Mechanism of post-weld heat treatment cracking in Rene 80 nickel based superalloy

摘要

Microstructural studies carried out on Rene 80 (approximate composition 60Ni - 14Cr- 9·5Co - 4Mo - 5Ti - 3A1 -0·17C - Zr - B, wt-percent) weldments before and after post-weld heat treatment (PWHT) revealed abundant evidence of constitutionally liquated and resolidified grain boundaries extending from the mushy zone into the heat affected zone (HAZ). While total dissolution of gamma ft occurred along such grain boundaries, a much lesser degree of gamma ft dissolution was noted in the adjacent material. During the PWHT, a high density of gamma ft precipitated out both within the mushy zone and in the constitutionally liquated and resolidified grain boundary regions in the HAZ. As the dissolution and reprecipitation of gamma ft occurred fairly uniformly throughout the mushy zone, the ensuing aging contraction stress/strain was fairly uniformly distributed in the region. In contrast, in the adjacent part of the HAZ, an extreme volume of gamma ft precipitation occurred locally along the grain boundary regions, a result of the highest concentration of gamma ft forming solutes and the complete dissolution of gamma ft during welding in these regions. This, combined with the much stronger adjacent grain matrix, caused the aging contraction stress and strain to become highly concentrated along the grain boundary regions in the HAZ. This promoted the formation of PWHT cracks along such grain boundaries, which then propagated along the grain boundary into the mushy zone and beyond.