Tensile properties and plastic strain distribution in a metastable Cr-Mn-N duplex stainless steel

The effect of stress-induced martensite on the work hardening behavior of ferrite-austenite duplex stainless steels has been investigated. A precise grid marker method was utilized to observe the plastic strain distribution in each phase during the tensile tests. Two kinds of duplex stainless steels containing 22Cr-5Mn-0.34N and 20Cr-5Mn-0.25N were employed. Stress-induced martensitic transformation did not occur in 22Cr-5Mn-0.34N steel while it occurred in 20Cr-5Mn-0.25N steel. The former uniform elongation is 0.33, but the latter is 0.61, indicating that the transformation-induced plasticity (TRIP) is apparent in the latter steel with metastable austenite due to the low nitrogen content. The precise grid marker method revealed the inhomogeneous plastic deformation in both steels. Particularly, a large difference of plastic strain was observed between the areas of newly formed martensite and the austenite adjacent to the martensite, although the plastic strain due to the martensitic transformation itself was not clearly detected. Vickers hardness tests were performed, and it was clarified not only that austenite phase is harder than ferrite phase but also that stress-induced martensite is harder than residual austenite. Based on these results, the contribution of the stress-induced martensite to the work hardening behavior in the duplex stainless steel was discussed from the view point of load transfer from the soft phase to the hard phase, i.e., the internal stress induced between the two phases.