Microstructures in a resistance spot welded high strength dual phase steel

The microstructure of a high strength dual phase steel resistance spot welded with tempering-pulse technology is characterized in this paper. In the fusion zone, there is a needle-like microstructure identified as acicular or side plate ferrite that has a cube-on-cube orientation relationship with respect to the surrounding martensite. In contrast to the microstructures produced by the lower cooling rate arc or laser welding techniques, the nucleation of this fine intragranular ferrite takes place independent of inclusions. Further, a leaf-like microstructure within the martensitic matrix is found to contain primitive orthorhombic Cr3C2 and face-centered cubic CrC chromium carbides, rather than Cr23C6 or Cr7C3 as is commonly observed in steel alloys. The formation histories of both the ferrite phase and the chromium carbides are analyzed.