Improvement of Surface Densification and Fatigue Strength by Tooth Root Rolling of Sintered Ferrous Alloy Gear Added Mo

Abstract

Surface rolling were carried out using P/M spur gears of 0.6Mo-0.2C partially alloyed sintered steel with the density of 7.18 g/cm3. A CNC form rolling machine of axis intersection type was employed, and an unmodified screw-shaped rolling tool was prepared. Then, porosity distributions in the surface layer of rolled gear teeth root were measured and the effects of the surface rolling on the selective surface densification behaviors were evaluated. Moreover, to obtain higher fatigue strength, reverse rolling was introduced to standard gears. As a result, the porosities in the tooth root were sufficiently reduced at 700 μm feed in rolling process. Furthermore, a profile shifted gear was prepared to increase the effect of surface rolling. After case hardening, the single tooth bending fatigue durability reached to about 83 % of the case hardened wrought steel gears of SCM415.