The influence of PWHT regime on microstructure and creep rupture behaviour of dissimilar T92/TP316H ferritic/austenitic welded joints with Ni-based filler metal

Abstract

In this paper the dissimilar welded joints between the ferritic (tempered martensitic) steel T92 and the non-stabilised austenitic steel TP316H were investigated in terms of microstructure and creep rupture behaviour characterisation. The welded joints were prepared by the TIG method using the Ni-based filler metal Nirod 600 (Inconel-type). After the welding, the individual weldments were subjected to the two different regimes of post-weld heat treatment (PWHT), either the "classical" PWHT (i.e. subcritical tempering - below Ac1 temperature) or the so-called "full" PWHT including the re-normalisation. The use of "classical" PWHT preserved microstructural gradient in the ferritic steel heat-affected zone (HAZ), which led to a premature creep failure by the "type IV cracking" mode. In contrast, the application of "full" PWHT eliminated the critical HAZ region of T92 steel and changed the creep failure mode to the "interfacial cracking" between the ferritic steel and the Ni-based weld metal. Consequently, this suppression of the "type IV cracking" failure led to a significant creep life increase of the studied ferritic/austenitic welded joints.