Effect of supercritical CO2 on the performance of 740H fusion welds

The supercritical carbon dioxide Brayton cycle is of significant interest due to the potential for high efficiency and low capital cost compared to Rankine power cycles. Maximum temperature and pressure conditions produce higher efficiencies, leading to the desire for materials with high strength at elevated temperatures, such as INCONEL 740H. This alloy has been found to have good corrosion properties, but the mechanical and weld performance in carbon dioxide has not been studied. This study has attempted to address this issue by testing the mechanical properties in both base material and transverse gas tungsten arc welded samples before and after exposure to supercritical carbon dioxide at 750 °C, 20 MPa for 1000 h. After exposure to CO2, base material showed an increase in strength with a moderate drop in ductility, while welded samples exhibited a decrease in both yield strength and elongation. This matched well with samples which were thermally aged in a neutral environment. Further analysis indicated the CO2 environment had little effect beyond a depth of 5 µm of the surface, and no effect beyond 10–15 µm. The evolution of mechanical properties of 740 H in this environment, therefore, is dominated by the effect of thermal aging, with only trace impact by the CO2 environment. The thermal aging effect was shown to be mechanically detrimental within the fusion zone of the weld due to micro-segregation during the welding process.