GAS-TUNGSTEN ARC WELDABILITY OF INCOLOY 800H WITH INCONEL 625 FILLER MATERIAL IN THE APPLICATION OF HTGRS

Abstract

Incoloy 800H is a potential core component material for high temperature gas-cooled reactors (HTGRs), with expected operating temperatures above 750 °C. However, creep rupture resistance of conventional Incoloy 800H welds is significantly degraded when the environmental temperature is higher than 475 °C. The objective of this work is to explore the weldability of Incoloy 800H using the gas tungsten arc welding (GTAW) technique with Inconel 625 filler material. Welding microstructure was characterized using optical microscopy, scanning electron microscopy, and transmission electron microscopy. Micro-hardness and high temperature tensile properties (conducted at 760 °C) of Incoloy 800H welds were measured. Incoloy 800H welded with Inconel 625 filler material showed superior high temperature tensile strength compared to the unwelded base metal (BM). The weld failure occurred outside the welding zones. The Incoloy 800H welds showed more than a 50% increase in yield strength compared to the BM. In contrast, the ultimate tensile strength is minimally affected by welding. Microstructural characterization shows that the welding process induced the diffusion of alloying elements across fusion boundaries and the formation of a large number of nano-precipitates in the heat-affected zone and fusion zone. As a result, the high temperature mechanical properties of Incoloy 800H were enhanced due to the effects of solid solution strengthening and precipitation strengthening.