POST-WELD HEAT TREATMENT CRACKING IN RENE 80 AND EFFECT OF PRE-WELD HEAT TREATMENT

Abstract

Edge build-up welding on Rene 80 nickel-based superalloy with In 625 filler metal was conducted by means of pulsed arc welding using a Liburdi Automated System- LAWS 500. The experimental results showed that, independent of pre-weld microstructures, five distinguished zones have been identified in the microstructure around the weldment. At the top of the weldment is the build-up bead characterized by coarse dendritic grains. This is followed by the mixed zone where partial mixing occurred between the filler and base materials. The next zone is the mushy zone in which partial melting took place. Next to it is the heat affected zone (HAZ), in which incipient melting of grain boundaries and dissolution of y' during welding are apparent. The final zone is the unaffected zone where no microstructural changes took place during welding. After the post-weld heat treatment (PWHT), the microstructure is characterized by non-homogenous re-precipitation of fine ?' along the incipient melted and re­ solidified grain boundaries and in between existing coarse ?' in the HAZ and mushy zone, and abundant fine ?' in the mixed zone and the build-up bead. Some topographical close-packed (TCP) phases are also present in the mixed zone. During PWHT, cracks initiated at the incipient melted and re-solidified grain boundaries in the HAZ, and propagated along such grain boundaries into the mushy zone and mixed zone. Both the higher welding residual stress and ageing contraction stress produced by the re-precipitation of ?' during the PWHT in HAZ and mushy zone are major contributing factors responsible for the formation of PWHT cracks in the material. After the cyclic overageing pre-weld heat treatment, the microstructure becomes coarser, more stable and homogenous, which gives rise to higher ductility of the base metal and narrower mushy zone and HAZ during welding. All these contribute collectively to reducing the susceptibility to PWHT cracking, hence improved weldability of Rene 80 alloy.