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.
This work proposes an analytical model developed from experimental data to describe the anisotropic dimensional change on sintering. Axial-symmetric iron parts differing for geometry and sintering conditions have been investigated, aiming at highlighting the influence of geometry. The specimens were measured in the green and sintered state by a coordinate measuring machine (CMM). The dimensional changes of height, external diameter and internal diameter were derived from measurement results. The anisotropy of the dimensional variations has been studied with reference to the isotropic dimensional change derived from the change in volume of the parts. The influence of geometry and sintering temperature was highlighted. To properly describe the dimensional variations in the compaction plane, the dimensional change of the external diameter versus the dimensional change of the internal one has been analysed. By means of the experimental data, a reliable analytical relationship has been found, dependent on the parts geometry. An anisotropy parameter has been identified, which allows relating the dimensional change in the compaction plane and in the axial direction to the isotropic dimensional change. This parameter depends both on geometry and on sintering conditions. By means of the anisotropy parameter an analytical model for the anisotropic behaviour has been developed.
The influence of die wall lubrication during warm die compaction on densification, microstructure and mechanical properties of three low alloy ferrous powders was investigated. Specimens were sintered at 1250°C. Die wall lubrication leads to higher green and sintered density and enhances the dimensional stability. It does not affect the microstructure of the matrix, while pores are smaller and more rounded than in bulk lubricated specimens. In TRS tests, both strength and deformation are higher in die wall lubricated specimens than bulk lubricated ones.