The effects of stress gradient and size effect on fatigue life are investigated based on the distribution of stress at the notch root of notched specimens of GH4169 alloy. The relationship between the life of notched specimens and smooth specimens is correlated by introducing the stress gradient impact coefficient, and a new life model of predicting notched specimens based on the Walker modification for the mean stress effect is established. In order to improve the prediction precision of life model with the equation parameters having a definite physical significance, the relationships among fatigue parameters, monotonic ultimate tensile strength, and reduction of area are established. Three-dimensional elastic finite element (FE) analysis of a vortex reducer is carried out to obtain the data of stress and strain for predicting its life. The results show that there is a high-stress gradient at the edge of the air holes of the vortex reducer, and it is thus a dangerous point for fatigue crack initiation. The prediction result of the vortex reducer is more reasonable if the mean stress, the stress gradient, and the size effect are considered comprehensively. The developed life model can reflect the effects of many factors well, especially the stress concentration. The life of notched specimens predicted by this model give a high estimation precision, and the prediction life data mainly fall into the scatter band of factor 2.

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