Mechanical and Microstructural Characterization of Inconel 625 Welds After Corrosion at Supercritical Water Conditions

Abstract

Inconel 625 is considered one of the candidate materials for fuel cladding in the Canadian supercritical water reactor. Gas tungsten arc welding (GTAW) is being evaluated as a joining technique for SCWR fuel cladding since this method is widely used to join components in the power and nuclear industry. The thermal cycle, during the GTAW process, produces different types of microstructures that affect the material's mechanical properties. The objective of this work was to study the effect of corrosion testing at supercritical water conditions on mechanical properties of Inconel 625 base material (BM) and weldments. Tubular Inconel 625 specimens were welded using a GTAW orbital process. Corrosion testing was conducted in an autoclave at 575 °C (848.15 K) and 23.5 MPa for 500 h (1.8 × 106 s). Weld characterization included mechanical tests, optical microscopy, and scanning electron microscopy-energy dispersive X-ray spectroscopy. The composition of second phase precipitates observed during microstructural characterization was elucidated using equilibrium phase diagrams. Tensile tests showed that both BM and welds displayed an increase in strength and a decrease in ductility after corrosion testing. The ultimate tensile strength increased about 2% and 9% for the BM and the weldment, respectively. The maximum specimen elongation decreased around 35% for the BM and 45% for the weldment. The microstructure showed indications of second phase precipitation and grain morphology changes produced by both welding and corrosion processes. Results show that ductility of Inconel 625 GTAW welds is significantly reduced after exposure to supercritical water for 500 h (1.8 × 106 s).