Here's something to chew on...
Post Weld Heat Treatment (in accordance with AWS D10.10M)
Post Weld Heat Treatment is performed after welding, generally at a higher temperature and with different objectives than Preheat/Interpass heating. PWHT may need to be applied without allowing the temperature to drop below the specified minimum for Preheat/Interpass heating. So in short you may have to apply the PWHT method with the work piece at temperatures up to 600° F (316° C).
Local PWHT of carbon and low alloy steels is typically performed below the lower critical transformation temperature and is therefore referred to as subcritical. The lower and upper critical transformation temperatures indicate where the crystal structure of steel begins and finally completes a change from body centered cubic to face centered cubic upon heating (the reverse upon cooling). And what this is saying, the molecules of the alloy will rearrange them self to a different configuration if we allow the temperature to reach the upper critical transformation. That will cause the properties of the metal to change, such as hardness and ductility.
There are several reasons why local supercritical PWHT (above the upper critical transformation temperature) such as annealing or normalizing is not desirable. First and foremost, the temperature gradients inherent to local PWHT would produce subcritical, intercritical and supercritical temperature regions. Depending upon the prior heat treatment of the material, this could result in a detrimental effect upon properties (tensile/yield strength and impact toughness) and/or local inhomogeneity (irregularity). Additionally, reduced material strength at supercritical temperatures creates a greater likelihood for distortion.
Post Weld Heat Treatment can have both beneficial and detrimental effects. Three primary benefits of PWHT are recognized. These are tempering, relaxation of residual stresses and hydrogen removal. Consequential benefits such as avoidance of hydrogen induced cracking, dimensional stability, and improved ductility toughness and corrosion resistance result from the primary benefits. It is important that PWHT conditions be determined based upon the desired objectives. With regard to local PWHT this is especially true for stress relaxation.
For reasons relating to carbide precipitation and the need for rapid cooling, localized solution annealing of austenitic alloys such as 300 series stainless steels is also generally not desirable. The discussion of PWHT below and in other parts of the document refers to subcritical PWHT, unless otherwise noted.
There's more here:http://www.superheatfgh.com/heat-treatment/heat-treatment-applications/post-weld-heat-treatment/
Enjoy the read!