Testing of High Endurance PM Steels for Automotive Transmission Gearing Components

Conventional powder metal (PM), at typical press and sinter densities of around 7.0 g/cm³, cannot be considered as a viable manufacturing technology for automotive transmission gears because of deficits in mechanical durability. The durability limitations are mainly a consequence of density restrictions. However, alloying element selection for hardenability and sintering condition selection are secondary but important factors to consider. It is shown that by development of PM densification and alloying technologies, gearing related mechanical properties can be achieved that closely match those of heat treated wrought alloy steels. It is now possible for PM to be considered as a substitution technology for automotive transmission gears. Traditionally, to satisfy stringent durability requirements, such gears are manufactured by costly extensive machining of steel forgings or nodular iron castings.

The mechanical properties that can be produced by the application of selective surface densification and by core density enhancement are reviewed. Data are provided from material tests and from actual component testing, which show that the advanced PM material and process systems give surface durability and bending fatigue endurance characteristics which are suitable for many transmission gearing applications.