Thermal Processing of Metal Alloys

1. Thermal Processing of Metal Alloys • Annealing, Stress Relief • More on Heat Treatment of Steels • Heat treatments of nonferrous alloys ; Precipitation Hardening

2. 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

3. 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 • Plastic deformation during machining • Nonuniform cooling • Phase transformations between phases with different densities • Annealing temperatures relatively low so that useful effects of cold working are not eliminated

4. 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

5. 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

6. 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 subsequent forming operations.

7. 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 structure depends on • composition • quenching conditions • size + shape of specimen

8. 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).

9. Jominy end-quench test of Hardenability Hardenability curve is the dependence of hardness on distance from the quenched end.

10. 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.

11. 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

12. 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)

13. 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 appreciable -  phase starts to form finely dispersed particles: ageing.

14. Heat Treatment for Precipitation Hardening (II) Discs of Cu atoms 1 or 2 monolayers thick Lattice Distortions No Lattice Distortions

15. Strength and ductility during precipitation hardening