Abstract
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 Δep. (2) The relation between the plastic strain energy ΔU (=Δσ·Δep) 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.
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