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不能产生某些几何特征，因此一些PM钢部件需要二次加工操作。大约40%的黑色粉末冶金零件需要加工。PM钢的加工性能不如锻造钢。PM钢的孔隙使润滑困难，减少了切削表面的热量传递。孔隙度会导致刀具的切割中断，导致刀具边缘的微冲击和疲劳状态。为了达到与锻造产品相似的强度，PM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件，其显微硬度显著提高，以抵消孔隙率的影响。然而，粉末冶金的优点是能够在铁合金中加入添加剂以改善可加工性。该项目的第一个目标是开发一种专门为PM钢设计的可加工性增强添加剂的新家族。这些添加剂将由预先与石墨合金的铜颗粒和预先与锰合金合金的Cu3Sn颗粒组成。本提案的第二部分涉及表征应变速率对PM镍钢中富镍区向马氏体转变的影响。富镍区是由Ni在Fe中的缓慢扩散带来的。然而，变形过程中发生的奥氏体向马氏体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PM镍钢构件抗疲劳性能方面的作用，文献中也存在矛盾。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，而另一些作者则指出，当裂纹靠近富镍区时，疲劳裂纹扩展速率更高。因此，本项目的第二个目标是从Ni浓度、Ni体积分数、显微组织、碳含量和应变速率等方面确定促进PM Ni钢构件疲劳裂纹偏转的富Ni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.