Theoretical analysis and experimental investigation of contact fatigue and surface damage in prealloyed and diffusion bonded sintered steels

The contact fatigue and surface damage of prealloyed (Fe-0.85Mo, Fe-1.5Mo) and diffusion bonded (Ni-free, low-Ni, high-Ni) powder metallurgy (PM) steels were investigated. Materials subjected to contact stress fail due to the nucleation of subsurface cracks (contact fatigue cracks), nucleation of brittle surface cracks, and surface plastic deformation. The occurrence of these contact damage mechanisms was predicted using theoretical models, which were developed by assuming that crack nucleation is preceded either by local plastic deformation (contact fatigue and surface plastic deformation) or local brittleness (brittle surface cracks ) of the metallic matrix. With reference to the mean yield strength of the matrix (mean approach) or the yield strength of soft constituents (local approach), the models predict the theoretical resistance of materials to the formation of damage mechanisms. The models were then verified using experimental evidence from lubricated rolling-sliding contact tests. In addition, the effect of compact density and microstructures of materials on the resistance to contact damage mechanisms was investigated. Density and microstructure were modified by varying green density, alloying elements, sintering temperature and time, and applying strengthening treatments: carburizing and shot peening on prealloyed (homogenous microstructure) and carburizing, sinterhardening and through hardening on diffusion bonded (heterogeneous microstructure) steels. The theoretical resistance to subsurface and surface crack nucleation in prealloyed materials was predicted using the mean approach since the microstructure is homogeneous. But the local approach is applied for diffusion bonded materials (Ni-free and low-Ni); exceptionally, the mean approach was applied for some homogeneous microstructure of Ni-free material sintered at a prolonged time. However, the models have a limitation in predicting the contact damage mechanisms in a high-Ni material. This issue may require further investigation to modify the model. Shot peening provides higher resistance to the nucleation of surface cracks. High compact density, high sintering temperature and time, and sinterhardening improve the resistance to contact damage mechanisms for Ni-free and low-Ni materials.