Microstructural investigation of white etching layer on pearlite steel rail

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Abstract

The microstructure of the white etching layer (WEL) on the contact surface of a head-hardened pearlitic rail was investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and a three-dimensional atom probe (3DAP). The WEL was confirmed to be composed of severely deformed pearlite lamellae as well as nanocrystalline martensite, austenite and cementite. The carbon atoms in the topmost surface of the WEL were distributed nearly uniformly. A few atomic percent of carbon was dissolved in the ferrite of the deformed pearlite lamellae. The maximum hardness was observed in the deformed pearlite region within the WEL rather than on the topmost surface layer. The microstructural feature of the WEL and the reason why the deformed layer shows a white contrast in optical microscopy are discussed.

Introduction

The rails in the modern railway system are subjected to intense use with fast train speed and large axle loads, which cause gradual surface structural modifications resulting from the serious wear and rail corrugation. The gradual structural changes near the rail–wheel contact surface of the rail is called white etching layer (WEL), which has drawn great research interest in the past decades, relating to the wear behavior or corrugation [1], [2], [3], [4], [5], [6], [7].

WEL is named according to its white featureless appearance under the optical microscope resulting from its higher corrosion-resistance against metallographic etching by Nital [1]. Due to its hard and brittle property with a high Vickers’ hardness up to 1200 HV [2], [3], WEL is usually believed to be the location of crack formation, thus it is widely believed to have a negative effect on the rail lifetime. An early investigation reported that WEL is martensite that was formed by the supersaturation of carbon during repeated severe plastic deformation [8], because the rail–wheel contact was unlikely to be heated to the austenization temperature. However, recent work [9] detected retained austenite and martensite in WEL and suggested that the martensite phase was formed from the austenite due to the considerable rise in temperature. With increasing evidence that severe plastic deformation such as high torsion straining [10], surface mechanical attrition treatment [11], [12], sliding and rolling motion [13] induced the nanocrystalline structure formation, Lojkowski et al. [14] proposed that WEL is composed of nanocrystalline α-Fe with the grain size ranging from 15 to 500 nm. They attributed the formation of the nanocrystalline structure to the severe plastic deformation at the wheel-rail contact zone, which is a solid state process analogous to a mechanical alloying process. Based on transmission electron microscopy (TEM) and advanced synchrotron X-ray diffraction (XRD) results, Österle et al. [15] confirmed that WEL is a martensite with high dislocation densities and occasional twins containing some fine undissolved cementite particles. Since the WEL is thought to be affected by temperature, accumulative strain, strain rate, environment, hydrostatic pressure, and cooling rate [7], [16], the features outlined above are believed to be specific for the WELs that were observed on various railway tracks.

In this study, we investigated the microstructural features of the WEL that was formed on an actual pearlite rail track, and found that the WEL is composed of severely deformed pearlite lamellar as well as martensite, retained austenite and cementite particles in spite of its uniform optical micrographic feature after Nital etching. The three-dimensional atom probe (3DAP) technique was applied to examine the carbon and silicon elemental distributions in different structures of WEL. The origin of the white etching contrast observed from the severely deformed surface layer is discussed based on the microstructural observation results.

Section snippets

Experimental

The sample consisted of a head-hardened pearlitic steel rail containing Fe–0.83 wt.% C–0.55 wt.% Si–1.2 wt.% Mn or Fe–3.7 at.% C–1.0 at.% Si–1.2 at.% Mn with a hardness of about 390 on the Brinell scale (≈410 HV) on the rail head surface before use. The tensile and yield strengths of the rail were 1340 and 896 MPa, respectively. The rail was installed and tested for 2 years at a 5° curve in the high tonnage loop (HTL) of the Facility for Accelerated Service Testing (FAST) at the Transportation

Results

Fig. 1(a) shows an optical micrograph and (b) the corresponding microhardness as a function of the distance from the topmost surface of the rail sample. A white etching layer (WEL) with a bright featureless contrast that is almost 30 μm thick is formed nearly uniformely on the rail top and gauge corner surface. Underneath the WEL, there is a transition layer of about 20 μm which is characterized by flattened grains. At a depth over 50 μm, the grains do not show any apparent deformation (undeformed

Structures of WEL

This investigation has shown that various microstructural features are contained within the WEL that was formed on the rail track surface. The topmost surface of the WEL was confirmed to be composed of largely nanocrystalline microstructure with some deformed pearlite regions. Austenite and cementite particles were observed within the nanocrystalline region. The XRD pattern showed an asymmetric screwed profile that was previously reported [15], and is indicative of the presence of some

Summary

We have characterized the microstructural features of the white etching layer that was formed on the surface of the head-hardened pearlitic rail tested at TTCI-HAL for 2 years with a total accumulated tonnage of 271 MGT by an axle load of 39 t. The WEL was confirmed to be composed of severely deformed pearlite as well as martensite, austenite and cementite particles. The 3DAP analysis confirmed that carbon with an average concentration of 4 at.% was distributed nearly uniformly in the topmost

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