Biaxial Low-Cycle Fatigue Behavior of Long-Term-Used Cr-Mo-V Steel at Elevated Temperature

Abstract

Low-cycle fatigue behavior of Cr-Mo-V steel used in a long-term under the biaxial strain condition at an elevated temperature was studied in this paper. A push-pull test, reversed torsion test and combined push-pull and reversed torsion test in Cr-Mo-V steel were performed at 823 K in air. The specimen was sampled from the high-temperature portion and coupling portion of 91 600 h experienced and 131 times start/stop operated turbine rotor. From this investigation, the following conclusions were obtained. (1) Low-cycle fatigue life of the high-temperature portion decreased in comparison with the coupling portion, particularly under the reversed torsion condition. (2) Cyclic yield stress obtained from the cyclic stress-strain curve decreased due to long-term use. (3) Crack propagation direction depends on the principal strain ratio and flaws in oxide film, and it was not affected by long-term use. (4) Flaw density of oxide film of the high-temperature portion was higher than that of the coupling portion. (5) Decrease in low cycle fatigue life of long-term-used Cr-Mo-V steel is explained by the decrease in cyclic yield stress and the increase in flaw density in oxide film.