Liquation Cracking in Heat Affected Zone in Ni Superalloy Welds

Precipitation hardened nickel-based superalloys are widely used in aero and industrial gas turbine engines due to their excellent high temperature strength and remarkable hot corrosion resistance. A drawback of many of these alloys is that they are very difficult to weld due to their high susceptibility to heat affected zone (HAZ) cracking, both during welding and post weld heat treatments (PWHT). Weld cracking in many of these alloys has been attributed mostly to constitutional liquatioin of grain boundary NbC precipitates. however, HAZ cracking has been observed in carbon-free superalloys as well, Therefore, research was initiated to examine grain boundary liquation and cracking in HAZs in a variety of Ni-based superalloys. It was found that intergranular cracking of grain boundaries involved liquation of several other phases, in addition to NbC, that were present in pre-weld microstructure of the alloy. These even included the primary strengthening phase,γ’, in a very widely used superalloy, Inconel 738. In addition, segregation of melting point depressant element B was also observed at gain boundaries in other superalloys, which also caused grain boundaries in HAZ to liquate and resulting in their cracking. An overview of microstructural aspects of different liquation phenomena involved and characteristics of the liquid film contributing to the HAZ microfissuring of nickel superalloys will be discussed in this presentation