Post weld heat treatment (PWHT) is a common practice to mitigate the propensity of weld residual stresses in pressure vessels and piping equipment. It is also often employed in the repair of welds on vessel components to minimize crack initiation and subsequent, catastrophic, failure due to unstable flaw propagation. In many instances, performing local or spot Pwht for vessels can be a more practical and cost effective alternative to treating the entire component as whole.
What is the PWHT procedure for pressure vessels?
However, when spot PWHT is applied, it is essential that the soak, heating and gradient control bands are properly engineered to avoid distortion of the weld and associated HAZ, and to reduce the likelihood of detrimental residual stress states that could increase in-service crack initiation and instability. Ideally, this is best accomplished with the assistance of advanced thermal-mechanical finite element analysis.
Although the limiting thickness beyond which PWHT is required for carbon and low-alloy steels in current design and inspection codes for pressure vessels, boilers and piping is relatively modest at 32mm, there is considerable divergence between different codes with regard to both welding procedures and inherent Charpy energy requirements. Moreover, for general structural steels used in bridges and buildings and offshore structures, there are considerably higher levels of permissible thicknesses in the as-welded condition, coupled with increasing toughness requirements linked to minimum Charpy impact energy values.
An Appendix in PD5500  permits local PWHT for pressure boundary regions following local weld repairs or alterations, but this philosophy is not widely accepted in other Codes. A material properties rather than code-based approach to determining the appropriate requirement for local PWHT might be more readily accepted, particularly with respect to the PWHT of thicker carbon and C-Mn steels, and it may facilitate greater scope for extending the boundaries of omission from PWHT.