Tracking heat-affected zone cracking susceptibility in standard and modified heat treated IN 738 superalloy welds

Abstract

The conditions influencing the phenomenon of heat affected zone (HAZ) cracking during tungsten inert gas (TIG) welding of a gamma-prime (γ') precipitation strengthened nickel-base superalloy, IN 738, is discussed here. The discussion is reinforced with the dependence of the cracking on particle size variation of the γ' precipitates and base alloy hardness. Microstructural analysis of the magnitude of HAZ cracking in the alloy indicates that while resistance to cracking susceptibility is poor in the standard heat treatment (SHT) condition, the modified heat treated (MHT) alloy exhibits significantly improved resistance to HAZ cracking. The improvement in resistance to HAZ cracking in the MHT alloy is attributed to the lower hardness of the alloy which permits stress relaxation in the base alloy compared to the SHT alloy. In contrast to what is expected based on the recommended manufacturer SHT procedure, preweld microstructural modification presents a viable way of limiting cracking susceptibility in TIG welded IN 738 superalloy.

Effect of Pre-Post TIG Welding Heat Treatment on Cast NI Superalloy

“…The high hardness values might also be due to the very dense or very fine γ′ precipitates in the base metal and in HAZ, together with more γ′ precipitates in the weld filler near the HAZ [25]. It was found that these γ′ precipitates become coarser due to more dispersion by other elements such as aluminium and/or titanium from the base metal into FZ, followed by the formation of a coarser size of γ′ precipitates in this area [26]. Fig.…”