Microstructure-performance relationships in gas tungsten arc welded Hastelloy X nickel-based superalloy

Abstract

This study aims at understanding the metallurgical and mechanical response of Hastelloy X nickel-based superalloy to the thermal cycle of gas tungsten arc welding (GTAW), post welding solution treatment (ST), and aging. In the as-weld condition, the weldment was characterized by the formation of the M6C carbide, sigma phase, and micro-solidification cracks in the interdendritic zones of the weld metal, and constitutional liquation of M6C carbide in the heat-affected zone. It is demonstrated that neither the weld metal nor the heat-affected zone could adversely affect mechanical properties of the welded joint in as-weld condition. However, to alleviate the microstructural heterogeneity, tensile residual stresses, and non-equilibrium microstructure developed due to the welding thermal cycle, the weldment was subjected to the solution treatment cycle (1177 °C/5 min) as post-weld heat treatment. The post-weld heat treatment enabled complete elimination of sigma phase. The tensile and creep properties of the weldment after the solution treatment cycle were compared to that of the as-weld condition. Despite the 8% decrease in the strength of the weldment after the ST cycle, the sample not only still met the minimum mechanical requirements but also benefited from a homogenized and nearly equilibrium microstructure. Microstructural observations of the weldment after the aging cycle (800 °C/8h), as well as its mechanical response, revealed that the formation of continuous Cr-rich carbides at the grain boundaries of the base metal deteriorates the room temperature ductility and creep life of the joints.