Effect of partial and full post-weld heat treatments on microstructure and mechanical properties of IN738LC joint by electron beam welding

This work explored the microstructure, mechanical properties, and the related fracture mechanism of electron beam welded (EBW) IN738LC joints subjected to partial post-weld heat treatment (P-PWHT) and full post-weld heat treatment (F-PWHT). The two PWHT schedules produced a significant effect on the microstructure and mechanical properties of IN738LC joints. The fusion zone (FZ) presented a similar microstructure with BM, composed of duplex-sized γ′ particles, blocky MC carbides, and continuous M23C6 carbides. While F-PWHT resulted in the complete dissolution of primary γ′ particles and unimodal-spheroidal γ′ particles were obtained in IN738LC joint. The size and amount of γ′ particles in FZ and BM of the joints after F-PWHT were higher than that of IN738LC joints subjected to P-PWHT. In addition, F-PWHT also facilitated the dissolution of MC carbides, releasing more free C atoms. High amount of M23C6 carbides consequentially formed along grain boundaries (GBs) in IN738LC joints. Compared with F-PWHT, the formation of duplex-sized γ′ particles and less M23C6 carbides in FZ and BM weakened the tensile strength (900 °C) but improved the elongation (16.6%) of P-PWHTed joints. The retained primary γ′ particles and minor M23C6 carbides in FZ of IN738LC joints after P-PWHT retarded the GB sliding, bringing about a higher stress-rupture life, about 2.5 h (900 °C/350 MPa).