An Experimental Study of the Thermal Aspects of Sliding Contacts and Their Relation to the Unlubricated Wear of Steel

Abstract

Experiments are described in which low-alloy, medium-carbon steel (AISI 4340) pins were slid against discs of the same material under various loads, at a speed of 625 cm/s. The bulk temperatures of the pins and discs were measured continuously, as also were the frictional forces and the volumes of wear. Similar experiments were also carried out, at a much lower speed (about 5 cm/s), in which the bulk temperatures of the specimens were due almost entirely to externally supplied heat. As far as possible, the bulk temperatures, and the corresponding loads, were maintained at the same values as for the initial, frictionally heated, experiments. The wear debris produced during both sets of experiments was examined by X-ray diffraction in order to identify the various components and to analyse their relative proportions. The variation of the proportions of α-iron, rhombohedral oxide, and spinel oxide with measured bulk pin temperature was used to give an estimate of the temperatures occurring in the regions of true contact. This estimate was approximately of the same magnitude as those temperatures measured in recent dynamic thermocouple experiments.

The wear results are discussed in terms of oxidation of the real areas of contact at the temperatures indicated by the proportional analysis of the wear debris. It is shown that reasonable values of the activation energy can be obtained from a direct comparison between frictionally heated and externally heated experiments carried out under the same load and the same bulk temperatures. It is concluded that the contact temperature is the factor governing the wear behaviour of steels rather than the ambient temperature of the whole surface.