Study On Physical And Mechanical Properties Of 21-4N Valve Steel In Large Deformation At High Temperature

In the actual production and application, multi-physics coupling deformation processes widely exist. Metal resistivity change significantly affects the temperature distribution of the workpiece, as the temperature change the resistivity and other material properties, which will cause great difficulties for the test of the performance parameters. At present, systematic and in-depth studies on the material parameters of large deformation at high temperature are rarely reports. Based on the support by a National Natural Science Foundation of China granted project, i.e. Study on the large deformation mechanisms at high temperature in the electric upsetting, physical and mechanical properties and deformation structures of 21-4N austenitic valve steel were investigated under high temperature deformation, the thermal and electrical conductivity under multi-physics coupling large deformation were clarified, which was significant for further establishing material models in the multi-physics coupling of metallic materials.1. Isothermal compression experiments at high temperature were carried out on Gleeble-1500D thermal simulation testing machine. Deformation structures were analyzed by using metallurgical microscope (OM) and scanning electron microscope (SEM), phase composition were analyzed by using X-ray diffraction (XRD). The results of compression experiments at high temperature show that the flow stress and the peak stress increase with the decrease of deformation temperature and the increase of strain rate. When true strain is 0.7 and deformation temperature is 1000℃,1100℃, samples with the strain rate of 0.005 s-1 and 0.01 s-1 undergo dynamic recrystallization.2. The value of the four material constants were obtained by the interrelations of flow stress (σ), strain rate(ε) and deformation temperature (T) for 21-4N valve steel during the plastic deformation at high temperature, structure factor A=1.008×1018, Stress level parameter a is 0.0053, stress index n is 6.1945 and the deformation activation energy Q is 464.237kJ/mol; and the hot deformation equation is obtained by regression analysis as following:ε=1.008×1018[sinh(0.0053σ)]6.1945 exp(-464237/RT). 3. The electrical resistivities of different deformation degree (the true strain from 0.05 to 0.55) and different temperature (from 575 to 953℃) were measured by the dc four-probe method using a ZEM-3 apparatus (ULVAC-RIKO). The results show that the electrical resistivity of the same deformation degree has a slight increase with the increase of temperature; at the same temperature, when the deformation degree is smaller, the electrical resistivity has a little change, the electrical resistivity first increases, then decreases, and there is a peak value. This is related to the precipitation of second phase induced by deformation which make the aggregation of precipitated phase, depleted within grain and elongated grains.4. The specific heat capacity of different deformation degree (the true strain from 0.2 to 0.6) and different temperature (from 40 to 1000℃) were measured by the differential scanning calorimetry (DSC) using a STA-449C thermo-analyzer. The results show that at different deformation degree, the specific heat capacity of tested steel has the same variation trends with the increase of temperature (they are in accord with Debye model besides ofⅠ,Ⅱ,Ⅲthermal effects areas precipitating the second phase). When the temperature is the same, the larger the deformation degree, the smaller the specific heat capacity of the tested steel, and the decreasing trend becomes more obvious. Because the larger the deformation degree, the greater the dislocation density, and the more the energy is stored, the release of the energy result in the decrease of specific heat capacity in the heating process.