Effect of boron concentration and grain size on weld heat affected zone microfissuring in Inconel 718 base superalloys

Microfissuring in the heal affected zone (HAZ) around electron beam (EB) welds was studied in two Inconel 718 based superalloys containing the minimum possible concentrations of C S and P, and 11 and 43 wt-ppm concentrations of B. The microfissuring susceptibility was evaluated by measuring the microcracks formed in the HAZs of EB welds and by examining the hot ductility behaviour of the material. Bead on plate type EB welding, and on cooling hot ductility tests in a Gleeble 1500 thermomechanical simulator were performed on the two alloys with different grain sizes. The results of the hot ductility tests and crack measurements on EB welded samples correlated very well with each other. Using both techniques it was observed that the HAZ microfissuring of Inconel 718 increased with an increase in B concentration and an increase in grain size. The effect of B was attributed to its segregation to the grain boundaries, which resulted in a reduction in the melting point of the grain boundary material. This rendered them susceptible to liquation in the HAZ, and formation of microfissures due to the introduction of stresses during the cooling portion of the welding thermal cycle.