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

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 gamma' particles, fanlike gamma-gamma' 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 gamma' 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 gamma' 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 gamma' particles. All the joints fractured in the BM at RM and 900 degrees C. The as-cast and water quenched (WQ) joints failed with a quasi-cleavage fracture mode at RM and 900 degrees C while the tensile fracture of furnace cooled (FC) joint showed a ductile rupture at RM and 900 degrees C.