抄録
In the thermal fatigue caused by transient temperature gradients in cross-section, the strain range and the degree of thermal strain restraint seem to be affected by the thermal cycling rate. The following study was performed to make clear the effects of heating and cooling rate on the thermal fatigue life and crack growth behavior of 18-8 and 13Cr steels under conditions of transient temperature gradient.
The results obtained are summarized as follows:
(1) When the heating rate is 100°C/sec (average heating rate to 600°C), the thermal fatigue life N01 decreases from 10% to 70% of the rate at 200°C/sec. This decrease is understood as the result of the growing tensile strain during the cooling process occasioned by the rising internal temperature of the specimen.
(2) When the heating rate is 300°C/sec (or 250°C/sec for 13Cr steel), the crack growth ratio rises by the increasing degree of thermal strain restraint, but the thermal fatigue life has about the same value as the rate at 200°C/sec. This is the effect of the tensile stress at elevated temperature.
(3) In the case of air cooling, the plastic strain range and the crack growth ratio decrease compared with water cooling, but the thermal fatigue life does not increase perceptively.
(4) From these results, it is evident that under conditions of transient temperature gradient the thermal fatigue life is affected not only by the plastic strain range, but also by the working temperature while the strain and thermal stress act on it, and the remarkable thermal fatigue damage is brought about by the tensile stress introduced on the heating surface at the maximum temperature.
