High Nitrogen Steels. Microstructure and Deformation Behavior of High Nitrogen Duplex Stainless Steels.

Abstract

The deformation of duplex structures in general and duplex stainless steels (DSS) in particular is very complex. The existence of the massive second phase leads to numerous unexpected features, i.e. the microstructure is the most decisive influence parameter on the deformation behavior of duplex structures. In the case of DSS additionally the chemical composition has to be taken into account. With increasing rolling deformation at room temperature several deformation mechanisms occur, e.g. shear relaxation, twinning of austenite, deformation induced martensitic transformation of the austenitic phase, crack formation (and crack healing accompanied by the refinement of the microstructure) and dynamic recovery. In α/γ DSS additionally the phase boundaries (PB) are obstacles for deformation. Therefore, here large deformation zones were built up during deformation which contribute to the complex deformation behavior. Since nitrogen reduces the SFE and, thereby, hardens the austenitic phase and promotes planar slip which is not homogeneously distributed in the austenitic grains but localized, ferrite becomes the more ductile phase in DSS. Furthermore, as a very strong austenite stabilizing element, N causes the change of the matrix phase form ferrite to austenite and leads to the ductile to brittle transition of austenite which also influences the deformation behavior. It occurred that there are equal deformation modes like the hindrance of shear band formation, shear band cracking and selective phase boundary sliding, which are obviously valid for all duplex structures, and other ones, due to the nitrogen content (e.g. brittleness) or the existence of the second phase (e.g. increased strain hardening rate).