Improving the machining behaviour of powder metallurgy steels through development and optimized utilization of machinability enhancing additives

钢构件的粉末冶金（PM）一直面临着近净成形工艺的悖论，其生产的零件中有40%至50%需要机械加工来确定其几何形状。此外，由于在其最终微观结构中存在孔隙，PM钢部件被认为比那些由锻造钢生产的部件更不容易加工。对于热处理或烧结硬化的粉末冶金零件，这种行为会加剧。为了提高可加工性，粉末冶金和零件生产商在很大程度上依赖于最初为锻造钢开发的策略，即利用硫化锰（MnS）。然而，MnS在PM中，特别是在其混合形式下，并不总是产生理想的结果。为了提高零件的可加工性，掺加MnS的粉末冶金零件的力学性能有所下降。这是由于MnS的吸湿性，导致其氧化以及生锈

Powder metallurgy (PM) of steel components is continually facing the paradox of being a near net shape process that produces parts of which 40 to 50% require a machining operation to finalize their geometry. Moreover, due to the presence of porosity in their final microstructure, PM steel components are recognized as being significantly less machinable than those produced from wrought steels. This behaviour is exacerbated for heat-treated or sinter-hardened PM parts. In order to improve machinability, PM powder and parts producers largely rely on strategies initially developed for wrought steels, i.e. the utilization of manganese sulphide (MnS). Nevertheless, MnS in PM, especially in its admixed form, does not always yield desirable results. It is typical to observe a decrease of mechanical properties in PM parts containing admixed MnS to improve their machinability. This is due to the hygroscopic nature of MnS that leads to its oxidation as well as rusting of parts. The work summarized in this proposal is based on 20+ years of research on improving machinability of PM steel parts. Recently, a new approach was developed at Université Laval to develop novel machinability enhancing additives that eliminate the problems inherent to the utilization of MnS. This research aims at building on recent successes to minimize the differences in terms of machinability between PM and wrought steels. Obviously, to be successful such improvement should not be achieved at the expense of lower static and dynamic mechanical properties. Therefore, extensive characterization of tensile and fatigue properties will be carried out to optimize machinability as a function of maximum mechanical properties. Achievement of these goals will help increase the competitiveness of the PM process by significantly reducing production costs related to machining.