Weldability and Liquation Cracking Characteristics on Resistance-Spot-Welded High-Mn Austenitic Steel

In the present paper, characterizations of resistance spot weldability of high-Mn austenitic twinning-induced plasticity (TWIP) steel were performed in terms of nugget and button sizes, microstructure, and mechanical properties. Owing to rich chemistry which leads to complicated microstructure, and void formation in fusion zone (FZ) and eventually prone to early expulsion in case of high-Mn steels. Cross-tensile test (CTT) failure characteristics were observed in the sequences of: strain localization of both sheets, crack initiation at notch tip, crack followed fusion boundary, and finally ductile shear fracture along thickness direction. The welding imperfections were investigated using optical microscopy (OM) and scanning electron microscopy (SEM). It was observed that resistance-spot-welded high-Mn steel have high susceptibility of heat-affected zone (HAZ) liquation cracking; cracks were appears in the HAZ when weld current above 5.0 kA. Among the other factors, welding current was identified most influencing factors in liquation crack formation. In order to quantify the segregation of elements in cracking zone, electron-probe microanalysis line scanning and elements mapping were performed. High segregation of alloy elements were detected in liquation cracking zone which were predicted to combine with each other to form MC (carbides, M = metal). The eutectic phase and carbide formation was analyzed using thermo-calc phase diagram. Furthermore, an optimized double pulse schedule was adopted with combination of upslope and downslope condition to minimize HAZ liquation cracks and produced crack free weldment. Finally, a finite element software application (SORPAS^®) was used to simulate the effects of welding schedules on weldability, nugget formation, and cracking characteristics; and comparison was made with experimental results.

Keywords: High-Mn Austenitic steels; Resistance spot welding; HAZ liquation Crack; Simulation; Metal Carbide; Thermo-Calc.