A study on heat affected zone softening in resistance spot welded dual phase steel by nanoindentation

The base metal (BM) and the heat affected zone (HAZ) of a resistance spot welded dual phase steel have been evaluated by nanoindentation hardness testing. Three different surface conditions have been explored on the BM for assessing the nanohardness response. Softening has been investigated along the sub-critical HAZ by making nanoindentations on individual phases such as ferrite and tempered martensite (TM) at various distances from the line of lower critical temperature Ac1. A broken appearance accompanied with sub-micron particles were consistently found on TM at 100 μm from the Ac1 line suggesting carbide precipitation along with partial recovery of martensite. The morphology of TM kept on changing while moving away from Ac1 towards the BM as the fraction of broken appearance was reduced and the sub-micron particles became finer. SEM observations resulted in good agreement with the nanohardness of the TM phase along the sub-critical HAZ. In contrast, microhardness results suggested the termination of tempering at a shorter distance with respect to Ac1 and hence a reduced extension of the softening region. The improved resolution for assessing softening through nanoindentation was due to the possibility of avoiding the contribution of the phase boundaries because of the smaller size of the indentation; this also permitted evaluation of TM at low peak temperatures far from Ac1 where early stages of tempering took place.