Studies On Fatigue And Fracture Mechanical Behavior Of A Stainless Steel Used For The Last-stage Long Blade In A New Nuclear Steam Turbine

The rapid developments of nuclear power put forward higher requirements of the height of last stage blade in steam turbine. The longer the blade is, the more efficient the generator unit is, which may lead higher possibility of fatigue failures at blade root. Nowadays, the conventional approach which is based on crack initiation of blade root and the damage tolerance approach which is based on crack propagation of blade root are the two main methods in steam turbine blade anti-fatigue design. Due to the demands of anti-fatigue design for a certain domestic long blade, fatigue tests of blade root in longitudinal and transverse directions were carried out in this study to provide complete data and models for anti-fatigue design and fatigue life prediction of the long blade.According to the conventional approach, high cycle fatigue tests of smooth and notch specimens from XCrXNiXXX blade root longitudinal and transverse directions were carried out under R=-1 firstly, and the S-N curves and fatigue limits were obtained. Basquin model, three-parameter model and Langer model were also obtained by fitting, and three-parameter model has the best fitting effect. The contrast between S-N curves of the two directions shows that the fatigue strength of longitudinal and transverse directions is almost same, although the fatigue strength in longitudinal direction is slightly higher than that in transverse direction especially within long life ranges. The fracture morphologies of both smooth and notch specimens were examined. And the results indicate that single crack initiate at the surface of smooth specimens while fatigue steps are observed on the fracture of notch specimens, which means multi-fatigue crack initiation occurs in notch specimens. The comparison of S-N curves between smooth and notch specimens shows that the notch has more influence on fatigue strength with the increasing fatigue life. The relationships between Kf and N? as well as q and Nf were obtained experimentally. And the results indicate that the fatigue notch factors of longitudinal and transverse directions both increase and gradually reach to Kt, and q is close to 1, with the decrease of applied stress level, which means XCrXNiXXX stainless steel will be sufficiently sensitive to notch at low stress level. According to the finite element elastic-plastic strain analysis of the notch root, the accuracy of a fatigue notch factor calculation model was examined. The results show that the calculated value is close to the experimental value and the error is not more than 10% at four stress levels (including high and low stress level). So we can use the model to estimate fatigue notch factor. According to the damage tolerance approach, fatigue crack propagation tests were also carried out. Fatigue threshold value and the crack propagation rate curves were obtained. The results indicate that the threshold value of transverse direction is slightly higher than that in longitudinal direction, and the fatigue crack propagation rates of both directions, which can be fitted well by Paris model and Walker model, are almost same. Matsueda model which establish the relationship between fatigue limit and fatigue threshold was used to estimate the fatigue threshold of XCrXNiXXX, and the results imply that the error between the calculated values and experimental values is not more than 15%.So we can use Matsueda model to estimate fatigue threshold value for primary design.