Creep Performance and Microstructural Characterization of Electron-Beam Welded 316LN SS-Grade 91 Steel Dissimilar Joint

Microstructural features and creep-rupture behaviour of a dissimilar weld joint (DWJ) between Grade 91 (G91) steel and 316LN stainless steel prepared by Electron beam (EB) welding are investigated. The SEM-EDAX and EBSD observations in the fusion zone revealed compositional variations and distinct microstructural features, depending on the extent of mixing of both base metals. Further, at the interface between the fusion zone and G91 steel, nickel enrichment was observed, which lowered the Ac-1 temperature, resulting in fresh martensite and higher hardness upon PWHT. In the creep-rupture tests conducted at 873 K and 110 to 175 MPa, failure location shifted from G91 base metal to intercritical HAZ of G91 steel above 2500 hours of creep exposure. As against the interface failures, which are considered the life-limiting factor in DWJs, the ruptured specimens in the present study revealed better interface stability with insignificant cavitation even after creep exposure of over 10,000 hours. The composition at the ferritic-weld interface exhibits almost equivalent carbon activity to G91 steel which improved the interface stability by resisting the coarsening of carbides. It is important to highlight that the EB weld joint exhibited higher rupture life and weld strength reduction factor than the joints made by conventional multi-pass arc welding.