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Grain Boundary Engineering for Intergranular Stress Corrosion Resistance in Austenitic Stainless Steel
Dirk Lars Engelberg
[Thesis].The University of Manchester;2006.
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Abstract
Austenitic stainless steels are frequently used for engineering applications in aggressive environments. Typical sources of component failures are associated with localized attack at grain boundaries, such as intergranular corrosion and stress corrosion cracking. To prevent premature failures, structural integrity assessments are carried out, with the aim of predicting the maximum likelihood of cracking that may develop. For accurate predictions it is of great importance to know the interaction of parameters involved in life-determining processes.This PhD thesis investigates the effect of microstructure and stress on intergranular stress corrosion cracking in Type 302 / Type 304 austenitic stainless steels. High-resolution X-ray tomography has been successfully applied to examine, for the first time in 3-dimensions, in-situ, the interaction between microstructure and crack propagation. The development and subsequent failure of crack bridging ligaments has been observed and correlated with regions of ductile tearing persistent on the fracture surface. These ductile regions were consistent with the morphology of low-energy, twin-type grain boundaries, and are believed to possess the capability of shielding the crack tip.Following this observation, a new grain bridging model has been developed, in order to quantify the effect of static stress and crack bridging on the maximum likely crack length. The model was compared and evaluated with in the literature available percolation-like models.Intergranular stress corrosion tests in tetrathionate solutions have been designed and carried out to validate the new model. The assessment comprised,(i) a thorough examination of the microstructure and analysis parameters employed,(ii) the determination of the degree of sensitisation with subsequent crack path investigations,(iii) the identification of a suitable test system with associated grain boundary susceptibility criteria,(iv) the application of Grain Boundary Engineering (GBE) for microstructure control,(v) statistical crack length assessments of calibrated IGSCC test specimens.The results of these tests showed that the new model successfully predicts the magnitude of stress and the effect of grain boundary engineering on the maximum crack lengths.