Examining Stress Relaxation in a Dissimilar Metal Weld Subjected to Postweld Heat Treatment

Abstract

Dissimilar metal welds are often required in nuclear power plants to join components made from austenitic steels to those from ferritic steels, particularly in fast breeder reactor plants to join the intermediate heat exchanger to the steam generator. The process of welding alters the microstructure of the base materials and also causes residual stresses to form, both due to the change in the microstructure and the differing thermal histories in various regions. Post-weld heat treatment (PWHT) is required to relieve the residual stresses and achieve preferable microstructural gradients across the weld joint. Therefore, in order to arrive at the optimal PWHT process, it is necessary to investigate the effects of heat treatment on the joint integrity, microstructure and residual stress relaxation in the welds.To investigate this effect of PWHT on the residual stress relaxation and corresponding alteration of microstructure across a welded joint, a dissimilar weld between modified 9Cr-1Mo (P91) steel and austenitic stainless steel AISI 316LN was made using autogenous electron beam (EB) welding. To achieve this, the welding process was first modelled numerically using finite element analysis and the residual stress predictions were validated with experimental investigation using neutron diffraction. The validated model was then used to study the residual stress relaxation through the simulation of PWHT. The predicted stress relaxation was compared with contour method measurement of residual stresses in the actual welded plate subjected to PWHT. The results indicate that albeit some relaxation of residual stresses during PWHT, there is still a significant portion of highly localised residual stresses left in the specimen.

Decisive impact of Filler-free joining processes on the Microstructural evolution, tensile and impact properties of 9Cr-1Mo-V-Nb to 316 L(N) dissimilar joints

“…17 However, the post-weld heat treatment (PWHT) of dissimilar ferritic martensitic/austenitic weld configurations is very complex due to the location dependant compositional variations, carbon diffusion and softening, sensitization of austenitic stainless steel at the temperature range of PWHT, a significant loss in hardness in 9Cr-1Mo-V-Nb side HAZ. 18,19 Hence it is crucial to select appropriate PWHT procedure to achieve the properties at the desired level.…”