Crystallography and Kinetics of Stress Corrosion Cracking in Type 316 Stainless Steel Single Crystals

Abstract

Crystallography, morphology, and kinetics of stress corrosion cracking (SCC) in austenitic stainless steels exposed to boiling, aqueous 45% magnesium chloride solution were studied on single crystal specimens of a Type 316 stainless steel. Notched specimens of selected orientations were stressed in bending, using a self relieving loading system. The variations of load during crack propagation were recorded and analyzed to obtain a relation between the crack propagation rate and other parameters. A single surface analysis of crack traces on the specimen sides and a two surface trace analysis on sections through flat regions of fracture surfaces were used in the determination of the crystallographic orientation of the cracking plane. Fracture surface topography and corrosion attack on slip steps were examined in a scanning electron microscope. The results of the crystallographic analysis show that the stress corrosion cracks propagated on (210) oriented crystal planes. After an initial period, the cracks in each specimen studied propagated at a constant rate, independent of crack length and nominal stress. The average crack propagation rate in the constant rate period was 0.121 mm/hour. Scanning electron micrographs are presented showing typical fracture surface features and evidence of corrosion attack on slip steps. It is shown that the cracks may be initiated by the surface attack but that they do not follow the slip planes. Models of SCC mechanisms are discussed in view of the results obtained. It is concluded that the evidence introduced does not fit the present dissolution models. The models involving hydrogen as critical species and the stress sorption model seem to be in best accord with the findings.