A Kinetic Model for Tribological Surface Transformations Occuring on the Railroads: Irreversible Near-Surface Metallurgical Transformations

During the manufacturing processes or the operational phases, some materials are liable to undergo metallurgical phenomena known as irreversible solid-solid phase transformations or Tribological Surface Transformations (TSTs). The treads of several rails in the French railroad network have been affected by TSTs. The kinetic model presented in this paper describes the initiation of TSTs and their development near the rail surface. 1. Introduction For the last twenty years, the weight and velocity of freight trains have been increasing steadily, resulting in the development of “Tribological Surface Transformations” (TSTs) or “White Etching Layers” (WELs) [1–4] (see Figure 1(a)). TSTs are the irreversible solid-solid phase transformations which occur both at the surface and in depth, in the immediately vicinity of the wheels of passing trains. These processses, which mainly occur in straight parts of the rails, are induced by heavy corrugation [5–7] (see Figure 2), which generates an alternating pattern of troughs, consisting of the initial material (ferrito-pearlitic steel) and crests, forming the damaged part, where the material is transformed into “quasi-martensitic” steel (see Figure 1(b)) at depths ranging from several nanometers to more than 100？ m [8]. The values obtained in microhardness tests on TSTs in steel rails generally range from 700 to 1000？ (corresponding to a martensitic phase), as compared with the initial hardness of ferrito-pearlitic rails, which is about 300？ [9]. Rails with TSTs may, in the worst cases, develop cracks due to strong strain incompatibilities between the hard “white” phase (martensite) and the soft “untransformed” phase (ferrite-pearlite), see Figure 3 [10]. Although the origin of TSTs has not yet been clearly established, it seems to involve contact fatigue processes [11, 12]. The repeated mechanical loads applied (normal and shear stresses) combined with the highly localized thermal effects occurring near the surface at the wheel/rail contact points as the result of the friction may be largely responsible for these transformations [13]. Figure 1: (a) Macroscopic view on the tread of a rail (S54/900A steel) showing TSTs or “White Etching Layer”. (b) Transverse cross-section of the upper part of a rail (a). The parent phase (grey part) is ferrite pearlite, which still remains in the rail when TSTs have developped and the daughter phase (white part), occurring at the surface, is “quasi-martensite” (extract from [ 8]). Figure 2: Rail corrugation: alternating of the troughs (grey part: ferrito-pearlitic phase) and crests