Aging effects on the mechanical properties of alumina-forming austenitic stainless steels

Isothermal aging and tensile evaluation were conducted for recently developed alumina-forming austenitic stainless steels (AFAs). Microstructural observation reveals that NiAl-type B2 and Fe₂(Mo,Nb)-type Laves phase precipitates form as dominant second phases in the austenitic matrix during aging at 750 °C. At room temperature these precipitates increase the strength but decrease the ductility of the AFA alloys. However, when tested at 750 °C, the AFA alloys did not show strong precipitation hardening by these phases, moreover, the elongation to fracture was not affected by aging. Fracture surface and cross-sectional microstructure analysis after tensile testing suggests that the difference of mechanical behaviors between room temperature and 750 °C results from the ductile-brittle transition of the B2 precipitates. At room temperature, B2 precipitates are strong but brittle, whereas they become weak but ductile above the ductile-brittle transition temperature (DBTT).