| 0Cr16Ni5Mo martensitic stainless steel called super martensitic stainless steel,becsuse of its excellent comprehensive mechanical properties and corrosion resistance, this steel are widely applied in many fields.The microstructure characters, morphology of precipitates and retained austenite,the volume fraction of retained austenite and mechanical properties of 0Cr16Ni5 Mo stainless steel under different heattreatment were characterized by optical microscope(OM), transmission electron microscope(TEM), X-ray diffraction(XRD) and mechanical testing mechine. Meanwhile, the flow stress behavior of 0Cr16Ni5 Mo super-low carbon martensitic stainless steel was studied by the isothermal compression of cylindrical specimens at 900~1150℃ with the strain rate of 0.01~10s-1on a Gleeble-3800 simulated machine; The main results we obtained are as follows:The microstructure of the tested steel after quenched at 900~1050℃ are quenching martensite, δ-ferrite and retained austenite. With the quenching temperature increase from 900 to 1050℃, the lath of martensite become thick and the austenite crystal grain size is increased from 18μm to 68μm. The carbides almost all dissolve into the matrix after quenched at 950℃. The effect of quenching temperature on mechanical properties is very complex. With the quenching temperature changes from 900 to 1050℃, the tensile strength and hardness of the steel show little change.The impact energy of the tested steel after quenched are all exceed 160 J.Through the reasearch of tempering treatment, the microstructure of tested steel after tempered at different temperatures are tempered martensites which still exhibits the lath morphology. With the temperature increase from 450 to 700℃, the amount of M23C6 carbides increase gradually, and precipitates along the martensite lath boundaries. The content of retained austenite first increases and then decreases with the temperature increase, exhibits the maximum amount of austenite at 600 ℃. The morphology of retained austenite also change with the tempering temperature. To further understand the stability of reversed austenite, the Ni content in reversed austenite was measured using EDS. Results show a significant difference in nickel concentrations between reversed austenite and martensite, that means the Ni element enrichment during austenite forming process, its forming mechanism is diffusion phase transformation. Upon tempering from 450 to 700 ℃, the tensile strength andhardness first decrease and then increase with tempering temperature, while the ductility and impact energy showed the reverse change trend. The tensile strength and hardness value of HRC reaches a minimum value at 600℃, whereas the ductility and impact energy reaches a peak value at this temperature. The viaration of tensile strength and hardness is 885~1167MPa, 25.2~37.3,respectively. In general, the variation of mechanical properties with tempering temperature is in agreement with the tendency of variation in the retained austenite content. Fracture morphology of the steel is a dimple pattern with features of plastic fracture, the torn edges are visible,and the second-phase particles distribute on the dimples.During the isothermal compression process, the flow stress decrease with the increase of deformation temperature and decrease of strain rate, the deformation condition have a great influence on the material microstructure. The relations of the thermo mechanical parameters with strain were obtained using the hyperbolic-sine mathematics model and the hot deformation constitutive relationship was established.The relationship between deformation parameters and strain can be expressed using four polynomial fitting. Processing map was also established based on the dynamic materials model. An optimum processing parameters of hot deformation for this steel can be obtained by the maps, in which the temperature is 980~1150℃ and the strain rate is 0.01~0.2s-1. |
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