Cyclic overaging pre-weld heat treatment of Rene 80: effect of solution treatment and end aging temperatures

摘要

The present work investigated the effect of a cyclic overaging pre-weld heat treatment proposed by Lim (US Patent no. 5 509 980, 1996) on the microstructure, tensile ductility, and weldability of Rene 80 nickel based superalloy (approximate composition 60Ni-14Cr-9·5Co-4Mo-5Ti-3Al-0·17C-Zr-B, wt-percent), with a focus on the role of solution treatment and end aging temperatures. The results showed that maximum ductility corresponds to a microstructure consisting of very coarse and widely spaced gamma ft, with fine gamma ft not resolvable even at a magnification of X 10 000. This was achieved by cyclic cooling the material from the solution temperature to a temperature in the range 500 - 750 deg C followed by fast furnace cooling to suppress the precipitation of fine gamma ft. The solution treatment temperature was found to control the number density of coarse gamma ft, which, in turn, slightly influenced the optimum end aging temperature. After the optimum cyclic overaging heat treatment with a solution temperature of 1080 deg C and an end aging temperature of 550 deg C, the material exhibited a tensile type fracture path with a high registered tensile ductility of approx 30 percent in elongation. Fractographic studies revealed that the necked gamma ligaments were thicker and taller, with an increased frequency of cleaved coarse gamma ft occurring at different levels in the material. Precision welding tests showed that the weldability of the optimally overaged alloy was significantly improved, owing to both the resultant soft and ductile base metal and the reduced heat affected zone size.