Development of additives to improve strength and machinability of Powder Metallurgy steel components

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Although Powder Metallurgy is a near-net shape process, several PM steel components require secondary machining operations since certain geometrical features cannot be generated during compaction. Approximately 40 % of all ferrous PM parts undergo machining. The machining behaviour of PM steels is inferior to that of their wrought counterparts. The porosity in PM steels makes lubrication difficult and reduces heat transfer from the cutting surface. Porosity produces an interrupted cut with the tool, resulting in micro-impacts and a fatigue condition at the edge of the cutting tool. To achieve similar strengths as wrought products, PM steels contain higher levels of carbon and alloying elements. As the apparent hardness of a PM part approaches that of a wrought product, its micro-hardness is significantly higher to offset the effect of porosity. Nevertheless, PM presents the advantage of being able to admix additives into ferrous alloys to improve machinability. The first objective of this project is to develop a new family of machinability enhancing additives specifically designed for PM steels. These additives will consist of copper particles pre-alloyed with graphite as well as Cu3Sn particles pre-alloyed with MnS. The second part of this proposal involves the characterization of the effect of strain rate on the transformation of Ni-rich areas into martensite in PM Ni steels. Ni rich areas are brought about by the slow diffusivity of Ni in Fe. Nevertheless, the austenite to martensite transformation that takes place during deformation increases strength for standard strain rates. Moreover, there is a contradiction in literature regarding the role of Ni-rich areas in improving or decreasing fatigue resistance of components made from PM Ni steels. Indeed, some authors indicate that Ni-rich areas retard fatigue-crack-growth through fatigue crack deflection, while others indicate that the fatigue crack-growth rate is higher when cracks are in the vicinity of Ni-rich areas. Thus, the second objective of this project is to identify the optimum utilization of Ni-rich additives that promote fatigue crack deflection in PM Ni steel components in terms of: Ni concentration, Ni volume fraction, microstructure, carbon content and strain rate.