A study of the wear of steel under heavy loads with lubricants containing sulphur-based additives

Abstract

A four-ball friction machine is described. It was employed to study the wear of hardened steel at considerable contact stresses in highly purified dearomatized bright stock and solutions of sulphur and dibenzyldisulphide in the latter. Profilometric measurements were carried out, microhardnesses were determined and a metallographic study was made in the friction zone. It is shown that estimation of the anti-wear properties of lubricants by scar diameters is only of qualitative value. The depth of the wear scars depends on the nature of the lubricant and on the sliding speed. It decreases under the action of anti-welding additives and when the sliding speed is lowered. Under seizure conditions non-etching surface layers of great hardness up to 20 or 30 μ, thick are formed. They probably arise in the process of rubbing as a result of diffusion to the surface of the alloy components and enrichment of the surface layers in carbon, because the hydrocarbon lubricant may act as a carburizer. The formation of very hard surface layers alleviates the heavy friction conditions at seizure. Reaching down to a considerable depth below the nonetching layer is a layer of partially annealed steel, which forms under the action of the heat flow from the surface. The appearance of this layer facilitates plastic deformation of the steel in the friction zone and the development of adhesion between the rubbing surfaces, resulting in seizure. The theory has been put forward that on seizure in the presence of sulphur-containing anti-welding additives submicroscopic relatively soft films of iron sulphides form first on the very hard surface layers. The structural changes that occur in the surface layers of steel worked for a long time under heavy friction conditions in a hydrocarbon medium are different from those occurring when wear is produced in the same hydrocarbon medium in the presence of anti-welding additives. The specific nature of heavy friction conditions is defined by the formation of a multi-layer structure in the friction zone of the steel.