Chapter11cor2

8/2/2019 Chapter11cor2

http://slidepdf.com/reader/full/chapter11cor2 1/14

Introduction to Materials Science, Chapter 11, Thermal Processing of Metal Alloys

University of Virginia, Dept. of Materials Science and Engineering 1

Lower critical temperature A1

below which austenite does not exist

Upper critical temperatures A3 and Acm

above which all material is austenite

Annealing of Fe-C Alloys (I)





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

Annealing of Fe-C Alloys (II)





Full annealing: austenizing + slow cooling (severalhours) Produces coarse pearlite (and possible

proeutectoid 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)





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

Heat Treatment of Steels





Hardenability is the ability of Fe-C alloy to hardenby forming martensite

Hardenability (not ³hardness´): Qualitative

measure of rate at which hardness decreases withdistance from surface due to decreased martensite

content

High hardenability means the ability of the alloy toproduce a high martensite content throughout the

volume of specimen

Hardenability measured by Jominy end-quenchtest performed for standard cylindrical specimen,

standard austenitization conditions, and standard

quenching conditions (jet of water at specific flow

rate and temperature).

Hardenability





Hardenability curve is the dependence of hardness

on distance from the quenched end.

Jominy end-quench test of Hardenability





Hardenability Curve

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 hardnesscurve is relatively flat.

Less Martensite





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 it

is accompanied by large thermal gradients

Shape and size: Cooling rate depends uponextraction 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





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)





Heat Treatment for Precipitation Hardening (I)

Solution heat treatment: To solute atoms A

dissolved to form a single-phase (E) solution.

R apid cooling across solvus to exceed solubility

limit. Leads to metastable supersaturated solid

solution at T1. Equilibrium structure is E+ F, but

limited diffusion does not allow F to form.

Precipitation heat treatment: supersaturatedsolution heated to T2 where diffusion is

appreciable - F phase starts to form finely





Discs of Cu atoms 1 or 2

monolayers thick

Lattice Distortions No Lattice Distortions

Heat Treatment for Precipitation Hardening (II)





Strength and ductility during precipitation hardening





Summary

Annealing

Austenitizing

Full annealing

Hardenability

Jominy end-quench test

Overaging

Precipitation hardening

Precipitation heat treatment

Process annealing

Solution heat treatment

Spheroidizing

Stress relief

Make sure you understand language and concepts:





R eading for next class:

Chapter 12: Structure and Properties of Ceramics

Crystal Structures

Silicate Ceramics

Carbon

Imperfections in Ceramics

Optional reading: 12.7 ± end

Documents

Chapter11cor2