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，但由于在压实过程中无法产生某些几何特征，因此一些粉末冶金钢部件需要二次加工操作。大约40%的铁基粉末冶金零件都要经过机加工.粉末冶金钢的机械加工性能不如锻造钢。粉末冶金钢中的多孔性使得润滑困难，并减少了从切削表面的热传递。多孔性会使刀具产生中断切削，从而导致切削刀具边缘处的微冲击和疲劳状况。为了达到与锻造产品相似的强度，粉末冶金钢含有更高水平的碳和合金元素。由于粉末冶金零件的表观硬度接近锻造产品的表观硬度，其显微硬度显著更高以抵消孔隙率的影响。然而，粉末冶金的优点是能够将添加剂混入铁合金中以改善可加工性。该项目的第一个目标是开发一系列专门针对粉末冶金钢设计的新的可加工性增强添加剂。这些添加剂将由与石墨预合金化的铜颗粒以及与MnS预合金化的Cu3Sn颗粒组成。该建议的第二部分涉及表征应变速率对PM Ni钢中富Ni区域转变为马氏体的影响。富Ni区是由Ni在Fe中的缓慢扩散引起的。然而，在变形过程中发生的奥氏体到马氏体的转变增加了标准应变速率下的强度。此外，在文献中关于富镍区在改善或降低由PM Ni钢制成的部件的抗疲劳性方面的作用存在矛盾。事实上，一些作者指出，富镍区通过疲劳裂纹偏转延缓疲劳裂纹扩展，而另一些作者指出，当裂纹位于富镍区附近时，疲劳裂纹扩展速率较高。因此，本项目的第二个目标是确定富镍添加剂的最佳利用，促进疲劳裂纹偏转在PM Ni钢部件方面：Ni浓度，Ni体积分数，显微组织，碳含量和应变速率。

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.