Part VII : Miscellaneous topics
Module 1 : Recovery and recrystallisation
 
Part VII Miscellaneous topics
In this part, we discuss the formation of microstructures and their evolution during heat treatment, aspects of practical heat treatment (such as furnaces, fixtures, atmosphere and quenching media), and some important heat treatment processes. The discussion is cursory. The interested reader should refer to Boyer and Dossett [1] for details on the practical aspects and Porter et al [2] for the phase transformations aspects.
1 Recovery and recrystallisation
1.1 Motivation
Why is it not possible to use work hardened structural materials in high temperature applications?
1.2 Cold working

There are many ways in which materials can be strengthened; work or strain hardening is one of the strengthening mechanisms. To work harden a material, it is cold worked: that is, it is deformed at low temperatures - typically at the ambient temperature, but any temperature which is below 0.3 to 0.5 of the melting temperature of the material will qualify as low temperature. Cold working introduces defects in the material (typcially point defects and dislocations) which act as impediments to the movement of dislocations and hence the material is strengthened. The defects generated during cold wokring can store about 1 to 10 % of the energy of plastic deformation.

A cold worked material, however, cannot be used at high temperatures; typically, 0.3 to 0.5 of the melting temperature of the material is considered as high service temperature. In this temperature range, the microstructure of a cold worked material undergoes changes which reduce the strength of the material. In this module we study these microstructural changes, namely, recovery and recrystallisation.