Micro and Nanoscale Characterization of Complex Multilayer-Structured White Etching Layer in Rails

Abstract

Micro- to nano-scale characterization of the microstructures in the white etching layer (WEL), observed in a Dutch R260 Mn grade rail steel, was performed via various techniques. Retained austenite in the WEL was identified via electron backscatter diffraction (EBSD), automatic crystallographic orientation mapping in transmission electron microscopy (ACOM-TEM), and X-ray diffraction (XRD). EBSD and ACOM-TEM methods were used to quantify grains (size range: 50 nm–4 μm) in the WEL. Transmission electron microscopy (TEM) was used to identify complex heterogeneous microstructural morphologies in the WEL: Nano-twinning substructure with high dislocation density in the WEL close to the rail surface and untransformed cementite and dislocations in the WEL close to the pearlite matrix. Furthermore, atom probe tomography (APT) revealed a heterogeneous through-thickness distribution of alloying elements in the WEL. Accordingly, the WEL is considered a multi-layered martensitic microstructure. These findings are supported by the temperature calculations from the shape analysis of the manganese profile from APT measurements, related to manganese diffusion. The deformation characteristics of the WEL and the pearlite beneath the WEL are discussed based on the EBSD measurements. The role of deformation in the martensitic phase transformation for WEL formation is discussed.

In situ study on fracture behaviour of white etching layers formed on rails

“…Most of the previous studies suggest that the WELs are primarily consisting of martensite in their microstructure [3,7,46]. Thus, we compared the fracture toughness of the WELs to that of a martensitic microstructure.…”

“…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].…”

“…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.…”

“…The KAM maps in Fig.5show that the average KAM increases from 0.82 ± 0.54° to 0.85 ± 0.56° before and after fracture (± 0.54° and ± 0.56° are the standard deviation values). These KAM values are used to calculate the Geometrically Necessary Dislocation (GND) density using(7) where α is a constant (α = 3 for mixed dislocations[52]), θ is the average KAM angle, u is the distance between misoriented points, which is the step size in EBSD map (i.e. 4 x 10 -8 m), b is the magnitude of the Burgers vector (i.e.…”

SLOZhNAYa PRIRODA BELOGO SLOYa V REL''SAKh PRI DLITEL''NOY EKSPLUATATsII

“…Тонкое хрупкое покрытие толщиной 10-20 мкм [1] образуется на поверхности рельсов при их длительной эксплуатации. Покрытие получило название «белый слой», т.к.…”