Effects of silicon addition and process conditions on ¿-phase sintering, sinter hardening, and

Abstract

Alpha phase sintering, sinter hardening, and mechanical properties of prealloyed Fe-1.5Mo base powder with and without additions of elemental Si, ferrosilicon, and carbon under various process conditions have been investigated. Liquid paraffin, as a new lubricating agent, was found to be useful in reducing segregation, interparticle and die wall frictions, as well as reducing ejection forces and die and tool wear. It was found that addition of Si to the base powder enhanced the sintering process by stabilisation of the ¿-phase and formation of two kinds of liquid phase at ~1045 and ~1180°C, corresponding to the solidus and liquidus temperatures, respectively. This addition increased the tensile strength of the as sintered Fe-1.5Mo from 174 to 445 MPa owing to massive solid solution strengthening effect of Si. An optimum sinter hardenable alloy, of composition Fe-1.5Mo + 3Si + 1.2C, provided a high sintered density of 7.55 g cm-3, tensile and bend strengths of 764 and 1405 MPa, respectively, with 2.5% elongation, after sintering at 1250°C for 1 h under hydrogen or vacuum using moderate cooling rates of ¿ 20 K min-1. Faster cooling rates caused brittleness and very low UTS for the high carbon steel. Full heat treatment improved the UTS by 200 MPa which was useful only for the high carbon steel with high cooling rates ¿ 30 K min-1. Depending on the cooling rate, the as sintered microstructures consisted of mainly fine or coarse pearlite, bainite, martensite, and some retained austenite with hardness in the range 250-720 HV10. Some proeutectoid grain boundary cementites were also present in the as sintered high carbon steel. This work, therefore, has shown that high densities with acceptable microstructures and good mechanical properties are achievable with single stage compaction and single sintering operations by using the optimum process conditions and alloying composition without the need for a post-sintering heat treatment.