Effect of δ-Ferrite Content on the Stress Corrosion Cracking Behavior of Cast Austenitic Stainless Steel in High-Temperature Water Environment

The slow strain rate tests (SSRT) at a nominal strain rate 1× 10−6/s were conducted to evaluate the mechanical properties of CF8A cast austenitic stainless steel (CASS) in air and high-temperature water environments. CF8A CASS is mainly composed of a matrix of γ phase dispersed with δ ferrites and is affected slightly by aging heat treatment. The hardness value of γ phase did not change with aging time, while that of δ ferrite increased with increasing the aging time. The SSRT results showed that a loss in elongation of a high δ-ferrite specimen was more significant than that of the low δ-ferrite specimens. The SCC paths of the low δ-ferrite specimen were along the links between δ ferrites dispersed in the matrix. For the high δ-ferrite specimen, most of the cracks were observed to propagate through the chromium depletion regions in the ferrite grains, which were preferentially oxidized during the SSR test in water. However, the elongation did not decrease further after aging time was longer than 2,400 h, which was consistent with the trend of hardness measurements.