Post-weld heat treatment cracking in autogenous GTA welded cast Inconel 738LC superalloy

Abstract

The post-weld heat treatment (PWHT) cracking in autogenous gas tungsten arc (GTA) welded Inconel 738LC superalloy, which was given two different preweld heat treatments, was studied. One of the preweld heat treatments, designated as SHT, consisted of solution heat treatment at 1120°C for 2 h in vacuum followed by argon quenching. The second preweld heat treatment, designated as UMT, consisted of solution treatment at 1120°C for 2 h followed by air cooling and then aging at 1025°C for 16 h followed by water quenching. The welded specimens were given the same conventional PWHT, which consisted of SHT at 1120°C for 2 h in vacuum followed by argon quenching and subsequent aging at 845°C for 24 h in vacuum. Microstructural examination of the welded SHT and UMT treated material showed that intergranular microfissuring occurred during welding only in the heat affected zone (HAZ) with some cracks extending into the adjoining base metal (BM), whereas after the PWHT microfissures were observed in the fusion zone (FZ), HAZ and the BM far removed from the HAZ. The crack width ranged from 5 to 10 μm in the PWHT specimens as compared with 1–2 μm in the as welded sections. Although similar type of cracks was observed in samples given the two preweld heat treatments, the UMT preweld heat treatment was found to result in a significant reduction in average total crack length and average crack length, both during welding and during the subsequent PWHT. After PWHT, SHT samples had ∼43% more cracking than the UMT samples. It is suggested that a larger particle size of γ′ precipitates in the HAZ and a smaller size of HAZ in the as welded samples, combined with a softer BM of the UMT material (hardness 280 ± 12 HV10, as compared with 380 ± 10 HV10 of the SHT material) resulted in an improved capability of the material to absorb the strain–aging stresses, and hence a reduced incidence of cracking during PWHT.