Abstract
There is increasing evidence in the literature that the cyclic crack-growth rate (da/dN) for A533B pressure vessel steel under fatigue loading in deozygenated water at 288 C is a strong function of the sulfur content of the steel. Explanations of this effects has been qualitative in the past and has centered around either hydrogen-assisted cracking or oxidation-related models of crack propagation. The objective of this investigation was to propose a mechanism which explained quantitatively this compositional effect on the corrosion fatigue behavior of low alloy steels. An oxidation related model of environmentally-enhanced cracking was used as a working-hypothesis.
Measurements of oxidation rates on bared surfaces in aqueous environments expected at the tip of advancing cracks have indicated that these rates are strongly dependent on the anion present. Provided sulfur-rich anions can be created in the crack tip enclave (by, for instance, dissolution of MnS precipitates), and provided that these can be retained (by, for instance, stagnant-flow conditions adjacent to the specimen), then the observed crack propagation rates can be predicted quantitatively by an oxidation-related model. The model leads to implications relating to a critical strain rate at the crack tip (controlled by frequency, stress intensity amplitude, etc.) which is necessary for environmentally-enhanced cracking in this alloy/environment system.
