Cu

Solid, 𝑊S = 70%Cu

Step 3: For 𝑊0 = 27% Nickel, project up to tie line

𝑊S − 𝑊0 30% 𝑁𝑖 − 27%𝑁𝑖 3
𝑋L = = = = 0.3
𝑊S — 𝑊L 30%𝑁𝑖 − 20%𝑁𝑖 10

Is the fraction of alloy liquid.

𝑊0 − 𝑊L 27% 𝑁𝑖 − 20%𝑁𝑖 7
𝑋S = = = = 0.7
𝑊S — 𝑊L 30%𝑁𝑖 − 20%𝑁𝑖 10

Is fraction of the alloy that is solid

𝑋L + 𝑋S = 1 OK!

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 EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

Example: Consider the lead-tin (Pb-Sn) diagram shown in Figure 20. For an alloy consisting of
40 w/o Sn determine the composition and amount of the α and β phases present at 100o C. Also,
determine the relative amount of material formed from the eutectic reaction.

Figure 20: Lead-Tin Phase Diagram

Solution: At T1 (100o C), the composition of the α phase is

5 w/o Sn, 95 w/o Pb

The amount of the α phase present is

(A1/A2) = (99 - 40)/(99 - 5) = 62.8% by weight

The composition of the β phase is

99 w/o Sn, 1 w/o Pb

The amount of the β phase present is

(A3/A2) = (40 - 5)/(99 - 5) = 37.2 % by weight

To determine the amount of material that experienced the eutectic reaction, we note that just above
the eutectic temperature of 183o C, the remaining liquid had the eutectic composition 62 w/o Sn
and 38 w/o Pb. The amount of remaining liquid was (40 - 19)/(62 - 19) = 48.8%. On cooling
below the eutectic temperature, this liquid experienced the eutectic reaction l α + β. The
amount of eutectic material present at 100 C is 48.8 % (by weight). The amount of α and β phase
o

present in the eutectic material at 100o C is (99 - 62)/(99 - 5) = 39.4 % α, 60.6 % β.

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 EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

Example : Shown in Figure 21 is the phase diagram for the H2O - NaCl system. Knowing that
seawater is 3.5 w/o NaCl, determine:
(a) the amount of NaCl is 100 lbs of seawater,
(b) the relative amount of ice and brine at 10o F, and
(c) the composition of the ice and brine at 10o F.

Figure 21: Phase Diagram for Seawater

Solution: (a) the amount of NaCl in 100 lbs of seawater is

(100)(.035) = 3.5 lbs

(b) the amount of ice and brine at 10o F is

(16.5 - 3.5)/(16.5) = 78.8% ice
3.5/16.5 = 21.2% brine

(c) the compositions are

0% NaCl in ice and 16.5 % NaCl in the brine.

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8.7 Metal Processing

The manufacturing processes can alter crystal microstructure and thus influence the strength,
ductility, and impact toughness of a material. While a variety of finishing processes are suited to
specific applications, we will focus on casting, hot and cold rolling, extrusion, forging, and
drawing.

Casting:
Molten metals can be cast into large ingots in a direct chill casting unit. These large ingots are
typically rolled into sheets or plates, or else extruded into structural shapes like I-Beams. Metals
can also be cast into smaller, complex shapes using metal or sand molds.

Fig 10 Casting

Hot and Cold Rolling:

To make large sheets or plates, large ingots are often hot-rolled initially (to allow for a larger
thickness change). A schematic is shown below. The hot rolling process continues until the plate
reaches its desired thickness or until it cools such that rolling becomes impossible. Plates will be
reheated and rolled until they reach the desired thickness.

Fig. 11 Hot Rolling

After hot rolling, plates are usually cold rolled. Cold rolling hardens and strengthens the metal
by introducing internal stresses (dislocations) in the material microstructure. To re-soften the
material, remove internal stresses, and improve toughness, the material may be reheated in a
process called annealing.

The amount of cold working is often expressed as a percentage related to the change in a material’s
thickness, i.e.:

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 EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

tho–thf
%𝐶𝑊 = × 100%
tho

Where %CW indicates the percent cold work done on a material, tho and thf indicate the initial and
final thickness of the material, respectively.

Extrusion:
Extrusion is the forming of a material through plastic deformation by forcing it though a die under
high pressure. Cylindrical rods and hollow tubes of most metals are fabricated this way. This
process is often used to form a wide variety of cross sections for aluminum, copper, and their
alloys.

Fig 12 Extrusion

Forging:
Forging is the process of a material being pressed or formed into its desired shape, which can be
irregular (unlike rolling or extruding). The temperature of forging is important for the final
strength, hardness, and ductility of the material. Typically forged metals are tougher and more
durable than cast metals. Forging involves hot and cold working by nature, and can act to reduced
voids in the metal’s microstructure.

Fig 13 Forging

Drawing:
Drawing is a cold working process typically used to make wire, in which a metal rod (such as
copper) is pulled through a carbide nib. Deep drawing is a form of drawing in which a punch and
die are used to form cup-shaped objects from sheet metal. Aluminum cans are formed by deep
drawing.

Drawing is a form of cold work, and the percentage of cold work can be expressed in a 2-D analogy
to the equation for rolling:
A –A
%𝐶𝑊 = xc,o xc,f × 100%
Axc,o

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Where Axc,o and Axc,f indicate the initial and final cross-sectional areas of the metal rod,
respectively.

Fig 15 Drawing

8.8 Work Hardening and Annealing

When a material is plastically strained the yield stress is increased. In many of the manufacturing
techniques listed earlier, the material is plastically deformed in order to fabricate the desired shape.
As Figure 16 shows, although this may significantly increase the yield strength of the material, it
also makes it more brittle. This change is a result of the dislocation density, or imperfections in
the lattice, increasing as a result of the deformation.

Figure 16: Work Hardening

If we take a work-hardened material and subject it to a sufficiently high temperature for a specified
time, we can reduce the dislocation density and the yield stress will return to its initial value. This
is known as annealing the material, which is a form of heat treatment. Due to other aspects, such
as recrystallization, subjecting the work-hardened material to a high temperature for too long a

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time, may result in a material whose yield stress is less than it was before the material was work-
hardened.

8.9 Precipitation Hardening of Alloys

Consider the phase diagram below in Figure 17. Suppose we have material held at temperature T2
and then suddenly quench it to room temperature, Tr. There is insufficient time for the β phase to
form and we therefore have an unstable α phase existing at room temperature. Because it is
thermodynamically unstable, the β phase would eventually form at room temperature, although it
would take many years. If we now heat the material to a temperature T1, the β will begin to form.

As the β phase begins to form, it transforms the unstable α phase into a stable α phase forming a
matrix surrounding numerous extremely small β grains. After some time, the maximum hardness
is reached and the material can again be quenched to room temperature, thus yielding a
precipitation hardened material (Figure 18).

If the material is held at the treatment temperature for too long, the material will begin to soften.
The most important reason for this is that the numerous small β grains which produced the
maximum hardness continue to grow. Some of the β grains grow into larger β, obviously
decreasing the number of total β grains. Other factors including the strain-energy present and grain
boundary depletion are also at work here.

Figure 17: Phase Diagram of Mg-Sn System

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Figure 18: Alloy Hardness vs. Precipitation Time at Elevated Temperatures

8.10 Iron-Carbon Systems

Alloys of iron and carbon for the system for common steels and cast irons. Generally, steels have
a carbon content between 0 and 2 % by weight, while cast irons have a carbon content between 2
and 5 % by weight. The iron-carbon phase diagram is shown below in Figure 19.

The phases are defined as follows:

(a) Ferrite α phase: BCC crystalline phase
(b) Austenite γ: FCC crystalline phase
(c) δ phase: high temperature BCC crystalline phase
(d) Cementite or carbide Fe3C: mixture
(e) Pearlite: layered two phase mixture generated by transforming austenite to ferrite and
carbide by the eutectoid reaction γ → α + Fe3C

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(f) Martensite: a non-equilibrium phase in steels formed by rapid cooling of austenite. It

arises because, with extremely rapid cooling, there is insufficient time for the carbon atoms
to realign themselves from their locations within the FCC structure of austenite to their
locations in the BCC structure of ferrite. As a result, the structure is "trapped" between
FCC and BCC, and actually results in a body centered tetragonal lattice. Martensite is very
hard and brittle, and by itself, has very few practical applications. Its value lies in
combinations of ferrite and martensite obtainable by various heat treatments.
(g) Bainite: mixture of ferrite and cementite formed by isothermal transformation of
austenite under selected conditions.

Figure 19: Iron-Carbon Diagram

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Martensite

Coarse Pearlite Fine Pearlite

Upper Bainite Lower Bainite

Fig 20

8.11 Eutectoid Point for Iron-Carbon Systems

The eutectic and eutectoid points on the phase diagram are important for heat treatment. The
eutectoid point for Iron-Carbon is at a temperature of 727 C and 0.77% carbon by weight. See the
figure below (fig 21) for an enlarged view of this region of the phase diagram.

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• Eutectoid steel has exactly 0.77% carbon. When cooling below the eutectoid temperature
(727 C) it becomes 100% Pearlite. The resulting steel is high strength and wear resistant,
used for things like music wire or railway track.
• Hyper-Eutectoid has more than 0.77% carbon content. When cooling below the eutectoid
temperature, it forms a combination of cementite and pearlite. This is more brittle.
• Hypo-Eutectoid has less than 0.77% carbon content. When cooled below the eutectoid
temperature, it forms a combination of pearlite and ferrite. This is more ductile.
The lever rule, discussed earlier, can be used to determine the proportions.

Fig 21

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8.12 Time-Temperature-Transformation (TTT) Diagrams for Iron-Carbon Systems

Suppose we rapidly cool steel of eutectoid composition (0.8 % C) from the austenitic range down
to some temperature where α and Fe3C are the stable phases. If we could observe the resulting
transformation process, we would find that, initially, the material is 100% unstable austenite. After
some time, the ferrite and cementite would begin to form (either as pearlite or bainite) and after
sufficient time, the transformation of the austenite would be complete. This process is represented
by the so-called Time-Temperature- Transformation (TTT) diagram shown in Figure 22.

Notice that, if we suddenly cool the austenite to a temperature Ms (martensite start) or less, some
martensite will form. The remaining austenite will then transform to bainite. If we suddenly cool
the material to temperature Mf (martensite finish), we will have 100% martensite and no further
transformation will occur.

Figure 22: TTT Diagram for Steel

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8.12 Continuous-Cooling Transformation Diagram for Iron-Carbon Systems

The TTT diagram refers strictly to the cases where the steel is suddenly cooled from the austenite
(γ) range down to some new temperature. In most instances, the cooling is not instantaneous, but
rather occurs at a finite rate. As an approximation, the TTT diagram may also be used in this case,
but a more accurate Continuous-Cooling Transformation (CCT) Diagram should be used when
available. This diagram is shown in Figure 23 for a eutectoid steel.

Figure 23: Continuous Cooling Transformation for Steel

If we suddenly cool the material at such a rapid rate, that when plotted on the TTT diagram the
time vs temperature line does not intersect the γ + α + carbide phase (also known as "missing the
knee" but instead goes straight from the γstable through the γunstable region to a temperature Mf
(martensite finish), we will have martensite with no additional phase. When the cooling rate is
such that both the beginning and end curves are reached before reaching the martensitic zone, no
(or very little) martensite will form.

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 EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

8.13 Heat Treatment of Steel

By varying the amount of martensite present in the material, the mechanical properties of the steel
can be varied. Martensite alone is very hard and strong but also is extremely brittle. A compromise
between strength and ductility can be reached by varying the amount of martensite present in
equilibrium with ferrite and carbide.

Fig. 24 Heat Treatment of Eutectic Steel

The procedure for forming this mixed material is called "heat treatment." First, the material is
rapidly cooled (quenched) so as to form nearly 100% martensite. At room temperature, this non-
equilibrium phase is, for all practical purposes, stable and will not further transform to ferrite and
carbide. If we then take the 100% martensite and heat it to an elevated temperature (say 700o C)
and hold it there for some length of time (tempering), the martensite can be softened (stress
relieved) and decompose. The changes that occur are temperature dependent and range from
merely a reduction in strain-energy and dislocation density to microstructural changes within the
martensite itself. The end result is a material that retains some of the high strength and hardness
characteristics of pure martensite while restoring some ductility.

As shown in Figure 25, the hardness obtainable with martensite is dependent on the carbon content.
Since it is the "trapping" of the carbon atoms which forms martensite, and thus hardens the metal,
pure iron can not be hardened by quenching

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 EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

Figure 25: Hardness

8.14 Specifics of Quenching Process for Iron-Carbon Systems

The rate of cooling affects the resulting grain structure and thus the hardness, ductility, and strength
of the steel specimen. Quenching is typically achieved in one of the following mediums. From
fastest to slowest, steel can be quenched in brine (fastest), water (second fastest), air, or molten
salt (slowest). We often use Time-Temperature-Transformation curves (TTT Curves), described
earlier, to predict a steel’s properties after heat treatment. The following discussion uses figure
26, below.

The first step in any heat treatment process is to heat the material above 727.⁰C. This step is called
austenitizing.

The dark blue curved lines on the diagram indicate the start and end of the transformation from
austenite to pearlite. Each of the lines labeled (a)—(g) indicate idealized cooling curves (remember
that the rate of cooling affects the transformation rate):

Curve (a) shows a steel sample rapidly quenched in brine or water. The idealized cooling
curve never approaches the transformation to pearlite, and therefore all austenite (ϒ) is
converted to Martensite. Martensite is an extremely hard and strong crystal structure with

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 EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

a very low ductility. It is rarely used structurally without tempering. The temperature at
which austenite will transform to martensite is around 250⁰C

Curve (b) shows a steel that is heated to 727 ⁰C, hot quenched (in molten salt) to ~695 ⁰C,
and held at that temperature for over 2 hours to form 100% coarse pearlite. 100% coarse
pearlite is very ductile as hot quenching removes strain-induced stresses, but it is also soft
and weak.

Curve (c) shows a steel that is heated to 727 ⁰C, hot quenched to 610 ⁰C, and held at that
temperature for over 3 minutes to form 100% fine pearlite. 100% fine pearlite has a
uniform structure and higher strength than coarse pearlite. The cutoff between coarse and
fine pearlite varies but is around 650 ⁰C.

Curve (d) shows a steel that is heated to 727 ⁰C, hot quenched to 580 ⁰C, held for ~5
seconds then water quenched to form a structure of 50% fine pearlite and 50%
martensite.

Curve (e) shows a steel that is heated to 727 ⁰C, quenched to ~475⁰C, and held for over 3
minutes to form upper bainite. Bainite is an intermediate structure between pearlite and
martensite. It forms between 250 - 550 ⁰C. Upper Bainite is distinguished by its coarser
cementite particles. IT forms between 330 – 550 ⁰C.

Curve (f) shows a steel that is heated to 727 ⁰C, quenched to ~295⁰C, and held for over 2
hours to form lower bainite. Lower bainite is characterized by its finer cementite
particles.

Curve (g) shows a steel that is heated to 727 ⁰C, cooled to ~300 ⁰C, and held for 30 minutes
then rapid quenched (in water) to form 50% lower bainite and 50% martensite.

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 EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

Fig 26: TTT Diagram of Eutectoid Steel

8.15 Case Hardening of Steel

Case Hardening is when metal is placed inside a carbon-rich environment. It is heated to very
high temperatures to introduce carbon to the outside of the structure. This results in a hard outer
surface, but a tough and strong inner surface. The process involves diffusion, following Fick’s
Second Law.

Fick’s second law tells us that when case hardening steel, the carbon concentration in air at the
surface, Cs, the carbon concentration at some distance x into the surface, Cx, and the initial carbon
concentration of the steel, Co, can be related according to:

𝐶s − 𝐶x 𝑥
= 𝑒𝑟𝑓 ( )
𝐶s − 𝐶o 2√𝐷𝑡
where D is the diffusivity constant in m2/s and t is time.

A plot of the error function (erf) is shown in the Figure 27 below. Typically, this is tabulated for
increasing values of z as shown in the following table.

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EN380 Naval Materials Science and Engineering Course Notes, U.S. Naval Academy

Figure 27: erf(x) vs. x

Tabulated values for erf(z)

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STAINLESS
- stainless steels and their properties

by
Béla Leffler

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Table of Contents
Introduction ...................................................................................................................................................... 3
Use of stainless steel ............................................................................................................................. 3
How it all started ................................................................................................................................... 4
Stainless steel categories and grades ................................................................................................................. 5
The effects of the alloying elements ...................................................................................................... 5
Corrosion and corrosion properties ................................................................................................................... 9
PASSIVITY.................................................................................................................................................................................9
AQUEOUS CORROSION........................................................................................................................................................ 10
General corrosion ..........................................................................................................................10
Pitting and crevice corrosion .........................................................................................................11
Stress corrosion cracking ...............................................................................................................14
Intergranular corrosion ..................................................................................................................16
Galvanic corrosion ........................................................................................................................18
HIGH TEMPERATURE CORROSION ................................................................................................................................... 19
Oxidation .......................................................................................................................................19
Sulphur attack (Sulphidation) .......................................................................................................20
Carbon pick-up (Carburization).....................................................................................................21
Nitrogen pick-up (Nitridation) ......................................................................................................21
Mechanical properties ......................................................................................................................................23
Room temperature properties ...............................................................................................................23
The effect of cold work ........................................................................................................................27
Toughness ............................................................................................................................................27
Fatigue properties .................................................................................................................................29
High temperature mechanical properties ..............................................................................................30
Precipitation and embrittlement .......................................................................................................................32
475°C embrittlement ............................................................................................................................32
Carbide and nitride precipitation ..........................................................................................................32
Intermetallic phases ..............................................................................................................................32
Physical properties ...........................................................................................................................................34
Property relationships for stainless steels.........................................................................................................36
Martensitic and martensitic-austenitic steels ........................................................................................36
Ferritic steels ........................................................................................................................................36
Ferritic-Austenitic (Duplex) steels .......................................................................................................37
Austenitic steels ...................................................................................................................................37
References .......................................................................................................................................................38
Attachment: US, British and European standards on stainless steels ........................................41

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Introduction

Iron and the most common iron alloy, steel, are from a corrosion viewpoint relatively poor materials since they
rust in air, corrode in acids and scale in furnace atmospheres. In spite of this there is a group of iron-base alloys,
the iron-chromium-nickel alloys known as stainless steels, which do not rust in sea water, are resistant to
concentrated acids and which do not scale at temperatures up to 1100°C.
It is this largely unique universal usefulness, in combination with good mechanical properties and manufacturing
characteristics, which gives the stainless steels their raison d'être and makes them an indispensable tool for the
designer. The usage of stainless steel is small compared with that of carbon steels but exhibits a steady growth, in
contrast to the constructional steels. Stainless steels as a group is perhaps more heterogeneous than the
constructional steels, and their properties are in many cases relatively unfamiliar to the designer. In some ways
stainless steels are an unexplored world but to take advantage of these materials will require an increased
understanding of their basic properties.
The following chapters aim to give an overall picture of the "stainless world" and what it can offer.

Use of stainless steel

Steel is unquestionably the dominating industrial constructional material.

160

140

120

100
109£ 80

60

40

20

0
Steel Stainless Cast iron Aluminium Copper Polym

Figure 1. World consumption of various materials in the middle of the 1980's.

The annual world production of steel is approximately 400 million, and of this about 2% is stainless.
The use and production of stainless steels are completely dominated by the industrialised Western nations and
Japan. While the use of steel has generally stagnated after 1975, the demand for stainless steels still increases by 3-
5% per annum.

Figure 2. Steel production in western Europe 1950-1994.

The dominant product form for stainless steels is cold rolled sheet. The other products individually form only a
third or less of the total amount of cold rolled sheet.

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Usage is dominated by a few major areas: consumer products, equipment for the oil & gas industry, the chemical
process industry and the food and beverage industry. Table 1 shows how the use of stainless steel is divided
between the various applications.

Table 1. Use of stainless steel in the industrialised world, divided into various product forms and application
categories.
PRODUCT FORMS APPLICATION CATEGORIES
Cold rolled sheet 60 % Consumer items 26 %
Bar and wire 20 % Washing machines and dishwashers 8%
Hot rolled plate 10 % Pans, cutlery, etc. 9%
Tube 6% Sinks and kitchen equipment 4%
Castings and other 4% Other 5%
Industrial equipment 74 %
Food industry and breweries 25 %
Chemical, oil and gas industry 20 %
Transport 8%
Energy production 7%
Pulp and paper, textile industry 6%
Building and general construction 5%
Other 5%

The most widely used stainless grades are the austenitic 18/9 type steels, i.e. AISI 304 * and 304L, which form
more than 50% of the global production of stainless steel. The next most widely used grades are the ferritic steels
such as AISI 410, followed by the molybdenum-alloyed austenitic steels AISI 316/316L. Together these grades
make up over 80% of the total tonnage of stainless steels.
*American standard (AISI) designations are normally used throughout this article to identify grades. If a certain grade does
not have a standard designation, a trade name, e.g. ‘2205’, is used. See Attachment 1 for chemical compositions.

How it all started

In order to obtain a perspective of the development of stainless steels, it is appropriate to look back to the
beginning of the century; stainless steels are actually no older than that. Around 1910 work on materials problems
was in progress in several places around the world and would lead to the discovery and development of the
stainless steels.
In Sheffield, England, H. Brearly was trying to develop a new material for barrels for heavy guns that would be
more resistant to abrasive wear. Chromium was among the alloying elements investigated and he noted that
materials with high chromium contents would not take an etch. This discovery lead to the patent for a steel with 9-
16% chromium and less than 0.70% carbon; the first stainless steel had been born.
The first application for these stainless steels was stainless cutlery, in which the previously used carbon steel was
replaced by the new stainless.
At roughly the same time B. Strauss was working in Essen, Germany, to find a suitable material for protective
tubing for thermocouples and pyrometers. Among the iron-base alloys investigated were iron-chromium-nickel
alloys with high chromium contents. It was found that specimens of alloys with more than 20% Cr did not rust
even after having been left lying in the laboratory for quite some time. This discovery lead to the development of a
steel with 0.25% carbon, 20% chromium and 7% nickel; this was the first austenitic stainless steel.
Parallel with the work in England and Germany, F.M. Becket was working in Niagara Falls, USA, to find a cheap
and scaling-resistant material for troughs for pusher type furnaces that were run at temperatures up to 1200°C. He
found that at least 20% chromium was necessary to achieve resistance to oxidation or scaling. This was the
starting point of the development of heat-resistant steels.
However, it was not until after the end of World War II that the development in process metallurgy lead to the
growth and widespread use of the modern stainless steels.

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Stainless steel categories and grades

Over the years since the start of the development of stainless steels the number of grades has increased rapidly.
The table in Attachment 1 shows the stainless steels that are standardised in the US and Europe. The table clearly
shows that there are a large number of stainless steels with widely varying compositions. At least at some time all
of these grades have been sufficiently attractive to merit the trouble of standardisation. In view of this 'jungle' of
different steels grades, a broader overview may be helpful.
Since the structure has a decisive effect on properties, stainless steels have traditionally been divided into
categories depending on their structure at room temperature. This gives a rough division in terms of both
composition and properties.
Stainless steels can thus be divided into six groups: martensitic, martensitic-austenitic, ferritic, ferritic-austenitic,
austenitic and precipitation hardening steels. The names of the first five refer to the dominant components of the
microstructure in the different steels. The name of the last group refers to the fact that these steels are hardened by
a special mechanism involving the formation of precipitates within the microstructure. Table 2 gives a summary of
the compositions within these different categories.

Table 2. Composition ranges for different stainless steel categories.

Steel category Composition (wt%) Hardenable Ferro-

magnetism

C Cr Ni Mo Others
Martensitic ›0.10 11-14 0-1 - V Hardenable Magnetic
›0.17 16-18 0-2 0-2
Martensitic- ‹0.10 12-18 4-6 1-2 Hardenable Magnetic
austenitic
Precipitation 15-17 7-8 0-2 Al, Hardenable Magnetic
hardening 12-17 4-8 0-2 Al,Cu,Ti,Nb
Ferritic ‹0.08 12-19 0-5 ‹5 Ti Not Magnetic
‹0.25 24-28 - - hardenable
Ferritic-austenitic ‹0.05 18-27 4-7 1-4 N, W Not Magnetic
(duplex) hardenable
Austenitic ‹0.08 16-30 8-35 0-7 N,Cu,Ti,Nb Not Non-
hardenable magnetic

The two first categories, martensitic and martensitic-austenitic stainless steels are hardenable, which means that it
is possible to modify their properties via heat treatment in the same way as for hardenable carbon steels. The
martensitic-austenitic steels are sometimes also referred to as ferritic-martensitic steels. The third category, the
precipitation hardening steels, may also be hardened by heat treatment. The procedures used for these steels are
special heat treatment or thermo-mechanical treatment sequences including a final precipitation hardening and
ageing step. The precipitation hardening steels are sometimes also referred to as maraging steels. The last three
steel categories, ferritic, ferritic-austenitic and austenitic are not hardenable, but are basically used in the as-
received condition. The ferritic-austenitic stainless steels are often referred to as duplex stainless steels. It may be
noted that there is only one category of stainless steels that is non-magnetic: the austenitic steels. All the others
are magnetic.

The effects of the alloying elements

The alloying elements each have a specific effect on the properties of the steel. It is the combined effect of all the
alloying elements and, to some extent, the impurities that determine the property profile of a certain steel grade. In
order to understand why different grades have different compositions a brief overview of the alloying elements and
their effects on the structure and properties may be helpful. The effects of the alloying elements on some of the
important materials properties will be discussed in more detail in the subsequent sections. It should also be noted
that the effect of the alloying elements differs in some aspects between the hardenable and the non-hardenable
stainless steels.

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Chromium (Cr)
This is the most important alloying element in stainless steels. It is this element that gives the stainless steels their
basic corrosion resistance. The corrosion resistance increases with increasing chromium content. It also increases
the resistance to oxidation at high temperatures. Chromium promotes a ferritic structure.
Nickel (Ni)
The main reason for the nickel addition is to promote an austenitic structure. Nickel generally increases ductility
and toughness. It also reduces the corrosion rate and is thus advantageous in acid environments. In precipitation
hardening steels nickel is also used to form the intermetallic compounds that are used to increase the strength.
Molybdenum (Mo)
Molybdenum substantially increases the resistance to both general and localised corrosion. It increases the
mechanical strength somewhat and strongly promotes a ferritic structure. Molybdenum also promotes the
formation secondary phases in ferritic, ferritic-austenitic and austenitic steels. In martensitic steels it will increase
the hardness at higher tempering temperatures due to its effect on the carbide precipitation.
Copper (Cu)
Copper enhances the corrosion resistance in certain acids and promotes an austenitic structure. In precipitation
hardening steels copper is used to form the intermetallic compounds that are used to increase the strength.
Manganese (Mn)
Manganese is generally used in stainless steels in order to improve hot ductility. Its effect on the ferrite/austenite
balance varies with temperature: at low temperature manganese is a austenite stabiliser but at high temperatures it
will stabilise ferrite. Manganese increases the solubility of nitrogen and is used to obtain high nitrogen contents in
austenitic steels.
Silicon (Si)
Silicon increases the resistance to oxidation, both at high temperatures and in strongly oxidising solutions at lower
temperatures. It promotes a ferritic structure.
Carbon (C)
Carbon is a strong austenite former and strongly promotes an austenitic structure. It also substantially increases
the mechanical strength. Carbon reduces the resistance to intergranular corrosion. In ferritic stainless steels carbon
will strongly reduce both toughness and corrosion resistance. In the martensitic and martensitic-austenitic steels
carbon increases hardness and strength. In the martensitic steels an increase in hardness and strength is generally
accompanied by a decrease in toughness and in this way carbon reduces the toughness of these steels.
Nitrogen (N)
Nitrogen is a very strong austenite former and strongly promotes an austenitic structure. It also substantially
increases the mechanical strength. Nitrogen increases the resistance to localised corrosion, especially in
combination with molybdenum. In ferritic stainless steels nitrogen will strongly reduce toughness and corrosion
resistance. In the martensitic and martensitic-austenitic steels nitrogen increases both hardness and strength but
reduces the toughness.
Titanium (Ti)
Titanium is a strong ferrite former and a strong carbide former, thus lowering the effective carbon content and
promoting a ferritic structure in two ways. In austenitic steels it is added to increase the resistance to intergranular
corrosion but it also increases the mechanical properties at high temperatures. In ferritic stainless steels titanium is
added to improve toughness and corrosion resistance by lowering the amount of interstitials in solid solution. In
martensitic steels titanium lowers the martensite hardness and increases the tempering resistance. In precipitation
hardening steels titanium is used to form the intermetallic compounds that are used to increase the strength.
Niobium (Nb)
Niobium is both a strong ferrite and carbide former. As titanium it promotes a ferritic structure. In austenitic steels
it is added to improve the resistance to intergranular corrosion but it also enhances mechanical properties at high
temperatures. In martensitic steels niobium lowers the hardness and increases the tempering resistance. In U.S. it
is also referred to as Columbium (Cb).

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Aluminium (Al)
Aluminium improves oxidation resistance, if added in substantial amounts. It is used in certain heat resistant alloys
for this purpose. In precipitation hardening steels aluminium is used to form the intermetallic compounds that
increase the strength in the aged condition.
Cobalt (Co)
Cobalt only used as an alloying element in martensitic steels where it increases the hardness and tempering
resistance, especially at higher temperatures.
Vanadium (V)
Vanadium increases the hardness of martensitic steels due to its effect on the type of carbide present. It also
increases tempering resistance. Vanadium stabilises ferrite and will, at high contents, promote ferrite in the
structure. It is only used in hardenable stainless steels.
Sulphur (S)
Sulphur is added to certain stainless steels, the free-machining grades, in order to increase the machinability. At
the levels present in these grades sulphur will substantially reduce corrosion resistance, ductility and fabrication
properties, such as weldability and formability.
Cerium (Ce)
Cerium is one of the rare earth metals (REM) and is added in small amounts to certain heat resistant temperature
steels and alloys in order to increase the resistance to oxidation and high temperature corrosion.

The effect of the alloying elements on the structure of stainless steels is summarised in the Schaeffler-Delong
diagram (Figure 3). The diagram is based on the fact that the alloying elements can be divided into ferrite-
stabilisers and austenite-stabilisers. This means that they favour the formation of either ferrite or austenite in the
structure. If the austenite-stabilisers ability to promote the formation of austenite is related to that for nickel, and
the ferrite-stabilisers likewise compared to chromium, it becomes possible to calculate the total ferrite and
austenite stabilising effect of the alloying elements in the steel. This gives the so-called chromium and nickel
equivalents in the Schaeffler-Delong diagram:
Chromium equivalent = %Cr + 1.5 x %Si + %Mo
Nickel equivalent = %Ni + 30 x (%C + %N) + 0.5 x (%Mn + %Cu + %Co)
In this way it is possible to take the combined effect of alloying elements into consideration. The Schaeffler -
Delong diagram was originally developed for weld metal, i.e. it describes the structure after melting and rapid
cooling but the diagram has been found to give a useful picture of the effect of the alloying elements also for
wrought and heat treated material. However, in practice, wrought or heat treated steels with ferrite contents in the
range 0-5% according to the diagram contain smaller amounts of ferrite than that predicted by the diagram.
It should also be mentioned here that the Schaeffler-Delong diagram is not the only diagram for assessment of
ferrite contents and structure of stainless steels. Several different diagrams have been published, all with slightly
different equivalents, phase limits or general layout. The effect of some alloying elements has also been the subject
of considerable discussion. For example, the austenite-stabilising effect of manganese has later been considered
smaller than that predicted in the Schaeffler-Delong diagram. Its effect is also dependent on temperature.

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N i-e q u iv a l e n t = % N i+30 ( % C + % N ) +0 .5 ( % M n + % C u + % C o )
26
”904L”
Austenitc
24
5%F
Ferritic - A u s t e n itic 310S
22
Ferritic
10% F
20 A
M a rtensitic
316LN 0 % ferrite in wrought,
anneled mat erial
18
M a rte n s itic-A u s tenitic 317 L
16
304LN
3 1 6 H i gh Mo
20% F
14 31 6 L o w M o
”2507”
304
12 40% F

60% F
10 ”2205”
”2304”
A+M
M A+F
8
420L
80 % F
6 410
18-2FM F 100% F
4 M+ F
430
2 405
444
0
12 14 16 18 20 22 24 26 28 30
C r- e q u i v a l e n t = % C r + 1 , 5 % S i+% M o

Figure 3. The Schaeffler-Delong diagram.(1)

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Corrosion and corrosion properties

The single most important property of stainless steels, and the reason for their existence and widespread use, is
their corrosion resistance. Before looking at the properties of the various stainless steels, a short introduction to
corrosion phenomena is appropriate. In spite of their image, stainless steels can suffer both "rusting" and corrosion
if they are used incorrectly.

PASSIVITY
The reason for the good corrosion resistance of stainless steels is that they form a very thin, invisible surface film
in oxidising environments. This film is an oxide that protects the steel from attack in an aggressive environment.
As chromium is added to a steel, a rapid reduction in corrosion rate is observed to around 10% because of the
formation of this protective layer or passive film. In order to obtain a compact and continuous passive film, a
chromium content of at least 11% is required. Passivity increases fairly rapidly with increasing chromium content
up to about 17% chromium. This is the reason why many stainless steels contain 17-18% chromium.
Corrosion rate, mm/year

0.25

0.2

0.15

0.1

0.05

0
0 2 4 6 8 10 12 14

% Chromium

Figure 4. The effect of chromium content on passivity (2).

The most important alloying element is therefore chromium, but a number of other elements such as molybdenum,
nickel and nitrogen also contribute to the corrosion resistance of stainless steels. Other alloying elements may
contribute to corrosion resistance in particular environments - for example copper in sulphuric acid or silicon,
cerium and aluminium in high temperature corrosion in some gases.
A stainless steel must be oxidised in order to form a passive film; the more aggressive the environment the more
oxidising agents are required. The maintenance of passivity consumes oxidising species at the metal surface, so a
continuous supply of oxidising agent to the surface is required. Stainless steels have such a strong tendency to
passivate that only very small amounts of oxidising species are required for passivation. Even such weakly oxidising
environments as air and water are sufficient to passivate stainless steels. The passive film also has the advantage,
compared to for example a paint layer, that it is self-healing. Chemical or mechanical damage to the passive film can
heal or repassivate in oxidising environments. It is worth noting that stainless steels are most suitable for use in
oxidising neutral or weakly reducing environments. They are not particularly suitable for strongly reducing
environments such as hydrochloric acid.
Corrosion can be roughly divided into aqueous corrosion and high temperature corrosion:
• Aqueous corrosion refers to corrosion in liquids or moist environments at temperatures up to 300 oC, usually in
water-based environments.
• High temperature corrosion denotes corrosion in hot gases at temperatures up to 1300 oC.
The following sections contain a brief description of the various forms of aqueous and high temperature corrosion,
the factors which affect the risk for attack and the effect of steel composition on corrosion resistance.

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AQUEOUS CORROSION

The term aqueous corrosion refers to corrosion in liquids or moist gases at relatively low temperatures, less than
300 oC. The corrosion process is electrochemical and requires the presence of an electrolyte in the form of a liquid
or a moisture film. The most common liquids are of course water-based solutions.

General corrosion
This type of corrosion is characterised by a more or less even loss of material from the whole surface or relatively
large parts of it. This is similar to the rusting of carbon steels.
General corrosion occurs if the steel does not have sufficiently high levels of the elements which stabilise the
passive film. The surrounding environment is then too aggressive for the steel. The passive film breaks down over
the whole surface and exposes the steel surface to attack from the environment.
General corrosion of stainless steels normally only occurs in acids and hot caustic solutions and corrosion
resistance usually increases with increasing levels of chromium, nickel and molybdenum. There are, however,
some important exceptions to this generalisation. In strongly oxidising environments such as hot concentrated
nitric acid or chromic acid, molybdenum is an undesirable alloying addition.
The aggressivity of an environment normally increases with increasing temperature, while the effect of
concentration is variable. A concentrated acid may be less aggressive than a more dilute solution of the same acid.
A material is generally considered resistant to general corrosion in a specific environment if the corrosion rate is
below 0.1 mm/year. The effect of temperature and concentration on corrosion in a specific environment is usually
presented as isocorrosion diagrams, such as that shown in Figure 5. In this context it is, however, important to
note that impurities can have a marked effect on the aggressivity of the environment (see Figure 7).

Figure 5.Isocorrosion diagram for pure sulphuric acid, 0.1 mm/year (3).

From the isocorrosion diagram in Figure 5 it is apparent that the aggressivity of sulphuric acid increases with
increasing temperature, also that the aggressivity is highest for concentrations in the range 40-70%. Concentrated
sulphuric acid is thus less aggressive than more dilute solutions. The grade ‘904L’, with the composition 20Cr -
25Ni-4.5Mo-1.5Cu, exhibits good corrosion resistance even in the intermediate concentration range. This steel
was specifically developed for use in sulphuric acid environments.
The effect of the alloying elements may be demonstrated more clearly in another way. In Figure 6 the limiting
concentrations in sulphuric acid, i.e. the highest concentration that a specific steel grade will withstand without
losing passivity, are shown for various stainless steels. The beneficial effect of high levels of chromium, nickel and
molybdenum is apparent, as is the effect of copper in this environment.

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Limiting concentration (mol/l H2SO4, 25 oC)

Steel Composition (%) 0,0001 0,001 0,01 0,1 1 10 100

Grade Cr Ni Mo

410S 13 - -
440C 17 - -
304 18 9 -
316 17 12 2,7
329 25 5 1,5
‘2205’ 22 6 3
‘254 SMO’ 20 18 6,2
‘904L’ 20 25 4,5+Cu

Figure 6. Limiting concentrations for passivity in sulphuric acid for various stainless steels.

The aggressivity of any environment may be changed appreciably by the presence of impurities. The impurities
may change the environment towards more aggressive or towards more benevolent conditions depending on the
type of impurities or contaminants that are present. This is illustrated in Figure 7 where the effect of two different
contaminants, chlorides and iron, on the isocorrosion diagram of 316L(hMo) in sulphuric acid is shown. As can be
clearly seen from the diagram, even small amounts of another species may be enough to radically change the
environment. In practice there is always some impurities or trace compounds in most industrial environments.
Since much of the data in corrosion tables is be based on tests in pure, uncontaminated chemical and solutions, it
is most important that due consideration is taken of any impurities when the material of construction for a certain
equipment is considered.

Figure 7. The effect of impurities on the corrosion resistance of 316L (2.5% Mo min.) in sulphuric acid.

Pitting and crevice corrosion

Like all metals and alloys that relay on a passive film for corrosion resistance, stainless steels are susceptible to
localised corrosion. The protective passive film is never completely perfect but always contains microscopic
defects, which usually do not affect the corrosion resistance. However, if there are halogenides such as chlorides
present in the environment, these can break down the passive film locally and prevent the reformation of a new
film. This leads to localised corrosion, i.e. pitting or crevice corrosion. Both these types of corrosion usually occur
in chloride-containing aqueous solutions such as sea water, but can also take place in environments containing
other halogenides.
Pitting is characterised by more or less local points of attack with considerable depth and normally occurs on free
surfaces. Crevice corrosion occurs in narrow, solution-containing crevices in which the passive film is more readily
weakened and destroyed. This may be under washers, flanges, deposits or fouling on the steel surface. Both forms
of corrosion occur in neutral environments, although the risk for attack increases in acidic solutions.

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Figure 8. Pitting on a tube of AISI 304 used in brackish water.

Figure 9.Crevice corrosion under a rubber washer in a flat heat exchanger used in brackish water.

Chromium, molybdenum and nitrogen are the alloying elements that increase the resistance of stainless steels to
both pitting and crevice corrosion. Resistance to localised corrosion in sea water requires 6% molybdenum or
more.
One way of combining the effect of alloying elements is via the so-called Pitting Resistance Equivalent (PRE)
which takes into account the different effects of chromium, molybdenum and nitrogen. There are several equations
for the Pitting Resistance Equivalent, all with slightly different coefficients for molybdenum and nitrogen. One of
the most commonly used formula is the following:
PRE = %Cr + 3.3 x %Mo + 16 x %N
This formula is almost always used for the duplex steels but it is also sometimes applied to austenitic steels.
However, for the latter category the value of the coefficient for nitrogen is also often set to 30, while the other
coefficients are unchanged. This gives the following formula:
PRE = %Cr + 3.3 x %Mo + 30 x %N
The difference between the formulas is generally small but the higher coefficient for nitrogen will give a difference
in the PRE-value for the nitrogen alloyed grades.

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Table 3. Typical PRE-values for various stainless steels

Grade 304L 316L ‘SAF 317L ‘2205’ ‘904L’ ‘SAF ‘254 ‘654
2304’ 2507’ SMO’ SMO’
PRE16xN 19 26 26 30 35 36 43 43 56
PRE30xN 20 26 30 37 46 63

The effect of composition can be illustrated by plotting the critical pitting temperature (CPT) in a specific
environment against the PRE-values for a number of steel grade, see Figure 10. The CPT values are the lowest
temperatures at which pitting corrosion attack occurred during testing.

Figure 10. Critical pitting temperature (CPT) in 1 M NaCl as a function of PRE values.
Since the basic corrosion mechanism is the same for both pitting and crevice corrosion, the same elements are
beneficial in combatting both types of corrosion attack. Due to this there is often a direct correlation between the
CPT- and CCT-values for a certain steel grade. Crevice corrosion is the more severe of the two types of corrosion
attack and the CCT-values are lower than the CPT-values for any stainless steel grade. This is illustrated in Figure
11 where the critical pitting temperature (CPT) and the critical crevice corrosion temperature (CCT) in 6% FeCl3
has been plotted against the PRE-values for a number of stainless steels. Again the CPT and CCT values are the
lowest temperatures at which corrosion attack occurred.

Figure 11. Critical pitting temperature (CPT) and critical crevice corrosion temperature (CCT) in ferric chloride
for various stainless steels.

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As can be seen from the diagrams in Figures 10 and 11 there is a relatively good correlation between the PRE-
values and the CPT and CCT. Consequently the PRE-value can be used to group steel grades and alloys into
rough groups of materials with similar resistance to localised corrosion attack, in steps of 10 units in PRE-value or
so. However, it can not be used to compare or separate steel grades or alloys with almost similar PRE values.
Finally, it must be emphasised that all diagrams of this type show comparisons between steel grades and are only
valid for a given test environment. Note that the steel grades have different CPT’s in NaCl (Figure 10) and FeCl3
(Figure 11). The temperatures in the diagrams cannot therefore be applied to other environments, unless there
exists practical experience that shows the relation between the actual service conditions and the testing conditions.
The relative ranking of localised corrosion resistance is, however, often the same even in other environments. The
closer the test environment is to the “natural” environment, i.e. the closer the test environment simulates the
principal factors of the service environment, the more can the data generated in it be relied on when judging the
suitability of a certain steel grade for a specific service environment. A test in sodium chloride is consequently
better than a test in ferric chloride for judging whether or not a certain grade is suitable for one of the neutral pH,
chloride containing water solutions which are common in many industries.
In order to obtain a good resistance to both pitting and crevice corrosion, it is necessary to choose a highly alloyed
stainless steel with a sufficiently high molybdenum content. However, choosing the appropriate steel grades is not
the only way to minimise the risk for localised corrosion attack. The risk for these types of corrosion attack can be
reduced at the design stage by avoiding stagnant conditions and narrow crevices. The designer can thus minimise
the risk for pitting and crevice corrosion both by choosing the correct steel grade and by appropriate design of the
equipment.

Stress corrosion cracking

This type of corrosion is characterised by the cracking of materials that are subject to both a tensile stress and a
corrosive environment. The environments which most frequently causes stress corrosion cracking in stainless
steels are aqueous solutions containing chlorides. Apart from the presence of chlorides and tensile stresses, an
elevated temperature (>60°C) is normally required for stress corrosion to occur in stainless steels. Temperature is
a very important parameter in the stress corrosion cracking behaviour of stainless steel and cracking is rarely
observed at temperatures below 60 oC. However, chloride-containing solutions are not the only environments that
can cause stress corrosion cracking of stainless steels. Solutions of other halogenides may also cause cracking and
caustic solutions such as sodium and potassium hydroxides can cause stress corrosion cracking at temperatures
above the boiling point. Sensitised 18-8 stainless steels are also susceptible to intergranular stress corrosion
cracking in the steam and water environments in boiling water reactors if the stress level is sufficiently high.
Cracking may also occur in high strength stainless steels, such as martensitic or precipitation hardening steels.
This type of cracking is almost always due to hydrogen embrittlement and can occur in both environments
containing sulphides and environments free of sulphides.

Figure 12. Stress corrosion cracking adjacent to a weld in a stainless pipe exposed to a chloride-containing
environment at 100°C.

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The risk for stress corrosion cracking is strongly affected by both the nickel content and the microstructure. The
effect of nickel content is apparent from Figure 13. Both high and low nickel contents give a better resistance to
stress corrosion cracking. In the case of the low nickel contents this is due to the structure being either ferritic or
ferritic-austenitic. The ferrite phase in stainless steels with a low nickel content is very resistant to stress corrosion
cracking.
For high strength steels the main factor affecting the resistance to hydrogen embrittlement is the strength. The
susceptible to hydrogen embrittlement will increase with increasing strength of the steel.

Time to failure (h)

1000

100

10

0 10 20 30 40 50
Nickel content (%)
Figure 13. Stress corrosion cracking susceptibility in boiling MgCl 2 as a function of
nickel content (4).
In applications in which there is a considerable danger of stress corrosion cracking, steels that either has a low or a
high nickel content should be selected. The choice could be either a ferritic or ferritic-austenitic steel or a high-
alloyed austenitic steel or nickel-base alloy. Although about 40% nickel is necessary to achieve immunity to
chloride-induced stress corrosion cracking, the 20-30% nickel in steel grades such as ‘654 SMO’, ‘254 SMO’,
‘904L’ and ‘A 28 (commonly known by the Sandvik tradename SANICRO 28). is often sufficient in practice.

Tim e t o failure (h )
0 100 200 300 400

"904 L"

"254 S MO"

AISI 3 1 6 L (hMo)

AISI 3 0 4 L

"220 5"

Figure 14. Comparison of stress corrosion cracking resistance of some austenitic stainless steels. Drop-
evaporation method testing with loading to 0.9 x Rp0.2.

In this context it should, however, be noted that nickel content is not the only factor that governs resistance to
stress corrosion cracking: the entire composition of the alloy is important. Molybdenum has been found to have a
considerable effect on resistance to stress corrosion cracking. However, more than 4% molybdenum is required to
obtain a significant effect, as is apparent from a comparison of ‘904L’ and ‘254 SMO’ in Figure 14. Selecting a
stainless steel for service in an environment that can cause stress corrosion cracking cannot just be done on the
basis of nickel content.

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Stress corrosion cracking can only occur in the presence of tensile stresses. The stress to which a stainless steel
may be subjected without cracking is different for different steel grades. An example of the threshold stresses for
different steel grades under severe evaporative conditions is given in Figure 15.
100 in % of Rp0.2 at 200 deg. C actual threshold stress in MPa 350

300
80
250

60
200
% MPa
150
40

100
20
50

0 0

316(hMo) 'SAF2304' '2205' '904L' 'SAF2507' '254SMO' '654SMO'

Figure 15. Threshold stresses for chloride stress corrosion cracking under severe evaporative conditions. Drop
evaporation test.
For ‘654 SMO’ 100% of Rp0.2 was the highest stress level tested. The threshold stress is above that
level in this test.

As can be seen in the diagram in Figure 15 high alloy austenitic stainless steels have a very high resistance to
chloride stress corrosion cracking in contrast to the lower alloyed grades of this category.
In this type of diagram the threshold stress level is often given as a percentage of the yield strength at a certain
temperature, here 200 oC, which is related to the testing temperature. Due to the varying strengths of the different
steel grades the actual maximum stress levels will vary. The threshold stress level gives a good indication of the
stress corrosion cracking resistance of a certain grade but an adequate safety margin must also be incorporated in
any design based on these threshold stresses. The reason for this is that the actual service conditions may deviate
from the test conditions in many ways, for example regarding maximum temperatures, chloride levels, the effect of
residual stresses, etc.

Intergranular corrosion
This type of corrosion is also called grain boundary attack and is characterised by attack of a narrow band of
material along the grain boundaries.

Figure 16. Intergranular corrosion adjacent to welds in a hook of AISI 316 used in a pickling bath of sulphuric
acid.

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Intergranular corrosion is caused by the precipitation of chromium carbides in the grain boundaries. Earlier this
type of corrosion caused large problems in connection with the welding of austenitic stainless steels. If an
austenitic or ferritic-austenitic steel is maintained in the temperature range 550 - 800°C, carbides containing
chromium, iron and carbon are formed in the grain boundaries. The chromium content of the carbides can be up to
70%, while the chromium content in the steel is around 18%. Since chromium is a large atom with a low diffusion
rate, a narrow band of material around the carbides therefore becomes depleted in chromium to such an extent
that the corrosion resistance decreases. If the steel is then exposed to an aggressive environment, the chromium-
depleted region is attacked, and the material along the grain boundaries is corroded away. The result is that grains
may drop out of the steel surface or in severe cases that the grains are only mechanically locked together as in a
jigsaw puzzle while the stiffness and strength of the material have almost disappeared. Ferritic stainless steels are
also sensitive to intergranular corrosion for the same reason as the austenitic and duplex steels, although the
dangerous temperatures are higher, generally above 900 - 950oC.

Temperatures that can lead to sensitisation, i.e. a sensitivity to intergranular corrosion, occur during welding in an
area 3-5 mm from the weld metal. They can also be reached during hot forming operations or stress relieving heat
treatments.
The risk for intergranular corrosion can be reduced by decreasing the level of free carbon in the steels. This may
be done in either of two ways:
• by decreasing the carbon content.
• by stabilising the steel, i.e. alloying with an element (titanium or niobium) which forms a more stable carbide
than chromium.
The effect of a decrease in the carbon content is most easily illustrated by a TTS-diagram (time- temperature-
sensitisation), an example of which is shown in Figure 17. The curves in the diagram show the longest time an
austenitic steel of type 18Cr-8Ni can be maintained at a given temperature before there is a danger of corrosion.
This means that for standard low-carbon austenitic steels (L-grades) the risk for intergranular corrosion cracking
is, from a practical point of view, eliminated. All high alloyed austenitic and all duplex grades intended for
aqueous corrosion service have carbon contents below 0.03% and are consequently “L-grades”. Due to the low
solubility of carbon in ferrite the carbon content will have to be extremely low in ferritic stainless steels if the risk
of intergranular corrosion is to be eliminated. In ferritic stainless steels stabilising and extra low carbon contents
are often used is to eliminate the risk for intergranular attack after welding or other potentially sensitising
treatments.
o
C
0,08%C
900

800 0,06%C
0,05%C
700
0,03%C

600

500
0,1 1 10 100
Time (min)
Figure 17. TTS (Time-Temperature-Sensitization) diagram for 18Cr-9Ni type steels with different carbon
contents. The curves are based on the Strauss test (1).

Addition of titanium or niobium to the steel, so-called stabilisation, means that the carbon is bound as titanium or
niobium carbides. Since titanium and niobium have a greater tendency to combine with carbon than does
chromium, this means that carbon is not available to form chromium carbides. The risk for intergranular corrosion
is therefore appreciably reduced. There is, however, a disadvantage associated with stabilisation. In the area
closest to a weld, the temperature during welding can be so high that titanium or niobium carbides are dissolved.
There is then a danger that they do not have time to re-precipitate before the material has cooled sufficiently to
allow the formation of chromium carbides in the grain boundaries. This leads to so-called knife line attack in
which a narrow zone of material very close to the weld suffers intergranular corrosion. Since the carbon level in
stabilised steels is often quite high (0.05-0.08%) this can give serious attack.

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A sensitised microstructure can be fully restored by adequate heat treatment. In the case of austenitic and ferritic-
austenitic duplex stainless steels a full quench anneal heat treatment is necessary. For ferritic stainless steels an
annealing treatment is normally used.
It should also be mentioned that many high temperature steels, which have high carbon contents to increase the
strength, are sensitive to intergranular corrosion if they are used in aqueous environments or exposed to
aggressive condensates.

Galvanic corrosion
Galvanic corrosion can occur if two dissimilar metals are electrically connected together and exposed to a
corrosive environment. The corrosive attack increases on the less noble metal and is reduced or prevented on the
more noble metal, compared to the situation in which the materials are exposed to the same environment without
galvanic coupling.

Stainless Mild steel

Figure 18.Galvanic corrosion on mild steel welded to stainless steel and exposed to sea water.

The difference in "nobility", the ratio of the area of the noble metal to the area of the less noble metal in the
galvanic couple and the electrical conductivity of the corrosive environment are the factors that have the largest
influence on the risk for galvanic corrosion. An increase in any of these factors increases the risk that corrosion
will occur.
Magnesium
Aluminum alloys Zinc

Low-alloy steel Mild steel

Aluminium bronze
Brass
Tin
Copper
90Cu-10Ni AISI 410, 416
AISI 430
70Cu-30Ni
Inconel 600
Silver
AISI 304, 321
Monel 400
Alloy 20 AISI 316, 317
Hastelloy B Titanium
Hastelloy C = active
Platinum
Graphite

0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -1.0 -1,2 -1.4 -1.6 -1.8

Potential E (V versus SCE)

Figure 19. Corrosion potentials for various materials in flowing sea water(after 5).
The risk of galvanic corrosion is most severe in sea water applications. One way of assessing whether a certain
combination of materials is likely to suffer galvanic corrosion is to compare the corrosion potentials of the two
materials in the service environment. One such "electrochemical potential series" for various materials in seawater
is given in Figure 19. The larger the difference between the corrosion potentials, the greater the risk for attack of
the less noble component; small differences in corrosion potential have a negligible effect.

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Stainless steels are more noble than most of the constructional materials and can therefore cause galvanic
corrosion on both carbon steels and aluminium alloys. The risk for galvanic corrosion between two stainless steel
grades is small as long as there is not a large difference in composition such as that between AISI 410S and AISI
316 or ‘254 SMO’. Galvanic effects to be operative when one of the materials in the galvanic couple is corroding.
This means that galvanic corrosion is rarely seen on alloys that are resistant to the service environment.

HIGH TEMPERATURE CORROSION

In addition to the electrochemically-based aqueous corrosion described in the previous chapter, stainless steels can
suffer attack in gases at high temperatures. At such high temperatures there are not the distinct forms of corrosion
such as occur in solutions, instead corrosion is often divided according to the type of aggressive environment.
Some simpler cases of high temperature corrosion will be described here: oxidation, sulphur attack (sulphidation)
carbon uptake (carburization) and nitrogen uptake (nitridation). Other more complex cases such as corrosion in
exhaust gases, molten salts and chloride/fluoride atmospheres will not be treated here.

Oxidation
When stainless steels are exposed to atmospheric oxygen, an oxide film is formed on the surface. At low
temperatures this film takes the form of a thin, protective passive film but at high temperatures the oxide thickness
increases considerably. Above the so-called scaling temperature the oxide growth rate becomes unacceptably high.
Chromium increases the resistance of stainless steels to high temperature oxidation by the formation of a chromia
(Cr2O3) scale on the metal surface. If the oxide forms a contiuous layer on the surface it will stop or slow down
the oxidation process and protect the metal from further. Chromium contents above about 18% is needed in order
to obtain a continuous protective chromia layer. The addition of silicon will appreciably increase the oxidation
resistance, as will additions of small amounts of the rare earth metals such as cerium. The latter also increase the
adhesion between the oxide and the underlying substrate and thus have a beneficial effect in thermal cycling i.e. in
cases in which the material is subject to large, more or less regular, variations in temperature. This is, at least
partly, due to the fact that the addition of Ce promotes a rapid intial growth of the oxide. This leads to a rapidly
formed thin and tenacious protective oxide. The scale is then thin and the chromium depleated zone below is also
thin which makes reformation of the oxide rapid if cracks form in it during thermal cycling. High nickel contents
also have a benefical effect on the oxidation resistance. The scaling temperatures for various stainless steels are
shown in Table 4. It is worth noting that the ranking in resistance to localized corrosion is not applicable at high
temperatures and that an increase in molybdenum content does not lead to an increased scaling temperature.
Compare, for example, 304L - 316 - 317L.
Table 4. Scaling temperature in air for various stainless steels.
Steel grade Composition (%) Scaling temperature
AISI C Cr Ni Mo N Other (°C) (approx.)
410 0.08 13 - - - 830
431 0.12 17 1 - 850
18-2Ti 0.01 18 - 2 0.01 Ti 1000
446 0.12 26 - - - N 1075
304H 0.05 18 9 - 0.06 850
321H 0.05 17 9 - 0.01 Ti 850
316 0.04 17 12 2.7 0.06 850
‘2205’ 0.02 22 5 3 0.17 1000
‘904L’ 0.02 20 25 4.5 0.06 Cu 1000
310S 0.05 25 20 - 0.06 1150
‘153 MA’ 0.05 18 9 - 0.15 Si, Ce 1050
‘253 MA’ 0.09 21 11 - 0.17 Si, Ce,N 1150
‘353 MA’ 0.05 25 35 - 0.15 Si, Ce 1175
Under certain conditions heat resisting steels can suffer very rapid oxidation rtes at relatively low temperatures.
This is referred to as catastrophic oxidation and is associated with the formation of liquid oxides. If a liquid oxide
is formed it will penetrate and disrupt the protective oxide scale and expose the metal to rapid oxidation.

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Catastrophic oxidation generally occurs in the temperature range 640 - 950 oC in the presence of elements whose
oxides either melt or form eutectics with the chromium oxide (Cr2O3) scale. For this reason molybdenum, which
forms low-melting-point oxides and oxide-oxide eutectics, should be avoided in steels designed for high
temperature applications. The presence of some other metals in the environment may cause catastrophic oxidation.
Vanadium, which is a common contaminat in heavy fuel oils, can easily cause rapid or catastrophic oxidation due
to its low melting point oxide,V 2O5, which melts at 690 oC. Some other metals, such as lead and tungsten, may
also act in this way.

Sulphur attack (Sulphidation)

At high temperatures sulphur compounds react with stainless steels to form complex sulphides and/or oxides.
Sulphur also reacts with nickel and forms nickel sulphide which, together with nickel, forms a low melting point
eutectic. This causes very severe attack unless the chromium content is very high. Steels with low nickel contents
should be used in environments containing sulphur or reducing sulphur compounds. For this reason the chromium
steels exhibit good resistance to sulphidation.
In reducing environments such as hydrogen sulphide or hydrogen sulphide/hydrogen mixtures, stainless steels are
attacked at even relatively low temperatures compared to the behaviour in air. Table 5 shows examples of the
corrosion rate for some stainless steels in hydrogen suphide at high temperatures. Table 6 shows corresponding
data for some austenitic stainless steels in a mixture of hydrogen sulphide and hydrogen. The beneficial effect of a
high chromium content is clear from the tables.
In oxidizing - sulphidizing environments such as sulphur dioxide (SO2) the relative performance of stainless steels
is similar to that in air, but the attack is more rapid and therefore more serious. The scaling temperature typically
decreases by 70-125°C compared to that in air. The decrease is smallest for the chromium steels (5).

Table 5. Corrosion rates for different steel grades in 100%H2S at atmospheric pressure and
two different temperatures (5).

Steel grade Composition Corrosion rate

(%) (mm/year)
Cr Ni 400°C 500°C
5%Cr steel 5 - 6.1 25.4
9%Cr steel 9 - 5.1 17.8
403 13 - 3.3 10.2
431 17 - 2.3 5.1
446 26 - no attack 2.5
304 18 9 2.0 5.1
310S 25 20 1.5 2.5

Table 6. Corrosion rate of some austenitic stainless steels in 50% H2 - 50% H2S at
atmospheric pressure and different temperatures (5).

Steel grade Composition Corrosion rate

AISI (%) (mm/ year)
Cr Ni Mo 500°C 600°C 700°C
304 18 9 - 1.1 3.0 10.2
316 17 11 2.2 1.5 4.4 10.8
310S 25 20 - 0.9 2.8 8.9

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Carbon pick-up (Carburization)

If a material is exposed to gases containing carbon, e.g. in the form of CO, CO 2 or CH4, it can pick up carbon.
The degree of carburisation is governed by the levels of carbon and oxygen in the gas, also the temperature and
steel composition. The carbon which is picked up by the steel will largely form carbides, primarily chromium
carbides.
Carbon pick-up causes embrittlement of stainless steel because carbides, or even a network of carbides, form in
the grain boundaries as well as within the grains. The formation of a large amount of chromium carbides causes
chromium depletion and thus a reduced resistance to oxidization and sulphidation. The resistance to thermal
cycling is reduced and, since carburization leads to an increase in volume, there is a danger of cracks developing in
the material.
Carbon pick-up can occur even at relatively low temperatures (400-800°C) in purely reducing - carburizing
atmospheres and gives rise to catastrophic carburisation or metal dusting. Attack is severe and characterized by
"powdering" of the steel surface due to the breakdown of the protective oxide layer and inward diffusion of
carbon which forms grain boundary carbides. The increase in volume on carbide formation means that grains are
rapidly broken away from the steel surface, giving rapid and serious attack.
Chromium, nickel and silicon are the alloying elements which most improve resistance to carburization. Table 7
shows carburization of some stainless steels in carburizing atmospheres. Note the beneficial effect of silicon,
apparent from a comparison of Type 304 and 302B. Also note the high level of carburization in Type 316. In
materials selection it is however necessary to consider both carburization and the effect of an increased carbon
content on mechanical properties. In general, austenitic stainless steels can tolerate an increased carbon content
better than other types of stainless steel.

Table 7. Carburization after 7340 hour at 910°C in an atmosphere of 34% H 2 14% CO,
12.4% CH4, 39.6% N2 (6)
Steel grade Composition (%) Carbon uptake
AISI Cr Ni Other (%)
304 18 9 2.6
302B 18 9 2.5 Si 0.1
321 18 10 Ti 1.5
347 18 10 Nb 0.2
316 17 11 2.0 Mo 1.0
309S 23 13 0
310S 25 20 0
314 25 20 2.5 Si 0
330 15 35 0.9

Nitrogen pick-up (Nitridation)

Stainless steels and other high temperature materials can pick up nitrogen if exposed to nitrogen-containing
atmospheres such as nitrogen, nitrogen mixtures and cracked ammonia. During nitrogen pick-up nitrides and other
brittle compounds of chromium, molybdenum, titanium, vanadium and aluminium are formed. Atmospheric
oxygen, even at relatively low levels, reduces the risk for nitridation. At low temperature, 400-600°C, a layer of
nitrides are formed at the steel surface; at higher temperatures nitrogen uptake and nitride formation occur
throughout the material. Nitridation i.e. nitride formation, causes chromium depletion and reduced oxidation
resistance in the same way as carburization. This can lead to catastrophically high oxidation rates on the outer
surface of equipment which is subjected to a nitriding atmosphere on the inside - for example the muffles in
annealing furnaces. Nitrogen pick-up can also cause embrittlement due to surface or internal nitride formation.
Nickel is the alloying element which provides the greatest protection against nitridation, due to the fact that nickel
does not form stable nitrides. This is illustrated by Figure 20 which shows the nitrided depth for some austenitic
high-temperature alloys after exposure to nitrogen with traces of oxygen at 825°C. If oxygen is present, i.e. in
oxidising conditions, strong oxide formers such as chromium and silicon are beneficial.

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Figure 20. Nitrided depth for some stainless steels after exposure to nitrogen gas containing approximately 200
ppm oxygen at 825°C for 400 hours(7).

In view of the effect of nickel, it is inadvisable to use martensitic, ferritic-austenitic or ferritic stainless steels in
nitriding atmospheres at temperature above approximately 500°C. More suitable materials are austenitic stainless
steels or nickel-base alloys.

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Mechanical properties

Stainless steels are often selected for their corrosion resistance, but they are at the same time constructional
materials. Mechanical properties such as strength, high-temperature strength, ductility and toughness, are thus also
important.
The difference in the mechanical properties of different stainless steels is perhaps seen most clearly in the stress-
strain curves in Figure 21. The high yield and tensile strengths but low ductility of the martensitic steels is
apparent, as is the low yield strength and excellent ductility of the austenitic grades. Ferritic-austenitic and ferritic
steels both lie somewhere between these two extremes.
Stress (MPa)
1250
Martensitic (420); quenched and tempered
1000
Martensitic-austenitic , quenched and tempered

750 Ferritic-austenitic (”2205”)

500
Ferritic (444Ti) Austenitic (316)

250

0
0 10 20 30 40 50 60 70
Strain (%)

Figure 21. Stress-strain curves for some stainless steels.

The ferritic steels generally have a somewhat higher yield strength than the austenitic steels, while the ferritic -
austenitic steels have an appreciably higher yield strength than both austenitic and ferritic steels. The ductility of
the ferritic and ferritic-austenitic steels are of the same order of magnitude, even if the latter are somewhat
superior in this respect.

Room temperature properties

In terms of mechanical properties, stainless steels can be roughly divided into four groups with similar properties
within each group: martensitic and ferritic-martensitic, ferritic, ferritic-austenitic and austenitic. Table 8 gives
typical mechanical properties at room temperature for a number of stainless steels in plate form.

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Table 8. Typical mechanical properties for stainless steels at room temperature.

Steel grade Rp0.2 Rp1.0 Rm A5

ASTM AvestaPolarit (MPa) (MPa) (MPa) (%)
Martensitic
410S 540 690 20
“420L” 780 980 16
431 690 900 16
Ferritic-martensitic
- 248 SV 790 840 930 18
446 - 340 540 25
444 Elit 18-2 390 560 30
Ferritic-austenitic (Duplex)
S32304 SAF2304 470 540 730 36
S31803 2205 500 590 770 36
S32750 SAF2507 600 670 850 35
Austenitic
304 18-9 310 350 620 57
304L 18-10L 290 340 590 56
304LN 18-9LN 340 380 650 52
304N 18-8N 350 400 670 54
321 18-10Ti 280 320 590 54
316L 17-11-2L 310 350 600 54
316Ti 17-11-2Ti 290 330 580 54
316 17-12-2.5 320 360 620 54
316L 17-12-2.5L 300 340 590 54
317L 18-13-3L 300 350 610 53
S31726 17-14-4LN 320 360 650 52
N08904 904L 260 310 600 49
S31254 254 SMO 340 380 690 50
S32654 654 SMO 520 560 890 55
Austenitic (heat resistant steels)
310S 25-20 290 330 620 50
S30415 153 MA 380 410 700 50
S30815 253 MA 410 440 720 52
S35315 353 MA 360 400 720 50

Stress values have been rounded off to the nearest 10MPa. Standard deviations are normally 17-20MPa for Rp0,2,
Rp1,0 and Rm; 3% for A5. More detailed information can be found in reference (8).
Martensitic and ferritic-martensitic steels are characterised by high strength and the fact that the strength is
strongly affected by heat treatment. The martensitic steels are usually used in a hardened and tempered condition.
In this condition the strength increases with the carbon content. Steels with more than 13% chromium and a
carbon content above 0.15% are completely martensitic after hardening. A decrease in the carbon content causes
an increase in the ferrite content and therefore a decrease in strength. The ductility of the martensitic steels is
relatively low. The ferritic-martensitic steels have a high strength in the hardened and tempered condition in spite
of their relatively low carbon content, and good ductility. They also possess excellent hardenability: even thick
sections can be fully hardened and these steels will thus retain their good mechanical properties even in thick
sections.
The mechanical properties of martensitic stainless steels are heavily influenced by the heat treatments to which the
steels are subjected. A brief description of the general heat treatment of martensitic stainless steels and the effect
on the mechanical properties is given below. Further information on the effects of various factors on the
mechanical properties of the different martensitic stainless steels may be found in references 5 and 9.
In order to obtain a useful property profile martensitic stainles steels are normally used in the hardened and
tempered condition. The hardening treatment consists of heating to a high temperature in order to produce an
austenitic structure with carbon in solid solution followed by quenching. The austenitizising temperature is

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generally in the range 925 - 1070 oC. The effect of austenitizising temperature and time on hardness and strength
varies with the composition of the steel, especially the carbon content. In general the hardness will increase with
austenitizising temperature up to a maximum and then decrease. The effect of increased time at the austenitizising
temperature is normally a slow reduction in hardness with increased time. Quenching, after austenitizising, is done
in air, oil or water depending on the steel grade. On cooling below the Ms - temperature, the starting temperature
for the martensite transformation, the austenite transforms to martensite. The M s - temperature lies in the range
300 - 70 oC and the transformation is finished of about 150 - 200 oC below the Ms - temperature. Almost all
alloying elements will lower the Ms - temperature with carbon having the greatest effect. This means that in the
higher alloyed martensitic grades the microstructure will contain retained austenite due to the low temperature
(below ambient) needed in order to finish the transformation of the austenite into martensite.

Figure 22. Effect of tempering temperature on the mechanical properties of AISI 431. Hardening treatment:
1020oC/30m/Oil quench

In the hardened condition the strength and hardness are high but the ductility and toughness is low. In order to
obtain useful engineering properties martensitic stainless steels are normally tempered. The tempering temperature
used has a large influence on the final properties of the steel. The effect of tempering temperature on the
mechanical properties of a martensitic stainless steel (AISI 431) is shown in Figure 22. Normally, increasing
tempering temperatures below about 400 oC will lead to a small decrease tensile strength and an increase in
reduction of area while hardness, elongation and yield strength are more or less unaffected. Above this
temperature there will be a more or less pronounced increase in yield strength, tensile strength and hardness due to
the secondary hardening peak, around 450 - 500 oC. In the temperature range around the secondary hardening
peak there is generally a dip in the impact toughness curve. Above about 500 oC there is a rapid reduction in
strength and hardness, and a corresponding increase in ductility and toughness. Tempering at temperatures above
the AC1 temperature (780 oC for the steel in Figure 22) will result in partial austenitizising and the possible
presence of untempered martensite after cooling to room temperature.

Ferritic steels have relatively low yield strength and the work hardening is limited. The strength increases with
increasing carbon content, but the effect of chromium content is negligible. However, ductility decreases at high
chromium levels and good ductility requires very low levels of carbon and nitrogen.

Ferritic-austenitic (duplex) steels have a high yield stress with increases with increasing carbon and nitrogen
levels. An increased ferrite content will, within limits, also increase the strength of duplex steels. Their ductility is
good and they exhibit strong work hardening.

Austenitic steels generally have a relatively low yield stress and are characterised by strong work hardening. The
strength of the austenitic steels increases with increasing levels of carbon, nitrogen and, to a certain extent, also
molybdenum. The detrimental effect of carbon on corrosion resistance means that this element cannot be used for
increasing strength. Austenitic steels exhibit very high ductility: they have a high elongation and are very tough.
Some austenitic stainless steels with low total content of alloying elements, e.g. Type 301 and 304, can be
metastable and may form martensite either due to cooling below ambient temperature or through cold deformation

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or a combination of both. The formation of martensite will cause a considerable increase in strength, as illustrated
in Figure 23. The temperature below which  martensite will form is called the Md temperature. The stability of
the austenite depends on the composition, the higher the content of alloying elements the more stable will it be. A
common equation for relating austenite stability and alloy composition is the M d30, which is defined as the
temperature at which martensite will form at a strain of 30% (10):
Md30 = 551-462(C+N)-9.2Si-8.1Mn-13.7Cr-29(Ni+Cu)-18.5Mo-68Nb-1.42(GS-8.0) (oC)
where GS = grain size, ASTM grain size number
This type of equation gives a good idea of the behaviour of lean austenitic stainless steels but it must be noted that
it is only approximate since interactions between the alloying elements are not taken into account.

Figure 23. The effect of strain on martensite and yield strength of AISI 301. (5)

The effect of alloying elements and structure on the strength of austenitic and ferritic-austenitic steels is apparent
from the following regression equations:
Rp0,2 = 120 + 210 N + 0.02 + 2Mn + 2Cr + 14Mo + 10Cu + (6.15 − 0.054) +
(MPa)
→→ +(7 + 35(N + 0.2))d−½

Rp1,0 = Rp0,2 + 40 (MPa)

Rm = 470 + 600(N+0.02) + 14Mo + 1.5  + 8d-½ (MPa)
where N, Mn, etc. denote the level of the alloying elements in wt%.
 is the ferrite content in %.
d is the grain size in mm.
These regression equations can be used to estimate the strength of an austenitic and ferritic-austenitic steel with an
uncertainty of approximately 20MPa (11).
In contrast to the constructional steels, austenitic steels do not exhibit a clear yield stress but begin to deform
plastically at a stress around 40% of Rp0.2.
It may be noted that although the different elements are included in the equation through rather simple
expressions, the actual strengthening mechanism may be more complex. Both chromium and nitrogen work
through more complex effects than may be seen at first sight. At chromium contents over 20% an austenitic steel
with 10% Ni will contain  -ferrite which in turn causes a smaller grain size and this will increase both the yield
strength and the tensile strength. Nitrogen is an element that has a strong strengthening effect but it is also a
powerful austenite stabiliser. In duplex stainless steels the strengthening effect of nitrogen is to a certain extent
countered by the increased austenite content caused by the addition of nitrogen.

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Stainless steels will harden during deformation. The amount of hardening depends on both the composition and
the type of steel. The work hardening exponent (n) defined as

 = K . n
where  and  are true stress and true strain respectively gives a simple measure of the tendency to work harden.
Ferritic steels have a work hardening exponents of about 0.20. For austenitic steels the work hardening exponent
is strain dependent. For the stable grades it lies in the range 0.4 to 0.6 and for the unstable grades, i.e. those that
form martensite at large deformations, it lies in the range 0.4 to 0.8. The higher values are valid for higher strains.
Nickel, copper and nitrogen tend to reduce the work hardening. Most other elements will increase the work
hardening.

The effect of cold work

The mechanical properties of stainless steels are strongly affected by cold work. In particular the work hardening
of the austenitic steels causes considerable changes in properties after, e.g. cold forming operations. The general
effect of cold work is to increase the yield and tensile strengths and at the same time decrease the elongation.
Figure 24 shows cold work curves for some stainless steels.
S tress E lo n g a t i o n
(M P a ) (%)
R p 0.2 Rm A5 60
1250

‘248SV ’ 50

1000 ‘2205’

40
316LN
750 316L
30

316L
500
316LN 20

250 ‘2205’
10

‘248SV ’

0 0
0 5 10 15 20 25 30 35
S train ( % )
Figure 24. Effect of cold work on some stainless steels.

The work hardening is larger for austenitic steels than for ferritic steels. The addition of nitrogen in austenitic
steels makes these grades particularly hard and strong: compare 316L and 316LN. The strong work hardening of
the austenitic steels means that large forces are required for forming operations even though the yield strength is
low. Work hardening can, however, also be deliberately used to increase the strength of a component.

Toughness
The toughness of the different types of stainless steels shows considerable variation, ranging from excellent
toughness at all temperatures for the austenitic steels to the relatively brittle behaviour of the martensitic steels.
Toughness is dependent on temperature and generally increases with increasing temperature.
One measure of toughness is the impact toughness, i.e. the toughness measured on rapid loading. Figure 25 shows
the impact toughness for different categories of stainless steel at temperatures from -200 to +100 °C. It is
apparent from the diagram that there is a fundamental difference at low temperatures between austenitic steels and
martensitic, ferritic and ferritic-austenitic steels.

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Impact strength (KV), (J)

250

200
Austenitic

Ferritic
150

Duplex
100

Martensitic
50

0
-200 -150 -100 -50 0 +50 +100
o
Temperature ( C)

Figure 25. Impact toughness for different types of stainless steels.

The martensitic, ferritic and ferritic-austenitic steels are characterised by a transition in toughness, from tough to
brittle behaviour, at a certain temperature, the transition temperature. For the ferritic steel the transition
temperature increases with increasing carbon and nitrogen content, i.e. the steel becomes brittle at successively
higher temperatures. For the ferritic-austenitic steels, an increased ferrite content gives a higher transition
temperature, i.e. more brittle behaviour. Martensitic stainless steels have transition temperatures around or slightly
below room temperature, while those for the ferritic and ferritic-austenitic steels are in the range 0 to - 60°C, with
the ferritic steels in the upper part of this range.
The austenitic steels do not exhibit a toughness transition as the other steel types but have excellent toughness at
all temperatures. Austenitic steels are thus preferable for low temperature applications.

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Fatigue properties
During cyclic loading stainless steels, as other materials, will fail at stress levels considerably lower than the tensile
strength measured during tensile testing. The number of load cycles the material can withstand is dependent on the
stress amplitude. The life time, i.e. the number of cycles to failure, increases with decreasing load amplitude until a
certain amplitude is reached, below which no failure occurs (Figure 26). This stress level is called the fatigue limit.
In many cases there are no fatigue limit but the stress amplitude shows a slow decrease with increasing number of
cycles. In these cases the fatigue strength, i.e. the maximum stress amplitude for a certain time to failure (number
of cycles) is called the fatigue strength and it is always given in relation to a certain number of cycles.

500
Stress amplitude, S, (MPa)

f = 90 Hz Rm = 620 MPa

400

300

200
10 105 106 107
4

Number of cycles, N

Figure 26. S-N curve (Wohler curve) for an austenitic stainless steel of Type 316(hMo) in air.

The fatigue properties, described by the Wohler or S-N curve with a fatigue limit (So = load amplitude) at a
lifetime of 106-107 load cycles, of ferritic-austenitic and austenitic stainless steels can be related to their tensile
strength as shown in Table 9. The relation between the fatigue limit and the tensile strength is also dependent on
the type of load, that is the stress ratio (R). The stress ratio is ratio of the minimum stress to the maximum stress
during the loading cycle (compressive stresses are defined as negative).

Table 9. Fatigue properties of stainless steels, relation between tensile

strength and fatigue strength.

Steel category So/Rm Maximum

stress
Stress ratio
R = -1 R = 0
Ferritic 0.7 0.47 Yield strength
Austenitic 0.45 0.3 Yield strength
Ferritic-austenitic 0.55 0.35 Yield strength

The fatigue strength is sensitive to the service environment and under both cyclic loading and corrosive
conditions, corrosion fatigue, the fatigue strength will generally decrease. In many cases there is no pronounced
fatigue limit, as observed in air, but a gradual lowering of the fatigue strength with increasing number of load
cycles. The more aggressive the corrosive conditions and the lower the loading frequency the higher the effect of
the environment. During very high frequency loads there is little time for the corrosion to act and the fatigue
properties of the material will mostly determine the service life. At lower frequencies the corrosive action is more
pronounced and an aggressive environment may also cause corrosion attacks that will act as stress concentrations
and thus contribute to a shorter life. As can be seen from Fig. 27, a lower pH, i.e. a more aggressive condition,
gives a lower fatigue strength. Comparison of the two austenitic steels shows that the higher alloyed grade,
316LN, that has the higher corrosion resistance also has a higher corrosion fatigue strength.

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Figure 27. Effect of environment on fatigue strength for some stainless steels (12).Fatigue strength at 40 oC and
rotating bending stress at 100Hz. Tested in air and 3% NaCl at various pH.

High temperature mechanical properties

The high temperature strength of various steel grades is illustrated by the yield strength and creep rupture strength
curves in Figure 28.
Yield stress (Rp0,2)
Creep strength (Rkm 100000)
(MPa)

1100

1000

900

800

700 Martensitic

600

500

400 Duplex

300
Ferritic
Creep strength

200
Austenitic

100

0
0 100 200 300 400 500 600 700

Temperature ( oC)

Figure 28. Elevated temperature strength of stainless steels.The dashed line shows the yield stress of some very
high alloyed and nitrogen alloyed austenitic steels.

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Martensitic and martensitic-austenitic steels in the hardened and tempered condition exhibit high elevated
temperature strength at moderately elevated temperatures. However, the useful upper service temperature is
limited by the risk of over-tempering and embrittlement. The creep strength is low. This type of stainless steel is
not usually used above 300°C but special grades are used at higher temperatures. The wide range of elevated
temperature strength shown in Figure 28 is due to the wide range of strength levels offered by different grades and
heat treatments.
Ferritic steels have relatively high strength up to 500°C. The creep strength, which is usually the determining
factor at temperatures above 500°C, is low. The normal upper service temperature limit is set by the risk of
embrittlement at temperatures above 350°C. However, due to the good resistance of chromium steels to high
temperature sulphidation and oxidation a few high chromium grades are used in the creep range. In these cases
special care is taken to ensure that the load is kept to a minimum.
The ferritic-austenitic (duplex) steels behave in the same way as the ferritic steels but have higher strength. The
creep strength is low. The upper service temperature limit is normally 350°C due to the risk of embrittlement at
higher temperatures.
Most austenitic steels have lower strength than the other types of stainless steels in the temperature range up to
about 500 oC. The highest elevated temperature strength among the austenitic steels is exhibited by the nitrogen
alloyed steels and those containing titanium or niobium. In Figure 28 the elevated temperature strengths of most
of the ordinary austenitic steels fall within the marked area. The dashed line represents the elevated temperature
strength of a few high alloyed and nitrogen alloyed austenitic steels. In terms of creep strength the austenitic
stainless steels are superior to all other types stainless steel (see Figure 29).
300

200
Creep
rupture Austenitic
strength
Rkm10000
100
(MPa)
Ferritic

0
500 600 700 800 900 1000

Temperature (oC)

Figure 29. Creep strength for austenitic and ferritic steels.

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Precipitation and embrittlement

Under various circumstances, the different stainless steel types can suffer undesirable precipitation reactions which
lead to a decrease in both corrosion resistance and toughness. Figure 30 gives a general overview of the
characteristic critical temperature ranges for the different steel types.

475°C embrittlement
If martensitic, ferritic or ferritic-austenitic steels are heat treated or used in the temperature range 350-550°C, a
serious decrease in toughness will be observed after shorter or longer times. The phenomenon is encountered in
alloys containing from 15 to 75 % chromium and the origin of this embrittlement is the spinodal decomposition of
the matrix into two phases of body-centered cubic structure,  and ´. The former is very rich in iron and the
latter very rich in chromium. This type of embrittlement is is usually denoted 475°C embrittlement.

Carbide and nitride precipitation

If ferritic steels are heated to temperatures above approximately 950°C, they suffer precipitation of chromium
carbides and chromium nitrides during the subsequent cooling, and this causes a decrease in both toughness and
corrosion resistance. This type of precipitation can be reduced or eliminated by decreasing the levels of carbon and
nitrogen to very low levels and at the same time stabilizing the steel by additions of titanium as in 18Cr-2Mo-Ti.
Carbide and nitride precipitation in the austenitic and ferritic-austenitic steels occurs in the temperature range 550-
800°C. Chromium-rich precipitates form in the grain boundaries and can cause intergranular corrosion and, in
extreme cases, even a decrease in toughness. However, after only short times in the critical temperature range, e.g.
in the heat affected zone adjacent to welds, the risk of precipitation is very small for the low-carbon steels.

Intermetallic phases
In the temperature range 700-900°C, iron alloys with a chromium content above about 17% form intermetallic
phases such as sigma phase, chi phase and Laves phase. These phases are often collectively called “sigma phase”
and all have the common features of a high chromium content and brittleness. This means that a large amount of
the precipitated phase leads to a drop in toughness and a decrease in resistance to certain types of corrosion. The
size of the deterioration in properties is to some extent dependent on which of the phases that actually is present.
Alloying with molybdenum and silicon promotes the formation of intermetallic phases, so the majority of ferritic,
ferritic-austenitic and austenitic steels show some propensity to form "sigma phase". Intermetallic phases form
most readily from highly-alloyed ferrite. In ferritic and ferritic-austenitic steels, intermetallic phases therefore form
readily but are on the other hand relatively easy to dissolve on annealing. In the austenitic steels, it is the highly
alloyed grades which are particularly susceptible to intermetallic phase formation. Austenitic steels which have low
chromium content and do not contain molybdenum require long times to form intermetallics and are therefore
considerably less sensitive to the precipitation of these phases.

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o
C
Martensitic Ferritic Duplex Austenitic

1000

500

Hardening Tempering 475o-

embrittlement

Carbides Intermetallic Carbides and

phases intemetallic
phases

Figure 30. Characteristic temperature ranges for stainless steels.

Finally, it should be noted that all types of precipitates can be dissolved on annealing. Re-tempering martensitic
steels and annealing and quenching ferritic, ferritic-austenitic or austenitic steels restores the structure. Relatively
long times or high temperatures may be required for the dissolution of intermetallic phases in highly alloyed steels.

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Physical properties
In terms of physical properties, stainless steels are markedly different from carbon steels in some respects. There
are also appreciable differences between the various categories of stainless steels. Table 10 and Figures 31 -33
shows typical values for some physical properties of stainless steels.

Table 10. Typical physical properties for various stainless steel categories.
Type of stainless steel
Property Martensiti Ferritic Austenitic Ferritic-
c* austenitic
Density 7.6-7.7 7.6-7.8 7.9-8.2 .8
(g/cm3)
Young's modulus 220,000 220,000 195,000 200,000
(N/mm²) or (MPa)
Thermal expansion 12-13 12-13 17-19 13
(x 10-6/°C) 200-600°C
Thermal conductivity 22-24 20-23 12-15 20
(W/m°C) 20°C
Heat capacity 460 460 440 400
(J/kg°C) 20°C
Resistivity 600 600-750 850 700-850
(nm) 20°C
Ferromagnetism Yes Yes No Yes
* in the hardened and tempered condition

Elastic Modulus Stainl ess Steel

220

200
Modulus (kN/mm2)

180

160

140

120

100
0 200 400 600 800
T em p er at u r e (oC)

Figure 31 Elastic Modulus of Austenitic Stainless Steels .

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10 −6 / oC (
  10 6 20 − t oC )
22
20
20 19
18
k (W/m,oC)

18 17
16
16 15
14
14
13
12 Austenitics
12 Austenitics
11 Duplex
Duplex
10
10
0 100 200 300 400 500 600
0 100 200 300 400 500 600
Temperature (deg. C) Temperature (deg.C)

Figure 32 Thermal Conductivity for Austenitic and Figure 33 Mean Linear Thermal Expansion for
Duplex Stainless Steels. Austenitic and Duplex Stainless Steels.

The austenitic steels generally have a higher density than the other stainless steel types. Within each steel category,
density usually increases with an increasing level of alloying elements, particularly heavy elements such as
molybdenum.

The two important physical properties that show greatest variation between the stainless steel types, and are also
markedly different for stainless steels and carbon steels, are thermal expansion and thermal conductivity.
Austenitic steels exhibit considerably higher thermal expansion than the other stainless steel types. This is can
cause thermal stresses in applications with temperature fluctuations, heat treatment of complete structures and on
welding. Thermal conductivity for stainless steels is generally lower than for carbon steels and decreases with
increasing alloying level for each stainless steel category. The thermal conductivity decreases in the following
order: martensitic steels, ferritic and ferritic-austenitic steel and finally austenitic steels which have the lowest
thermal conductivity.

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Property relationships for stainless steels

Using one stainless steel grade in each group or category as a starting point, i.e. regarding it as the archetype for
the category, it is now possible to see how the other steel grades within the category have evolved or how they are
related. In this way the full range of stainless steels may be systematised. The property and alloying relationships
between the different grades in the group are shown in the overview in Figures 34 and 35.

Martensitic and martensitic-austenitic steels

The steels in this group are characterised by high strength and limited corrosion resistance.
An increased carbon content increases strength, but at the expense of lower toughness and considerable
degradation of weldability. Strength thus increases in the series: AISI 420R, 420L and 420 while toughness and
weldability decrease. The martensitic 13% chromium steels with higher carbon contents are not designed to be
welded, even though it is possible under special circumstances. In order to increase high temperature strength,
alloying with strong carbide formers such as vanadium and tungsten are used in 13Cr-0.5Ni-1Mo+V. An increase
in the nickel content also increases toughness and leads to the martensitic-austenitic steels 13Cr-5Ni and 16Cr-
5Ni-1Mo. These are characterised by high strength, good high temperature strength and, because of the low
carbon content in the martensite, good toughness even when welded. In contrast to the martensitic steels, the
martensitic-austenitic steels do not have to be welded at elevated temperatures except in thick section, even then
only limited preheating is required.
An increased chromium content increases corrosion resistance, while an increased carbon content has the opposite
effect due to the formation of chromium carbides. Alloying with molybdenum improves corrosion resistance and it
is molybdenum, in combination with the higher chromium content, which gives 16Cr-5Ni-1Mo superior corrosion
resistance to the other hardenable stainless steels. The martensitic stainless steels are resistant to damp air, steam,
freshwater, alkaline solutions (hydroxides) and dilute solutions of organic and oxidising inorganic acids. The
martensitic-austenitic steels, in particular 16Cr-5Ni-1Mo, exhibit better corrosion resistance than the other steels
in the group. 16Cr-5Ni-1Mo can be used in the same environments as the martensitic steels with 13% or 17%
chromium, but can withstand higher concentrations and higher temperatures. The martensitic steels have poor
resistance to pitting and crevice corrosion but are largely immune to stress corrosion cracking. They should not
normally be used in sea water without cathodic protection. 16Cr-5Ni-1Mo is comparable with the low alloyed
austenitic stainless steels in terms of resistance to pitting and crevice corrosion in sea water but is not susceptible
to stress corrosion cracking.
The areas of use of martensitic and martensitic-austenitic steels are naturally those in which the high strength is an
advantage and the corrosion requirements are relatively small. The martensitic steels with low carbon contents
(AISI 410S and 410) and the martensitic-austenitic steels are often used as stainless constructional materials. In
addition, AISI 410S is used for, among other things, tubes for heat exchangers in the petrochemical industry,
while AISI 410 is used for stainless cutlery. The martensitic steels with a high carbon content (AISI 420L and
420) are used for springs, surgical instruments and for sharp-edged tools such as knives and scissors. The higher
chromium content in AISI 431 means that it is often used for marine fittings and for components in the nitric acid
industry.

Ferritic steels
The ferritic steels are characterised by good corrosion properties, very good resistance to stress corrosion
cracking and moderate toughness.
The toughness of ferritic stainless steels are generally not particularly high. Lower carbon and nitrogen levels, as in
AISI 444, give a considerable improvement in both toughness and weldability, although toughness is limited for
thicker dimensions. Consequently ferritic steels are usually only produced and used in thinner dimensions.
The ferritic steels exhibit good corrosion resistance: AISI 444 is comparable to the austenitic AISI 316 in this
respect. However, the ferritic steels are also very resistant to stress corrosion cracking. Higher levels of chromium
yield better oxidation resistance and the absence of nickel results in good properties in sulphur-containing
environments at high temperatures. This is one of the major areas of use of AISI 446.
Use of AISI 430 and AISI 444 includes piping, heat exchanger tubes, vessels and tanks in the food, chemical and
paper industries. AISI 444 can also be used in water with moderately high levels of chlorides in applications where
there is a danger of stress corrosion cracking. Low alloyed ferritic stainless steels are also used in mild
environments where freedom from maintenance is sought or where a ‘non-rusting’ material is required.

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Ferritic-Austenitic (Duplex) steels

The modern duplex steels span the same wide range of corrosion resistance as the austenitic steels. The corrosion
resistance of the duplex steels increases in the order “2304” (23Cr-4Ni) — “2205” (22Cr-5Ni-3Mo) — “2507”
(25Cr-7Ni-4Mo). Duplex equivalents can be found to both the ordinary austenitic grades, such as 316L, and to
the high alloyed austenitic grades, such as ‘254 SMO’. The corrosion resistance of “2304” type duplex is similar
that of 316L while “2205” is similar to “Type” 904L and “2507” is similar to the high alloyed austenitic grades
with 6% molybdenum, such as ‘254 SMO’.
The ferritic-austenitic (duplex) steels are characterised by high strength, good toughness, very good corrosion
resistance in general and excellent resistance to stress corrosion cracking and corrosion fatigue in particular. An
increased level of chromium, molybdenum and nitrogen increases corrosion resistance, while the higher nitrogen
level also contributes to a further increase in strength above that associated with the duplex structure.
Applications of ferritic-austenitic steels are typically those requiring high strength, good corrosion resistance and
low susceptibility to stress corrosion cracking or combinations of these properties. The lower alloyed “2304 type”
is used for applications requiring corrosion resistance similar to 316L or lower and where strength is an
advantage. Some examples of such applications are: hot water tanks in the breweries, pulp storage towers in the
pup and paper industry, tanks for storage of chemical in the chemical process industry and tank farms in tank
terminals in the transportation industry. The higher alloyed “2205 type” is for example used in pulp digesters and
storage towers in the pulp and paper industry where it is rapidly becoming a standard grade. It is also used in
piping systems, heat exchangers, tanks and vessels for chloride-containing media in the chemical industry, in
piping and process equipment for the oil and gas industry, in cargo tanks in ships for transport of chemicals, and
in shafts, fans and other equipment which require resistance to corrosion fatigue. High alloyed grades, e.g.
“2507”, are used in piping and process equipment for the offshore industry (oil and gas) and in equipment for
environments containing high chloride concentrations, such as sea water.
Austenitic steels
The austenitic steels are characterised by very good corrosion resistance, very good toughness and very good
weldability; they are also the most common stainless steels.
Resistance to general corrosion, pitting and crevice corrosion generally increases with increasing levels of
chromium and molybdenum, while high levels of nickel and molybdenum are required to increase resistance to
stress corrosion cracking. Resistance to pitting and crevice corrosion thus increases in the order: AISI 304 / 304L
- 316 / 316L -317L - ‘904L’ - ‘254 SMO’ — ‘654 SMO’. The low-carbon grades exhibit good resistance to
intergranular corrosion and consequently the higher alloyed steels are only available with low carbon contents.
The stabilised steels (AISI 321, 347 and 316Ti) and the nitrogen-alloyed steels (304LN and 316LN) have roughly
the same corrosion properties in most environments as the equivalent low-carbon grades: 304L and 316L
respectively. There are however, exceptions to this rule so it should be treated with some caution. Austenitic
steels are generally susceptible to stress corrosion cracking; only the highly alloyed steels ‘904L’, ‘254 SMO’ and
‘654 SMO’ exhibit good resistance to this type of corrosion. An increased level of chromium and silicon, in
combination with additions of rare earth metals (cerium), gives an increased resistance to high temperature
corrosion, which is exploited in ‘153 MA’, ‘253MA’ and ‘353MA’.
The austenitic stainless steels are used in almost all types of applications and industries. Typical areas of use
include piping systems, heat exchangers, tanks and process vessels for the food, chemical, pharmaceutical, pulp
and paper and other process industries. Non-molybdenum alloyed grades, e.g. 304 and 304L, are normally not
used in chloride-containing media but are often used where demands are placed on cleanliness or in applications in
which equipment must not contaminate the product. The molybdenum-alloyed steels are used in chloride-
containing environment with the higher alloyed steels, ‘904L’, ‘254 SMO’ and ‘654 SMO’, being chosen for
higher chloride contents and temperatures. Grades such as ‘254 SMO’ and ‘654 SMO’ are used to handle sea
water at moderate or elevated temperatures. Applications include heat exchangers, piping, tanks, process vessels,
etc. within the offshore, power, chemical and pulp & paper industries.
The low alloyed grades, especially 304, 304LN and 304N but also 316LN, are used in equipment for cryogenic
applications. Examples are tanks, heaters, evaporator and other equipment for handling of condensed gases such
as liquid nitrogen.
Another use is in high temperature applications or equipment designed for elevated temperature service. In these
cases both the good creep resistance and the good oxidation resistance of the austenitic steels are exploited. High
carbon grades (AISI 304H) and stabilised steels (AISI 321, 347 and 316Ti) or nitrogen-alloyed steels (AISI
304LN and 316LN) are used at elevated and moderately high temperatures depending on the service temperature
and environment. At higher temperatures (above about 750 oC) special high temperature or heat resistant grades
are needed, such as 310, ‘153 MA’, ‘253MA’ and ‘353MA’. Typical applications for the heat resistant steels are
furnace components, muffles, crucibles, hoods, recuperators, cyclones and conveyor belts working at high

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temperatures. The high alloyed heat resistant grades, such as ‘353MA’, are used in aggressive high temperature
environments, such as those encountered in waste incineration.
Finally it is worth mentioning that austenitic stainless steels are often used in applications requiring non-magnetic
materials since they are the only non-magnetic steels.

420F 446
13Cr+0.2S 17Cr-2Mo+0.2S 26Cr
13Cr-0.5Ni-1Mo+V
Duplex
Increased C content High S-content for Increased Cr content stainless
alloying with Mo, V better machinability for better oxidation
for increased high resistance
temperature strength

MARTENSITIC FERRITIC
Decreased Cr Increased Ni
410
content and 430 content for
(13Cr) (17Cr) better toughness
increased
C content for
hardenability

Increased C Increased Cr content

Increased Ni content content for high for better corrosion Increased Mo content
for better toughness strenth resistance. Increased Ni better corosion resistance
for better toughness

420R
FERRITIC-
13Cr-0.12C
MARTENSITIC 431 439
17-2 18Cr-2Mo

13Cr-5Ni 420L
13Cr-0.2C

Low C content and

stabilization to
420 improve toughness
Increased Cr and Mo 13Cr-0.3C and weldability.
content for better
corrosion resistance.
Low C content for
better weld
properties. 444
18Cr-2Mo+Ti
(CODE: Cr-Ni-Mo)
16-5-1

Figure 34 Compositional and property relations for martensitic and ferritic stainless steels.

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"253MA"
21-11-Ce
310H
DUPLEX 25-20-Si

Ce for increased
oxidation resistance
17-12-2.5+0.2S

Ferritic Increased Cr, Ni and Si

Increased Mo content contents for better 303
Decreased Ni content
for better corrosion oxidation resistance. 18-9+0.2S
for better resistance
resistance to stress corrosion
cracking and higher
strength. Oxidation High S content for
"2205" Stress better machinability .
22-5-3 corrosion
AUSTENITIC
Increased Mo och N
content for better Cold work to
301
corrosion resistance. increase strength
17-7

"2507"
25-7-4
Intergranular
corrosion Strength
General corrosion,
pitting and
crevice corrosion Increased N
Ti and Nb for Low C content for higher
better weld for better weld strength.
properties properties.

347 321 304L 304LN

18-10Nb 18-10Ti 18-10L 18-10LN

Increased Cr och Mo content for better corrosion resistance

316Ti 316L 316 316LN

17-11-2Ti 17-11-2L 17-11-2 17-12-2.5LN

Figure 35. 316L high Mo 316 high Mo

17-12-2.5L 17-12-2.5
Compositional and property relations
for austenitic and duplex stainless
steels. 317L
18-13-3L

904L
20-25-4.5L

"254 SMO"
20-18-6LN

"654 SMO"
24-22-7.3LN

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References
1. MNC Handbok nr 4, Rostfria stål
Metallnormcentralen
Stockholm, Sweden, 1983
2. Design Guidelines for the Selection and Use of Stainless Steel.
Specialty Steel Industry of the United States.
Washington, D.C., USA
3. Avesta Sheffield Corrosion Handbook.
Avesta Sheffield AB, 1994.
4. A J Sedriks
Corrosion of Stainless Steels.
John Wiley & Sons, 1979
5. D Peckner, I M Bernstein
Handbook of Stainless Steels.
McGraw-Hill, 1977
6. Corrosion Resistance of the Austenitic Chromium-Nickel Stainless Steels in High Temperature
Environments.
International Nickel.
7. Sandvik Steel
8. H. Nordberg, K. Fernheden (ed.)
Nordic Symposium on Mechanical Properties of Stainless Steels.
Avesta Research Foundation, 1990.
9. Metals Handbook (9:th ed), Vol. 4
American Society for Metals. 1981
10 K.-J. Blom
Press formability of stainless steels
Stainless Steel 77
11. H. Nordberg
Mechanical Properties of Austenitic and Duplex Stainless Steels.
in Stainless Steel 93. Innovation Stainless Steel, Florens, 1993
12. R.E. Johansson, H. L. Groth
Fatigue data for stainless steels.
In Nordic Symposium on Mechanical Properties of Stainless Steels.
H. Nordberg, K. Fernheden (ed.)
Avesta Research Foundation, 1990.

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Chemical composition and US, European and British Standard designations for Stainless Steels

The composition ranges given are valid for the European standards (EN) and the US standards (AISI/ASTM). The different standards should be consulted for detailed information regarding
compositions and composition ranges. Equivalent American and European grades are grouped together and marked with { . The BS grade designations are the equivalents or the closest
available equivalents to the AISI or EN grades.

ASTM EN C N Cr Ni Mo Others BS Avesta

(%) (%) (%)r (%) (%) (%) Polarit
grade
Ferritc and martensitic steels
409 < 0.08 10.5 - 11.75 < 0.5 Ti 409S19 409
1.4512 < 0.03 10.5 - 12.5 Ti 409S19 409
410S < 0.08 11.5 - 13.5 < 0.6 403S17 410S
1.4000 < 0.08 12.0 - 14.0 403S17 410S
410 < 0.15 11.5 - 13.5 < 0.75 410S21 393 HCR
1.4006 0.08 - 0.15 11.5 - 13.5 < 0.75 410S21
0.18 - 0.25 12.0 - 14.0 < 1.0 420S29 13XH
420 > 0.15 12.0 - 14.0 420S45 420
1.4028 0.26 - 0.35 12.0 - 14.0 420S45 420
“420L” 1.4021 0.16 - 0.25 12.0 - 14.0 420S29 420L
430 < 0.12 16.0 - 18.0 < 0.75 430S17 430
1.4016 < 0.08 16.0 - 18.0 430S17 430
431 < 0.20 15.0 - 17.0 1.25 - 2.5 431S29 16-2XH
1.4057 0.12 - 0.22 15.0 - 17.0 1.5 - 2.5 431S29 16-2XH
434 < 0.08 16.0 - 18.0 0.75 - 1.25
444 < 0.025 < 0.035 17.5 - 19.5 < 1.0 1.75 - 2.5 Ti ELI-T 18-2
1.4521 < 0.025 < 0.030 17.0 - 20.0 1.8 - 2.5 Ti ELI-T 18-2
446 < 0.20 < 0.25 23.0 - 27.0 - -
416 < 0.15 12.0 - 14.0 < 0.6 S* 416S21 416
1.4005 0.08 - 0.15 12.0 - 14.0 < 0.6 S* 416S21
*Sulfur addition (normally S = 0.20 - 0.30 %) ** Trademark owned by Sandvik AB

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ASTM EN C N Cr Ni Mo Others BS Avesta
(%) (%) (%)r (%) (%) (%) Polarit
grade
Martensitic-austenitic steels 316S33
1.4313 < 0.05 > 0.020 12.0 - 14.0 3.5 - 4.5 0.3 - 0.7
1.4418 < 0.06 > 0.020 15.0 - 17.0 4.0 - 6.0 0.8 - 1.5 248 SV
Ferritic-austenitic (Duplex) steels
329 < 0.080 23.0 - 28.0 2.5 - 5.0 1.0 - 2.0
S31500 < 0.030 18.0 - 19.0 4.25 - 5.25 2.5 - 3.0 3RE60
1.4460 < 0.05 0.05 - 0.20 25.0 - 28.0 4.5 - 6.5 1.3 - 2.0 25-5-1L
S32304 < 0.030 0.05 - 0.20 21.5 - 24.5 3.0 - 5.5 0.05 - 0.6 Cu SAF 2304**
1.4362 < 0.030 0.05 - 0.20 22.0 - 24.0 3.5 - 5.5 0.1 - 0.6 Cu SAF 2304**
S31803 < 0.030 0.08 - 0.20 21.0 - 23.0 4.5 - 6.5 2.5 - 3.5 318S13 2205
1.4462 < 0.030 0.10 - 0.22 21.0 - 23.0 4.5 - 6.5 2.5 - 3.5 318S13 2205
S32750 < 0.030 0.24 - 0.32 24.0 - 26.0 6.0 - 8.0 3.0 - 5.0 SAF 2507**
1.4410 < 0.030 0.20 - 0.35 24.0 - 26.0 6.0 - 8.0 3.0 - 4.5 SAF 2507**
*Sulfur addition (normally S = 0.20 - 0.30 %) ** Trademark owned by Sandvik AB

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ASTM EN C N Cr Ni Mo Others BS Avesta
(%) (%) (%) (%) (%) (%) Polarit
grade
Austenitic steels
301 < 0.15 < 0.10 16.0 - 18.0 6.0 - 8.0 301s21 17-7
1.4310 0.05 - 0.15 < 0.11 16.0 - 19.0 6.0 - 9.5 < 0.8 301s21 17-7
303 < 0.15 17.0 - 19.0 8.0 - 10.0 S* 303s31* 18-9S
1.4305 < 0.10 < 0.11 17.0 - 19.0 8.0 - 10.0 S* 303s31* 18-9S
304L < 0.030 < 0.10 18.0 - 20.0 8.0 - 12.0 304s11 18-9L
1.4307 < 0.030 < 0.11 17.5 - 19.5 8.0 - 10.0 304s11 18-9L
1.4306 < 0.030 < 0.11 18.0 - 20.0 10.0 - 12.0 304s11 19-11L
304 < 0.08 < 0.10 18.0 - 20.0 8.0 - 10.5 304s31 18-9
1.4301 < 0.07 < 0.11 17.0 - 19.5 8.0 - 10.5 304s31 18-9
304LN < 0.030 0.10 - 0.16 18.0 - 20.0 8.0 - 12.0 304s61 18-9LN
1.4311 < 0.030 0.12 - 0.22 17.0 - 19.5 8.5 - 11.5 304s61 18-9LN
321 < 0.08 < 0.10 17.0 - 19.0 9.0 - 12.0 Ti 321s31 18-10Ti
1.4541 < 0.08 17.0 - 19.0 9.0 - 12.0 Ti 321s31 18-10Ti
347 < 0.08 17.0 - 19.0 9.0 - 13.0 Nb 347s31 18-10Nb
1.4550 < 0.08 17.0 - 19.0 9.0 - 12.0 Nb 347s31 18-10Nb
316L < 0.030 < 0.10 16.0 - 18.0 10.0 - 14.0 2.0 - 3.0 316s11 17-11-2L
1.4404 < 0.030 < 0.11 16.5 - 18.5 10.5 - 13.0 2.0 - 2.5 316s11 17-11-2L
“316L(hMo)” 1.4432 < 0.030 < 0.11 16.5 - 18.5 10.5 - 13.0 2.5 - 3.0 316s13 17-12-2.5L
“316L(hMo)” 1.4435 < 0.030 < 0.11 17.0 -19.0 12.5 -15.0 2.5 - 3.0 316S13 17-12-2.5L
316 < 0.08 < 0.10 16.0 - 18.0 10.0 - 14.0 2.0 - 3.0 316s31 17-11-2
1.4401 < 0.07 < 0.11 16.0 - 18.5 10.0 - 13.0 2.0 - 2.5 316s31 17-11-2
“316(hMo)” 1.4436 < 0.05 < 0.11 16.5 - 18.5 10.5 - 13.0 2.5 - 3.0 316S33 17-12-2.5
316LN < 0.030 0.10 - 0.16 16.0 - 18.0 10.0 - 14.0 2.0 - 3.0 316s33 17-11-2LN
1.4406 < 0.03 0.12 - 0.22 16.5 - 18.5 10.0 - 12.0 2.0 - 2.5 316s33 17-11-2LN
1.4429 < 0.03 0.12 - 0.22 16.5 - 18.5 11.0 - 14.0 2.5 - 3.0 17-13-3LN
316Ti < 0.08 < 0.10 16.0 - 18.0 10.0 - 14.0 2.0 - 3.0 Ti 320s31 17-11-2Ti
1.4571 < 0.08 16.5 - 18.5 10.5 - 13.5 2.0 - 2.5 Ti 320s31 17-11-2Ti

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ASTM EN C N Cr Ni Mo Others BS Avesta
(%) (%) (%) (%) (%) (%) Polarit
grade
Austenitic steels (cont.)
317L < 0.030 < 0.10 18.0 - 20.0 11.0 - 15.0 3.0 - 4.0 317s12 18-14-3L
1.4438 < 0.030 < 0.11 17.5 - 19.5 13.0 - 16.0 3.0 - 4.0 317s12 18-14-3L
S31726 < 0.030 0.10 - 0.20 17.0 - 20.0 13.5 - 17.5 4.0 - 5.0 17-14-4LN
1.4439 < 0.030 0.12 - 0.22 16.5 - 18.5 12.5 - 14.5 4.0 - 5.0 17-14-4LN
N08904 < 0.020 19.0 - 23.0 23.0 - 28.0 4.0 - 5.0 Cu 904s13 904L
1.4539 < 0.020 < 0.15 19.0 - 21.0 24.0 - 26.0 4.0 - 5.0 Cu 904s13 904L
S31254 < 0.020 0.18 - 0.22 19.5 - 20.5 17.5 - 18.5 6.0 - 6.5 Cu 254 SMO
1.4547 < 0.020 0.18 - 0.25 19.5 - 20.5 17.5 - 18.5 6.0 - 6.5 Cu 254 SMO
S32654 < 0.020 0.45 - 0.55 24.0 - 25.0 21.0 - 23.0 7.0 - 8.0 Cu, Mn 654 SMO
1.4652 < 0.020 0.45 - 0.55 24.0 - 25.0 21.0 - 23.0 7.0 - 8.0 Cu, Mn 654 SMO
N08028 < 0.030 26.0 - 28.0 30.0 -34.0 3.0 - 4.0 Cu A 28
1.4563 < 0.020 < 0.11 26.0 - 28.0 30.0 - 32.0 3.0 - 4.0 Cu A 28
Heat resistant austenitic steels
304H 0.04 - 010 18.0 - 20.0 8.0 - 10.5
321H 0.04 - 010 17.0 - 19.0 9.0 - 12.0 Ti
309S 23-13
1.4833 23-13
310S < 0.08 24.0 - 26.0 19.0 - 22.0 310S16 25-20
1.4845 < 0.08 24.0 - 26.0 19.0 - 22.0 310S16 25-20
1.4828 20-12Si
S30415 0.04 - 0.06 0.12 - 0.18 18.0 - 19.0 9.0 - 10.0 Ce, Si 153MA
1.4818 < 0.08 18.0 - 20.0 9.0 - 11.0 Ce , Si 153MA
S30815 0.05 - 0.10 0.14 - 0.20 20.0 - 22.0 10.0 - 12.0 Ce, Si 253MA
1.4835 < 0.10 20.0 - 22.0 10.0 - 12.0 Ce, Si 253MA
S35315 0.04 - 0.08 0.12 - 0.20 24.0 - 26.0 34.0 - 36.0 Ce, Si 353MA
1.4854 < 0.08 24.0 - 26.0 34.0 - 36.0 Ce, Si 353MA
* Free machining steel, Sulfur addition (normally S = 0.20 - 0.30 %)

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