Reverse strain-induced martensitic transformation of the ferrite to austenite in duplex stainless steels

The possibility of a reverse strain-induced martensitic transformation (RSIMT) of ferrite into austenite was evaluated in two duplex stainless steels (DSS) solution treated at 1000, 1100, and 1200 °C. For each temperature, we evaluated the effect of stress–strain state (tensile tests—triaxial; compression tests—uniaxial, and plate rolling—biaxial) on the reverse martensitic transformation behavior. For each strain level, the ferrite volume fraction was measured with a ferritscope, and finite element analyses allowed the correlation of the stress–strain states to the total transformed ferrite. X-ray diffraction and metallographic analyses showed that microstructures of all samples are composed only by ferrite and austenite, and the general trend of lower ferrite contents associated to higher strains corroborates the hypothesis of RSIMT of the ferrite in austenite. The amount of martensitic transformation is proportional to equivalent strain or to the respective strain energy, but different stress–strain states lead to different transformation behaviors. In the triaxial stress state during necking, the amount of transformed ferrite by RSIMT in a specific strain energy level is higher than that obtained in the biaxial stress state of the plate rolling, which in turn is higher than that observed for uniaxial stress state generated by compression tests. It can be then concluded that different strain states, and not only the amount of plastic deformation, affect the behavior of the RSIMT of ferrite in austenite of DSS.