Low temperature tensile properties of microduplex stainless steel having different hardness of ferrite phase were investigated by means of transmission electron microscopy and X-ray diffraction. The hardness of ferrite in duplex stainless steel was changed by heat treatment.
The main results obtained were as follows;
(1) The austenite phase in the microduplex stainless steel was metastable. Therefore, martensite transformation was induced during tensile tests below room temperature.
(2) The elongation to fracture showed a peak at 201K in the elongation-test temperature curves. This phenomenon is closely related to the amount of strain necessary to induce α' martensite, the amout of α' martensite and the ductility of the ferrite phase.
(3) A discontinuity in flow stress-strain curves, namely the quasi-yield point phenomenon, was found buring tensile tests at 77K. This phenomenon seems to be associated with both necking arising from the formation of ε martensite and strengthening of that portion due to the formation of α' martensite.
(4) Md temperature was unchanged in spite of the variation of ferrite hardness in the duplex stainless steel. This fact may suggest that the deformation of ferrite and austenite phases follows the rule of mixture.