Low temperature aging of a duplex steel DIN 1.4462

Duplex stainless steels (DSS) are an important and expanding class of stainless steels with a noticeable combination of mechanical properties and resistance to corrosion, due to their two phase austenite-ferrite structure. The ferrite provides high mechanical and corrosion resistance, while the austenite supplies ductility and uniform resistance to corrosion. This family of steels, however, becomes brittle when exposed to intermediate temperatures, from 300oC to 500oC, which limits their applications. In the ferrite phase of these alloys the solution is to develop a spinodal decomposition with iron and chromium rich regions called '475oC embrittlement'. The sharp drop in the toughness and ductility that can occur in the material makes more research about its response to aging at intermediate temperatures necessary in order to determine their reliability under these conditions. There are studies on the effects of aging on the development of mechanical properties in different types of steels, but the effects on the properties of fatigue have not been studied extensively and the few existing reports focus on the development of the properties of a first generation aged DSS steel. This work presents a study on the behavior of a DIN 1.4462 steel submitted to aging at temperatures from 400oC to 515oC. Microscopy and hardness measurements determined the provisional evolution of the aging; while cyclic tests determined how this affects the material's life in fatigue. The austenite does not noticeably change in hardness, but the ferrite's hardness continuously increases at all temperature ranges studied, with a maximum of around 480oC. The fatigue properties with low numbers of cycles after aging for 100 hours at 475oC do not differ noticeably from the behavior without aging, while the cyclic hardening-softening curves show heavy hardening caused by the aging that increases the flow limit as well as the saturation stress(CW)