Surface integrity during rail grinding under wet conditions: Full-scale experiment and multi-grain grinding simulation

Highlights

• Surface integrity of a ground rail is improved under wet conditions.

• Grinding burn and WEL can be avoided under wet conditions.

• A grinding wheel model with hundreds of abrasive grains is established.

• Thermodynamic coupling is less significant under wet conditions.

Abstract

Rail grinding will cause poor surface integrity when the critical grinding temperature is exceeded. To improve the grinding quality and explore the feasibility of grinding operations in rainy days, this study first investigated the surface integrity during rail grinding under wet conditions, including surface roughness, grinding burn, phase transition, residual stress, etc. Moreover, a grinding wheel finite element model was established considering abrasive grains' actual geometry and distribution, providing a novel thermodynamic coupling analysis method. As a result, it is revealed that the surface integrity is significantly improved under wet conditions, especially the grinding burn and white etching layer can be avoided, and residual tensile stress can be reduced by up to 45% since the thermodynamic coupling becomes less apparent.

Introduction

Since the 1980s, rail grinding has been identified as a critical maintenance technique on the railway to eliminate rail damages and re-profile rail heads as reviewed by Magel and Kalousek [1]. Satoh and Iwafuchi [2] stated that the service life of rails could be observably extended, and the running safety and stability of trains could be increased through the periodic grinding procedures. Rail grinding operations are primarily performed at night by occupying a track and considering weather conditions as described by Singleton et al. [3]. Since the grinding process is accomplished by a cup grinding wheel without coolant addition, the material removal is significantly affected by grinding heat. It has been reported that grinding can change both surface topography and subsurface microstructure of a rail, even result in the formation of a white etching layer (WEL) and surface burn, which are regarded as unfavorable thermal damages [4]. Moreover, the success of a rail grinding operation is considerably related to the grinding parameters, grinding wheel, grinding conditions. Therefore, revealing the material removal mechanism under various conditions is essential to improve the grinding quality further.

Rail grinding was generally considered to have no adverse effect on the rail material at first. However, Cuervo et al. [5] found the rail material after a grinding procedure plastically deformed and was covered with a WEL with higher hardness. After that, some representative research studies were conducted by Steenbergen [6] and Rasmussen et al. [7] to investigate the effect of WEL on the initiation and propagation of fatigue cracks. It was found that the hard and brittle WEL would be pressed into a deeper rail matrix during repetitive wheel/rail rolling contacts, resulting in the fatigue cracks initiation. The microstructure characteristics and formation mechanics of rail WEL due to wheel/rail rolling contact [8], electrical leakage [9], etc. are widely discussed recently. It is generally believed that the thermal effect and mechanical stress are responsible for the rail WEL [10]. Besides, Wu et al. [11] described that the microstructure of a thermally induced WEL consists of predominate martensite accompanied with a few retained austenite and undissolved carbides. Furthermore, Carroll and Beynon [12] thought that high contact stress between wheel/rail could reduce the austenitizing temperature of rail steel. However, the formation mechanisms of WEL induced by rail grinding are hardly investigated, which will be discussed in the present study. On the other hand, according to the acceptance of reprofiling rails in track defined in BS EN 13231-3 [13], the surface roughness Ra after grinding should be within 10 µm, since the surface roughness and grinding scratches after rail grinding also affected the wear rate and friction coefficient of a wheel/rail rolling contact as found by Zhu et al. [14] and Khalladi et al. [15]. Besides, Fukagai et al. [16] found that a rougher rail surface could result in higher traction but a more significant work-hardening as well. Hu et al. [17] found that the increase in rail hardness would lead the wheel wear to increase. The numerical research performed by Nejad et al. [18] indicated that the residual tensile stress could significantly exacerbate the initiation and propagation of fatigue cracks in the rail during wheel-rail rolling contact. Therefore, the rail grinding-induced surface integrity played a vital role in its service performances, regarded as an evaluation index of grinding quality on the railway. It is necessary to clarify the effect of grinding processes with different parameters and conditions on the rail surface integrity.

In the last decade, several research studies were performed in the laboratory to investigate the effects of a rail grinding process on surface integrity. Gu et al. [19] and Uhlmann et al. [20] studied the influences of rotational speed and grinding depth on the surface roughness and hardness of rail material, respectively. They also found the ground rail covered with a WEL, and increasing the feed rate or decreasing the rotational speed reduced the WEL thickness. Zhou et al. [21] conducted a series of rail grinding experiments and systematically studied the material removal behavior. They thought the thermal effect of a rail grinding process was more significant than other types of grinding; consequently, the rail material was oxidized or phase transformed while being removed in the form of curled grinding chips. Moreover, the surface burn and WEL at the rail gauge corner were much more severe than those at the rail shoulder and crown under the same conditions [22]. Furthermore, Pereira et al. investigated the tribological and metallurgical behavior in a scratch test and found that the work-hardening and microstructure evaluations of pearlitic steel were closely concerned with the scratching parameters, including normal load [23] and scratch pass [24]. Other scholars tried to improve grinding quality and efficiency by utilizing high-performance grinding wheels; many new grinding wheels were therefore developed through adjusting abrasive material [25], granularity [26], hardness [27], porosity [28], and shape [29]. However, the rail grinding process under wet conditions has not been investigated. On the one hand, the field rail grinding operations are sometimes carried out in rainy weather, and the water will enter the grinding interface due to this non-artificial reason. On the other hand, in the grinding industry, it is widely believed that the grinding temperature can be reduced by adding coolant, thus avoiding the generation of thermal damage as reviewed by Gupta et al. [30]. In this case, the present study will clarify the surface integrity of rail material after grinding under wet conditions.

Since the formation of surface integrity, especially the grinding burn, phase transition, and residual stress, is closely related to the thermal effect during rail grinding, many scholars have studied the distribution and evolution of grinding temperature by combining experiment and simulation. Zhang et al. [31] first simulated the temperature field during rail grinding based on a moving heat source method. However, the model did not consider the actual shape of the grinding zone, and the heat flux was assumed to distribute uniformly. Next, Zhang et al. [32] calculated the temperature field during grinding with a small proportion rail and a grinding wheel specimen. However, the grinding wheel did not have abrasive grains, i.e., only the friction action between rail and grinding wheel was simulated. After that, Zhou et al. [33] established a numerical model with a parabolic distribution of heat flux which gave a more accurate description of the temperature field in the grinding zone. However, the temperature field models in the above researches can be understood as externally imposed rather than directly generated by the material removal process during grinding. It is also difficult to simulate the material removal process accompanying temperature field during conventional grinding, primarily because the shape and position distribution of the numerous abrasive grains on a grinding wheel surface is disordered. In this case, it is common to establish the grinding temperature and force model based on the single abrasive grain cutting as conducted by Zhang et al. [34] and Tang et al. [35], respectively. This study proposed a novel simulation method by establishing a grinding wheel model considering the actual geometry and distribution of abrasive grains, which provided a more accurate and convenient method to solve material removal, temperature, force, and residual stress during grinding.

This study aims at clarifying the rail grinding mechanisms under wet conditions. For this purpose, the full-scale rail grinding experiment was conducted, and the surface integrity, including surface roughness, surface burn, microstructure transformation, residual stress, and grinding chips, were analyzed in detail. Moreover, a thermo-mechanical coupling analysis method of the grinding process was first proposed by establishing a grinding wheel model based on the actual geometry and distribution of abrasive grains. The work is expected to be of significance for the field rail grinding operations on rainy days and the implementation of cooling measures in the future.