The dependences of deformation temperature on the strain-hardening characteristics and fracture behavior of Mn–N bearing lean duplex stainless steel

The tensile properties and fracture behavior of lean duplex stainless steel (LDSS) containing Mn–N was investigated through tensile deformation by varying the deformation temperature from 353 K to 213 K. The ultimate tensile strength (UTS) and yield strength (YS) of the test LDSS increased continuously as the temperature was lowered from 353 K to 213 K. The total elongation (TE) did not decline smoothly as the temperature decreased, instead, the TE peaked at the room temperature (293 K). This temperature-dependent mechanical response was mainly associated with the kinetics of strain-induced martensite transformation (SIMT) in the metastable austenite (γ). A quantitative kinetic model of temperature-dependent SIMT was established, which indicates a positive relationship that the lower the deformation temperature, the faster the SIMT rate. With respect to the nucleation of strain-induced martensite, three modes were clearly manifested by the nucleation mechanism during tensile deformation at different temperature ranges. The first case corresponded to the direct transformation of γ→α′ at 213 K. The second mode of γ→ε→α′ dominated the SIMT at the 213 K–293 K range. The α′-martensite nucleated at the mechanical twinning interactions at 293–333 K, i.e., γ→Twins→α’, which corresponded to the third mode. Notably, as the test temperature was lowered, a continuous fracture mechanism transition gradually emerges from fully ductile to brittle. Microstructural observations from in-situ tensile tests and EBSD showed that damages at the ferrite/martensite (α/α′) interface dominated at 293 K. At 213 K, besides the interfacial fractures that occurred at 293 K, cleavage-type fractures were also found inside ferrite and martensite. Void coalescence was mainly responsible for the ductile fractures over 353 K where SIMT was absent.