Effect of filler alloy composition on post-weld heat treatment cracking in GTA welded cast Inconel 738LC superalloy

Abstract

The effect of filler alloy composition on post-weld heat treatment (PWHT) cracking in Inconel (trade name of the Special Metals group of companies) 738LC (IN-738LC) superalloy was investigated. Five filler alloys (IN-625, IN-718, FM-92, C-263 and Rene-41), containing varying concentrations of Al and Ti, were used to gas tungsten arc (GTA) weld IN-738LC superalloy specimens which were given two different preweld heat treatments. Additionally, autogenous (without filler) welds were also made on the same two preweld heat treated materials. The preweld heat treatments consisted of the standard industrial solution heat treatment (SHT) at 1120°C for 2 h in vacuum followed by argon quenching and a new heat treatment (UMT), developed by the authors to improve the heat affected zone (HAZ) cracking resistance of IN-738LC. The latter heat treatment comprised of solution treatment at 1120°C for 2 h followed by aging at 1025°C for 16 h and then water quenching. The welded specimens were given a PWHT consisting of solution treatment at 1120°C for 2 h in vacuum followed by argon quenching and subsequent aging at 845°C for 24 h in vacuum. Intergranular cracking was observed in all the PWHT welds, regardless of the filler alloy; however, autogenous welds suffered the highest amount of cracking, and cracking susceptibility of the filler welds reduced with a decrease in concentration of the main hardening elements, Al and Ti, in the fillers. While cracks were observed only in the HAZ of the as welded samples, almost all the PWHT samples were found to have cracks in the fusion zone (FZ) and in the base metal (BM), in addition to those present in the HAZ. The severity of cracking in the SHT preweld heat treated material was, however, consistently higher than that observed in the UMT preweld treated material, in both autogenous and filler welds. The microstructure of FZ of the welds made with various filler alloys consisted of a unimodal distribution of small spheroidal γ' particles of 0˙10–0˙15 μm, while a bimodal distribution of γ' precipitates, consisting of large cuboidal particles of about 0˙5–0˙6 μm and fine spheroidal γ' particles of ∼0˙1 μm, was observed in FZ of the autogenous welds. Results of the microstructural analysis and crack measurements were correlated with the chemical composition and the precipitation kinetics of γ' in the filler alloys. It was observed that PWHT cracking in GTA welds of IN-738LC was reduced to a significant extent by using age hardenable fillers with (Al+Ti) concentration less than that in the BM. The PWHT cracking resistance of IN-738LC was further improved by the use of these fillers in conjunction with the UMT preweld heat treatment.