Effect of δ-ferrite on stress corrosion cracking of CF8A austenitic stainless steels in a simulated pressurised water reactor environment

摘要

The effect of δ-ferrite on the stress corrosion cracking (SCC) of CF8A austenitic stainless steels in a simulated pressurised water reactor environment is investigated from the viewpoint of oxidation. In constant-displacement tensile tests, a large number of entangled dislocations are formed in δ-ferrite due to the incompatibility of the slip system between the austenite and δ-ferrite. The activity of the metal atoms in dislocation-entangled nodules is much higher than that in other regions. Therefore, many Fe and Ni atoms break through the barrier of the inner Cr2O3oxide films and diffuse to react with the Cr2O3to form non-stoichiometric (Ni,Fe)(Fe,Cr)2O4spinel-structure oxides, and denser outer Fe3O4granular oxides. In addition, δ-ferrite–austenite grain boundaries are oxidised preferentially due to the accumulation of a high level of interfacial energy and plastic damage. However, SCC retardation is clearly observed because the presence of δ-ferrite changes the direction of SCC propagation and increases the length of the SCC propagation pathways.