As I understand welding most materials have time restraints between the time the joint is welded to completion and when Post Weld Heat Treatment is performed unless the WPS -Welding Procedure Specification- says something else. So, the general phrasing usually immediately after welding.
Depending of the material there is in selected cases creep and other performance degrading characteristics that require post heat treating to offset.
I do know that post weld heat treatment is required for Grade 91 steels. The holding range should be 1375 to 1425°F for a minimum of 2 hours. a long time at temperature post weld heat treating temperature is necessary to form the required weld structure, to ensure adequate toughness testing and to ensure adequate service life.