Metallurgical phase transformations in the rubbing of steels.

The formation of phase transformed material in the unlubricated rubbing of plain carbon steels using a crossed cylinders machine, in single and multiple rubs, has been investigated. Previous studies of the dry wear of steels had suggested that if the frictional flash temperatures reached values of approximately 750C, then a phase transformed material, akin to the martensitic structure of conventional ferrous metallurgy, appeared upon the rubbing surfaces. Exploratory experiments indicated that phase transformed material appears as a result of a single rub of the surface only under conditions so severe that the calculated flash temperatures are of the order of 1100C or more. The fundamental metallurgical considerations applicable to small volumes of material subject to a combination of high temperatures and pressures of short duration which are the special characteristics of flash temperatures, have therefore been re-examined. The formation of homogeneous austenite, which is the first stage in the production of transformed material, has been considered to occur in two temperature ranges 723 - 910C and 910C. Only above 910C can the transformation be completed in a typical flash temperature duration. Diffusion of carbon in austenite has been proposed as the rate determining process. Theoretical temperatures of approximately 1250C are calculated for the formation of homogeneous austenite from lamellar pearlite. It has been found that the pioneering results of Welsh (1965), in particular his T2 and T3 transitions can be given satisfactory interpretations in terms of the new metallurgical theory. An important element in these interpretations is the number of rubs to which any element of the surface is subjected. This factor has been deduced from wear theory. Further detailed experiments have confirmed the magnitude of flash temperatures required for the production of phase transformed material. The influence of the maximum temperature has been discussed. Metallurgical and physical analysis indicates phase transformed material to be a fine grained martensitic structure.