Carburizing increases the contact fatigue resistance of sintered steels, but the surface hardening may result the formation of surface brittle cracks due to the combined effect of high hardness and porosity. The effect of carburizing on the embrittlement of the case of a 7.3 g/cm3 1.5%Mo - 0.25%C sintered steel was studied. The phenomenon was analyzed theoretically and verified by experiments. The resistance of the carburized steel to surface brittle cracking increases with the load bearing surface and the decrease of the maximum pore size, of the surface microhardness and the friction coefficient. The theoretical analysis was implemented in a design procedure for parts subject to contact stresses.
The influence of the microstructure on density on the resistance to subsurface and surface damage, during lubricated rolling-sliding wear of a 1.5%Mo-2%Cu-0.65%C sintered and sinterhardened steel, was investigated. The nucleation of subsurface contact fatigue cracks and the occurrence of surface plastic deformation due to surface stresses were studied through a theoretical analysis and confirmed through wear tests. The expected positive effect of density and sinterhardening was confirmed. Moreover, the resistance of the steel to the two damage mechanisms investigated depends on the microstructural heterogeneity. The theoretical model for subsurface and for surface damage may be used to predict the resistance of the material but, depending on the distribution of the microstructural constituents, either a local or a mean approach in defining the properties of the metallic matrix has to be used.
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