Full length articleIn situ study on fracture behaviour of white etching layers formed on rails
Graphical abstract
Introduction
The design of sustainable new materials with well-controlled structural integrity requires a macro-to microscale understanding of degradation and failure of the conventional materials. Steels are one of the most commonly used conventional materials around the world. The study of failure in steels is crucial to ensure the safety in engineering applications such as construction, transport, energy, etc. In the transport industry, railway consists of 1,370,782 km length of rail track worldwide [1] and this length is still growing. In this entire rail network, the pearlitic steels are the most commonly used steels [2]. Failure in pearlitic railway steels is strongly affected by the detrimental microstructural changes on the rail raceway due to the wheel-rail contact. Conventional pearlitic steels are prone to the formation of White Etching Layers (WELs) on the rail raceway during the wheel-rail interaction [[3], [4], [5], [6], [7], [8], [9], [10], [11]]. Delamination and partial brittle fracture of these WELs cause micro-crack initiation in the rails [4,5,9,10,[12], [13], [14]]. These micro-cracks grow into the base material and this propagation finally leads to rail track failure, posing a safety threat to the passengers. Worldwide, the rail industries invest tens of million dollars per year to grind various in-service surface defects and to remove the WELs before extended cracks form [15,16].
In the literature, two hypotheses are proposed for the formation mechanism of the WELs. The first hypothesis suggests that the WELs form only due to severe plastic deformation during wheel-rail contact and stress assisted cementite dissolution leads to the formation of nano-crystalline ferrite in the WEL microstructure [10,11,13]. The second hypothesis suggests that WELs form due to the temperature rise above austenite start temperature – followed by fast cooling [3,5,9]. This temperature variation leads to the formation of martensite and austenite. However, recent insights have shown that WELs form by the combination of the temperature rise above the austenite start temperature and the plastic deformation at the rail raceway [[17], [18], [19]]. Still, there is considerable debate among the research community concerning the WEL formation mechanism. Typically, the WELs consist of complex microstructural features such as martensite, retained austenite and partially dissolved parent cementite [7,[17], [18], [19]]. In addition, the overall microstructural evolution varies in different studies because of the variation in rail-wheel contact conditions such as wheel profile, axle load, train speed and slip rate. In spite of having a detailed understanding on formation mechanism and microstructural evolution of the WELs, there is no detailed study available in literature, which focuses on the fracture behaviour of the WEL on microstructural scale. Most of the available studies only focus on macro scale fracture due to the WELs in rails [4,20].
The WELs are considered to be among the metallurgical causes for crack initiation and propagation in rails because of their brittle nature [4,5,9,14,18,19]. However to date, there is no quantification of the fracture toughness of WELs and their fracture behaviour has not been studied in detail. This lack of fracture properties is primarily due to the small size of in-service WELs. The fracture behaviour of such small features cannot be determined via conventional testing. Only small scale in situ fracture mechanics allow studying the fracture behaviour of these microscale features. Micromechanics also allows analysing special microstructural features such as single crystals, grain boundaries, phase boundaries, coatings and multilayer microscale systems [[21], [22], [23], [24], [25], [26], [27], [28]].
In the present study, we show in situ microscale fracture experiments on the WELs. The fracture toughness values of the WELs can be of utmost importance for modelling the failure in rails due to the presence of WELs at the rail surface. They can also enable the estimation of quantities such as critical WEL thickness in rails and consequently the required grinding intervals in rails to avoid failure and also to minimize the grinding costs. For brittle materials, microscale fracture experiments use notched micro-cantilevers and apply Linear Elastic Fracture Mechanics (LEFM) [[21], [22], [23], [24], [25], [26],[28], [29], [30], [31], [32]] to quantify the fracture toughness. In these conditions, the size of the plastic zone at the crack tip should be significantly smaller than the specimen dimensions [33]. Thus, the LEFM approach is useful primarily for brittle materials, which fracture without any plastic deformation. Application of LEFM for ductile and semi-brittle materials will lead to the underestimation of the fracture toughness. We use both the LEFM and Elastic-Plastic Fracture Mechanics (EPFM) approach [33,34] to calculate the fracture toughness of the WELs.
In this study, the fracture toughness of the WELs is compared with those of undeformed pearlite, quenched martensite (same chemical composition as pearlitic steels in this work), heavily drawn nanocrystalline pearlite, iron (Fe) and nanocrystalline Fe thin films in order to put the quantified fracture behaviour of the WELs into context. We discuss the effect of microstructural features such as the presence of austenite, grain size, dislocation density and carbon (C) segregation at the dislocations and the grain boundaries in the WELs on the fracture response. Additionally, we investigate the strain-induced transformation of austenite to martensite during crack growth in the WELs.
Section snippets
Materials and experimental methods
The specimens containing WEL were cut from an in-service curved rail track with 400 m radius. The approximate load passage was 200 Mt with an axial load ranging from 120 to 180 kN [19]. The chemical composition of the studied R350HT steel is Fe-0.72C-1.1Mn-0.56Si-0.11Cr (wt.%), or Fe-3.25C-1.09Si-1.1Mn-0.11Cr (at.%). These steels were produced by 6 pass hot rolling at 1000 °C into the form of a rail, followed by annealing at 900 °C for 3 h and cooling the rail in the accelerated air flow which
Failure in rails due to the White Etching Layers (WELs)
Fig. 1(a) is an optical micrograph showing the brittle failure of the WEL followed by fatigue crack propagation into the deformed and undeformed pearlitic matrix. The crack in the matrix grows at an angle of around 38° to the loading direction (opposite to the X direction in Fig. 1(a)), which is close to the direction of maximum resolved shear stresses (i.e. 45°). However, the crack in the WEL seems unaffected by the maximum resolved shear stresses and grows by brittle cleavage (Fig. 1(a)). The
Conclusions
The WELs formed in the pearlitic microstructure of rail steels are considered as one of the reasons for micro-cracking in rails. These micro-cracks grow inside the rail and cause failure. The current study shows the first quantification of WEL fracture toughness and its correlation to the WEL microstructure. Results from the current study are of importance for modelling and understanding of wheel-rail contact in the presence of the WELs. The results from WEL models can be used to estimate the
Acknowledgements
This research was carried out under project number F91.10.12475b in the framework of the Partnership Program of the Materials innovation institute M2i (www.m2i.nl) and the Foundation for Fundamental Research on Matter (www.fom.nl), which is part of the Netherlands Organisation for Scientific Research (www.nwo.nl). We would like to express our gratitude to Dr. Stefan Zaefferer and Mrs. Monika Nellessen for providing assistance during the EBSD measurement on the microcantilevers. We thank Mr.
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