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What is the difference between stress relief and PWHT processes?

The objective of both the process is one and the same likewise their phenomenon however the term stress relief is used when you have to relieve the stress after some cold working on the part whereas Post Weld Heat Treatment is used where you relieve the residual stresses after welding or other hot working.

But sometimes they are also used interchangeably.

I agree with Irish as where welding is not involved the term used is "Stress Relieving".
And both the terms can be used interchangeably also as the process and objective are the same.