In the world of pressure vessels (like those governed by ASME Section VIII), the terms “stress relieved during fabrication”, Cold-Formed Heads, and Hot-formed Heads relate to heat, but they are very different techniques used in PV construction. PWHT is about treating the material after it’s been worked, while “hot formed heads” is about shaping the material in the first place.
First, let’s see what CGA 2.1 says about NH3 storage tanks: