Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Chapter 11
Sections 11.7 to end
Thermal Processing of Metal Alloys
Will learn more on how thermal processing techniques
can be used to control microstructure and mechanical
properties of metal alloys
 Annealing, Stress Relief
 More on Heat Treatment of Steels
 Precipitation Hardening
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Annealing
Stages of annealing:
• Heating to required temperature
• Holding (“soaking”) at constant
temperature
• Cooling
Soaking time at the high temperature needs to be long
enough to allow desired transformation to occur.
Cooling is done slowly to avoid warping/cracking of due
to the thermal gradients and thermo-elastic stresses
within the or even cracking the metal piece.
Purposes of annealing:
• Relieve internal stresses
• Increase ductility, toughness, softness
• Produce specific microstructure
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Examples of heat treatment
Process Annealing –
effects of work-hardening (recovery and
recrystallization) and increase ductility.
Heating limited to avoid excessive grain
growth and oxidation
Stress Relief Annealing –
minimizes stresses due to
o Plastic deformation during machining
o Nonuniform cooling
o Phase transformations between phases
with different densities
Annealing temperatures relatively low
so that useful effects of cold working
are not eliminated
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Annealing of Fe-C Alloys (I)
• Lower critical temperature A1
below which austenite does not exist
• Upper critical temperatures A3 and Acm
above which all material is austenite
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Annealing of Fe-C Alloys (II)
Normalizing: annealing heat treatment just above
upper critical temperature to reduce grain sizes (of
pearlite and proeutectoid phase) and make more
uniform size distributions.
Austenitizing complete transformation to austenite
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Annealing of Fe-C Alloys (III)
Full annealing: austenizing + slow cooling (several
hours) Produces coarse pearlite (and possible
proeutectoid phase) that is relatively soft and
ductile. Used to soften pieces which have been
hardened by plastic deformation, but need to
undergo subsequent machining/forming.
Spheroidizing: prolonged heating just below the
eutectoid temperature, results in the soft spheroidite
structure. This achieves maximum softness needed in
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Heat Treatment of Steels
Martensite has strongest microstructure.
Can be made more ductile by tempering.
Optimum properties of quenched and tempered
steel are realized with high content of martensite
Problem: difficult to maintain same conditions
throughout volume during cooling:
Surface cools more quickly than interior,
producing range of microstructures in volume
Martensitic content, and hardness, will drop from
a high value at surface to a lower value inside
Production of uniform martensitic
depends on
• composition
• quenching conditions
• size + shape of specimen
structure
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Hardenability
Hardenability is the ability of Fe-C alloy to harden
by forming martensite
Hardenability (not “hardness”): Qualitative
measure of rate at which hardness decreases with
distance from surface due to decreased martensite
content
High hardenability means the ability of the alloy to
produce a high martensite content throughout the
volume of specimen
Hardenability measured by Jominy end-quench
test performed for standard cylindrical specimen,
standard austenitization conditions, and standard
quenching conditions (jet of water at specific flow
rate and temperature).
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Jominy end-quench test of Hardenability
Hardenability curve is the dependence of hardness
on distance from the quenched end.
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Hardenability Curve
Less Martensite
 Quenched end cools most rapidly, contains most
martensite
 Cooling rate decreases with distance from
quenched end: greater C diffusion, more
pearlite/bainite, lower hardness
 High hardenability means that the hardness
curve is relatively flat.
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Influence of Quenching Medium, Specimen Size,
and Geometry on Hardenability
Quenching medium: Cools faster in water than air
or oil. Fast cooling  warping and cracks, since it
is accompanied by large thermal gradients
Shape and size: Cooling rate depends upon
extraction of heat to surface. Greater the ratio of
surface area to volume, deeper the hardening effect
Spheres cool slowest, irregular objects fastest.
Radial
hardness
profiles of
cylindrical
steel bars
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Precipitation Hardening
• Inclusion of a phase  strengthens material
• Lattice distortion around secondary phase
impedes dislocation motion
• Precipitates form when solubility limit exceeded
• Precipitation hardening called age hardening
(Hardening over prolonged time)
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Heat Treatment for Precipitation Hardening (I)
• Solution heat treatment: To  solute atoms A
dissolved to form a single-phase () solution.
• Rapid cooling across solvus to exceed solubility
limit. Leads to metastable supersaturated solid
solution at T1. Equilibrium structure is +, but
limited diffusion does not allow  to form.
• Precipitation heat treatment: supersaturated
solution heated to T2 where diffusion is
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Heat Treatment for Precipitation Hardening (II)
Discs of Cu atoms 1 or 2
monolayers thick
Lattice Distortions No Lattice Distortions
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Strength
and ductility during precipitation hardening
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Summary
Make sure you understand language and concepts:
 Annealing
 Austenitizing
 Full annealing
 Hardenability
 Jominy end-quench test
 Overaging
 Precipitation hardening
 Precipitation heat treatment
 Process annealing
 Solution heat treatment
 Spheroidizing
 Stress relief
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Reading for next class:
Chapter 12: Structure and Properties of Ceramics
 Crystal Structures
 Silicate Ceramics
 Carbon
 Imperfections in Ceramics
Optional reading: 12.7 – end
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