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CHAPTER
9
Engineering
Alloys
9-1

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Production of Iron and Steel

• Production of pig iron

Fe2O3 + 3CO 2Fe + 3CO2

Ore Coke
Pig iron
(Liquid)

Blast Furnace

Figure 9.1
9-2 After A. G. Guy,”Elements of Physical Metallurgy,”2nd ed., !959, Addision-Wesley, Fig. 2-5, p.21.

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Steel Making
• Pig iron and 30% steel crap is fed into refractory
furnace to which oxygen lane is inserted.
• Oxygen reacts with liquid bath to form iron oxide.
• FeO + C Fe + CO
• Slag forming fluxes
are added.
• Carbon content and
other impurities are
lowered.
• Molten steel is
continuously cast and Figure 9.2
formed into shapes.

9-3 Courtesy of Inland Steel

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Iron Carbide Phase Diagram

• Plain carbon steel 0.03% to 1.2% C, 0.25 to 1%

Mn and other impurities.
• α Ferrite: Very low solubility
of carbon. Max 0.02 % at 7230C
and 0.005% at 00C.
• Austenite: Interstitial solid
solution of carbon in γ
iron. Solubility of C is
2.08% at 11480C and 0.8%
at 00C.
• Cementite: Intermetallic compound. Figure 9.6

6.67% C and 93.3% Fe.

9-4

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Invariant reactions

• Peritectic reaction:
14950C
Liquid (0.53%C) + δ (0.09% C) γ (0.17% C)

• Eutectic reaction:
0
Liquid (4.3% C) 1148 C γ austenite (2.08%C) + Fe3C ( 6.67%C)

• Eutectoid reaction:
0
γ Austenite (0.8%C) 723 C α Ferrite(0.02%C) + Fe3C ( 6.67%C)

Hypoeutectoid Less than 0.8% 0.8% C More than 0.8% Hypereutectoid

Steel Eutectoid Steel Steel

9-5

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Slow Cooling of Plain Carbon Steel

• Eutectoid plain carbon steel: If a sample is heated up

to 7500C and held for sufficient time, structure will
become homogeneous austenite.

• Below eutectoid temperature,

layers of ferrite and cementite
are formed. Pearlite.

Figure 9.7 Figure 9.8

9-6 After W. F. Smith, “The Structure and Properties of Engineering Alloys,” 2nd ed.,McGraw-Hill, 1981, p.8

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Slow Cooling of Plain Carbon Steel (Cont..)

• Hypoeutectoid plain carbon steel: If a sample of 0.4%

C is heated up to 9000C, it gets austenitized.
• Further cooling gives rise to α and pearlite.
Pearlite

Figure 9.9 Figure 9.10

9-7 After W. F. Smith, “The Structure and Properties of Engineering Alloys,” 2nd ed.,McGraw-Hill, 1981, p.10

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Slow Cooling of Plain Carbon Steel (Cont..)

• Hypereutectoid plain carbon steel: If a 1.2% C sample

is heated up to 9500C and held for sufficient time, it
entirely gets austenitized.
• Further cooling results results in eutectoid cementite
and pearlite.

Figure 9.11
9-8 After W. F. Smith, “The Structure and Properties of Engineering Alloys,” 2nd ed.,McGraw-Hill, 1981, p.12.

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Heat treatment of plain carbon steels.

• Heating and cooling properties of steels vary

mechanical properties.
• Martensite: Metastable phase consisting of super
saturated solid solution of C in BCC or BCC tetragonal
iron.
• Caused by rapid cooling of austenitic steel into room
temperature (quenching).
Ms temperature of martensite start.
Mf temperature of martensite finish.

9-9

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Microstructure of Fe – C Martensites
• Lath martensite: Less than 0.6% C and consists of
domains of lathe of different orientation.
• Plate martensite: More than 0.6% C and have fine
structure of parallel twins.

Lath type Plate type

Figure 9.14

Figure 9.13

9-10 After A. R. Marder and G. Krauss, as presented in “Hardenebility Concepts with Applications to Steel,” AIME, 1978, p. 238.

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Martensite (Cont..)

• Transfer to martensite is diffusionless.

• No change of relative position of carbon atoms after
transformation.

Figure 9.17

• Strength and hardness increases

with carbon content.
• Strength is due to high dislocation
concentration and interstitial solid
solution strengthening. Figure 9.19

9-11After E. R. Parker and V. F. Zackay Strong and Ductile Steels, Sci.Am.,November 1968, p.36; Copyright by Scientific
American Inc; all rights reserved

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Isothermal decomposition of Austenite.

• Several samples are first austenitized above eutectoid
temperature and rapidly cooled in sand bath to desired
temperature in a salt bath and then quenched in water at various
time intervals.

Repeat
procedure
at

Figure 9.20 progressive

lower
temperatures

Figure 9.22

Figure 9.21
9-12 After W. F. Smith, “The Structure and Properties of Engineering Alloys,” McGraw-Hill, 1981, p.14

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Isothermal decomposition of Austenite (Cont..)

• If hot quenching temperature is between 5500C to

2500C, an intermediate structure Bainite is produced.
• Bainite contain nonlamellar eutectoid structure of α
ferrite and cementite.
• Upper Bainite Between 5500C and 3500C
• Lower Bainite Between 3500C and 2500C

Upper Bainite Lower Bainite Figure 9.24

9-13 After H. E. McGannon(ed.), “The Making Shaping and Treating of Steel,” 9th ed., United States Steel Corp., 1971

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Continuous Cooling-Transformation Diagram

• In continuous cooling transformation from martensite

to pearlite takes place at a range of temperature.
• Start and finish lines shifted to longer time.
• No transformation below 4500C.

Figure 9.27

Figure 9.26
9-15 After R. A. Grange and J. M. Kiefer, “Alloying Elements in Steel,” ASM 2nd ed., 1966, p.254.

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Annealing and Normalizing

• Full annealing: Sample heated to 400C above austenite

ferrite boundary, held for necessary time and cooled
slowly.
• Process annealing: Used for stress
relief. Applied to hypoeutectoid
steel at eutectoid temperature.
• Normalizing: Steel heated in
austenite region and cooled
in still air.
• Makes grain structure
uniform
• Increases strength
Figure 9.28

9-16 After T. G. Diggers et al., “ Heat Treatment and Properties of Iron and Steel,” NBS Monograph 88, 1966, p. 10

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Tempering of Plain Carbon Steel

• Martensitic steel is heated at a temperature below

eutectic temperature.
• Makes steel softer and ductile.
• Carbon atoms, in low carbon
steels, segregate themselves on
tempering.
Figure 9.29
Tempering
Temperature Structure

Below 2000C Epsilon Carbide

200 – 7000C Cementite (rod-like)
400 – 7000C Cementite (Spheroidite)

Figure 9.31
9-17 From “ Suiting the heat Treatment to the job,” United States Steel Corp., 1968, p.34.

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Effects of Tempering

• Hardness decreases as temperature increases above

2000C
• This is due to diffusion of
carbon atoms from interstitial
sites to iron carbide precipitates.

Figure 9.32
9-18 After JE. C. Bain, and H. W. Paxton, “Alloying Elements in Steel, “ 2nd ed., American Society for Metals, 1996 p.38.

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Martempering and Austempering

• Martempering (Marquenching): Austinitizing,

quenching at around Ms, holding in quenching media
until temperature is uniform, removing before Bainite
forms and cooling at a moderate rate.
• Austempering: Same as martempering but held at
quenching media till austenite to Bainite
transformation takes place. Table 9.2

9-19 Source: “Metals Handbook,” vol. 2, 8th ed., American Society for Metals, 1964.

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Martempering (marquenching)
• austenitizing the steel,
• quenching it in hot oil or molten salt at a temperature just slightly above the Ms
temperature
• holding the steel in the quenching medium until the temperature is uniform throughout and
stopping this isothermal treatment before the austenite-to-bainite transformation begins
• cooling at a moderate rate to room temperature to prevent large temperature differences.
• The steel is subsequently tempered by the conventional treatment.

Austempering
1. austenitizing the steel,
2. quenching it in a molten salt at a temperature just above the Ms temperature
3. holding the steel isothermally to allow treatment the austenite-to-bainite
transformation take place
4. cooling at a moderate rate to room temperature in air
5. The final structure of an austempered eutectoid plain-carbon steel is bainite.

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Example:
0.25 mm thick strips of 1080 steel heated at 850 C for 1 h and then the tratment
listed below. Determine the microstructure of the sample after each heat treatment.
a) Water quench at room temp.; b) Hot quench in molten salt to 690 C and hold 2 h, water
quench.; c) Hot quench to 610 C and hold 3 min, water quench.; d) Hot quench to 580 C and
hold 2 sec, water quench.; e) Hot quench to 450 C and hold 1 h, water quench.; f) Hot
quench to 300 C and hold 30 min, water quench.; g) Hot quench to 300 C and hold 5 h,
water quench.

a) all matensite, b) all coarse pearlite, c) all fine pearlite, d) approximately 50% fine pearlite and 50% martensite, e) all
upper bainite, f) approximately 50% lower bainite and 50% martensite, g) all lower bainite

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Calssification of Plain Carbon Steel

• Four digit AISI-SAE code.

• First two digits, 10, indicate plain carbon
steel.
• Last two digits indicate carbon content in
100th wt%.
• Example: 1030 steel indicate plain carbon
steel containing 0.30 wt% carbon.
• As carbon content increase, steel becomes
stronger and ductile.

9-20

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Low Alloy Steels

• Limitations of plain carbon steels:

 Cannot be strengthened beyond 690 MPa without
loosing ductility and impact strength.
 Not deep hardenable.
 Low corrosion resistance
 Rapid quenching leads to crack and distortion.
 Poor impact resistance at low temperature.

• Alloy steels: Up to 50% alloying elements like

manganese, nickel, chromium, molybdenum and
tungsten.

9-21

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Classification of Alloy Steels

• First two digits: Principle alloying element.

• Last two digits: % of carbon.

9-22 Source: “Alooy Steel: Semifinished; Hot-Rolled and Cold-Finished Bars,” American Iron and Steel Institute, 1970.

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Effects of Alloying Element on Eutectoid Temperature

• Mn and Ni lower eutectoid temperature.

• They act as austenite stabilizing
element.
• Tungsten, molybdenum
and titanium raise
eutectic temperature.
• They are called ferrite
stabilizing elements.

Figure 9.35
9-24 Source: “Metals Handbook,” vol. 2, 9th ed., American Society for Metals, 1973.

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Hardenability
• Hardenability determines the depth and distribution of
hardness induced by quenching.
• Hardenability depends on
 Composition
 Austenitic grain size
 Structure before
quenching
• Joming hardenability test:
 Cylindrical bar (1 inch dia and 4
inch length with 1/16 in flange
at one end is austenitized and one
end is quenched.
 Rockwell C hardness is measured Figure 9.36b
up to 2.5 inch from quenched end.

9-25 After H. E. McGannon(ed.), “The Making Shaping and Treating of Steel,” 9th ed., United States Steel Corp., 1971, p.1099

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Hardenability (cont..)

• For 1080 plain carbon steel, the hardness value at

quenched end is 65 HRC while it is 50 HRC at 3/16
inch from quenched end.
• Alloy steel 4340 has high
hardenability and has
hardness of 40 HRC 2 inches
from quenched end. Figure 9.37
• In alloy steel, decomposition
of austenite to ferrite is delayed.
• Cooling rate depends on bar dia,
quenching media and bar cross
section.
Figure 9.38
9-26 After H. E. McGannon(ed.), “The Making Shaping and Treating of Steel,” United States Steel Corp., 1971, p.1139.

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Aluminum Alloys

• Precipitation Strengthening : Creates fine dispersion

of precipitated particles in the metal and hinder
dislocation movement.
• Basic steps :
 Solution heat treatment: Alloy sample
heated to a temperature between solvus and
solidus and soaked at that temperature.
 Quenching: Sample then quenched to room
temperature in water.
 Aging: Solutionized and quenched sample is
then aged to form finely dispersed particles.

9-28

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Effects of Aging on Strength

• Aging curve: Plot of strength or hardness versus aging

time.
• As aging time increases
alloy becomes stronger
harder and less ductile.
• Overaging decreases
strength and hardness.

Figure 9.43

9-30

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Wrought Aluminum Alloys

• Primary Fabrication: Usually semiconsciously cast by
direct chill method.
• Scalping: ½ inch metal is removed from hot rolled
surface for good finishing.
• Ingots are homogenized and rolled. Table 9.7

• Classification: According to
major alloying elements.
• Four digits: First digit -
major group of alloying
elements.
• Second digit: Impurity limits.
• Last 2 digits: Identify
aluminum alloy.
9-34

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Temper Designations

• Temper designations are designated by hyphen.

• Example: 2024-T6

T1 – Naturally aged
T3 – Solution heat treated.
F – as fabricated H1 – Strain hardened T4 – Solution heat treated
O – Annealed alloy. and naturally aged.
H – Strain hardened. H2 – Strain hardened T5 - Cooled and artificially
T – Heat treated to and partially aged.
T6 - Solution heat treated
produce stable annealed.
and artificially aged.
temper H3 - Strain hardened T7 - Solution heat treated
an annealed and stabilized.
T8 - Solution heat treated,
cold worked and then
artificially aged.

9-35

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Non Heat Treatable Aluminum Alloys

• 1xxx alloys : 99% Al + Fe + Si + 0.12% Cu

Tensile strength = 90 MPa
Used for sheet metals
• 3xxx alloys : Manganese is principle alloying element.
Al 3003 = Al 1100 + 1.25% Mn
Tensile strength = 110 MPa
General purpose alloy
• 5xxx alloys: Al + up to 5% Mg
Al5052 = Al + 25%Mg + 0.2% Cr
Tensile strength = 193 MPa
Used in bus, truck and marine sheet metals.

9-36

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Heat Treatable Aluminum Alloys

• 2xxx alloys : Al + Cu + Mg
Al2024 = Al + 4.5% Cu + 1.5% Mg +0.6%Mn
Strength = 442 MPa
Used for aircraft structures.
• 6xxx alloys: Al + Mg + Si
Al6061 = Al + 1% Mg + 0.6%Si + 0.3% Cu + 0.2% Cr
Strength = 290 MPa
Used for general purpose structure.
• 7xxx alloys: A + Zn + Mg + Cu
Al7075 = Al + 5.6% Zn + 2.5% Mg + 1.6% Cu + 0.25% Cr
Strength = 504 MPa
Used for aircraft structures.

9-37

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Aluminum Casting
• Sand Casting: Simple and used for small
quantities and complex jobs.
• Permanent mold casting: Molten metal is
poured into permanent metal mold.
 Finer grain structure and strength due to fast cooling.
 Less shrinkage and porosity.
 More shrinkage and simple parts only.
• Die casting: Molten metal forced into molds
under pressure.
 Almost finished parts, automatic.
 Good tolerance and surface finish.
 Fine grain structure.

9-38

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Aluminum Casting Alloy Composites

• Composition of casting alloys differs greatly from

wrought alloys
• Casting properties and Table 9.9
mechanical properties
are of primary interest.
• Denoted as 4 digits
with a period between
last two digits.

9-39

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Copper Alloys

• General properties of Copper: Good electrical and

thermal conduction, ease of fabrication, corrosion
resistance, medium strength.
• Production of copper:
 Copper sulfide concentrates are smelted.
 Copper sulfide is converted to blister copper by
blowing air through matte.
 Impurities in blister copper removed as slag in
refining furnace tough pitch copper.
 Tough pitch copper is further refined
electrolytically.

9-40

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Classification of Copper Alloys

• Numbers C10100 to C79900 designate wrought alloys.

• Numbers C80000 to C99900 designate casting alloys.
Table 9.10

9-41

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Other Copper Alloys

• Copper-Tin Bronzes: 1 to 10% tin with Cu to form

solid solution strengthened alloys.
 Stronger and less corrosive than Cu-Zn bronzes.
 Up to 16% Sn is added to alloys that are used for
high strength bearings.
• Copper beryllium alloys: 0.6 to 2% Be and 0.2 – 2.5 %
Cobalt with copper.
 Can be heat treated and cold worked to produce
very strong (1463 MPa) bronzes.
 Excellent corrosion resistance and fatigue
properties.
 Used in springs, diaphragms, valves etc.

9-44

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Stainless Steel
• Excellent corrosion resistance in stainless steel is due
to high (at least 12%) Chromium forming chromium
oxide on surface.
• Ferrite stainless steel :
 12-30% Cr
 Structure is mainly
ferritic (BCC α ).
 Cr extends α region
and suppresses γ region
forming γ loop.
Figure 9.55
 Low cost high strength (517 MPa) and hence used
in construction materials.

9-45 After “Metals Handbook,” vol. 8, 8th ed., American Society for Metals, 1973, p.291.

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Martensitic Stainless Steel

• 12 – 17% Cr and 0.15 – 1% C.

• Martensite formed from quenching from austenite
region.
• Poor corrosion resistance.
• Heat treatment: Same as plain carbon steel.
• Tensile strength : 517 MPa to 1966 MPa.
• Used for machine parts, pumps, bearings, and valve
parts.
• When carbon content is greater than 1%, α loop is
enlarged.

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Austenitic Region

• Iron-Chromium (16-25%) – Nickel (7-20%)

ternary alloy.
• Austenitic structure (FCC γ ) remains austenitic
at all temperature due to nickel.
• Better corrosion resistance than other steels.
• Tensile strength 559-759 MPa.
• Used for chemical equipment, pressure vessels
etc.
• Alloying element, columbium, prevents
intergranular corrosion if the alloy is to be used
for welding.

9-47

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Cast Iron

• General Properties: Contains 2-4% Carbon and 1-

3% Si.
• Easily melted, very fluid, low shrinkage, easily
machinable.
• Low impact resistance and ductility.
• Types of Cast Iron:
 White cast iron
 Gray cast iron
 Malleable cast iron
 Ductile cast iron

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White Cast iron

• Much of Carbon forms Iron Carbide instead of

graphite up on solidification.
• Fractured surface appears white and crystalline.
• Low carbon (2.5 – 3%) and silicon (0.5 – 1.5%)
content.
• Excellent wear resistance.
Iron Carbide

Pearlite

Figure 9.59

9-49 Courtesy of central Foundry

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Gray Cast Iron

• Carbon exceeds the amount that can dissolve in

austenite and precipitate as graphite flakes.
• Fractured surface appears gray.
• Excellent machinability, hardness and wear resistance,
and vibration damping capacity.
• 2.5 – 4% C and 1 – 3% Si (Promotes formation of
graphite).

Graphite
Flakes

Figure 9.60 Figure 9.61

9-50 After “Metals Handbook,” vol. 7, 8th ed., American Society for Metals, 1972, p.82.

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Ductile Cast iron

• Has processing advantages of cast iron and

engineering advantages of steel.
• Good fluidity, castability, machinability, and wear
resistance.
• High strength, toughness, ductility and hardenability
(due to spherical nodules of graphite).
• 3-4% C and 1.8 – 2.8 % Si and low impurities.
• Bull’s eye type microstructure.

Figure 9.63

9-51 After “Metals Handbook,” vol. 7, 8th ed., American Society for Metals, 1972, p.88.

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Ductile (nodular) Cast Iron

• Adding magnesium to the metal just before it is cast,

reduces to very low level the amount of sulfur and
oxygen. Therefore, spherical nodules are formed
• Structure consists of spherical graphite nodules with
envelopes of free ferrite around them in a matrix of
pearlite

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Malleable Cast Iron

• 2-2.6 % C and 1.1 – 1.6% Si.

• White cast iron is heated in malleablizing furnace to
dislocate carbide into graphite.
• Irregular nodules of graphite are formed.
• Good castability, machinability, moderate strength,
toughness and uniformity.

Figure 9.65

9-52 After “Metals Handbook,” vol. 7, 8th ed., American Society for Metals, 1972, p.95.

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Heat Treatment

• Heat treatment of white irons to produce malleable irons

are
 Graphitization: Castings heated above the eutectoid
temperature (9400C) and held for 3 to 20h depending on
the composition and structure.
white iron graphite and austenite.
 Cooling :
• Ferritic malleable iron: Fast cooled from 740-7600C and
then slowly cooled.
• Pearlitic malleable iron: Slowly cooled up to 8700C and
then air cooled.
• Tempered martensitic malleable iron: Casting cooled in
furnace to a quenching temperature and homogenized and
then quenched in agitated oil.

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Magnesium, Titanium and Nickel Alloys

• Magnesium Alloys:
 Low density metal, high cost, low castability, low
strength, poor creep, fatigue and wear resistance.
 Two types: wrought alloys (sheet, plate, extrusion)
and casting alloys (casting).
 Designated by two capital letters and two or three
numbers.
 First two letters indicate two major alloying
elements.
 The numbers indicate wt% of alloying elements.

9-54

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Structure and Properties of Magnesium Alloys

• Limited cold working due to HCP structure.

• Usually hot worked.
• Al and Zn are added to increase strength.
• Alloying with rare earth elements (cerium)
produces rigid boundary network.
• Tensile strength 179 – 310 MPa.
• Elongation – 2 to 11%

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Titanium Alloys

• Low density and high strength

• Expensive – used for aircraft applications.
• Superior corrosion resistance.
• Special technique needed to work with metal.
• HCP at room temperature. Transforms to BCC
at 8830C.
• Al and O increase transformation temperature.
• Tensile strength – 662 to 862 MPa

9-56

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Nickel Alloys

• Expensive, good corrosion resistance and high

formability.
• Commercial Nickel and Monel alloys: good weldability,
electrical conductivity and corrosion resistance.
• Nickel + 32% Cu Monel alloy (strengthens nickel).
• Nickel based super alloys: High temperature creep
resistance and oxidizing resistance for gas turbine parts.
• 50 -60 % Ni + 15-20% Cr + 15-20% Co + 1-4% Al + 2-
4% Ti.
• 3 phases – Gamma austenite, gamma prime, carbide
particles.

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Titanium Alloys

• Easily formed, outstanding corrosion resistance

• Low elastic modulus, highly biocompatible
• Pure Ti is used in low strength applications
• Alpha-beta alloys of Ti like Ti-6Al-4V (F1472)
are strengthened by solution heat treatment.
• Poor wear resistance and notch sensitivity
• Beta alloys have low elastic modulus
• Ion implantation improves wear resistance

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Issues in Orthopaedic Applications

• High yield strength, fatigue strength and hardness of

implants is desired.
 Implant should support healing bone
• Low elastic modulus is desired
 Implant and bone should carry proportionate amount of load
 Implant should not shield the bone from load
 Stress shielding stops remodeling of bone and weakens it.
 Elastic modulus of bone is only 17 GPa while most alloys have
elastic modulus greater than 100 GPa.
• Wear causes metallic toxicity
 Co-Cr alloys have good wear resistance

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