Thermal Fatigue and Cyclic Strain Fatigue at Elevated Temperature of 18-8 Cb Steel and 2.25Cr-1 Mo Steel

Abstract

In the previous paper the authors discussed the prediction of lifetime for thermal fatigue of heat-resistant metals from the information of cyclic strain fatigue at high temperature basing on the analysis and the experiments. In the present paper, interests were placed on confirming the criterion on prediction of lifetime for thermal fatigue, from the tests of both thermal fatigue and cyclic strain fatigue at elevated temperature on AISI 347 type stainless steel and 2.25Cr-1 Mo steel, and the following conclusions were obtained from the present study ; (1) In order to relate thermal fatigue strength with cyclic strain fatigue strength, an equivalent temperature was introduced as steady temperature at which the material endures the same strain amplitude, for the same lifetime as under thermal stress cycling. The equivalent temperature for AISI 347 type stainless steel was approximately equal to the mean temperature of cyclic temperature and on the other hand, for 2.25Cr-1 Mo steel it was approximately equal to the upper temperature. (2) According to the analysis, material subjected to cyclic thermal stress at different frequencies fractures after the same number of thermal strain cycles. However, from the tests of AISI 347 type stainless steel, it was found that the lifetime at 0.1c/min was slightly smaller than that at 1c/min.