Evaluation of failure lives of mod. 9Cr-1Mo steel under high temperature multiaxial creep-fatigue using strain energy; Effect of maximum stress history and stress relaxation on life correlation

Ogawa, Fumio*; Imagawa, Yuya ; Hashidate, Ryuta ; Hiyoshi, Noritake*; Ito, Takamoto*

In recent years, the demand for environmental protection has led to a trend toward shorter startup and shutdown intervals in thermal power plants. In fast breeder reactors (FBRs), more stringent operating environments (temperature and sodium environment) are also being considered. Modified 9Cr-1Mo steel is used in thermal power plants and fast breeder reactors because of its excellent mechanical properties at high temperatures. In actual machines, Modified 9Cr-1Mo steel is used in a creep-fatigue type with complex stress conditions, which may cause more damage than the uniaxial condition, so it is necessary to measure the multiaxial creep-fatigue life and establish a life evaluation method. In this study, axial and torsional multiaxial creep-fatigue tests were conducted in 550C air to accumulate property data and to evaluate the life. The results will be described later, but the strain rate and retention time dependence showed peculiarities different from those in the past. Next, the strain energy corresponding to the hysteresis loop area between stress and strain was calculated to evaluate the life, and the same energy was theorized to compare the actual and theoretical life. Specifically, a new life evaluation method based on strain energy was studied, which can evaluate the peculiar strain rate, retention time dependence, and multiaxial state observed in the proportional loading test.