Brazed–Resin Composite Grinding Wheel with CBN Segments: Fabrication, Brazing Mechanism, and Rail Grinding Performance

Abstract

To enhance the grinding performance and service life of rail grinding wheels, a novel brazed–resin composite wheel was developed by embedding brazed CBN (cubic boron nitride) segments into a resin working layer. The brazed CBN segments were fabricated using a Cu–Sn–Ti + WC (tungsten carbide) composite filler via a cold-press forming–vacuum brazing process. Microstructural and phase analyses revealed the formation of Ti–B and Ti–N compounds at the CBN–filler interface, indicating metallurgical bonding, while the incorporation of WC reduced excessive wetting, enabling precise shape retention of the segments. Comparative laboratory and field grinding tests were conducted against conventional resin-bonded wheels. Under all tested pressures, the composite wheel exhibited lower grinding temperatures, generated predominantly strip-shaped chips with lower oxygen content, and produced fewer spherical oxide-rich chips than the resin-bonded wheel, confirming reduced thermal load. Field tests demonstrated that the composite wheel matched the resin-bonded wheel in grinding efficiency, extended service life by approximately 28.8%, and achieved smoother rail surfaces free from burn-induced blue marks. These results indicate that the brazed–resin composite grinding wheel effectively leverages the superior hardness and thermal conductivity of CBN abrasives, offering improved thermal control, wear resistance, and surface quality in rail grinding applications.

1. Introduction

Railway tracks, primarily manufactured from high-strength carbon–manganese (C–Mn) steel, are subject to progressive surface damage and structural defects due to cyclic loading, wheel–rail friction, and braking forces during service [1,2,3]. To restore rail profiles and ensure operational safety, abrasive grinding remains an essential maintenance technique [4,5,6]. To improve grinding efficiency, reduce grinding costs, and enhance grinding quality, current research in rail grinding focuses mainly on two aspects: optimizing grinding parameters to enhance efficiency and surface integrity [5,6,7], and improving grinding wheel performance through material and structural innovations [8,9,10]. Despite these efforts, conventional resin-bonded alumina grinding wheels present inherent limitations. The dry grinding of high-strength rail steel under heavy loads generates extreme temperatures in the absence of coolant. Resin bonds have low thermal resistance, making the wheels prone to thermal degradation, burning, or volatilization, which accelerates wear and shortens service life [11]. Severe wheel consumption also generates dust, fumes, and harmful gases, polluting the environment [12]. Additionally, alumina grains dull rapidly, leading to reduced sharpness, increased grinding temperatures, and excessive heat transfer to the workpiece [13], which can result in grinding burns. These drawbacks limit both the consistency of grinding quality and the further development of rail grinding technology. Therefore, developing grinding wheels with superior grinding performance is crucial for the enhancement of rail grinding technology.

As a super-hard abrasive, cubic boron nitride (CBN) exhibits high hardness, excellent wear resistance, and chemical inertness toward ferrous metals [14]. Its application in rail grinding can significantly enhance the quality of the grinding process. Brazed super-abrasive tools have emerged as a promising alternative, offering high grain protrusion, strong grain retention, long service life, and superior grinding efficiency [15]. Such a single-layer brazed super-abrasive is typically produced by metallurgically bonding diamond or cubic boron nitride (CBN) grains to the substrate using brazing alloys such as Cu–Sn–Ti, Ni–Cr, or Ag–Cu–Ti [16,17,18,19,20,21]. Furthermore, in previous studies, multi-layer brazed CBN abrasive segments have been successfully fabricated, which can further enhance the service life of the tool [22]. However, their potential in heavy-duty rail grinding remains underexplored, particularly in the form of hybrid wheel structures that combine brazed superabrasives with conventional wheel matrices.

To address this gap, the present study integrates brazed CBN technology with traditional hot-press wheel manufacturing to develop a novel composite grinding wheel containing brazed CBN segments. This design aims to overcome the thermal and wear limitations of resin-bonded wheels while maintaining impact resistance for field use. Comparative grinding tests were performed against conventional resin-bonded wheels on both a rail grinding test platform and in actual railway service. Performance was evaluated comprehensively in terms of grinding temperature, wheel durability, post-grinding surface quality, and grinding chip morphology. The results provide a pathway toward more efficient and durable solutions for rail maintenance.

2. Materials and Methods

2.1. Preparation of Brazed CBN Segments

The brazed CBN segments were fabricated using a Cu–Sn–Ti alloy filler (Cu—70 wt%, Sn—18 wt%, and Ti—12 wt%) with a particle size of 125 μm. Uncoated synthetic CBN grains (300–400 μm, Zhongnan Jete, Zhengzhou, China) served as the abrasives, while tungsten carbide (WC) particles (10–100 μm) acted as skeletal reinforcement. In the experiment of preparing brazed CBN segments, the contents of WC were 0 wt%, 15 wt%, 20 wt%, and 25 wt%, respectively. CBN grains and WC particles were ultrasonically cleaned in acetone to remove surface contaminants. The Cu–Sn–Ti alloy powder and WC particles were mixed in a 3D mixer for 30 min, after which CBN grains and a paraffin-based forming agent were added and uniformly blended. Then, the mixture was cold-pressed in a mold to form the blank to be brazed (Figure 1). Vacuum brazing was conducted in a high-vacuum furnace (<10⁻² Pa) at a heating rate of 10 °C/min, with a peak temperature of 960 °C, and the holding time was 18min. Following brazing, the furnace was cooled to room temperature, yielding brazed CBN segments. After brazing, the cross-section of the brazed CBN segment sample was polished and ground. The microstructure of the brazing interface was observed using a JSM-6360LV scanning electron microscope (SEM, JEOL, Shanghai, China), and the elemental distribution at the interface was analyzed with an EDAX energy-dispersive spectrometer (EDS, AMETEK, Shanghai, China). The phase composition of the brazing interface was examined using an XD-3A X-ray diffractometer (XRD, SHIMADZU, Shanghai, China), and the scanning range was 20°~90°, with a scan speed of 0.1 °/s.

2.2. Fabrication of the Composite Grinding Wheel

The composite grinding wheel was produced via sequential mixing, feeding, segment embedding, hot-press forming, glass fiber reinforcement, and thermal curing (Figure 2). Eight brazed CBN segments were embedded per wheel, each pre-treated with KH550 silane coupling agent to enhance adhesion between the phenolic resin bond and the brazed segments, thereby reducing cracking risk. First, a wetting agent was added to the proportioned phenolic resin bond and alumina abrasives, and the mixture was then thoroughly blended in a mixing machine. The uniformity of the mixture significantly impacts the quality of the grinding wheel; to ensure homogeneity, the mixing time was set to 24 h. Next, the mixed material was loaded into a grinding wheel mold, and the pre-fabricated brazed CBN segments were embedded into the mixture in the mold according to the predetermined quantity and arrangement angle using specialized tooling equipment. Subsequently, the mixture was hot-pressed and shaped under a temperature of 180 °C and a pressure of 20–40 N/mm², with a holding time of 40 min. After pressing, the material was demolded to obtain a green grinding wheel. Then, a layer of glass fiber was wound around the periphery of the wheel. Known for its excellent mechanical properties and thermal stability, the glass fiber layer helps prevent cracking during use. Finally, the green wheel wrapped with glass fiber was trimmed to remove minor deformations that occurred during the pressing process. Although the hot-pressed green wheel already possesses considerable strength, it still requires thermal curing before becoming a finished product. The curing process was carried out in a thermal curing oven at a temperature generally close to that used during hot pressing (180 °C) for a duration of 36 h. The final brazed–resin composite grinding wheel is shown in Figure 3.

2.3. Grinding Test Platform

Comparative grinding tests were conducted on a self-developed rail grinding test platform, as shown in Figure 4. The platform consists primarily of a vertical-shaft grinding machine and a computer-based control console. A custom-fabricated annular high-strength rail steel specimen was mounted onto a rotary table driven by an electric motor, providing a linear workpiece speed equivalent to the operating speed of rail grinding trains in field conditions. Above the workpiece, a grinding wheel spindle motor was positioned to perform grinding at preset pressure and rotational speed. The grinding pressure, wheel speed, and workpiece linear velocity were adjustable via the control cabinet, and their values were displayed on a digital readout.

The grinding temperature was measured using the thermocouple method. A separate temperature-measurement setup was installed on the rail grinding test platform, as shown in Figure 5. The rail specimen was made of U71Mn steel. A K-type armored thermocouple, together with a mica sheet, was clamped within the rail specimen and securely fixed to the temperature-measurement platform. The thermocouple was connected to a computer via a UNI-T325 (UNI-T, Hong Kong, China) signal acquisition unit, with a sampling frequency of 10 Hz. The recorded temperature data were directly stored on the computer. In the grinding temperature measurement tests, each grinding cycle lasted 60 s, with the grinding wheel engaging the rail specimen for 6 s and remaining disengaged for the rest of the cycle to enable natural cooling of the rail. The novel composite grinding wheel was designated as No. 1, while the conventional resin-bonded grinding wheel was designated as No. 2. The grinding temperatures of rail specimens were measured for different types of grinding wheels under various grinding pressures. The experimental parameters are listed in

Table 1. Grinding test parameters of the test platform.

Project	Parameters Title 3
Test grinding wheels	No. 1, No. 2
Rotational speed of grinding wheels (r/min)	3600
Grinding pressures (N)	280, 440, 600, 760
Grinding time (s)	6
Grinding cycle (s)	60

.

Field grinding tests were conducted by mounting the experimental wheels on a GMC-96X rail grinding train (Figure 6). This model is equipped with 96 grinding spindle motors, with 48 grinding heads installed on each side of the train. The grinding wheels engage the rail at different angles, ensuring that the combined grinding tracks fully cover the entire rail profile. The novel brazed–resin composite grinding wheels and conventional resin-bonded wheels were mounted on opposite sides of the grinding train and operated under identical working conditions for comparative evaluation. Under identical grinding conditions, different types of grinding wheels were used to continuously grind a 2 km section of rail. The ground rail surfaces were then photographically documented and tracked for comparative observation, enabling an evaluation of the grinding quality of each wheel based on the appearance of the ground rails.

3. Results and Discussion

3.1. Wetting Characteristics of the Cu–Sn–Ti + WC Filler Alloy on CBN

CBN segments were fabricated using Cu–Sn–Ti + WC composite brazing alloys with varying WC contents, and the post-brazing morphologies are shown in Figure 7. It is evident that when the WC particle content did not exceed 15 wt%, the segments failed to retain their intended shape, exhibiting pronounced collapse. When the WC content reached 20 wt%, the segments maintained their designed shape well. For the segments brazed using pure Cu–Sn–Ti alloy filler, severe alloy flow and extensive interfacial reactions occurred, leading to significant deformation and collapse, making it essentially impossible to retain the designed shape. In contrast, WC particles do not melt at the peak brazing temperature of 960 °C and act as a structural skeleton within the segment. The incorporation of WC skeletal particles partially suppresses the excessive reaction between the filler alloy and CBN grains, while also reducing the alloy’s fluidity [18]. As a result, the post-brazing shape integrity of the CBN segments was significantly improved.

As shown in Figure 8, the cross-sectional microstructure of a CBN segment brazed with the Cu–Sn–Ti + WC composite filler (20 wt% WC) reveals that the CBN grains are embedded within the brazing alloy layer and exhibit intimate interfacial bonding. Some pores are observed within the segment interior, which are attributed to the flow of molten brazing alloy toward the CBN grain boundaries, as well as the escape of volatile substances, such as the paraffin-based forming agent, during high-temperature brazing. The presence of such pores increases the accommodation space for chips within the brazed segment, which is beneficial for chip evacuation during grinding.

The interfacial morphology between the CBN and the Cu–Sn–Ti + WC alloy is shown in Figure 9. The filler alloy exhibits good wettability on the CBN, without obvious cracks or pores observed at the interface. Elemental line-scan analysis was performed from the CBN abrasive toward the filler alloy. Clear slopes of Ti, B, and N elements are observed at the boundary of CBN, indicating that Ti–B and Ti–N compounds formed at the interface during brazing. To further investigate the phase composition of the newly formed compounds within the brazed CBN segment, the region inside the CBN segment was analyzed via X-ray diffraction (XRD), and the corresponding diffraction pattern is shown in Figure 10. The phases present in the segment include CBN, TiN, TiB₂, WC, and TiC. These results indicate that TiN and TiB₂ compounds are produced by the reaction between the composite brazing alloy and the CBN abrasive, while TiC forms through the reaction between WC particles and the brazing alloy.

The presence of active Ti enhanced the wettability of the filler alloy on the CBN surface, leading to strong metallurgical bonding and superior abrasive grain retention. The interfacial chemical reactions between the filler alloy and CBN can be described as follows [20]:

2BN + 3Ti = 2TiN + TiB₂

(1)

The Gibbs free energies of TiN and TiB₂ formation reactions are −307.5 and −319.8 J/mol, respectively [22]. The negative values of Gibbs free energy indicate that the reaction can proceed spontaneously.

The Ti–W binary phase diagram [22] indicates that W and Ti are mutually soluble. At the brazing temperature, W precipitates and simultaneously dissolves Ti from the alloy. The resulting reactions between WC and the active alloy are as follows:

WC + Ti = TiC + W

(2)

The Gibbs free energy of TiC formation is −217.8 J/mol. And the negative values of Gibbs free energy indicate that the reaction can occur spontaneously.

3.2. Grinding Temperature

A comparative experimental study on grinding temperature was conducted using two different types of grinding wheels. The novel composite grinding wheel was designated as No. 1, while the conventional resin-bonded grinding wheel was designated as No. 2. The variation in grinding zone temperature during each grinding cycle is shown in Figure 11. It can be observed that, under different grinding pressures, the temperature evolution exhibits a broadly similar trend. During grinding, the temperature in the grinding zone rises rapidly and reaches a peak value. And during the subsequent natural cooling stage, the interfacial temperature first drops sharply and then decreases at an increasingly slower rate. A sharp peak is formed at the highest grinding temperature. This phenomenon can be explained as follows: the grinding heat generated during material removal is conducted into both the grinding wheel and the workpiece, causing a rapid temperature rise in the grinding zone. Once grinding is completed, the large initial temperature difference between the workpiece and the surrounding environment leads to a high heat transfer rate and a rapid temperature drop. As the surface temperature decreases and the temperature difference diminishes, the heat transfer rate falls, resulting in a slower cooling rate.

Figure 12 presents the peak grinding temperatures per cycle for different types of grinding wheels under various grinding pressures. For both wheel types, the peak grinding temperature increases with increasing grinding pressure. However, at all tested pressures, the novel composite grinding wheel consistently exhibits lower peak temperatures than the conventional resin-bonded wheel.

3.3. Morphologies of Grinding Chips

For rail grinding trains equipped with 96 grinding heads, the most commonly applied grinding pressure is around 600 N. To accurately replicate field grinding conditions, this study collected chips produced during the grinding of rail specimens using the novel composite grinding wheel and the resin-bonded wheel under a grinding pressure of 600 N, followed by observation and analysis. Figure 13 presents SEM micrographs of the chips generated by the two types of grinding wheels.

The composite wheel predominantly produces strip- and ribbon-like chips, whereas grinding with the resin-bonded wheel generates a large quantity of spherical chips. When the grinding wheel is in a sharp state, the abrasives have good cutting edge exposure, resulting in relatively low grinding temperatures. Under the action of these sharp cutting edges, the rail material is removed in the form of strips or flakes. In contrast, when the wheel loses sharpness and the abrasive grains become dulled, the grinding force increases, leading to higher grinding temperatures. At excessively high temperatures, the chips react with oxygen in the air and partially melt, solidifying into spherical shapes upon cooling. Such spherical chips typically contain Fe oxides, and a higher oxygen content indicates a higher grinding temperature. EDS area scans were performed on the two types of chips, and the corresponding spectra are shown in Figure 14. The results indicate that the oxygen content in chips produced by the composite wheel is lower than that in chips produced by the resin-bonded wheel. Additionally, X-ray diffraction (XRD) analysis was performed on the spherical chips, and the results are shown in Figure 15. It can be observed that the spherical chips primarily consist of iron and iron oxides. The grinding chips underwent oxidation and melting at high temperatures, followed by solidification into fine spherical particles, forming typical spherical chips. Thus, iron oxides are present in the spherical chips. From the chips analysis perspective, this further confirms that the composite wheel generates lower grinding temperatures during rail grinding and offers superior machining performance compared with the resin-bonded wheel.

3.4. Processing Efficiency and Service Life

Comparative tests on the processing efficiency and service life of the grinding wheels were carried out under field railway conditions. The novel composite grinding wheel was designated as No. 1, while the conventional resin-bonded grinding wheel was designated as No. 2. A total grinding length of 200 m was selected, comprising five measurement points. The material removal amount of the rails was measured using an RS2015-2W-S-142 rail profile measuring instrument developed and manufactured by the Shanghai Railway Research Institute, as shown in Figure 16. The principle of the measurement is to compare the rail profiles before and after grinding, using the reduction in railhead height as the evaluation criterion. The average value from the five measurement points was taken as the test result, as summarized in

Table 2. Processing efficiencies of the grinding wheels.

Types of the Grinding Wheels	Material Removal Amount/mm
No. 1	0.314
No. 2	0.317

.

The measurements show that the variation in material removal between the two types of grinding wheels is within 4%, indicating that, under commonly used rail grinding conditions, the grinding efficiencies of the two wheels are approximately the same.

The service life of grinding wheels is generally evaluated in terms of wheel durability, which can be calculated using Equation (3):

Ws = Ls/hs

(3)

where Ws (km/mm) is the wheel durability, Ls (km) is the total grinding distance, and hs (mm) is the corresponding wheel thickness consumed. Three repeated service life tests were conducted, and for each test, the average service life of each group of 48 grinding wheels was taken as the final result. The results are presented in

Table 3. The durability of different grinding wheels.

Types of the Grinding Wheels	Thickness Before Grinding (mm)	Thickness After Grinding (mm)	The Consumption of Grinding Wheels (mm)	The Distance of Grinding (km)	The Service Life (km/mm)
No. 1	90	79.3	10.7	38	3.54
No. 2	90	75.67	14.33	38	2.74

. It can be seen that, under identical grinding conditions, the average service life of the composite wheel is approximately 28.8% higher than that of the imported resin-bonded wheel.

During rail grinding with resin-bonded wheels, the alumina abrasives on the wheel surface gradually wear down, leading to a loss of sharpness and the onset of abrasive grain passivation. This condition increases grinding heat, which in turn softens the resin bond, reducing its holding strength and causing abrasive grains to dislodge more easily, thereby accelerating wheel wear [23]. In contrast, the novel composite wheel contains CBN superabrasives with a hardness significantly greater than that of alumina abrasives. The passivation rate of CBN grains is lower, allowing them to maintain high sharpness over prolonged grinding periods, generate less grinding heat, and slow the softening of the resin bond, thus reducing the wheel’s wear rate. Furthermore, the brazed CBN segments possess higher hardness than the resin bond and can act as structural supports during wheel wear, helping to resist rapid material loss. Therefore, the service life of the novel composite grinding wheel is superior to that of the resin-bonded wheel.

3.5. The Appearance Condition of the Rail

Modern electrified railway lines impose strict requirements on the surface appearance of rails. According to China’s grinding quality acceptance standards, the ground areas on the rail head surface should be free of continuous blue tempering marks [24]. Figure 17 shows sequential photographs of the rail surface appearance after continuous grinding over 2 km with the two types of wheels. It can be observed that the novel composite wheel produces a grinding band on the rail surface that is smoother and finer, with no visible burn-induced blue marks. In contrast, the resin-bonded wheel produces a comparatively rougher grinding band, within which randomly distributed burn-induced blue marks can be clearly observed.

Based on the previous analysis, it is evident that after prolonged grinding operations, the super-abrasives in the novel composite wheel can still maintain high sharpness, resulting in lower grinding temperatures compared with the resin-bonded wheel. In addition, the CBN abrasives have a smaller particle size than alumina abrasives, enabling a secondary polishing effect. Even if burn-induced blue marks occur, the secondary polishing action of CBN can remove the burned layer. In summary, according to the rail surface appearance evaluation criteria, the grinding performance of the novel composite wheel is superior to that of the resin-bonded wheel.

4. Conclusions

In order to improve the performance of the rail’s grinding wheel, a novel composite grinding wheel containing brazed CBN segments was developed, and experimental research on the processing performance of grinding wheels was carried out. The following conclusions were drawn:

(1)

To exploit the advantages of CBN superabrasives in machining rail materials, brazed CBN segments were embedded into the resin working layer to fabricate a composite grinding wheel. And the brazed CBN segments were produced using a Cu–Sn–Ti + WC composite filler alloy. Microstructural and phase analyses of the brazed CBN revealed that chemical metallurgical bonding was achieved between the Cu–Sn–Ti + WC composite filler and the CBN, with the formation of Ti–B and Ti–N phases. The introduction of WC reduced the excessive wetting aggressiveness of the filler alloy on CBN, enabling precise shape retention of the brazed CBN segments.

(2)

Under all tested pressures, the grinding temperature of the novel composite wheel was lower than that of the resin-bonded wheel. Grinding with the composite wheel predominantly produced strip-shaped chips, with fewer spherical chips, and the oxygen content in the chips was relatively low. In contrast, grinding with the resin-bonded wheel generated a larger proportion of spherical chips with higher oxygen content. The elevated oxygen levels result from the reaction of Fe with O at high temperatures to form iron oxides, further confirming that the grinding temperature of the novel composite wheel is lower than that of the imported resin-bonded wheel.

(3)

Field grinding tests demonstrated that the grinding efficiency of the novel composite wheel is comparable to that of the resin-bonded wheel, while its service life is approximately 28.8% longer. Rails ground with the composite wheel exhibited lower surface roughness than those ground with the resin-bonded wheel, and no burn-induced blue marks were observed on the rail surface.