On the Metallurgical Design of Carburised Gears

Abstract

All automotive power transmission gears, with few exceptions, are carburized and this has become a universal practice today. Carburizing of the gear produces a hard case with relatively tougher core. The common benefits resulting from carburizing are the good wear resistance of the tooth surface and resistance to failure under impact and fatigue load. However, considering the stress systems acting on a gear tooth during its rotation in mesh with another gear, the other properties of a gear that assume importance are : 1. High contact load carrying capacity of the tooth flank surface, i.e.,contact fatigue resistance. 2. High bending fatigue resistance arising from the repetitive bending load at the tooth fillet. The development of each of these properties to a maximum level is not possible, because of the opposing influence of hardness on toughness and increase in contact fatigue resistance on bending fatigue properties. What can be aimed at and achieved, is an optimization of these prop-erties in relation to the type of gear and its applicat-ions. For this purpose, however, a careful designing of all metallurgical parameters, including process para-meters, are necessary. In this lecture, the important metallurgical factors that influence the process of optimization have been pointed out, and it has been shown tha9the development of residual compressive stresses on the surface is the most important single factor that determines the success of carburizing a gear. The conditions and controls necessary for the ultimate development of residual surface comp-ressive stresses have been discussed, and data on IS-20MnCrl carburizing steel have been presented as a means for optimizing the desired metallurgical properties in a gear.