Effect of pre-weld heat treatment on the microstructure and mechanical properties of electron beam welded IN738LC joint

Abstract

Influence of pre-weld heat treatment on the microstructure and mechanical properties of electron beam welded IN738LC superalloy was investigated systematically. The microstructure of as-cast base metal (BM) mainly consisted of “ogdoadically” diced cube γ′ particles, fanlike γ-γ′ eutectics, MC carbides, Cr–Mo borides and Ni–Zr intermetallics. Grain-boundary liquation cracking was observed in the HAZ of as-cast joint, resulting from the segregation of elemental boron. Pre-weld heat treatment had a significant influence on the size, volume fraction and distribution of γ′ particles. It also decreased the non-equilibrium segregation of boron during cooling, which inhibited the formation of grain-boundary liquid film and liquation cracking. The precipitation of MC carbides and nanoscaled γ′ particles in the fusion zone (FZ) improved the micro-hardness of the FZ. The micro-hardness of the BM and the HAZ were influenced by the volume fraction and the size of γ′ particles. All the joints fractured in the BM at RM and 900 °C. The as-cast and water quenched (WQ) joints failed with a quasi-cleavage fracture mode at RM and 900 °C while the tensile fracture of furnace cooled (FC) joint showed a ductile rupture at RM and 900 °C.