Effect of post weld heat treatments on microstructure and mechanical properties of dissimilar P92 steel-AISI 304H ASS A-TIG weld joint

In present work, the efficacy of post weld heat treatment (PWHT) on microstructure and mechanical properties of dissimilar P92 steel-AISI 304H austenitic stainless steel (ASS) activated flux-tungsten inert gas (A-TIG) weld joint was studied. Based on the time temperature cycle, the post weld heat treatment was performed in two ways: (a) PWHT-1 (760 °C, 2 h) and (b) PWHT-2 (1160 °C, 2 h and 760 °C, 2 h). After PWHTs, the microstructure and mechanical properties (micro-hardness, tensile behavior and impact toughness) of weld joint were tested and compared with as-welded condition. PWHT-1 resulted in coarsening of existing precipitates and formation of new precipitates in weld zone. The average size of precipitates along the PAGB and inside the grain matrix was measured to be 294 ± 50 nm and 153 ± 50 nm, respectively. After PWHT-2, the microstructure of P92 FGHAZ, P92 CGHAZ and weld zone were observed to be quite similar due to the re- austenitisation during this PWHT regime. The hardness distribution shows the non-uniformity even after PWHT-1 however; PWHT-2 shows negligible variation in hardness of P92 side HAZs and weld zone. In tensile testing, the failure location has been changed to P92 steel base metal from AISI 304H ASS which suggests that P92 steel becomes weak after the post weld heat treatments. An increase in elongation % (PWHT-1: 37% and PWHT-2: 45%) was also obtained after PWHTs. Upon PWHTs the weld zone impact toughness also improved (PWHT-1: 59 ± 2 J and PWHT-2: 132 ± 2 J) and satisfy the minimum average impact toughness as per standard EN 1559:1999.