ASME doesnot say anything about stresses during PWHT. Deformation and stresses during PWHT may be different when 1)a completed vessel is heat treated in a furncae 2)during local PWHT. In either case, the stresses and deformation will depend on the supporting arrangement provided during heat treatment and geometry. Thus it is obvious that ASME will not talk about it. In eithe case, enough supporting arrangement shall be provided to avoid excessive permenant/plastic deformation due to self weight. At the same time, it is important to allow free thermal expansion to minimize the stresses.
This is a concern specially for thin vessels with high D/t ratio. You can go for elastic-plastic finite element analysis for such cases.