Surface Roiling of Sintered Gears

Predictive Modelling of P/M Gear Rolling

Selective surface densification (SSD) can greatly improve the mechanical properties of powder metallurgical (P/M) parts with low additional costs. This work proposes a new surface vibration extrusion process to make up for some drawbacks of the existing SSD processes as far as possible. The surface densification effect and mechanism, as well as mechanical properties of the surface extruded P/M Fe-Mo-Cu-C steel with and without vibration assistance, are comparatively investigated. Following a small surface extrusion processing via a simple extrusion die, the surface of the vibration extruded P/M steel is more thoroughly and uniformly densified under a lower extrusion load when compared with the directly extruded P/M steel. This can be attributed to the vibration assistance facilitating the dislocation motion and metal flow. The high surface-densified layer is achieved, with depth up to 530 μm slightly superior to some commercial SSD parts. Moreover, approximately twice the surface hardness is obtained compared with the hardness before the extrusion. Lastly, superior friction and wear properties are achieved. Results indicate that surface vibration extrusion can be used as a promising process for the surface densification of P/M parts. 

A new surface vibration extrusion process for surface densification and improvement of properties in powder metallurgical steel

The mechanical properties, particularly strength and toughness, of sintered steels are improved considerably by surface densifying treatments consisting of small reduction extrusion and rolling and subsequent full annealing. It has been found that a reduction of about 11% is most effective for extrusion and a reduction of about 0·2 mm, by a small reduction per pass with a small diameter roll, is most effective for rolling. The toughening mechanism may be ascribed mainly to an appropriate density gradient which is given by the small cold-reduction in forming. Also the collapsed pores in the surface layers are reduced in size and spheroidized, and the interparticle bonding is increased by the subsequent full annealing.

M. Kobayashi

Papers

Effect of Small Cold-Reductions on Mechanical Properties of Sintered Steels