Microstructure and flow behavior of Ni-Cr-Fe welds with Nb and Mo additions

This paper was intended to investigate the influence of niobium and molybdenum on the microstructure and flow behavior of Ni-Cr-Fe welds at room/elevated temperature. Microstructural examinations and mechanical tests were performed on the weld specimens of Alloy 52, Alloy 52M and two variants with higher contents of niobium and molybdenum. In Alloy 52 weld, without the additions of niobium and molybdenum, TiCs/TiNs were present in the matrix and Cr-carbides precipitated along the grain boundary. It was noted that niobium could strengthen the welds with the precipitation of small NbCs/(Nb,Ti)Cs, while molybdenum could not only precipitation-strengthen with small (Nb,Mo)/Cs but also solution-harden the welds. Both of niobium and molybdenum would increase the amount and size of Laves phases when their additions were higher. The average hardness, yield and tensile strength of the welds increased with the contents of niobium and molybdenum. Jerky flow was found on the stress-strain curves at 300 °C, except for the Alloy 52M weld. Ductile dimples were the dominant fracture feature of the welds with lower niobium contents. With the Nb/Mo additions, precipitation and the solution strengthening resulted in a decrease of slip lines in the matrix.