Keywords: wheel-rail contact, rolling contact, local hardening, normal pressure, slip, coefficient of traction, twin-disc testing.

During the last decade, the speed of railway vehicles has been rising, increasing the severity of the contact conditions between the wheel and rail surfaces. In order to improve the components performance and obtain a long service life, the correct selection of the materials in contact remains of paramount importance. These materials are chosen from their mechanical properties. Unfortunately, these properties begin to change once the vehicle initiates its movement. Due to the high stresses located on the contact path, a local plastic deformation may occur and a relatively fast hardening process may happen. The extreme conditions are repeated for every rotation of the wheel, therefore the materials in contact are working in unpredicted conditions. Hence, in order to deal correctly with this problem, a good knowledge of these changes is imperative. In this work it is investigated the effect of normal pressure and relative slip on the surface hardening in wheel and rail surfaces. To this end a series of wheel-rail contact tests are carried out in a twin-disc machine for different pairs of carbon-steel discs. These tests are done for four different pressure levels and seven relative slip values.

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