Deformation behavior of lean duplex stainless steels with strain induced martensitic transformation: Role of deformation mechanisms, alloy chemistry and predeformation

The design of duplex stainless steels (DSS) with strain induced martensite (SIM) transformation relies on the optimization of austenite stability. The goal is to achieve a distribution of SIM over a wide range of strain so as to get the best combination of strength and ductility. A systematic study of plastic deformation mechanisms, of the link between phase chemistry, SIM kinetics and related stress–strain behavior has been carried out by combining advanced characterization: orientation mapping in the transmission electron microscope, in-situ high energy X-ray diffraction, and microscopic digital image correlation. The role of Ni/N balance on controlling the austenite stability has been investigated in medium-Ni lean DSS steels containing ∼4 wt% Ni. Results show that SIM occurs in the DSS by a two-steps transformation: first some austenite transforms into ε-martensite and the latter subsequently transforms into α’-martensite at the intersections of ε-bands and further grow into the austenite. It is found that SIM formation occurs at a slower kinetics in the DSS as compared to its fully austenitic counterpart, however with a similar relationship to flow stress. An optimal level of mechanical behavior is shown to be related to an optimal rate of SIM formation during tensile testing. The yield strength can be improved by rolling with a limited impact on the phase transformation potential.