Effect of Post-weld Heat Treatment on the Cross Tensile Strength of Resistance Spot-Welded Medium Manganese Steel

The influence of the post-weld heat treatment (PWHT) on the cross tensile strength (CTS) of the resistance spot-welded medium manganese steel (MMS, 7Mn steel) is investigated in the present study. The results showed that the PWHT (tempering) could increase the CTS of the resistance spot-welded 7Mn steel to 5.4 kN, nearly 3 times compared with that for the as-welded specimen (~ 2.0 kN). The as-welded 7Mn steel contains fully dendritic martensite and revealed severe Mn segregation at the grain boundaries, leading to a weak zone adjacent to the centerline in the nugget. As a result, the fracture resulted in a complete intergranular fracture morphology, reflecting an interfacial fracture (IF) mode. On the other hand, the tempering of PWHT transformed the microstructure of 7Mn steel to tempered martensite. In addition, the PWHT introduced nano-sized Mn 23 C 6 -type carbides along the grain boundaries in the nugget, successfully relieved the Mn segregation and strengthened the grain boundaries. Consequently, the as-tempered 7Mn steel failed in partial interfacial fracture (PIF) mode with a large amount of dimples on the fracture surface. This experimental study shows the capability of PWHT to dramatically improve the CTS of 7Mn steel by microstructure manipulation.