Microstructure and Mechanical Properties of T91 Steel Welded Joint with Transient Liquid-Phase Bonding Two-step and Three-step Heating Process

T91 steel tube was bonded by transient liquid-phase bonding under two and three-step heating process, respectively. The microstructure, element distribution and mechanical properties of the joints were investigated by scanning electron microscopy, energy dispersive spectrometer, electronic universal testing machine, Charpy test device and bending testing machine. The results show that T91 steel joint had good formation quality under two heating processes. The joint under two-step heating process had the approximate line weld and brazing characteristics, and the Ni enrichment in weld resulted in the formation of black brittle phase. The weld of joint under three-step heating process was curved; Ni element distributed evenly; the microstructure and composition of the joint were the same with those of the base metal; the tensile strength was 660 MPa, the impact energy was 25.7 J and the joint did not cracked during bending 180°; the mechanical properties were improved compared with that under two-step heating process. The fracture mode of the joint under two-step heating process was ductile-brittle mixed fracture, and the fracture mode under three-step heating process was typical ductile fracture.