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Abstract
The effect of steel cleanness, core hardenability, and microstructure after carburising on bending fatigue strength is described for the standard steel grade SS 2506. Examination of the fatigue fracture surfaces revealed that initiation had occurred at the surface where an internally oxidised layer was present. Differences in the inclusion content were not a major cause of fatigue failure. The highest fatigue strength was obtained for the smallest grain size material, but the effect of grain size is small in comparison with the base composition. An explanation for the variation in fatigue strength between heats is given with reference to the difference in case depth and core hardenability, and their effect on the compressive residual stresses within the case. Optimum fatigue strength was achieved for the shortest case depth and a low core hardenability. The results indicate the importance of close control over these two parameters in producing the optimum fatigue strength.
MST/1239