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