A Design Procedure To Define The Optimum Hardness For Parts Subject To Contact Stresses

Martensite strengthening increases the contact fatigue resistance of sintered steels, but the microstructural hardening may result in the formation of surface brittle cracks due to the combined effect of high hardness and porosity. Therefore, there is a hardness threshold above which the achieved resistance to the formation of the subsurface Hertzian cracks is cancelled out by the surface brittle damage. Such a threshold depends on density and on the mechanical properties of the metallic matrix. Analytical models for the prediction of the nucleation of the subsurface fatigue cracks and of the brittle surface cracks in carburized steels were developed and verified experimentally. In this work, these models are described and implemented in a design procedure for sinterhardened parts subject to contact stresses, aiming at selecting the material and its heat treatment for applications where contact stresses may cause both contact fatigue and brittle surface.