Case-hardening Medium Carbon Steel for Tough and Long Life Bearing under Severe Lubrication Conditions

It is known these days that case-hardening bearings have a longer life than through-hardening ones under severe lubrication conditions (i.e. mixing-in of foreign particles in the lubrication oil). To explain this fact, we first presented the mechanism of stress relaxation at the debris dent edge [1]. According to test results, it was found that both retained austenite and hardness are the most important factors for a longer life material.

Such material has a longer life even under boundary lubrication conditions. Since a sufficient EHL oil film does not form under boundary lubrication conditions, metal contact occurs. The resulting damage is called “peeling” and it decreases the bearing life.

To realize ideal case hardening material for bearings, a new carbo-nitride heat treatment has been developed. Normally, it is extremely difficult to obtain a sufficient case depth using a traditional carbo-nitride heat treatment process. As an alternate, medium carbon steel was studied. The application of newly developed medium carbon steel has not only proved to make the creation of a sufficient case depth easier, but also provided economic benefits. Based on results from testing both the dimensional stability and fracture toughness, newly developed medium carbon steel can be used for case-hardening bearings.