M.

E (WELDING ENGINEERING): 1st SEMESTER

PHYSICAL METALLURGY

Lecture by

Dr.V.BALASUBRAMANIAN, M.E., Ph.D (IITM),

Professor & Director
Centre for Materials Joining & Research (CEMAJOR),
Department of Manufacturing Engineering,
ANNAMALAI UNIVERSITY
 UNIT - III

• Classification of steels
• Purpose of alloying
• Effect of important alloying elements
• Iron- Carbon equilibrium diagram
• Isothermal Transformation  (IT) diagram
• Time Temperature Transformation (TTT)
Diagram, 
• Continuous cooling transformation (CCT)
diagrams
 BASIS FOR CLASSIFICATION OF STEELS
•The composition, such as carbon, low-alloy or stainless steel.

•The manufacturing methods, such as open hearth, basic oxygen process, or

electric furnace methods.

•The finishing method, such as hot rolling or cold rolling

•The product form, such as bar plate, sheet, strip, tubing or structural shape

•The deoxidation practice, such as killed, semi-killed, capped or rimmed steel

•The microstructure, such as ferritic, pearlitic and martensitic

•The required strength level, as specified in ASTM standards

•The heat treatment, such as annealing, quenching and tempering, and

thermomechanical processing

•Quality descriptors, such as forging quality and commercial quality.

 BASIC CLASSIFICATION OF STEELS

• Carbon Steel
• Low Alloy Steel (Alloying Elements < 8%)
• High Alloy Steel (Alloying Elements > 8%)
 DEFINITION OF CARBON STEEL (AS PER AISI)

• Steel is considered to be carbon steel when no minimum

content is specified or required for chromium, cobalt,
columbium [niobium], molybdenum, nickel, titanium,
tungsten, vanadium or zirconium, or any other element to
be added to obtain a desired alloying effect;
• when the maximum content specified for any of the
following elements does not exceed the percentages noted:
manganese 1.65, silicon 0.60, copper 0.60.
 Types of Carbon Steels

• Low Carbon Steels ( < 0.3% C)

• Medium Carbon Steels (0.3% to 0.6% C)
• High Carbon Steels (0.6% to 1.0% C)
• Ultra High Carbon Steels (1.25% to 2.0% C)
 LOW CARBON STEELS
• Contains about 0.1 %C
• Not heat treated, as hardenability is low to produce
martensite
• Widely used in the form of cold rolled sheets
• Microstructure consists of essentially Ferrite, with a small
amount of Pearlite.
• Typical uses are in automobile body panels, tin plate, and
wire products.

Typical Mechanical Properties

• YS = 200 – 300 MPa
• TS = 300 – 370 MPa
• % Elongation = 28 – 40%
 MILD STEEL
• Contains 0.15 % C – 0.25 % C
• Not heat treated, but used in as rolled and air-cooled
conditions
• Micro-structure is about 25 % of fine Pearlite interspersed in
Ferrite Matrix
• Possess Excellent Weldablity
• These materials may be used for stampings, forgings,
seamless tubes, and boiler plate.

Typical Mechanical Properties

• YS = 300 – 350 MPa
• TS = 400 – 450 MPa
• % Elongation = 26 – 30 %
 MEDIUM CARBON STEEL
• These steels are similar to low-carbon
steels except that the carbon ranges
from 0.30 to 0.60% and the
manganese from 0.60 to 1.65%.
• Increasing the carbon content to
approximately 0.5% with an
accompanying increase in manganese
allows medium carbon steels to be
used in the quenched and tempered
condition.
• The uses of medium carbon-
manganese steels include shafts,
axles, gears, crankshafts, couplings Mechanical Properties:
and forgings. YS = 300 – 320 MPa
• Steels in the 0.40 to 0.60% C range TS = 500 - 550 MPa
are also used for rails, railway wheels % Elongation = 12-16 %
and rail axles.
 HIGH CARBON STEELS
• High-carbon steels contain from 0.60 to 1.00% C with
manganese contents ranging from 0.30 to 0.90%.
• High-carbon steels are used for spring materials and high-
strength wires

Mechanical Properties:
YS = 450-490 MPa
TS = 600-635 MPa
% Elongation = 8-10 %
 ULTRA HIGH CARBON STEELS
• These steels are experimental alloys containing
1.25 to 2.0% C.
• These steels are thermomechanically processed to produce
microstructures that consist of ultrafine, equiaxed grains of
spherical, discontinuous proeutectoid carbide particles.
 TYPES OF LOW ALLOY STEELS
• Low-carbon quenched and tempered (QT) steels,
• Medium-carbon ultrahigh-strength steels,
• Bearing steels,
• Heat-resistant chromium-molybdenum steels
 LOW-CARBON QUENCHED AND TEMPERED (QT) STEELS

• Combine high yield strength (from

350 to 1035 MPa) and high tensile
strength with good notch toughness,
ductility, corrosion resistance, or
weldability.
• The various steels have different
combinations of these characteristics
based on their intended applications.
• However, a few steels, such as HY-80
and HY-100, are covered by military
specifications.
• The steels listed are used primarily
as plate.
• Some of these steels, as well as
other, similar steels, are produced as
forgings or castings.
 MEDIUM-CARBON ULTRA HIGH-STRENGTH
STEELS
• These steels are structural
steels with yield strengths that
can exceed 1380 MPa.
• Many of these steels are
covered by SAE/AISI
designations or are
proprietary compositions.
• Product forms include billet,
bar, rod, forgings, sheet,
tubing, and welding wire.
 BEARING STEELS

• Comprised of low carbon (0.10 to 0.20% C) case-hardened

steels and high carbon (-1.0% C) through-hardened steels.
• Many of these steels are covered by SAE/AISI designations.
• Used for ball and roller bearing applications
 HEAT-RESISTANT CHROMIUM-
MOLYBDENUM STEELS
• These steels contain 0.5 to
9% Cr and 0.5 to 1.0% Mo.
• The carbon content is usually below
0.2%.
• The chromium provides improved
oxidation and corrosion resistance,
and the molybdenum increases
strength at elevated temperatures.
• They are generally supplied in the
normalized and tempered,
quenched and tempered or
annealed condition.
• Chromium-molybdenum steels are
widely used in the oil and gas
industries and in fossil fuel and
nuclear power plants.
 HIGH STRENGTH LOW ALLOY (HSLA) STEEL

• Carbon 0.15 % - 0.25 % (same as MS)

• Small quantities of additional alloying elements

Microstructure
Ferrite and Bainite / Ferrite and Martensite

Typical Mechanical Properties

• YS = 400-700 MPa
• TS = 500 – 800 MPa
• % Elongation = 18–25%
 TYPES OF HSLA STEELS
 Weathering steels: designated to exhibit superior atmospheric corrosion
resistance
 Control-rolled steels: designed to develop a highly deformed austenite
structure by hot rolling that will transform to a very fine equiaxed ferrite
structure on cooling
 Pearlite-reduced steels: strengthened by very fine-grain ferrite and
precipitation hardening but with low carbon content and therefore little
or no pearlite in the microstructure
 Microalloyed steels: with very small additions of such elements as
niobium, vanadium, and/or titanium for refinement of grain size and/or
precipitation hardening
 Acicular ferrite steel: very low carbon steels with sufficient hardenability
to transform on cooling to a very fine high-strength acicular ferrite
structure rather than the usual polygonal ferrite structure
 Dual-phase steels: processed to a micro-structure of ferrite containing
small uniformly distributed regions of high-carbon martensite thus
providing a high-strength steel of superior formability
 MICRO-ALLOYED STEELS
• To the basic composition of mild steel, small amounts of
strong carbide forming elements such as Niobium,
Vanadium, and Titanium are added.
• Total concentration of micro-alloying elements is less than
0.2 %
• Fine Ferrite grain size (ASTM 12-14)
• Yield Strength increases with decreasing grain size
• Fine Dispersion of alloy carbides result in Precipitation
hardening
• Impact Transition Temperature (ITT) decreases with
decreasing grain size
Typical Mechanical Properties
• YS = 400-500 MPa
• TS = 600 – 650 MPa
• % Elongation = 20–22%
 MARAGING STEELS
• Ultra high strength group
• Martensite + Aging = Maraging
• C- 0.03 %; Ni-18%; Co-3.8%; Mo-3.8%; a fraction of Ti and Al
• Ys = 1800 MPa and FT 120MN-3/2
• Weldability is Good due to softness of Martensite
 AISI - SAE CLASSIFICATION SYSTEM
AISI XXXX
American Iron and Steel Institute (AISI)
• classifies alloys by chemistry
• 4 digit number
– 1st number is the major alloying element
– 2nd number designates the subgroup alloying
element OR the relative percent of primary
alloying element.
– last two numbers approximate amount of carbon
(expresses in 0.01%)
Examples:

2350
2550
4140
1060
 STANDARDS

INDIAN BRITISH BRITISH AISI – SAE

(Old) (New)

55 Cr 70 En 11 526 M 60 5155

40Ni2Cr1Mo25 En 24 817 M 40 4340

C100 En 44 060 A 96 1095

50Cr1V23 En 47 735 A 50 6150

 PURPOSE OF ALLOYING
• Increases Hardenability
• Improve Strength at ordinary temperatures
• Improve mechanical properties at either high
or low temperatures
• Improve toughness at any minimum hardness
or strength
• Increases Wear Resistance
• Increase Corrosion Resistance
• Improve Magnetic Properties
 TYPES OF ALLOYING ELEMENTS

GROUP I
• Elements which dissolve in ferrite

• GROUP II
• Elements which combine with carbon to form
carbides
 Effect of Sulfur
• It should be less than 0.05%
• Sulfur combines with Iron to form Iron Sulfide (FeS)
• FeS concentrates at Grain Boundaries
• If steel is forged or rolled at elevated temperature,
the steel will become brittle
• This phenomenon is known as ‘HOT SHORTNESS’
• Free Machining steels will contain 0.08%-0.35%
 Effect of Phosphorous
• It should be below 0.04%
• Dissolves in Ferrite
• Increases strength and hardness slightly
• 0.07% - 0.12% improves cutting properties
• Larger quantities, reduces ductility.
• Phosphur combines with Iron and forms Iron
Phosphide
• During cold working, the steels get cracked
• This phenomenon is known as “COLD SHORTNESS”
 Effect of Mn and Si
• Mn will be 0.03 to 1.0%
• Counteract illeffects of Sulfur by forming MnS
• 2 to 8% times Sulfur is required
• Soundness or Steel Casting Improves

• Si will be 0.05 to 0.3%

• Si dissolves in Ferrite, increasing the strength of the
Steel
• Promotes Oxidation
 STAINLESS STEELS
• All SS will contain Cr which gives corrosion
resistance
• Minimum Cr required is 12%
• Cr shuld be in the form of solute solution and
not in the form of intermetallics
• Most of the SS are dominated by single phase
• Austenite Stabilisers: C, N, Mn, Ni
• Ferrite Stabilisers: Cr, Mo, V, W, Ti, Nb, Al
 Stainless Steels

 The essential property of stainless steels is their resistance to

corrosion, especially in saline solutions, under oxidizing
conditions.
 These properties are the result of a thin adhesive film of Cr2O3,
which forms on the surface at room temperature and which is self
healing when scratched or otherwise damaged.
 In general a minimum concentration of 12% Cr is required to
obtain a film that completely covers the exposed surface of a
sample
 The Cr2O3 in the steel is very stable against attack by a number of
chemicals and electrolytic corrosion actions.
 Types of Stainless Steels

In general, there are five types of stainless steels based on their crystal
structure and strengthening mechanisms. They are:

• Ferritic stainless steels (FSS)

• Austenitic stainless steels (ASS)
• Duplex Stainless Steels (DSS)
• Martensitic stainless steels (MSS)
• Precipitation-hardened stainless steels (PHSS)
 FERRITIC STAINLESS STEEL

• Cr:12-30%; C:0.10%; Fe: Remaining

• BCC
• DBT occurs above zero deg C
• SCC resistance is good
• Cheaper (No Nickel)
• If Cr is 12-16%...Corrosion Resistance
• If Cr is above 16%...Heat Resistance
FSS
 Austenitic Stainless Steel
• Cr: 12-18%; Ni:8%; C:0.10%; Fe: Balance
• FCC
• Absence of DBT
• Easily formable and weldable
• Good at elevated and low temperatures
• Excellent corrosion resistance
• Expensive because of Ni
• YS is not so high
• SCC resistance is not so good
• More than 90% used
ASS
 DUPLEX STAINLESS STEEL
• 50% Austenite and 50% Ferrite
• C:0.02%; Cr:22%; Ni:5%; Mo:3%; N:0.15%; Fe:
Remaining
• More easily weldable than FSS
• Not as easily weldable than ASS
• Decreased DBT than FSS
• YS is higher than ASS
• Widely used in Paper and Pulp industries,
Petrochemical industries
• Excellent material for Pitting corrosion resistance
DSS
Effect of Nickel
 Martensitic Stainless Steel

• C:0.2%; Cr:12%; Mo:1%; Fe:Balance

• Hardest of all stainless steel
• Creep resistance is very good
• Least corrosion resistance
• High temperature applications
• Surgical applications
MSS
 PH STAINLESS STEEL
• Precipitation Hardenable SS
• C:0.1%; Cr:17%; Ni:4%; Fe: Remaining
• Used in Aircraft applications
• High strength is derived from Precipitation
• UTS is 1200-1400 MPa

• AISI 300….Austenitic Stainless Steel

• AISI 400….FSS and MSS
17-7 PHSS
IRON-CARBON EQUILIBRIUM DIAGRAM
Iron-Carbon Diagram
 FERRITE ()
• Interstitial solid solution of
carbon in BCC iron
• It is stable over temperature
range -273 0C to 912 0C
• Size of the largest atom that
can fit into this site is 0.91A
• Solubility is very limited
• Maximum Solubility is 0.025
wt % at 727 0C
• It is the softest structure in Fe-
C Diagram
• TS = 280 MPa (approx)
• Elongation 40 % in 50 mm
• Hardness : less than RC0 or RC
B 90
 FERRITE (δ )
• Interstitial solid solution
(BCC)
• It nucleates from the melt
of the steel that contains
less than 0.53 % C
• Stable from 1394 0C to 1539
0
C (MP)
• Maximum solubility of
Carbon is 0.10 % at 1495 0C
 AUSTENITE
• Interstitial solid solution of
carbon in FCC iron
• Stable from 912 0C to 1394
0
C
• Size of the largest atom A

that can fit into this site is

0.52A
• Solubility is higher when
compared to ferrite
• Maximum Solubility is 2.11
wt % at 1146 0C
 CEMENTITE (Fe3C)

• It is an intermetallic
compound
• Contains 6.67 wt % carbon
• Hardest structure in Fe-C
diagram
• Low tensile strength and
High Compressive strength
• Crystal structure is
orthorhombic (12 Fe + 4C in
unit cell)
 PEARLITE
• Eutectoid mixture of
ferrite + cementite
• Ferrite matrix and thin
plates of cementite
• Typical Elongation is 20
% in 50 mm
• Hardness Rc 20
 LADEBURITE

• Eutectic mixture of
Austenite and Cemetite
• Forms from liquid that
contain 4.3% C during slow
cooling at 1130 °C
• Not stable below 723 C
because where Austenite
of Ladeburite transforms
into Pearlite.
• Then the structure is called
‘Transformed Ladeburite’.
ISOTHERMAL TRANSFORMATION
(IT) DIAGRAM
 EXPERIMENTAL PROCEDURES

• Step1: Prepare large no of samples cut from same bar

• Step2: Place sample at austenizing temperature
• Step3: Place sample at molten salt bath at constant
temperature.
• Step4: After varying the time, quench the sample in
cold water
• Step5: After cooling check hardness and study
microscopically.
• Step6: Repeat the same at different sub critical
temperatures.
SCHEMATIC REPRESENTATION OF EXPERIMENTAL SET UP
SCHEMATIC REPRESENTATION OF
SPECIMEN ANALYSIS
ISO-THERMAL TRANSFORMATION CURVE
Transformation V/s
logarithm of Time
plot

Isothermal
Transformation
diagram plot
 Transformation Transformation
starts/begins ends
Stable Austenite

Coarse Pearlite
Unstable Austenite

Fine Pearlite
400 C -600 C range
0 0

Feathery Bainite

Unstable Austenite Acicular Bainite

Ms ≈ 2500C

M 90% at 1100C Austenite + Martensite

Mf ≈-500C
Martensite Time-Temperature Transformation Curves
TIME-TEMPERATURE-TRANSFORMATION (TTT) DIAGRAM
 Transformation Transformation
starts/begins ends

A- fine Pearlite

Continuous cooling Transformation

EFFECT OF
DIFFERENT
COOLING RATES
A - 50C/sec(normalising)

B- M + little Au B - Rapid 4000C/sec (water quench)

C D
C - 1400C/sec- Martensitic
Continuous Cooling D - 500C/sec- P + M + Au
Transformation diagram
 Moderately rapid
[1] and slow
cooling [2] curves
[2] superimposed on
Full
continuous
Annealing cooling
transformation
diagram for
EUTECTOID
[1]
Normalising
Dependence of final
microstructure on
transformations
occurring during
cooling;
Martensite will form
for quenching rates
greater than the
critical
POSSIBLE TRANSFORMATIONS
 SEMINAR
No. Student Name Seminar Topic
1. APPURAJA ANNEALING TREATMENT
2. ARUNKUMAR SPEROIDIZING & NORMALISING TREATMENTS
3. DHARMASEELAN AUSTEMPERING & MARTEMPERING
4. HARIPRASATH HARDENABILITY OF STEEL
5. MELWIN MELLO CARBURIZING
6. MOHAN NITRIDING
7. RAJPRADEEP CYANIDING
8. SADHASIVAM CARBONITRIDING
9. SATHISH FLAME HARDENING
10. SILAMBARASAN INDUCTION HARDENING
11. VIJAYAKUMAR VACUUM HARDENING
12. ARUN PRASATH CRYOGENIC TREATMENT