応力除去焼鈍に関する研究(第5報)

Abstract

The stress relief heat treatment process may be divided into three stages. The first stage is heating process, in which residual stress decreases due to temperature dependence of Young's modulus and yield strength. The second stage is isothermal relaxation process, in which residual stress decreases due to creep. The third stage is cooling process, in which residual stress increases due to temperature dependence of Young's modulus.In order to study the relation between isothermal relaxation and creep, a short time creep test and an isothermal relaxation test were carried out using HT60.Creep curves might be represented by equation (I). Applying the strain hardening theory, creep rate-stress relation was expressed by equation (5). Combining this with relaxation condition, equation (7) indicating time-residual stress relation was obtained.The calculated data by equation (7) were in good agreement with experimental data for test temperatures 500°C and 550°C. But for 600°C and 650°C, the calculated data differed from experimental data. In this case, plastic strain rate in relaxation became independent of initial strain amplitude and dependent only on stress amplitude. So the creep rate theory of steady state creep indicated by equation (8) was adopted. Combining this with relaxation condition, equation (9) indicating time-residual stress relation was obtained for 600°C and 650°C. This time, calculated data coincided sufficiently well with experimental data. For HT60, the estimation of residual stress might be done with sufficient accuracy using equation (7) for 500°C, 550°C and equation (9) for 600°C, 650°C.The temperature dependence of creep rate might be expressed by equation (10). Applying this to relaxation phenomena, equations showing time-residual stress relation at various temperature were obtained in the forms of equation (12) and (13). The values of constants in these equations could be obtained only from creep and high temperature tension test data. So isothermal relaxation curves for arbitrary temperature and initial strain could be calculated from creep and tension test data.The limiting temperature at which the applicable equation differed from one to another was not precisely confirmed in this study. It might be dependent on steel properties and high temperature properties.