Creep behavior of dissimilar metal weld joints between P91 and AISI 304

Dissimilar welds between ferritic creep-resistant steels and austenitic stainless steels in power plant service are known to fail prematurely due to a large difference in their thermal coefficient of expansion. To address this problem, in the current work, modified 9Cr-1Mo steel (P91) and austenitic stainless steel (AISI 304) transition joints were produced using friction welding employing three interlayers: Inconel 625, Inconel 600, and Inconel 800 H. These interlayers were friction welded one over another on P91 alloy, which was finally friction welded to AISI 304. The joints produced with single and three interlayers were subjected to creep rupture tests at different temperatures and stress levels. The creep test results demonstrated that the proposed approach can help realize striking improvements in creep performance of P91/AISI 304 dissimilar metal welds. Further, analysis of creep resulted in a stress exponent (n) of 3 by incorporating the concept of threshold stress which suggested the creep deformation mechanism to be viscous glide due to solute drag. Creep rupture data was also analyzed using Monkman-Grant relationship and creep damage tolerance factor (A). Results indicated that the damage mechanism is due to cavity growth by the combined effect of power law and diffusion creep.