Enhancement of mechanical properties through modified post-weld heat treatment processes of T91 and Super304H dissimilar welded joint

Abstract

The work is focused on examining the influence of heat treatment modification during post-weld heat treatment on microstructure and mechanical properties of dissimilar welds of T91 and Super304H. Tungsten inert gas (TIG) welding was employed with ERNiCr-3 as filler to prepare the dissimilar metal welds. Following that, two post-weld heat treatments (PWHTs) were performed, viz. post-weld normalizing and tempering (PWNT) and post-weld direct tempering (PWDT). The latter was performed at 760 °C for 1 h, while the former was carried at 1050 °C for 30 min (normalizing) and then tempered at 760 °C for 1 h. The microstructural examination using scanning electron microscopy and electron-backscattered diffraction suggested a uniform microstructure after PWNT compared to PWDT. For instance, the grain size range in heat-affected zone for T91 side and Super304H side was 5–80 μm and 10–140 μm, respectively, after PWDT, while it was respectively reduced to 5–35 μm and 10–60 μm after PWNT. Further, the PWDT heat treatment resulted in coarse Cr-carbides at grain boundaries (~ 671 nm), indicating sensitization on the Super304H side, whereas the PWNT heat treatment ensured small Cr-carbides (~ 504 nm), indicating that the sensitization phenomenon was delayed. Further, on the T91 side, the vulnerability of fine-grained heat-affected zone for type IV cracking has been identified and eliminated through PWNT treatment. The improved microstructure of PWNT against PWDT heat treatment correlated well with the enhanced mechanical properties, where 0.2% proof stress, tensile strength, % elongation and toughness for PWDT were 324 MPa, 649 MPa, 20%, and 109 MJ/m3, respectively, while after PWNT, these were improved to 384 MPa, 691 MPa, 22%, and 132 MJ/m3, respectively. For both treatments, failure was observed in the weld metal region due to coarse Nb/Ti intermetallic carbides.