1. Thermal Processing of Metal Alloys
Chapter 11
Sections 11.7 to end
Thermal Processing of Metal Alloys
Will learn more on how thermal processing techniques can be used to control microstructure and mechanical properties of metal alloys
Annealing, Stress Relief
More on Heat Treatment of Steels
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. Quenched end cools most rapidly, contains most martensite
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. Spheres cool slowest, irregular objects fastest.
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

16. Summary Make sure you understand language and concepts: Annealing
Austenitizing
Full annealing
Hardenability
Jominy end-quench test
Overaging
Precipitation hardening
Precipitation heat treatment
Process annealing
Solution heat treatment
Spheroidizing
Stress relief

17. Reading for next class:
Chapter 12: Structure and Properties of Ceramics
Crystal Structures
Silicate Ceramics
Carbon
Imperfections in Ceramics
Optional reading: – end