Austenitic stainless steel is not heat treatable in the sense of strengthening: it can be exposed to different heat treatment cycles, but no combination of temperature and time will increase its strength.
Solution annealing softens rather than strengthens
Exposure to elevated temperatures of around 1000degC, followed by an immediate quench, softens an austenitic stainless steel. This solution annealing process makes the composition consistent throughout the workpiece and produces a relatively coarse grain size.
Stress relieving at lower temperatures
Stress relieving is carried out at a much lower temperature - perhaps a few hundred degrees centigrade. Although austenitic stainless steels do not work harden in this context, internal stresses can remain from earlier forging, rolling or fabrication, and these can distort the workpiece during later processing.
This lower-temperature treatment is most often used before machining long or complex shapes, where distortion from the release of internal stresses would otherwise be a problem.