Weldability of Cast and Wrought Nickel Base Alloys 59, 617 and 625

Abstract

In this study, the hot cracking susceptibility of centrifugal and sand cast nickel base alloys was evaluated compared to the wrought products. Three solid-solution strengthened nickel base alloys—alloy 59, alloy 625 and alloy 617—were studied with respect to the formation and propagation of hot cracking in the cast and wrought microstructure. Hot cracking tests were performed by PVR (programmable deformation crack) test to rank the cracking susceptibility of the cast base metals against the wrought equivalents. Further investigation on the crack susceptible region in the base metal heat-affected zone (HAZ) of cast and wrought alloy 625 was conducted by hot ductility testing using a Gleeble^® system. PVR testing indicates a considerably higher hot cracking susceptibility of the cast nickel base alloys compared to the wrought forms of the same composition. The much more severe HAZ liquation cracking of the cast alloys is attributed to the presence of low melting constituents or constitutional liquation of constituents in the large grained and very segregated cast microstructure. Results of hot ductility testing showed that the brittle temperature range (BTR) of the cast alloy 625 is almost three times as wide as for the wrought equivalent. Indicating a much higher hot cracking susceptibility of the cast alloy, this is in good correlation to the PVR test results. Metallurgical and fractographic evaluation of hot ductility samples revealed that in the cast microstructure severe liquation initiates in the vicinity of MC (NbC) carbides. This carbide constitutional liquation reaction leads to the formation of grain boundary liquid films providing a strong driving force for HAZ liquation cracking in cast alloy 625.