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

虽然粉末冶金是一种近净成形工艺，但一些粉末冶金钢件需要二次加工，因为在压制过程中无法生成某些几何特征。大约40%的铁基粉末冶金零件经过机械加工。粉末冶金钢的机械加工性能比锻造钢差。粉末冶金钢中的孔洞使润滑变得困难，并减少了切割表面的换热。多孔性会使刀具产生断续的切割，导致刀具边缘产生微小冲击和疲劳。为了获得与锻造产品类似的强度，粉末冶金钢含有更高水平的碳和合金元素。当粉末冶金零件的表观硬度接近锻造产品的表观硬度时，其显微硬度显著较高，以抵消气孔的影响。然而，粉末冶金的优点是能够在铁合金中掺入添加剂以改善加工性。该项目的第一个目标是开发专门为粉末冶金钢设计的新的加工性增强添加剂家族。这些添加剂将包括与石墨预合金化的铜颗粒以及与MnS预合金化的Cu3Sn颗粒。本建议的第二部分涉及应变速率对PM-Ni钢中富镍区向马氏体转变的影响的表征。Ni在Fe中的缓慢扩散是形成富Ni区的主要原因。然而，在变形过程中发生的奥氏体到马氏体的转变提高了标准应变速率下的强度。此外，关于富镍区在提高或降低粉末冶金镍钢部件的疲劳抗力方面的作用，文献中也存在矛盾。的确，一些作者指出富镍区通过疲劳裂纹偏转来延缓疲劳裂纹扩展，而另一些作者则指出当裂纹位于富镍区附近时，疲劳裂纹扩展速率较高。因此，该项目的第二个目标是根据镍浓度、镍体积分数、显微组织、碳含量和应变速率来确定促进粉末冶金镍钢件疲劳裂纹偏转的富镍添加剂的最佳利用。

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.