Effect of the Cooling Time on the Cross Tensile Strength of the Resistance Spot Welded Medium Manganese Steel

Abstract

The effect of the cooling time between the welding pulses and post-heating pulses on the cross tensile strength (CTS) of the resistance spot welded medium manganese steel (RSW-MMS) was studied in this paper. Optical microscope (OM) and scanning electron microscope (SEM) were employed to analyze the microstructure evolution. The results showed that the microstructure in nugget with cooling time of 100 ms was martensite and the microhardness could reach ~530 HV. On the contrary, the microstructure of the sample cooled for 2000 ms was tempered martensite with the microhardness of ~ 370 HV. Accordingly, the strength increased from 1.8 to 3.4 kN accompanied with the failure mode transformed from interfacial fracture (IF) to partial interfacial fracture (PIF). The improvement of the CTS for the resistance spot welded 7 Mn MMS was mainly attributed to the higher ability of the tempered martensite to prevent the crack propagation.