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Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 1
Thermal Processing of Metal Alloys
Annealing, Stress Relief
More on Heat Treatment of Steels
Heat treatments of nonferrous alloys ; Precipitation
Hardening
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 2
Stages of annealing:• Heating to required temperature• Holding (“soaking”) at constant
temperature• Cooling
Soaking time at the high temperature needs to be longenough to allow desired transformation to occur.Cooling is done slowly to avoid warping/cracking of dueto the thermal gradients and thermo-elastic stresseswithin the or even cracking the metal piece.
Purposes of annealing:• Relieve internal stresses• Increase ductility, toughness, softness• Produce specific microstructure
Annealing
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 3
Process Annealing –
effects of work-hardening (recovery andrecrystallization) and increase ductility.Heating limited to avoid excessive graingrowth and oxidation
Stress Relief Annealing –
minimizes stresses due too Plastic deformation during machiningo Nonuniform coolingo Phase transformations between phases
with different densitiesAnnealing temperatures 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
University of Virginia, Dept. of Materials Science and Engineering 4
• Lower critical temperature A1
below which austenite does not exist
• Upper critical temperatures A3 and Acmabove which all material is austenite
Annealing of Fe-C Alloys (I)
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 5
Normalizing: annealing heat treatment just aboveupper critical temperature to reduce grain sizes (ofpearlite and proeutectoid phase) and make moreuniform size distributions.
Austenitizing complete transformation to austenite
Annealing of Fe-C Alloys (II)
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 6
Full annealing: austenizing + slow cooling (severalhours) Produces coarse pearlite (and possibleproeutectoid phase) that is relatively soft andductile. Used to soften pieces which have beenhardened by plastic deformation, but need toundergo subsequent machining/forming.
Spheroidizing: prolonged heating just below theeutectoid temperature, results in the soft spheroiditestructure. This achieves maximum softness needed insubsequent forming operations.
Annealing of Fe-C Alloys (III)
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 7
Martensite has strongest microstructure.Can be made more ductile by tempering.
Optimum properties of quenched and temperedsteel are realized with high content of martensite
Problem: difficult to maintain same conditionsthroughout volume during cooling:Surface cools more quickly than interior,producing range of microstructures in volumeMartensitic content, and hardness, will drop froma high value at surface to a lower value inside
Production of uniform martensitic structuredepends on
• composition• quenching conditions• size + shape of specimen
Heat Treatment of Steels
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 8
Hardenability is the ability of Fe-C alloy to hardenby forming martensite
Hardenability (not “hardness”): Qualitativemeasure of rate at which hardness decreases withdistance from surface due to decreased martensitecontent
High hardenability means the ability of the alloy toproduce a high martensite content throughout thevolume of specimen
Hardenability measured by Jominy end-quenchtest performed for standard cylindrical specimen,standard austenitization conditions, and standardquenching conditions (jet of water at specific flowrate and temperature).
Hardenability
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 9
Hardenability curve is the dependence of hardnesson distance from the quenched end.
Jominy end-quench test of Hardenability
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 10
Hardenability Curve
Quenched end cools most rapidly, contains mostmartensite
Cooling rate decreases with distance fromquenched end: greater C diffusion, morepearlite/bainite, lower hardness
High hardenability means that the hardnesscurve is relatively flat.
Less Martensite
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 11
Influence of Quenching Medium, Specimen Size, and Geometry on Hardenability
Quenching medium: Cools faster in water than airor oil. Fast cooling warping and cracks, since itis accompanied by large thermal gradients
Shape and size: Cooling rate depends uponextraction of heat to surface. Greater the ratio ofsurface area to volume, deeper the hardening effect
Spheres cool slowest, irregular objects fastest.
Radialhardnessprofiles of cylindricalsteel bars
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 12
Precipitation Hardening
• Inclusion of a phase strengthens material
• Lattice distortion around secondary phaseimpedes dislocation motion
• Precipitates form when solubility limit exceeded• Precipitation hardening called age hardening
(Hardening over prolonged time)
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 13
Heat Treatment for Precipitation Hardening (I)
• Solution heat treatment: To solute atoms Adissolved to form a single-phase () solution.
• Rapid cooling across solvus to exceed solubilitylimit. Leads to metastable supersaturated solidsolution at T1. Equilibrium structure is +, butlimited diffusion does not allow to form.
• Precipitation heat treatment: supersaturatedsolution heated to T2 where diffusion isappreciable - phase starts to form finelydispersed particles: ageing.
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 14
Discs of Cu atoms 1 or 2 monolayers thick
Lattice Distortions No Lattice Distortions
Heat Treatment for Precipitation Hardening (II)
Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys
University of Virginia, Dept. of Materials Science and Engineering 15
Strength and ductility during precipitation hardening