Tensile properties and deformation behavior of ferrite and austenite duplex stainless steel at cryogenic temperatures

The tensile properties and deformation and fracture behaviors of a commercial ferrite (body-centered cubic) and austenite (face-centered cubic) duplex stainless steel were investigated at cryogenic temperatures, and the mechanism underlying the improvement in the tensile properties of the steel at cryogenic temperatures was discussed. The strength of the steel increased continuously with lowering temperature, and the elongation hardly decreased with a decrease in temperature from 293 K to 77 K. At 40 K, the elongation of the steel decreased drastically, however, it increased significantly at 8 K. The serration of the steel was measured during its tensile test at 8 K. The volume fraction of the deformation induced martensite was high at 77 K and 8 K, while it was low at 40 K. The strain accumulated in the ferrite and austenite phases during the deformation process remained almost constant at all the test temperatures. The ductile fracture occurred 293 K, 200 K, 77 K, and 8 K, while brittle fracture occurred at 40 K. The low volume fraction of the deformation induced martensite and the occurrence of brittle fracture can explain the low elongation of the steel at 40 K. At 8 K, the occurrence of serration may increase the temperature in the deformation region, which led to an increase in the volume fraction of the deformation induced martensite, as well as the suppression of brittle fracture, resulting in high elongation. The occurrence of deformation induced martensitic transformation, simultaneous deformation of ferrite and austenite, ductile fracture contribute to improvement of tensile property of ferrite and austenite duplex stainless steel at cryogenic temperatures.