Fracture property degradation in cast stainless steels after long-term thermal aging

A research on the thermal degradation of cast austenitic stainless steels (CASSs) has been performed to evaluate degradation of mechanical properties during thermal aging and to enhance scientific understanding of aging mechanisms. This article is to summarize the thermal degradation of fracture properties given as functions of a newly-defined aging parameter, focusing on the dependence of property degradation on δ-ferrite content. Static fracture test results indicate that the change of fracture toughness during thermal aging is not monotonic. As in the ductile-brittle transition temperature of impact energy reported earlier, the static fracture toughness often showed a significant increase with increasing degree of aging before showing a steep decline. Within the range of aging parameter tested, only the CASS alloys with very high δ-ferrite contents (>25%) showed monotonic decreasing in fracture toughness. Here it is attempted to present and explain such a complex degradation behavior in the CASS materials. It is noted that, while significant degradation in fracture properties is observed for the highly-aged high-ferrite materials, no material is observed having its fracture toughness less than 100 MPa√m, regardless of aging and testing conditions. Further, the contour maps of fracture toughness on the aging parameter versus δ-ferrite content plane were constructed to search for an overarching thermal degradation behavior among the CASS alloys with a wide range of δ-ferrite contents. These maps clearly demonstrate that a moderate-ferrite, low-aged alloy (like CF8) will show optimum fracture toughness properties at room temperature while a reduced ferrite content is desirable in elevated temperature conditions.