Asymmetric Strain Fatigue Behavior and Life Prediction of 3.5NiCrMoV Steel and 20Cr13 Steel for Steam Turbine

Symmetric and asymmetric strain control fatigue tests were performed on 3.5NiCrMoV bainite steel and 20Cr13 martensite steel for steam turbine, and their fatigue behaviors were compared and studied. Morrow model, SWT model and Ellyin model based on symmetric strain fatigue parameters were used to predict asymmetric fatigue life, and the applicability of these models was discussed. The results show that the asymmetric fatigue life of 3.5NiCrMoV steel was not higher than its symmetric fatigue life, and the asymmetric fatigue life of 20Cr13 steel was higher than its symmetric fatigue life. There was a big difference between symmetric and asymmetric cyclic stresses of 3.5NiCrMoV steel, and the symmetric and asymmetric cyclic stresses of 20Cr13 steel were similar. The average stress reduction was a common phenomenon for asymmetric strain control fatigue, and the plastic strain change was the main factor affecting asymmetric fatigue life. The prediction accuracy of the Morrow model depended on the effect of strain ratio on fatigue life. The SWT model was not suitable for fatigue life prediction of 3.5NiCrMoV steel with a large difference between asymmetric and symmetric cyclic stress-strain curves. The Ellyin model had a good prediction effect on the asymmetric fatigue life of 20Cr13 steel with similar asymmetric and symmetric cyclic hardening trends.