Thermal and strain ageing

Strain ageing of steel was first observed during the 19th century when the maximum load carrying capacity of a test piece was increased after it had been retested after a previous series of testing in the plastic range.
General acceptance has been recognized that strain ageing is due to the diffusion of carbon and/or nitrogen atoms in solution to dislocations that have been generated by plastic deformation however the resulting characteristics are the result of a complex series of metallurgical reactions.

Strain-ageing is a type of behavior, usually associated with the yield-point phenomenon,in which. the strength of a metal is increased and the ductility is decreased on heating at a relatively low. temperature after cold working.

The behavior of steel is significantly affected by its chemical composition and the method of manufacture. Bauschinger (1887) observed an increase in the maximum load-carrying capacity of a piece of steel after it was tested in the inelastic range, left for some time and then retested. This observation is now known as strain-ageing. Physical basis of this phenomenon can be interpreted by time dependent diffusion of small elements such as carbon and nitrogen to crystal flaws (e.g. dislocations). Plastic deformation (yielding) is triggered by the movement of dislocations through the crystal, whereas the presence of small atoms (e.g. carbon and nitrogen) can markedly increase the sliding resistance between the atoms and hence increase the yield strength.