Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Chapter 11 Thermal Processing of Metal Alloys
• Designer Alloys: Utilize heat
treatments to design optimum
microstructures and mechanical
properties (strength, ductility,
hardness….)
• Strength in steels correlates with how
much martensite remains in the final
structure
• Hardenability: The ability of a
structure to transform to martensite
• Precipitation hardening
¾ Annealing, Stress Relief
¾ More on Heat Treatment of Steels
¾ Precipitation Hardening
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Annealing
Annealing - A heat treatment process in which a material
is heated to an elevated temperature, allowed to dwell there
for a set amount of time and then cooled with a controlled
rate.
Stages of annealing:
• Heating to required temperature
• Holding (“soaking”) at constant temperature
• Cooling
The time at the high temperature (soaking time) is long
enough to allow the desired transformation (diffusion,
kinetics) 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 - used to revert effects of workhardening (by recovery and recrystallization) and to
increase ductility. Heating is usually limited to avoid
excessive grain growth and oxidation.
Stress Relief Annealing – used to eliminate/minimize
stresses arising from
o Plastic deformation during machining
o Non-uniform cooling
o Phase transformations between phases with
different densities
Stress relief annealing allows these stresses to relax.
Annealing temperatures are 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)
eutectoid point
• Lower critical temperature A1 below which
austenite (γ) does not exist
• Upper critical temperature lines, A3 and Acm
above which all material is austenite (γ)
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Annealing of Fe-C Alloys (II)
eutectoid point
Normalizing: an annealing heat treatment just above the
upper critical temperature to reduce the AVERAGE grain
sizes (of pearlite and proeutectoid phase) and make more
uniform size distributions. After complete transformation
to austenite (austenitizing - γ) the treatment is completed
by cooling to the required microstructure.
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Annealing of Fe-C Alloys (III)
eutectoid point
Full annealing: austenizing (γ) and slow cooling (several
hours). Produces coarse pearlite -large grains (and possible
proeutectoid phase) that is relatively soft and ductile. Full
annealing is used to soften pieces which have been
hardened by plastic deformation, and which need to
undergo subsequent machining/forming.
Spheroidizing: prolonged heating just below the eutectoid
temperature, which results in the soft spheroidite structure
discussed in Sect. 10.5. This achieves maximum softness
of Tennessee,
Dept. of Materials
Science and Engineering
neededUniversity
in subsequent
forming
operations.
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Heat Treatment of Steels
Martensite has the strongest microstructure and can be
made more ductile by tempering. Therefore, the optimum
properties of quenched and tempered steel are realized
if a high content of martensite is produced.
Problem: It is difficult to maintain the same conditions
throughout the entire volume of steel during cooling: the
surface cools more quickly than interior, producing a range
of microstructures throughout. The martensitic content,
and the hardness, will drop from a high value at the surface
to a lower value in the interior of the specimen.
Production of uniform martensitic structure depends on
• composition
• quenching conditions
• size + shape of specimen
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Tempering - Hardness
Rockwell Hardness, Scale C
Martensite
Brinell Hardness Number
• Martensite is the
hardest / strongest
and most brittle of
the steel
microstructures
• Hardness is a
function of carbon
content
• Hardening
mechanism is solid
solution hardening
from interstitial C
• Enhance ductility
by tempering.
Anneal to
equilibrium ferrite
plus cementite
phases. Formation
by this route called
tempered
martensite
Tempered martensite
(tempered at 371 °C)
Fine Pearlite
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Hardenability (I)
Hardenability is the ability of the Fe-C alloy to be hardened
by forming martensite.
Hardenability is not “hardness”. It is a qualitative
measure of the rate at which hardness decreases with
distance from the surface because of decreased
martensite content.
High hardenability means the ability of the alloy to
produce a high martensite content throughout the volume
of specimen.
Hardenability is measured by the 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
Hardenability (II)
Jominy end-quench test of Hardenability
The “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 (III)
Hardenability Curve
Less Martensite
¾ Quenched end cools most rapidly and contains most
martensite
¾ Cooling rate decreases with distance from quenched end:
greater C diffusion, more pearlite/bainite, lower
hardness
¾ 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
Hardenability (IV)
• Alloying elements delay formation of
pearlite, bainite : more martensite
• Can also define hardenability in terms of
cooling rate (C/s)
• Alloys in figure above all have 0.40 wt% C,
but have different additional alloying
elements (see Callister F11.5)
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Hardenability (V)
• Hardenability also generally increases
with C content
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Hardenability (VI)
Influence of Quenching Medium, Specimen Size,
and Geometry on Hardenability
Quenching medium: Cooling is faster in water then oil,
slow in air. Fast cooling brings the danger of warping and
formation of cracks, since it is usually accompanied by
large thermal gradients.
The shape and size of the piece: Cooling rate depends
upon extraction of heat to specimen surface. Thus the
greater the ration of surface area to volume, the deeper the
hardening effect. Spheres cool slowest, irregularly shaped
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 (I)
• Small inclusions of secondary phases strengthen material
• Lattice distortions around these secondary phases impede
dislocation motion
• The precipitates form when the solubility limit is
exceeded
•
• Precipitation hardening is also called age hardening
because it involves the hardening of the material over a
prolonged time.
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Heat Treatment for Precipitation Hardening (II)
• Solution heat treatment: at To, all the solute atoms A
are dissolved to form a single-phase (α) solution.
• Rapid cooling across the solvus line to exceed the
solubility limit. This leads to a metastable supersaturated
solid solution at T1. Equilibrium structure is α+β, but
limited diffusion does not allow β to form.
• Precipitation heat treatment: the supersaturated
solution is heated to T2 where diffusion is appreciable - β
phase starts to form as finely dispersed particles: aging.
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Precipitation Hardening (III)
k
Solution treatment
100% k solid solution
(retained upon
quenching)
Quench
θ+k
“Equilibrium
microstructure”
- coarse θ
precipitates
at k grain
boundaries
Time
By quenching and then reheating an Al-Cu (4.5 wt%)
alloy, a fine dispersion of precipitates form within the k
grains. These precipitates are effective in hindering
dislocation motion and consequently, increasing alloy
hardness and strength. Known as precipitation or age
hardening.
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Heat Treatment for Precipitation Hardening (IV)
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
Precipitation Hardening (V)
Strength and Ductility
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Summary
Make sure you understand language and concepts:
¾ Annealing
¾ Austenitizing
¾ Full annealing
¾ Hardenability
¾ Jominy end-quench test
¾ Overaging
¾ Precipitation hardening
¾ Precipitation heat treatment
¾ Process annealing
¾ Solution heat treatment
¾ Spheroidizing
¾ Stress relief
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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
Reading for next class:
Skip Chapter 12: Metal Alloys
Chapter 13: Structure and Properties of Ceramics
¾ Crystal Structures
¾ Silicate Ceramics
¾ Carbon
¾ Imperfections in Ceramics
Optional reading: 13.6 – 13.10
Chapter 14: Applications and Processing of Ceramics
¾ Short review of glass/ceramics applications and
processing (14.1 - 14.7)
Optional reading: 14.8 – 14.18
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