Microstructure and mechanical performance of resistance spot-welded cold-rolled high strength austenitic stainless steel

Abstract

The microstructure, fracture and fatigue behavior of spot welds were characterized. The results showed that the improper welding parameters easily caused folding interface in HAZ between welded-sheets. Overload failure modes were interfacial fractures even if nugget diameters satisfied d > 5 t . The failure deformation introduced 29.8–32.7 (v.%) α′-martensite transformation at interfacial fractures. The hardness under the interfacial fracture surface was more than two times of that in the nugget. Fatigue specimens cracked through-thickness sheets. The stress concentration led to the actual stress exceeding the shear strength of annealed austenite in HAZ, and shear bands initiated under very low nominal stress. Fatigue cracks initiated 0.3–0.4 and 1.2–1.3 mm away from the nugget edge for spot-welded 1.5 and 4 mm sheets respectively. There was no detectable strain induced α′-martensite at fatigue fractures. The influence of the folding interface on fatigue performances depended on the folding location. When the folding coincided with the location of maximum stress concentration, it would exacerbate the stress concentration and reduce the fatigue limit remarkably. However, if the folding located outside the area of maximum stress concentration, while still the crack initiated at the interface fold, it did not reduce the fatigue limit significantly.