Effect of boron and phosphorus on HAZ microfissuring of Allvac 718 Plus superalloy

Heat affected zone (HAZ) microfissuring was investigated in electron beam welds of two Allvac 718 PLUS (718 Plus) alloys with different boron and phosphorus contents. Two preweld solution heat treatments were used and microfissuring susceptibility was evaluated by measuring the HAZ crack lengths in both alloys after bead on plate welding. The segregation behaviour of boron and phosphorus was studied before and after welding by secondary ion mass spectrometry (SIMS). Hot ductility behaviour of the alloys in the standard solution preweld heat treatment was also examined. The results of crack measurements, segregation studies and hot ductility tests correlated well with each other. It was observed that besides constitutional liquation of Nb rich MC type carbides, segregation of B and P largely influenced the microfissuring susceptibility of the alloy. Heat affected zone microfissuring increased with an increase in B+P concentrations and with an increase in the preweld solution heat treatment temperature. Segregation behaviour of B and P and its contribution to microfissuring in Allvac 718 Plus alloy were discussed.