Why is austenitic stainless steel not heat treatable? Stainless Steels
It is possible to expose austenitic stainless steels to different heat treatment cycles i.e. different temperatures and times. However, none of these combinations would result in an increase in strength.
Exposure to elevated temperatures of c. 1000degC would result in a softening of an austenitic stainless steel, assuming it is immediately quenched. This solution annealing process ensures the composition is consistent throughout the workpiece, with a relative coarse grain size.
A stress relieving heat treatment is conducted at a much lower temperature, perhaps a few hundred degrees centigrade only. Although austenitic stainless steels do not work harden, it is possible that there could be internal stresses from earlier forging, rolling or fabrication. These could result in distortion of the workpiece after further processing, so this lower temperature heat treatment.