FRACTURE PROPERTY DEGRADATION IN CAST STAINLESS STEELS AFTER LONG-TERM THERMAL AGING

摘要

A research on the thermal degradation of castaustenitic stainless steels (CASSs) has been performed toevaluate degradation of mechanical properties duringthermal aging and to enhance scientific understanding ofaging mechanisms. This article is to summarize thethermal degradation of fracture properties given asfunctions of a newly-defined aging parameter, focusing onthe dependence of property degradation on δ-ferritecontent. Static fracture test results indicate that thechange of fracture toughness during thermal aging is notmonotonic. As in the ductile-brittle transition temperatureof impact energy reported earlier, the static fracturetoughness often showed a significant increase withincreasing degree of aging before showing a steepdecline. Within the range of aging parameter tested, onlythe CASS alloys with very high δ-ferrite contents (>25%)showed monotonic decreasing in fracture toughness. Hereit is attempted to present and explain such a complexdegradation behavior in the CASS materials. It is notedthat, while significant degradation in fracture propertiesis observed for the highly-aged high-ferrite materials, nomaterial is observed having its fracture toughness lessthan 100 MPa√m, regardless of aging and testingconditions. Further, the contour maps of fracturetoughness on the aging parameter versus δ-ferrite contentplane were constructed to search for an overarchingthermal degradation behavior among the CASS alloyswith a wide range of δ-ferrite contents. These mapsclearly demonstrate that a moderate-ferrite, low-agedalloy (like CF8) will show optimum fracture toughnessproperties at room temperature while a reduced ferritecontent is desirable in elevated temperature conditions.