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Chapter 11 Thermal Processing of Metal Alloys
¾ Annealing, Stress Relief
¾ More on Heat Treatment of Steels
¾ Precipitation Hardening
- 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
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
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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
Annealing
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Process Annealing - used to revert effects of work-
hardening (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.
Examples of Heat Treatment
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
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- Lower critical temperature A 1 below which
austenite (γ) does not exist
- Upper critical temperature lines, A 3 and A (^) cm
above which all material is austenite (γ)
Annealing of Fe-C Alloys (I)
eutectoid point
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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
Heat Treatment of Steels
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
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hardest / strongest
and most brittle of
the steel
microstructures
function of carbon
content
mechanism is solid
solution hardening
from interstitial C
by tempering.
Anneal to
equilibrium ferrite
plus cementite
phases. Formation
by this route called
tempered
martensite
Martensite
Tempered martensite (tempered at 371 °C)
Brinell Hardness NumberFine Pearlite Rockwell Hardness, Scale C
Tempering - Hardness
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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).
Hardenability (I)
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
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The “Hardenability Curve” is the dependence of hardness on distance from the quenched end.
Jominy end-quench test of Hardenability
Hardenability (II)
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- Hardenability also generally increases with C content
Hardenability (V)
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of (^) 14
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
Hardenability (VI)
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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.
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
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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 T 1. Equilibrium structure is α+β, but limited diffusion does not allow β to form.
- Precipitation heat treatment: the supersaturated solution is heated to T 2 where diffusion is appreciable - β phase starts to form as finely dispersed particles: aging.
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Strength and Ductility
Precipitation Hardening (V)
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
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Summary
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
Make sure you understand language and concepts:
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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.
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.
