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Effects of pre- and post-weld heat treatment cycles on the liquation and strain-age cracking of IN939 superalloy

Performing heat treatment cycles on precipitation-hardened nickel-based superalloy before and after welding imposes significant contributions to reduced welding cracks. In this study, different pre- and post-weld heat treatment cycles were performed on IN939 alloy to study their effects on microstructure and mechanical properties of the weldments. Three types of pre-weld heat treatment cycles were performed to over age the microstructure and reduce the hardness of the base alloy. Results indicated that hardness of over-age heat-treated samples decreased to about 300 to 315 HV (∼35%). Standard post-weld heat treatments (PWHT) were applied on weldments for strain-age cracking examination. Microstructural investigations showed that the above-mentioned treatments impose significant effects on reducing the liquation and strain-age cracking in heat-affected zone (HAZ) of IN939 weld. Slow furnace cooling heat treatment was found to be the optimal pre-weld heat treatment for the tungsten inert gas (TIG) welding of IN939. Based on the results obtained upon mechanical and microstructural analyses, the four-stage PWHT cycle was found to be a suitable one for IN939 welding. A joint efficiency of 79% was achieved in the optimum pre-and post- weld heat treatment cycles.