Microstructure and high-temperature mechanical properties of TP310HCbN welded joint

The microstructure of TP310HCbN welded joints and its effect on the tensile and low-cycle fatigue properties were investigated in the temperature range 20-700 °C, particularly in the operating range of 500-700 °C. The results showed that the welded joint was fully austenitic; however, a significantly different microstructure was formed in each welding zone, inducing material inhomogeneity. The material inhomogeneity was most pronounced in the weld metal, wherein an austenitic columnar grain structure composed of dendrite cores and interdendritic regions was developed. A combined analysis of the nanoindentation, failure mechanism, and kernel average misorientation using electron backscatter diffraction revealed that the dendrite core was the softest region in the welded joint and the deformation was localized here, thereby serving as a crack nucleation site and propagation path. This promoted premature failure of the welded joint, resulting in a reduction in the tensile strength, ductility, and the fatigue resistance, in comparison to the base metal. As both the welded joint and base metal showed significant cyclic hardening behavior (more than two-fold increase in the tensile peak stress) during fatigue deformation, the plastic strain energy density was found to be a suitable fatigue parameter, which was invariant throughout the fatigue life. An energy-based unified fatigue life prediction model using material toughness was developed, and its validity was demonstrated by successfully predicting the temperature dependence of the fatigue life as well as the fatigue life reduction of the welded joint.