Microstructure and Fatigue Fracture of Spot Welded TRIP800 Steel

The fatigue property of a TRIP (transformation induced plasticity) steel after resistance spot welding (RSW) is investigated. It is found that the fatigue strength of the spot weld is only 8.9% of the original TRIP800 steel. This is a result of microstructure mismatch in heat affected zone (HAZ) due to different cooling rates during RSW. A maximum variation of about 220HV microhardness is observed at different areas from the relatively narrow HAZ. The fatigue degradation is attributed to the inhomogeneous distributions of microstructure and microhardness at HAZ and its adjacent zones. The fatigue crack initiated between HAZ and BM is a cleavage fracture as confirmed by scanning electron microscopy (SEM) observation.