Resistance spot welding of MS1200 martensitic advanced high strength steel: Microstructure-properties relationship

This paper addresses the microstructure and tensile-shear mechanical performance of MS1200 Giga-grade martensitic advanced high strength steel resistance spot welds. The key phase transformations in MS1200 welds were lath martensite formation in the fusion zone (FZ) and upper-critical heat affected zone (HAZ), new ferrite formation in the inter-critical HAZ and martensite tempering in the sub-critical HAZ. The MS1200 welds were featured by a near matching hardness in the fusion zone and under-matching hardness in the heat affected zone (HAZ) compared to the base metal. At certain process window a complete nugget pullout and separation was observed with high post-necking tearing energy. The interfacial to pullout failure mode transition was explained in the light of FZ hardness as well as the HAZ softening associated with martensite tempering in the sub-critical HAZ. The load bearing capacity of MS1200 welds failed at interfacial mode was strongly depends on the FZ size as well as the FZ hardness. However, the peak load of welds failed at pullout mode was a function of HAZ softening as well as the plastic constraint in the HAZ associated with the hard upper-critical HAZ/FZ and martensitic BM.