Influence of In Situ Thermal Processing Strategies on the Weldability of Martensitic Stainless Steel Resistance Spot Welds: Effect of Second Pulse Current on the Weld Microstructure and Mechanical Properties

Abstract

Martensitic stainless steel (MSS) welds are notorious for their susceptibility to low-energy failure due to the formation of brittle martensitic structure in the fusion zone. The unique approach to enhance the mechanical properties of MSS resistance spot welds during both the tensile-shear and the cross-tension loading is to improve the fracture toughness of the fusion zone. In the present study, the effect of double-pulse welding on the microstructure-mechanical properties relationship of the AISI420 MSS resistance spot welds is investigated. Depending on the second pulse current level, various metallurgical phenomena was observed including (i) rapid tempering of martensite in the fusion zone featured by hardness reduction and precipitation of nano-sized carbide precipitates which resulted in remarkable improvement of the load-bearing capacity and energy absorption capability of the welds, (ii) re-austenitization of the fusion zone and reformation of the un-tempered martensite without improvement of the weld mechanical properties, (iii) temper embrittlement of the nugget edge which resulted in very low-energy grain-boundary failure, and (iv) nugget edge re-melting/enlargement which resulted in slight improvement of weld mechanical properties. Therefore, to enhance the mechanical properties of the MSS resistance spot welds using a two-pulse welding strategy, the second pulse parameters should be precisely controlled.