Tensile and fatigue properties of Super304H welded joint at elevated temperatures

Abstract

Super304H is a promising candidate for heat exchangers in boiler systems used in thermal power plants operating at ultra-supercritical conditions, and the mechanical properties of its welded joint play a decisive role in the reliability of the thermal power plant. Herein, we report the tensile and fatigue properties of Super304H welded joint at elevated temperatures up to 750 °C, focusing on the operating temperature range of 500–700 °C. Our results showed that the tensile strength of the welded joint was almost the same as that of the base metal at all test temperatures, meeting the requirement for weld integrity, but a significant reduction in the elongation was induced in the welded joint due to non-uniform deformation that promotes deformation localization, leading to an early occurrence of necking. As for fatigue resistance, both the welded joint and base metal had comparable fatigue lives at high strain amplitudes (≥~0.4%) in the temperature range of 500–700 °C, whereas a drastic reduction in the fatigue resistance was observed in the welded joint at low strain amplitudes (<~0.4%), which was associated with the transition of the fatigue failure location (mechanism) from the base metal to the weld metal. As both the welded joint and base metal cyclically hardened during fatigue deformation, the plastic strain energy density was found to be a suitable fatigue parameter. To predict the fatigue life behavior and its temperature dependence, an energy-based unified life prediction model was developed, and its validity was demonstrated with both the welded joint and base metal.