Microstructural and mechanical characterisation of some sinter hardening alloys and comparisons with heat treated PM steels

Abstract

Four grades of sinter hardening materials have been compared, using industrial equipment. Three powder types were completely prealloyed; the last one was a hybrid, combining prealloying and diffusion bonding. Different amounts of Cu have been added by mixing. The lubricated mixes, containing 0.6% graphite, have been compacted at different pressures, to form gears at green densities ≥7.0 g cm⁻³; the compacts have been sintered at 1120°C, under endogas from methane and fast cooled (at least 7 K s⁻¹ within the range 850–400°C). The final step has been stress relieving, at 180°C, for 1 h. Material properties have been investigated, focusing on porosity, pore shape, hardness, microhardness, microstructure, local chemical composition and mechanical properties. For comparison, other gears, compacted in the same tool and at the same density level, but manufactured according to a more conventional cycle, i.e. starting from less alloyed powders and adding carbonitriding, quenching and stress relieving, have been used. The analysis of the different experimental results enabled the authors to find out and outline some criteria suitable for selecting sinter hardening materials and for choosing more reliable manufacturing conditions to fulfill specific application requirements.