高使用性能粉末冶金零件的表面致密化

Selective surface densification for high performance P/M components

为保证机构的可靠性,历史上一直在用锻钢合金批量生产高转矩变速器零件,诸如变速器齿轮、高性能链轮、单向离合器座圈及滚珠轴承座圈。虽然用粉末冶金工艺生产的零件成本低廉,但常规粉末冶金钢的静态、疲劳及耐磨性能的局限性,使粉末冶金零件不能应用于上述领域。为解决这些缺欠,近年来,一直在研发用选择性表面致密化(SSD)改进粉末冶金钢性能的各种生产工艺。特别是,PMG集团开发出了一种拥有专利权的表面致密化工艺——DensiForm。利用这种工艺可使零件表面关键部分形成深度达1mm的完全致密化表面层,而零件心部仍然是多孔性的。这种结合与粉末冶金材料合金化的灵活性,使生产的烧结钢零件的使用性能可与在高应力下使用的锻钢性能相比拟。本文将介绍2个例子,即变速器中应用的表面致密化螺旋齿轮和直齿链条的驱动链轮。本文说明了螺旋齿轮和驱动链轮的生产工艺与显微组织,介绍了轿车变速器用螺旋齿轮的性能(其中包括在三轴总成的成对试验台架上得到的滚动接触疲劳的数据)和表面致密化链轮的材料性能与使用性能。

Historically,wrought steel alloys have been employed in the series manufacturing of high torque transmitting components such as transmission gears,high performance sprockets,one-way clutch races and bearing races to insure long term system reliability.Although P/M technology offers lower cost components,limitations in static,fatigue and wear properties of conventional P/M steels preclude utilization of powder metallurgy parts in these applications. To address these shortcomings,various processes to improve the properties of P/M steels by selective surface densification（SSD） have been developed during the past years.Specifically,the PMG Group developed DensiForm,a proprietary densification process which generates components with a fully dense surface layer in critical areas up to 1 mm deep into the part and a highly dense yet still porous core.This combination and powder metal alloying flexibility allows the production of sintered steel components which match the performance of wrought steel in highly stressed applications.Two examples,i.e.a surface densified helical gear for a gear box application and a spur chain drive sprocket are described in this paper. After describing the processing and microstructure,properties of a helical P/M gear for a passenger car gear box are presented including rolling contact fatigue data obtained on a 3-shaft back to back test rig.Finally the properties and performance data of surface densified sprockets are described.