Ask a group of experienced engineers in ammonia refrigeration, ammonia fertilizer, and eletrical utilities if Post Weld Heat Treating (PWHT) is needed for vessels used in Anyhydrous Ammonia service and you will get a varying degree of answers.  I am certainly not one to ask, but we do recommend PWHT for any new vessels put into this type of service based on the benefits this treatment brings to Process Safety.  I turn to engineers with a special background in metalurgy for answers and one of the best articles I have seen on this topic and the problems that stem from Stress Corrosion Cracking (SCC) is by Nabeel Ahmed Qureshi, B.Sc Metallurgy & Materials Engineering.