High cycle fatigue properties of Mod.9Cr-1Mo steel at elevated temperatures

Mod.9Cr-1Mo steel has been widely used for elevated temperature components e.g. power boiler, chemical plant because of its superior strength properties at elevated temperatures. However some failure accident has recently happened in the weldments of power boiler steam pipes and it's due to the creep damage so-called Type IV damage. Many research have being carried out to prevent Type IV failures. On the other hand, Mod.9Cr-1Mo steel is also a candidate material for demonstration fast breeder reactor in Japan. A flow induced vibration is assumed in the piping due to the coolant liquid sodium flowing with high velocity inside the pipe. Therefore, the high cycle fatigue properties of this material at elevated temperatures have to be clarified to evaluate the integrity of the piping against a flow induced vibration. In this research, a series of high cycle fatigue tests were performed at room temperature, 400°C and 550°C. As a result, the fatigue limit appeared at room temperature and 400°C, while it didn't appear within 108 cycles at 550°C. This may be due to the suppression of crack initiation by the oxide film formed on the specimen surface at 550°C. Life prediction was also discussed beyond 108 cycles.