Fatigue Behavior of Carburized Steel

This paper represents the first part of a systematic investigation of factors affecting the fatigue resistance of surface-hardened steel components. The adopted approach is to view a surface-hardened member as a composite material consisting of a high strength, low ductility case and a lower strength, higher ductility core. A series of smooth axial specimens, representing case and core carbon contents, were prepared to determine cyclic stress-strain and strain-life curves. The cyclic properties obtained from these tests were then used to analyze and interpret the cyclic behavior of carburized specimens. Both axial and bend specimens were carburized and subjected to constant amplitude, strain-controlled fatigue tests. A discontinuity in the resulting strain-life curves, representing a shift from surface to subsurface crack initiation, is predictable from comparison of the strain-life curves for simulated case and core material. Finally, a relation is developed to determine optimum casexore ratios taking into account cyclic material properties, residual stresses, strain gradient, and applied stress or strain level.