Microstructure and Thermal Cracking Susceptibility of Dissimilar Resistance Spot Welded Austenitic and Mild Steels

Dissimilar resistance spot welding (RSW) of 301LN cold-rolled austenitic stainless steel and Q235B mild steel was carried out to study the microstructure and thermal cracking susceptibility. The microstructure of the fusion zone was columnar dendrites (CD) composed of martensite and a small amount of δ-ferrite and very thin inter-dendritic (ID) austenite, whose transformation path was L→L + δCD→L + δCD + γID→δCD + γID→(δ + γ)CD + γID→(δ + α′)CD + γID. The martensitic transformation could cross the interdendritic austenite to form large blocks, and the thin interdendritic austenite remained at room temperature. The protrusion area of the fusion zone under the lap surface of the 301LN plate was susceptible to thermal cracking, which can be prevented by adopting the RSW process with low welding current matched to high electrode force. The thermal cracking resulted in a reduction in ductility and fracture strength of the joint.