Abstract
The cross-sectional micrograph, microhardness profile, and microstructures of the resistance spot-weld (RSW) for QP980 steel are shown to reveal the effects of the content and tempering of the martensite in the fusion zone (FZ) and the heat-affected zone (HAZ) on the microhardness profile of the RSW. It is indicated that the transient peak temperature above the critical temperature during the welding thermal cycle induces the significant variations of material and mechanical properties in the FZ and HAZ. The quasi-static uniaxial compression stress-strain curve of the FZ martensite in the RSW for QP980 steel is obtained with digital image correlation (DIC) and compared with that of two micropillars of martensite phase in the base material (BM) of the RSW for QP980 steel which was given by Srivastava et al. It is attributed to the welding thermal cycle different from QP980 steel heat treatment that the flow stress of the FZ martensite in the RSW is higher than that of the martensite phase in the BM. The dynamic uniaxial compression experiments for the FZ martensite in the RSW for QP980 steel are performed on a modified split Hopkinson pressure bar (SHPB), in which the reflected and transmitted waves are improved. A dynamic compression constitutive equation is presented by analyzing the results of the quasi-static and dynamic uniaxial compression experiments. A Swift law for martensite phase is extended to high strain rates to describe the weak strain-rate dependence of the dynamic compression behaviors of the FZ martensite in the RSW for QP980 steel.
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