Delayed cracking of metastable austenitic stainless steels after deep drawing

Abstract

Certain metastable austenitic stainless steels, especially low-nickel grades, can be susceptible to delayed cracking after forming operations. Small amount of hydrogen is always present in stainless steels as an inevitable impurity. Delayed cracking results from a time-dependent hydrogen redistribution process driven by stress, either applied or residual. In this study, delayed cracking susceptibility of three metastable austenitic stainless steels 301, 301LN and 304 after deep drawing was examined by Swift cup tests. The objective was to clarify the role of alloy composition, strain-induced α'-martensite and residual stresses in delayed cracking. Hydrogen content of the stainless steels, tested in as-produced condition, was < 3 wppm. Detailed characterization of the Swift cups was performed using X-ray diffraction, magnetic permeability measurement, nanoindentation, FEG-SEM and EBSD. Stainless steel 304 with the highest Ni content was not susceptible to delayed cracking regardless of the presence of α'-martensite. Stainless steel 301LN with lower Ni content and lower austenite stability showed minor cracking at the highest drawing ratio. Stainless steel 301 was the most susceptible material, and had the highest residual stresses after deep drawing. Residual stress level in the stainless steels was found to be affected by the amount of α'-martensite and also by the hardness of the phases, both of which depend on chemical composition. Fracture mechanism in delayed cracking was predominantly transgranular quasi-cleavage and crack propagation occurred through α'-martensite. High residual stresses and presence of α'-martensite are essential in delayed cracking of austenitic stainless steels.