Paper 4: Study of the Effect of Material Combination and Hardness in Rolling Contact

Because of uneven stressing, environment, or lubricant effects, one element of a rolling mechanism may be prone to premature failure, and it has been found in some instances that the seemingly easy solution of replacing the element with one manufactured in a superior material has not been successful. By means of the simple rolling four-ball test, the effect of the combination of material in rolling contact has been studied. The effect on the incidence of rolling-contact fatigue of running different materials such as high-speed tool steels, stainless steels, and other wear resistant materials potentially suitable for rolling elements against one another and against conventional En 31 ball-bearing steel has been explored. The effect of various hardness combinations of En 31 steel balls has been determined. The choice of material combination is shown to be a major factor in enhancing or reducing the performance of either mating material. With En 31 steel there appears to be an optimum hardness range and the hardness of both surfaces is important to ensure a maximum rolling-contact fatigue life. Metallurgical investigations have been carried out to elucidate the mechanisms of failure, and ‘compatibility’ is discussed in the light of fundamental concepts of the initiation of surface fatigue by mechanisms such as reversed micro-plastic deformation and how these mechanisms, the contact area, and the stress distribution are influenced by the material properties such as hardness and by rolling and sliding action.