Kinetics Study on Low-Temperature Tempering of Martensitic Phase in Medium Mn Steel Weldment During Paint-Baking Heat Treatment

The kinetics of low-temperature tempering in a dissimilar resistance spot weldment between medium Mn transformation-induced plasticity (MT) steel and dual-phase (DP) steel were investigated to interpret the change of failure mode after a paint-baking process. After the isothermal heat treatments with various soaking conditions, the Johnson–Mehl–Avrami (JMA) model describing nucleation and growth kinetics was adopted. The results indicated that the tempering kinetics parameters were different for each area of the welds. Especially in the coarse-grained heat-affected zone (CGHAZ) in the MT steel, carbon diffusion from martensite to austenite acts as a tempering mechanism at an earlier stage of tempering. Thereafter, the tempering mechanism changes to epsilon carbide precipitation and coarsening. In comparison, the change was relatively slow or absent in the DP steel CGHAZ and fusion zone because of its lower kinetics parameters. Based on the results of this study, ways to improve the cross-tension strength of entire welds in automotive applications of MT steel are discussed.