The behavior of the stress and strain in the thermal fatigue of 18-8 Cb stainless steel is analysed and as a result, some of its theoretical information are obtained. The experimental observations, ...which the auther has reported already, are compared with the theoretical results and it is known that some of them can be explained satisfactorily from the theoretical view point.
A Thermal Fatigue Apparatus has been newly designed and made. With this apparatus, the effect of variations of the temperature cycle on the thermal fatigue of 18-8 Cb steel are studied. In this ...experiment, the temperature-range and the hold time are varied, maintaining the mean temperature 350°C constant. The following results are obtained. (1) The number of cycles for failure is the function of four tightly correlating factors : the temperature range, the hold time, the stress amplitude Δσ and the cyclic plastic strain Δεp. (2) The relation between the plastic strain energy ΔU (=Δσ·Δεp) and the number of cycles for failure N, ΔU·Nα=K, is obtained, and the value of α is constant independently of the change of the hold time, then the effect is negligible. (3) Very small deviations in the amount of chemical component of the steel affect seriously its thermal fatigue strength, and it is seen that one of the most important factors is the hot ductility.
Dynamic creep and fatigue tests were conducted on a 18-8 Mo-Cb steel at a temperature of 650°C. The results were discussed from the standpoint of the analysis which had been proposed by the authors ...previously to predict the dynamic creep and the fatigue strength from the static creep and rupture data together with reversed stress fatigue data. The prediction by the analysis showed a quite satisfactory agreement with the experimental results for the practical purpose, although a little discrepancy was observed due to the acceleration of the precipitation hardening by alternating stress. As a contribution to make clear the relation of the precipitation hardening characteristics of the material to its strength characteristics, especially the discrepancy between the analytical and experimental strengths, the following tests were also carried out, (1) the hardeness test of the material before and after the dynamic creep, (2) the observation of structure before and after the dynamic creep by an electron microscope, (3) the creep rupture test after the dynamic creep. According to the results of these tests, the variations of structure and of hardness during the dynamic creep process seemed not to be related directly with the discrepancy, but the results of the creep rupture test after the dynamic creep showed appreciably greater life than in the case of static creep rupture, and it was believed certain that the cause of the discrepancy consisted in strengthening of the material by the alternating stress related to the precipitation hardening.
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