Effect of Tempering Process on Microstructure and Properties of Resistance Spot-Welded Joints of δ-TRIP Steel

In this paper, a medium-frequency inverter spot welder was used for resistance spot-welding experiments on 980 MPa grade cold-rolled δ-TRIP(Transformation-induced Plasticity) steel. The effects of the tempering process morphology, microstructure, element distribution, and properties spot-welded joints were studied by Scanning Electron Microscope (SEM), Transmission Microscopy (TEM), Electron-Probe MicroAnalysis (EPMA). microstructure nugget zone obtained single-pulse δ ferrite, lath martensite, twin martensite. After adding under process, ferrite However, morphology still dendritic, which remained unchanged. tensile shear failure two processes an interface failure, fractures cleavage fractures. tempering, fracture surface presents kinds characteristics: outer cracks’ growth direction consistent with columnar crystal direction, inner cracks occurred in core finally grew along grain boundary. Due to significant hardness difference between (283 HV) martensite (533 HV), low-strength phase main channel crack propagation. distribution more uniform force increased (7.4 kN→8.5 kN).