Resistance spot welding of martensitic stainless steel: Effect of initial base metal microstructure on weld microstructure and mechanical performance

This paper addresses the microstructure-properties relationship during resistance spot welding of martensitic stainless steel (MSS). The effect of the initial base metal microstructure (ferritic microstructure in annealed condition vs. partially tempered martensitic in quench and tempered condition) on the weld microstructure evolution is highlighted. Regardless of the initial base metal microstructure, the fusion zone exhibited a predominantly martensitic structure plus some δ-ferrite. The heat affected zone (HAZ) in the quenched and partially tempered (Q-PT) sheet was featured by formation of martensite and carbide in upper-critical zone and tempering of martensite in sub-critical zone. The latter caused pronounced softening in the HAZ compared to the base metal. However, the HAZ in the annealed sheet exhibited hardening compared to the base metal due to the formation of a dual phase microstructure composed of martensite plus carbide. Nor high strength mismatch between fusion zone and base metal (when welding in the annealed condition), neither severe HAZ softening (when welding in the Q-PT condition) did not promote pullout failure mode. The consistently predominant interfacial failure of the MSS welds was governed by the low fracture toughness of the fusion zone.