Effect of Heating and Cooling Speed and Hold Time on Thermal Fatigue

The effect of a heating and cooling speed and that of a hold time at the highest temperature on the thermal fatigue life of an 18-8 stainless steel were investigated. The strain cycling was so applied to the specimen mechanically as to produce a tension at a higher temperature. Four different heating and cooling times ranging from 5 to 18 seconds and three different hold times ranging from 1 to 30 seconds were selected in order to see their effects, while the temperature was cycled between 300 and 700°C and between 200 and 500°C. Another emphasis was put on the study of the microstructures of the specimens which experienced various test conditions. With respect to the effect of the heating and cooling speed, little difference in the number of cycles to fracture was observed by changing the cooling speed from free cooling in the atmosphere to the forced cooling by the air jet. However, there were recognizable changes in the microstructures of the specimens which were tested under different cooling conditions. The hold time at the highest temperature had a different influence on the fatigue life owing to the magnitude of the applied mechanical strain ; for instance, when the highest temperature was 700°C, the longer the hold time, the larger the number of cycles to fracture.