Thermal fatigue strength of high-temperature structural materials for power plant

In order to investigate the basic thermal fatigue (TF) life properties of materials used for high-temperature components in a power plant, in-phase and out-of-phase TF tests as well as isothermal low-cycle fatigue tests at maximum temperature, Tsub(max), were carried out on low-alloy steels, austenitic steel and superalloys. Also, based on these test results, discussion was done on TF life prediction from other mechanical properties. Ter results obtained were summarized as follows: (1) The dependence of TF life on strain range could be classified into four categories, and two of them (i.e., I-and O-types) showed shorter in-phase and out-of-phase TF lives at low strain ranges. The I-type was due to creep-fatigue interaction in the in-phase TF, while the O-type was caused by the difference in crack initiation life between both phases. (2) The TF lives of low-alloy and stainless steels with low-strength and high-ductility, except a case of the I-type, were almost the same, i.e., approximately 500 and 5000 cycles at the total strain range, Δepsilon, of 1.5 and 0.5% respectively. In general the TF lives of superalloys with high-strength and low-ductility were shorter than those of the above steels, but approached to them at low strain ranges. (3) Good or safe prediction of TF life, except in-phase in the I-type, could be done by applying the Δepsilon-based relations, particularly the Δepsilon upsilons. life relation in isothermal fatigue at Tsub(max). (4) High-temperature low-cycle fatigue failure seemed to have the same stress-dependence as that appearing remarkably in the high-strength/low-ductility superalloys. (author)