Highly Enhanced Cross Tensile Strength of the Resistance Spot Welded Medium Manganese Steel by Optimized Post-heating Pulse

Abstract

The role of the post-heating pulse in the microstructure evolution and cross tensile strength (CTS) variation of the resistance spot welded 7Mn medium manganese steel (MMS) was studied. The results showed that the microstructure in the nugget fabricated by 4.0 kA was tempered martensite, yet the one by 4.5 kA was martensite. According to the cross tensile test results, the CTS was improved from 2.0 to 3.0 kN with the current of post-heating pulse increased from 2.5 to 4.0 kA, while the strength decreased to 1.8 kN when the current increased to 4.5 kA. In particular, the post-heating pulse over 4.0 kA to result in the nugget remelting and subsequently martensite formation were the crucial factors to decrease the CTS. The enhancement of CTS for the resistance spot welded 7Mn MMS could be attributed to the microstructure transition from martensite to tempered martensite.