An EBSD based comparison of the fatigue crack initiation mechanisms of nickel and nitrogen-stabilized cold-worked austenitic stainless steels

Abstract

Nitrogen additions are known to improve both the pitting potentials and fatigue response of high-nitrogen austenitic stainless steels (HNASS). HNASS alloys are of particular interest to the biomedical industry due to patient allergy concerns. Available HNASS fatigue studies in the literature were all performed on materials in the solution annealed condition, and not the 20–30% cold-worked condition often ordered for implant manufacturing. Previous studies in our laboratories evaluated the corrosion fatigue response of commercially available cold-worked 316L and HNASS implant grade steels, and found evidence of unusual cleavage-like facets in the crack initiation and early propagation regions of the fracture surfaces in the HNASS. The 316L steel showed a typical ductile fatigue striated fracture surface with no evidence of cleavage-like facets. These findings suggested the fatigue crack initiation mechanisms may be different between the two austenitic steels. The purpose of the present study was to compare and contrast fatigue crack initiation mechanisms in the cold-worked 316L and HNASS steels, using an SEM and EBSD based examination technique, under both low-cycle and high-cycle conditions. Under both fatigue conditions, cracks were shown to preferentially initiate along former annealing twin boundaries in the HNASS steel. In contrast, fatigue crack initiation was shown to occur primarily along extrusions or intrusions associated with trans-granular slip markings in the 316L steel.