Microstructure and mechanical properties of a new cast nickel-based superalloy K4750 joint produced by gas tungsten arc welding process

The microstructures and mechanical properties of gas tungsten arc-welded joints of superalloy K4750 that has been newly developed were examined. The microsegregation pattern during solidification of the fusion zone indicated that Fe was prone to segregate into the dendritic core, while Nb, Mo, Ti, W and Si were rejected into the dendritic boundary. The phase transformation sequence of the K4750 alloy fusion zone was LL+L+ +(Ti,Nb)C+(Ti,Nb)C. Heat-affected zone (HAZ) examination shows it consisted of partially melted zone with liquated MC carbides and liquated grain boundaries. Cracking-free HAZ microstructure suggested that K4750 alloy possess excellent HAZ cracking resistance properties. Relatively low concentration of Al and Ti results in the excellent cracking resistance properties of the alloy. Vickers microhardness of as-welded joint is lower than that of post-weld heat treatment (PWHT) joint across the whole joint due to the absence of 50nm spherical particles formed during PWHT. Tensile properties of as-welded joints are comparable to PWHT joints in terms of the ultimate tensile strength and elongation.