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The rail grinidng strategies
2024-10-28
Regular rail grinidng reduces or removes rail damage and extends their service life, which is widely recognized as one of the effective maintenance methods in rail transit systems. Based on the behavior of rail damage and combined with the needs of railway transportation capacity, rail grinidng technology and equipment are developing in a multi-level and diversified manner. The main strategies for rail grinding include pre-grinding, preventive grinding, and repairing grinding.
Pre-grinding. During the hot rolling process of rails, high temperatures intensify the oxidation of carbon and iron elements on the surface of the rails. When the oxidation rate of carbon is greater than that of iron, some of the cementite in pearlite transforms into ferrite, leading to the formation of a decarburization layer, as shown in Fig.1. The hardness, strength, and wear resistance of the decarburization layer on the rail are lower than those of the substrate [1]. During service, cracks mainly propagate along the ferrite, which is more likely to cause serious damage to the rail [2]. During the construction of the new line, frequent start/stop of construction vehicles can easily cause rail scratches and other issues. Rail scratches are not easily detected by inspection personnel in the early stage, but the friction heat between the wheel and rail causes metallographic transformation in the scratched area of the rail, which can quickly develop into serious damages in later service [3]. Therefore, for the construction of new lines and the laying of new rails in sections, pre-grinding should be carried out after commissioning and before opening for operation to remove the decarburization layer of about 0.2 mm on the surface and the damage caused by the construction process. For lines that have already been put into operation, it is required to complete pre-grinding before passing a total mass of 10 Mt.
Fig. 1. The decarburization layer on railhead.
Preventive grinding. For severe defects such as corrugation, delamination, lateral abrasion, and crushing, it is necessary to use a larger grinding depth to remove the defects, and at the same time correct and repair the rail profile. At this point, the amount of rail material removed and the depth of grinding are large, and this grinding strategy is called reparative grinding. Due to the design wear tolerance of the rail top, the frequency of rail replacement has increased during repair grinding, resulting in poor grinding economy.
China has made clear regulations on the repair grinding operation of rail transit lines: (a) Repair grinding should select appropriate grinding equipment and operating parameters based on the characteristics of rail defects, remove rail defects, and repair rail profiles; (b) In ordinary lines, for defects such as wave grinding depth or fat edge width exceeding 0.3 mm, timely repair grinding should be organized; (c) In high-speed railway lines, when there is high-speed train shaking, lateral acceleration alarm of the transverse frame, poor light strip in sections, fish scale depth of the turnout rail reaching 0.5mm, track impact response index exceeding the management value, or rail defects reaching the remediation limit, timely repair and grinding of the rail should be organized.
Repairing grinding. The rail is subjected to periodic loads from the wheels, and the rolling contact fatigue layer (RCF) is formed on the surface of the rail due to the "ratchet effect" [4,5]. In the rolling contact fatigue layer, the hardness and brittleness of the rail material increase significantly, and the microstructure shows an increase in dislocation density and microcrack initiation[5]. The lateral and longitudinal propagation of microcracks may cause damage such as delamination and rail breakage in rails [1]. In the crack development cycle (nucleation, initiation, and propagation), nucleation and initiation time account for a large proportion. Grinding is usually carried out when the depth of crack propagation does not exceed 0.2 mm, which is the best time to block the propagation of rail cracks and avoid further deterioration of the rail, as given in Fig.2. Based on the mechanism of rolling contact fatigue on the surface of rails and the law of crack propagation, a preventive grinding strategy is proposed, which involves regularly removing the rolling contact fatigue layer on the surface of rails. Preventive grinding has a smaller grinding depth, which is more conducive to extending the service life of rails and has better grinding economy compared to repair grinding.
China Railway Corporation has put forward clear requirements for the implementation of preventive grinding of railway rails. (a) For high-speed railways, preventive grinding should be carried out every 30-50 Mt of total weight, with a grinding interval of no more than 2 years. (b) For straight and large radius curved sections (>1200 m) of ordinary speed lines, a grinding process is usually carried out once for a total weight of 100 Mt; for small radius sections (<1200 m), polish every 30-50 Mt.
The evolution law of the service life of rails under different grinding strategies is shown in Fig. 2, and the results indicate that grinding can effectively extend the service life of rails. According to reports, the probability of defects occurring on the surface of rails after non grinding, repair grinding, and preventive grinding is 15%, 8%, and 4%, respectively [6], indicating that preventive grinding has a significant advantage in reducing the probability of rail defects. In addition, preventive grinding materials have a smaller removal thickness, which can significantly extend the service life of rails compared to repair grinding. Under the severe situation of rapid economic development and urgent demand for the transportation capacity of China's rail transit, adopting preventive grinding can effectively extend the rail replacement cycle and have higher grinding economy. Predictive preventive grinding is the mainstream development direction of future rail grinding technology.
Fig.2. The propagation behavior of rolling contact fatgiue cracks
Fig.3. The relationship between grinding strategies and service life of rails
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[2] ZHAO Xiangji, WANG Hengyu, Guo Jun, et al. The Effect of Decarburized Layer on Rolling Contact Fatigue of Rail Materials under Dry-wet Conditions[J]. Engineering Failure Analysis, 2018, 91: 58-71.
[3] LIU Fengshou, LI Chuang, TIAN Changhai. Research on Early Damage of Chinese High-speed Railway Rails. Railway Engineering, 2018, 58(1): 138-140.
[4] Reza Masoudi Nejad, Shariati Mahmoud, Farhangdoost Khalil. Effect of Wear on Rolling Contact Fatigue Crack Growth in Rails[J]. Tribology International, 2016, 94: 118-13.
[5] HU Yue, ZHOU Lang, DING Haohao, et al. Microstructure Evolution of Railway Pearlitic Wheel s under Rolling-sliding Contact Loading[J]. Tribology International, 2021, 154: 106685.
[6] ZAREMBSKI Allan M., PALESE Joseph. Does Rail Grinding Reduce Rail Defects[J]. Railway Track&Structure, 2011, 107(2): 32-35.