The structure design of grinding stone

One of the main drawbacks of domestically produced grinding stones at present is the tendency to burn steel rails [1]. During the process of rail grinding, the grinding effect of abrasives (sliding, plowing, cutting) and the friction between the binder and rail interface are the main sources of grinding heat [3]. Under the coupling effect of heat (grinding heat) and force (mechanical force), the pearlite in the rail material undergoes austenite transformation and subsequently forms martensite and ferrite during cooling, resulting in a high hardness and brittle white layer structure. Partial cracks will propagate at the boundary between the white layer and pearlite, causing premature failure of the rail [1], as shown in Figure 1 (a). During the polishing process, the surface of the steel rail undergoes varying degrees of oxidation, resulting in different colors of the polished rail. Yellow, blue, and purple are commonly referred to as "burns". Lin et al. [9] placed a semi artificial thermocouple in the steel rail to monitor the temperature of the polishing interface in real time under different polishing parameters. They compared the polishing temperature with the degree of burn on the surface of the steel rail and established a relationship model between the degree of burn (color change) and the polishing temperature, as shown in Figure 1 (b). On this basis, Zhou et al. [3] established a relationship model between temperature and the thickness and firing degree of the white layer during rail polishing, providing a new method for optimizing rail polishing parameters, as shown in Figure 1 (c). The above research results indicate that optimizing grinding parameters and reducing grinding heat are important methods for improving rail burns.

Figure.1 The grinding induced rail burning and white etching laywer （WEL）

Many scholars explore the mechanism of rail grinding burn from the perspective of grinding stone design. The research results of Zhang et al. [2] indicate that white corundum grinding stone has the best self sharpness and the most significant grinding effect, resulting in the highest grinding temperature and the largest white layer thickness. Yuan et al. [4] prefabricated a pore structure in the grinding stone, which is beneficial for the discharge of grinding debris, reduces grinding stone blockage, lowers grinding temperature, and improves the surface quality of the polished steel rail. Wang et al. [5] conducted a study on the influence of grinding stone hardness (N, R, P, T) on the surface quality of steel rails, and the results showed that the thickness of the white layer increased with the increase of grinding stone hardness. Therefore, reasonable regulation of grinding stone structure (pores, abrasive composition), hardness, etc. has a positive effect on improving rail burns.

The above research results indicate that grinding parameters and grinding stone performance are the two main factors affecting rail grinding burns. For existing polishing vehicles on the route, it is difficult to make significant adjustments to the operating parameters on the existing vehicle structure in order to ensure polishing efficiency. Therefore, the design and performance control of grinding stone structure is one of the effective ways to improve rail burns. Wu et al. [7, 8] implanted brazed diamond prefabricated blocks in a certain arrangement into the grinding stone, as shown in Figure 2 (a). The polishing results show that the composite grinding stone can effectively improve the efficiency of rail polishing, reduce the surface roughness of the polished rail, and improve rail burns. Zhao Jinbo et al. [9] bonded CaF2 with polyetheretherketone to form self-lubricating joint blocks, and prepared self-lubricating grinding stones by placing them in the grinding stone embryo, as shown in Figure 2 (b). The grinding results show that the self-lubricating joint block can continuously release at the interface between the grinding stone and the rail as the grinding stone wears, reducing grinding heat and improving rail burns. Implanting brazed prefabricated blocks, self-lubricating joint blocks, etc. into the grinding stone matrix results in uneven grinding stone structure and introduces a low strength interface (grinding stone matrix/implant block interface), thus ensuring the mechanical properties (rotational strength, dynamic balance, etc.) of the composite structure grinding stone is a key challenge. Wu et al. [10] designed a brazed CBN abrasive grinding wheel with a slit as shown in Figure 2 (c), which improved the burn of rail workpieces. However, the brazing layer used in the grinding stone has poor wear resistance during the rail grinding process, and the service life of the grinding stone is extremely short. Therefore, reasonable design/regulation of the grinding stone structure has a positive effect on reducing grinding heat and improving rail burns, but it is a prerequisite that must be fully considered to ensure that the grinding stone has good physical and chemical properties and workability.

(a)Pre set diamond block grinding stone [7,8]

(b) Pre set Self-lubricating block grinding stone[9](c)Slit structured grinding stone [10]

Figure 2. The structure design of grinding stone

Reference

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[2]GUO Shuai, ZHAO Xiangji, HE Chenggang, et al. Effects of Grinding Marks on Fatigue Damage of Rails under Water Conditions[J]. China Mechanical Engineering, 2019, 30(08): 889-895.

[3]36[3]    ZHOU Kun, DING Haohao, Steenbergen Michaël, et al. Temperature Field and Material Response as a Function of Rail Grinding Parameters[J]. International Journal of Heat and Mass Transfer, 2021, 175: 12366.

[4]YUAN Yongjie, ZHANG Wulin, ZHANG Pengfei, et al. Porous Grinding Wheels Toward Alleviating the Pre-fatigue and Increasing the Material Removal Efficiency for Rail Grinding[J]. Tribology International, 2021, 154: 106692

[5]WANG Ruixiang, ZHOU Kun, YANG Jinyu, et al. Effects of Abrasive Material and Hardness of Grinding Wheel on Rail Grinding Behaviors[J]. Wear, 2020, 454-455: 203332.

[6]57[6]    ZHANG Wulin, ZHANG Pengfei, ZHANG Jun, et al. Probing the Effect of Abrasive Grit Size on Rail Grinding Behaviors[J]. Journal of Manufacturing Processes, 2020, 53: 388-395.

[7]XIAO Bing, XIAO Haozhong, XIAO Bo, et al. Grinding Wheel for High-efficiency Rail Grinding and its Manufacturing Method: China, CN 108453638 A[P]. 2018-08-28.

[8]WU Hengheng, XIAO Bing, XIAO Haozhong, et al. Wear Characteristics of Brazed Diamond Sheets with Different Grinding Time[J]. Wear, 2019, 432-433: 202942.

[9]WU Hengheng, XIAO Bing, XIAO Haozhong, et al. Study on Wear Characteristics of Brazed Diamond Sheet for Rail’s Composite Grinding Wheel under Different Pressures[J]. Wear, 2019, 424-425: 183-192.

[10]LIN Bin, ZHOU Kun, GUO Jun, et al. Influence of Grinding Parameters on Surface Temperature and Burn Behaviors of Grinding Rail[J]. Tribology International, 2018, 122: 151-162.