Abstract
Today, most bearings for automotive transmission operate in an environment of severe debris contaminated lubrication. Thus, to develop the bearings that have a long life under the lubrication environment, micro-alloyed forging steel with a vanadium element based on medium-carbon steel and a carbonitriding heat-treatment process have been developed. Initially, the variation of grain size caused by an amount of aluminum and nitrogen was investigated. The hot deformation behavior was evaluated by means of a Gleeble tester. In general, it is extremely difficult to obtain sufficient nitrogen in the case-hardened layer by a conventional carbonitriding process. However, the application of the newly developed steel has not only the increment of nitrogen content in the case-hardened layer, but also the additional benefit that heat treatment costs could be cut effectively. In addition, we have investigated the retained austenite and precipitation behavior of V(CN), and the evaluated rolling contact fatigue life of a heat-treated specimen. As a result, we identified that the fatigue life of developed steel samples, under contaminated lubrication environment, is about five times better than comparative steel. The micro-structural degradation mechanism during fatigue testing was elucidated.
