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To investigate the effect of non-metallic inclusions on the fatigue properties of quenched and tempered 0.46% car-bon steel(HV 650)in the super-long life fatigue range(N≨5.0×10^8), tension-compression fatigue tests were car-ried out. The fatigue strength was discussed based on the √<area> parameter model.The results Pbtained are :(1)Fatigue fracture origins were mostly at a non-metallic inclusion.(2)However, the locations of the inclusion at fracture origin were not uniformly distributed over the specimen section due to the nonuniform distribution of residual stress induced by heat treatment.(3)The fatigue limit defined by the cycle N=5.0×10^8 can be predicted by the √<area> parameter model, i.e.with three parameters, the Vickers hardness, HV, of the matrix, the square root of the projected area of inclusions, √<area> and residual stress.(4)The expected value of inclusion size √<area> X_<max> of 0.46% carbon steel in a definite number of specimens can be estimated using the statics of extreme values. The lower bound of the scatter of fatigue strength was predicted with the combination of the √<area> parameter model and the value of √<area>X_<max>.
