A Novel Carbide-Free Bainitic Heavy-Haul Wheel Steel with an Excellent Wear-Resistance under Rolling-Sliding Condition

: With the rapid development of railways towards high speed and larger carrying capacity, the problem of wear and fatigue damage between wheel/rail is gradually becoming serious. However, traditional pearlite wheel/rail has reached the limit, which leads to more attention to developing a novel wheel/rail material. This study aims to report a novel carbide-free bainite wheel steel. The wear-resistance of novel steel was tested by a rolling-sliding wear experiment under heavy-haul condition and investigated the impacts of the running speeds on the damage mechanism of wear and fatigue. The results show that the yield strength of the bainite wheel was as high as 950 MPa and the hardness was 415 HV, which was superior to most of the reported typical wheel steel. During the process of wear, the surface damage of the wheel was mainly adhesive wear and fatigue damage, and the gradient strain layer (GS layer) was formed on the wheel surface. As the running speed increased, fatigue damage gradually became more serious than adhesive wear, and the shear stress and strain of the GS layer were enhanced. The higher thickness and hardening were produced on the GS layer, which is the main reason for the higher wear-resistance of the bainitic wheel under higher running speeds. In addition, the wear-resistance of the novel wheel steel was better than that of the reported wheel steel. This novel bainitic wheel is a promising wheel for heavy-haul condition applications, which could provide a guide in choosing bainitic wheel steel for the railway.


Introduction
Rolling contact fatigue and wear are the most important ways of causing train wheelrail failure [1][2][3][4], and it not only increases the transportation cost of the railway, but also directly endangers the safety of railways. Therefore, solving the failure problem caused by rolling contact fatigue wear has become the focus of research in the world today. With high-speed passenger lines and heavy-haul rail freight lines, the rolling contact fatigue damage between wheels and rails becomes more and more serious [5,6].
As one of the crucial components of the wheel-rail system, the wheel is required to transmit the force of the wheel-rail interface while carrying the load of the train [7]. The wheel is operated under strong frictional forces and different environmental coupling during rolling contact, and its frictional wear and fatigue damage behavior is bound to be more complex. Thus, the wear and rolling contact fatigue (RCF) resistance of wheel/rail has attracted much scientific interest from researchers. Guo et al. [8] investigated the effects of the slip ratio and contact pressure on the evolution of wear and damage of the CL60 wheel material were explored, which found that the wear and damage of the wheel material are milder under the wet conditions than under the dry conditions. Liu et al. [9] found that pre-wear resulted in an effective strengthening of the wheel surface, which improved the RCF life of wheel specimens under oil lubrication. Faccoli et al. [10] investigated the effect of desert sand on the wear and RCF performance of different wheel steels, which there are many studies on the damage behavior of wheel materials under harsh conditions, which provide valuable insights, extending the service life of wheel materials remains a challenge. Given the fact that bainitic steel is considered a potential alternative to pearlitic wheel/rail steel, the failure mechanism of bainitic wheel materials under harsh conditions has not been studied in great detail yet.
In view of the outstanding issue discussed above, the present work aims to extend the knowledge of the effect of running speed on the wear resistance and fatigue resistance of bainitic wheel steels. The wear and fatigue damage behavior of bainitic wheel steels was analyzed by wear rate, hardening rate, surface abrasion, fatigue cracking and plastic deformation of wheel specimens. This paper tends to provide a theoretical basis for exploring the mechanism of damage of new bainitic wheels, optimizing wheel materials, and improving the safety of trains in service.

Test Steel
The specimens used in the rolling-sliding wear experiment were two types of materials. The main specimen was taken from a novel bainitic wheel steel (BW), and the accompanying specimen was taken from a pearlitic rail steel (U75V, 310 HV, Chinese standard: GB/T 2585-2007). As illustrated in Figure 1a, the heat treatment process of the wheel steel consists of two main stages: Austenitizing and tempering. The specimens were heated to the austenitization temperature (900~920 • C) with a heating rate of 10 • C/s and held for 30 min under a vacuum. Afterwards, specimens were cooled (cooling rate: 5 • C/s) to room temperature and tempered for 2 h. Bainitic transformation occurred during continuous cooling. The cooling rate was enough to avoid the formation of other phases which exist prior to bainite transformation according to the CCT diagram ( Figure 1b). The chemical composition of the wheel steel is listed in Table 1. To provide insight into the yield strength and hardness of the novel BW presented in this study, which were listed in Table 2, alongside some representative wheel steel reported in previous studies ( Table 3). The yield strength and microhardness value of the BW are higher than those of most of the typical wheel steels from American, Chinese, and European Standards. suffered from damage with different degrees under various working conditions during the harsh service conditions. The wear rate of pearlitic materials increases with increasing axle load [7] and running speed [15], but decreases with the increase of curve radius. While there are many studies on the damage behavior of wheel materials under harsh conditions, which provide valuable insights, extending the service life of wheel materials remains a challenge. Given the fact that bainitic steel is considered a potential alternative to pearlitic wheel/rail steel, the failure mechanism of bainitic wheel materials under harsh conditions has not been studied in great detail yet.
In view of the outstanding issue discussed above, the present work aims to extend the knowledge of the effect of running speed on the wear resistance and fatigue resistance of bainitic wheel steels. The wear and fatigue damage behavior of bainitic wheel steels was analyzed by wear rate, hardening rate, surface abrasion, fatigue cracking and plastic deformation of wheel specimens. This paper tends to provide a theoretical basis for exploring the mechanism of damage of new bainitic wheels, optimizing wheel materials, and improving the safety of trains in service.

Test Steel
The specimens used in the rolling-sliding wear experiment were two types of materials. The main specimen was taken from a novel bainitic wheel steel (BW), and the accompanying specimen was taken from a pearlitic rail steel (U75V, 310 HV, Chinese standard: GB/T 2585-2007). As illustrated in Figure 1a, the heat treatment process of the wheel steel consists of two main stages: Austenitizing and tempering. The specimens were heated to the austenitization temperature (900~920 °C) with a heating rate of 10 °C/s and held for 30 min under a vacuum. Afterwards, specimens were cooled (cooling rate: 5 °C/s) to room temperature and tempered for 2 h. Bainitic transformation occurred during continuous cooling. The cooling rate was enough to avoid the formation of other phases which exist prior to bainite transformation according to the CCT diagram ( Figure 1b). The chemical composition of the wheel steel is listed in Table 1. To provide insight into the yield strength and hardness of the novel BW presented in this study, which were listed in Table 2, alongside some representative wheel steel reported in previous studies ( Table 3). The yield strength and microhardness value of the BW are higher than those of most of the typical wheel steels from American, Chinese, and European Standards. Figure 2 shows that the microstructure of the wheel steel, which consists of carbonenriched retained austenite (film-like) embedded in exceptionally fine plates of carbidefree bainitic ferrite, as the precipitation of carbides is suppressed by the high silicon content (more than 1.5 wt%).     Figure 2 shows that the microstructure of the wheel steel, which consists of carbonenriched retained austenite (film-like) embedded in exceptionally fine plates of carbide-free bainitic ferrite, as the precipitation of carbides is suppressed by the high silicon content (more than 1.5 wt%).

Microstructural Characterization
The microstructure of the specimens was characterized by optical microscope (OM, ZEISS Axiovert 40MAT, Oberkochen, Germany), field-emission scanning electron microscope (FESEM, Tescan MIRA3 XMU, Brno, Czech Republic), and transmission electron microscope (TEM; Tecnai G2 F20 S-TWIN, Hillsboro, OR, USA). Specimens for FESEM were prepared by mechanical polishing and then etched with 4% nitric acid alcohol. The cross-section of samples for microstructure observation was perpendicular to the wear surface and parallel to the rolling direction.

Dry Rolling-Sliding Wear Test
The rolling-sliding tests were performed on a twin-disc wear testing machine (Wear Tester, CQHH-RCF-1, Chongqing, China), which was previously used in wheel-rail contact studies under dry conditions. The test specimens were cylindrical rings with Ø60 mm outer diameter, Ø30 mm inner diameter and 20 mm thickness. The contact width between the two discs was 5 mm. The upper one machined from the wheel steel and the lower one machined from the standard rail steel. The schematic diagram of the tester is shown in Figure 3. The maximum contact stress between the two samples in point contact mode was calculated using Hertz contact theory and simulation criteria. The method is shown in Equation (1) [38]: where ρ is the curvature at the contact point between the twin disc determined by the sample radius size (unit: mm −1 ). σ max is the maximum contact stress (unit: MPa). α and β are point contact deformation coefficients determined by the auxiliary parameter cosτ, obtained from YB/T 5345-2014 standard [39]. F is the vertical load applied to the sample (unit: N). The operation condition of a heavy-haul train with the axle load of 35 t and the wheel diameter of 840 mm was simulated. The maximum contact stress simulated in the experiment between the wheel and rail was 1200 MPa. According to Equation (1), the pressure load was calculated to be 3035 N. The actual angular velocity of wheels (ω) could be calculated with the following formula: where ω wheel is the actual angular velocity of wheels, V represented the actual operation speed of heavy-haul trains (80, 120, 150 km/h), and R represented the radius of wheels. Therefore, the experimental rotational speed could be calculated in the following way: where N exp is the experimental rotational speed of wheels, and N wheel is the actual rotational speed of wheels. From Formula (3), the rotational speeds were 546, 740, and 950 r·min −1 , and the rolling cycles were 30,000. The slippage ratio of wheel/rail specimens was about 10%. All experiments were conducted at room temperature. The main test parameters are shown in Table 4.

Wear Resistance
The specific wear rate (SWR) is the volume loss per unit load and the distance traveled by a point on the perimeter during the wear test.
where V is volume loss, Sd is wearing distance, and FN is load. Figure 4 shows the wear loss of the wheel specimens with increasing running speed. The mass loss of wheel roller under higher rotational speed (80 km/h) is higher than that under lower speed (150 km/h), with the similar trend of the SWR. Therefore, it is clear that the wear loss of the wheel declines with the increase in running speed.  Figure 5 shows the microscopic morphology of the wear surface of the specimens at different speeds. It can be seen that the surface of the wheel specimen is dominated by adhesive wear and fatigue cracks. The cracks, delamination and peeling off of the specimen surface were observed at the low speed (80 km/h) ( Figure 5(a1,a2)). Cracks are caused by cyclic stresses expanding towards the interior of the matrix and folding back towards

Wear Resistance
The specific wear rate (SWR) is the volume loss per unit load and the distance traveled by a point on the perimeter during the wear test.
where V is volume loss, S d is wearing distance, and F N is load. Figure 4 shows the wear loss of the wheel specimens with increasing running speed. The mass loss of wheel roller under higher rotational speed (80 km/h) is higher than that under lower speed (150 km/h), with the similar trend of the SWR. Therefore, it is clear that the wear loss of the wheel declines with the increase in running speed.

Wear Resistance
The specific wear rate (SWR) is the volume loss per unit load and the distance traveled by a point on the perimeter during the wear test.
where V is volume loss, Sd is wearing distance, and FN is load. Figure 4 shows the wear loss of the wheel specimens with increasing running speed. The mass loss of wheel roller under higher rotational speed (80 km/h) is higher than that under lower speed (150 km/h), with the similar trend of the SWR. Therefore, it is clear that the wear loss of the wheel declines with the increase in running speed.  Figure 5 shows the microscopic morphology of the wear surface of the specimens at different speeds. It can be seen that the surface of the wheel specimen is dominated by adhesive wear and fatigue cracks. The cracks, delamination and peeling off of the specimen surface were observed at the low speed (80 km/h) ( Figure 5(a1,a2)). Cracks are caused by cyclic stresses expanding towards the interior of the matrix and folding back towards  Figure 5 shows the microscopic morphology of the wear surface of the specimens at different speeds. It can be seen that the surface of the wheel specimen is dominated by adhesive wear and fatigue cracks. The cracks, delamination and peeling off of the specimen surface were observed at the low speed (80 km/h) ( Figure 5(a1,a2)). Cracks are caused by cyclic stresses expanding towards the interior of the matrix and folding back towards the surface, in which cases spalling to form severe peeled blocks [7]. In addition, a small number of pits were found on the surface of the specimen, the wear mechanism of this surface specimen is not only fatigue wear and adhesive wear, but also a small amount of pitting fatigue flaking when the degree of wear is serious. As the speed increases, the surface of specimen became more densely packed with pitting pits and a large number of pockmarks appeared, indicating that the wear mechanism of specimen changes to a mainly pitting fatigue spalling at 120 km/h, where the wear was reduced, but the fatigue damage was increased ( Figure 5(b1,b2)). Finally, almost no pitting pits were observed on the specimen surface at 150 km/h, and there is a large amount of flattened puckering ( Figure 5(c1,c2)), which indicated that the specimen surface was dominated by fatigue wear.

The Morphology of Wear Surface
Metals 2023, 13, x FOR PEER REVIEW 7 of the surface, in which cases spalling to form severe peeled blocks [7]. In addition, a sm number of pits were found on the surface of the specimen, the wear mechanism of th surface specimen is not only fatigue wear and adhesive wear, but also a small amount pitting fatigue flaking when the degree of wear is serious. As the speed increases, t surface of specimen became more densely packed with pitting pits and a large number pockmarks appeared, indicating that the wear mechanism of specimen changes to mainly pitting fatigue spalling at 120 km/h, where the wear was reduced, but the fatig damage was increased ( Figure 5(b1,b2)). Finally, almost no pitting pits were observed the specimen surface at 150 km/h, and there is a large amount of flattened puckering (F ure 5(c1,c2)), which indicated that the specimen surface was dominated by fatigue wea

Microstructure of the Gradient Strain Layer
Plastic deformation of the material occurs under cyclical load, which gradually acc mulates and eventually forms a gradient strain layer on the wheel surface due to t ratchet effect [10,40,41]. Figure 6 shows the original morphology of plastic deformation

Microstructure of the Gradient Strain Layer
Plastic deformation of the material occurs under cyclical load, which gradually accumulates and eventually forms a gradient strain layer on the wheel surface due to the ratchet effect [10,40,41]. Figure 6 shows the original morphology of plastic deformation in the cross-section of the specimens. After the wear test, a gradient strain layer (GS layer) was formed on the surface of the specimen. As the speed increased, the plastic deformation of the wheel became more severe. With the running speeds increasing from 80 km/h to 150 km/h, the thickness of the GS layer (T GS ) increased from 42 µm to 110 µm. the cross-section of the specimens. After the wear test, a gradient strain layer (GS layer) was formed on the surface of the specimen. As the speed increased, the plastic deformation of the wheel became more severe. With the running speeds increasing from 80 km/h to 150 km/h, the thickness of the GS layer (TGS) increased from 42 μm to 110 μm. In order to further investigate the microstructure evolution of the wheel steel, the surface of the specimens can be divided into three zones, as shown in Figure 7. As shown in the above results, the topmost surface of the specimens was the severe plastic deformation (SPD) zone with the deepest bainite refinement, almost parallel to the surface of the specimens (Figure 7a (I)). The light plastic deformation (LPD) zone was closed to the matrix, with shallow plastic deformation and curved bainite flow lines, while the direction of the bainite slats in the matrix region was randomly arranged (Figure 7a (II)). Thus, the degree of bainite lath and the refined grain size gradually increase from the matrix to the topmost surface. Further observation of the topmost surface layer (SPD zone) showed that the bainite was extruded and fragmented into submicron grains and thin lath. The degree of grain refinement and grain size in the topmost surface layer of the wheel specimens were similar for different rotational speed conditions (Figure 7(a1,b1,c1)). In order to further investigate the microstructure evolution of the wheel steel, the surface of the specimens can be divided into three zones, as shown in Figure 7. As shown in the above results, the topmost surface of the specimens was the severe plastic deformation (SPD) zone with the deepest bainite refinement, almost parallel to the surface of the specimens (Figure 7a (I)). The light plastic deformation (LPD) zone was closed to the matrix, with shallow plastic deformation and curved bainite flow lines, while the direction of the bainite slats in the matrix region was randomly arranged (Figure 7a (II)). Thus, the degree of bainite lath and the refined grain size gradually increase from the matrix to the topmost surface. Further observation of the topmost surface layer (SPD zone) showed that the bainite was extruded and fragmented into submicron grains and thin lath. The degree of grain refinement and grain size in the topmost surface layer of the wheel specimens were similar for different rotational speed conditions (Figure 7(a1,b1,c1)).

The Distribution of Shear Strain and Hardness
The materials on the contact surfaces are subjected to positive and shear stresses [4 which will lead to a rheological structure. The degree of bending of the bainite can oft be used to reflect the intensity of plastic deformation [43]. The shear strain can be calc lated by the displacement field of the plastic flow line, which will reflect the degrees the bending of bainitic lath, as shown in Figure 8a. The calculation equation of equivale shear strain is as follows [44]:

The Distribution of Shear Strain and Hardness
The materials on the contact surfaces are subjected to positive and shear stresses [42], which will lead to a rheological structure. The degree of bending of the bainite can often be used to reflect the intensity of plastic deformation [43]. The shear strain can be calculated by the displacement field of the plastic flow line, which will reflect the degrees of the bending of bainitic lath, as shown in Figure 8a. The calculation equation of equivalent shear strain is as follows [44]: where θ is the angle of shear at different depths on the plastic flow line, ε is the equivalent shear strain, and tan(θ) is the slope of the tangent line of the rheological curve.
y(x) =y 0 +A × exp Rx (6) where y(x) is the shear stain, x is the depth from surface, y 0 , and A and R are parameters.
where θ is the angle of shear at different depths on the plastic flow line, ε is the equivalent shear strain, and tan(θ) is the slope of the tangent line of the rheological curve.
y(x) = + A × exp Rx (6) where y(x) is the shear stain, x is the depth from surface, y0, and A and R are parameters. According to Equations (5) and (6), the distribution of the shear strain along the depth from the surface (Ds) under different rotational speed conditions can be given in Figure  8b. It can be found that the shear stain (ε) and depth (Ds) of the plastic flow line displacement field satisfy the exponential relationship. As the Ds increases, the shear strain decreased monotonically at the surface and it finally tended to 0 in the matrix. Compared to 80 km/h, the shear strain increased significantly at 150 km/h. The hardening of the wheel specimens was analyzed using microhardness. The hardness distribution of the gradient strain layer is shown in Figure 9, which also satisfy the exponential relationship according to the Equation (6). The hardness of the specimen profile was distributed in a gradient, which was similar to the shear strain curve (Figure 8b). Furthermore, the hardening rate of the GS layer was ~1.08 times higher at high running speeds (150 km/h) than at low running speeds (80 km/h).  According to Equations (5) and (6), the distribution of the shear strain along the depth from the surface (D s ) under different rotational speed conditions can be given in Figure 8b. It can be found that the shear stain (ε) and depth (Ds) of the plastic flow line displacement field satisfy the exponential relationship. As the D s increases, the shear strain decreased monotonically at the surface and it finally tended to 0 in the matrix. Compared to 80 km/h, the shear strain increased significantly at 150 km/h. The hardening of the wheel specimens was analyzed using microhardness. The hardness distribution of the gradient strain layer is shown in Figure 9, which also satisfy the exponential relationship according to the Equation (6). The hardness of the specimen profile was distributed in a gradient, which was similar to the shear strain curve (Figure 8b). Furthermore, the hardening rate of the GS layer was~1.08 times higher at high running speeds (150 km/h) than at low running speeds (80 km/h).
where θ is the angle of shear at different depths on the plastic flow line, ε is the equivalent shear strain, and tan(θ) is the slope of the tangent line of the rheological curve. (6) where y(x) is the shear stain, x is the depth from surface, y0, and A and R are parameters. According to Equations (5) and (6), the distribution of the shear strain along the depth from the surface (Ds) under different rotational speed conditions can be given in Figure  8b. It can be found that the shear stain (ε) and depth (Ds) of the plastic flow line displacement field satisfy the exponential relationship. As the Ds increases, the shear strain decreased monotonically at the surface and it finally tended to 0 in the matrix. Compared to 80 km/h, the shear strain increased significantly at 150 km/h. The hardening of the wheel specimens was analyzed using microhardness. The hardness distribution of the gradient strain layer is shown in Figure 9, which also satisfy the exponential relationship according to the Equation (6). The hardness of the specimen profile was distributed in a gradient, which was similar to the shear strain curve (Figure 8b). Furthermore, the hardening rate of the GS layer was ~1.08 times higher at high running speeds (150 km/h) than at low running speeds (80 km/h).

Fatigue Cracks Damage
When there is an inhomogeneous plastic deformation locally on the material surface, the ratcheting effect occurs as the plastic deformation accumulates, eventually leading to crack sprouting [45][46][47]. As can be seen above, as the running speed increases, the thickness of the GS layer on the surface of material and the degree of accumulation of plastic deformation gradually increased [48]. Figure 10 shows the fatigue cracking of the specimens at different running speeds. When the accumulation of plastic deformation reaches its limit, fatigue cracks start to appear on the surface or subsurface of material, forming a variety of cracks, such as main cracks, multilayer cracks, subsurface cracks, and branching cracks, and propagating along the matrix in the direction of plastic deformation [49]. However, these cracks do not always increase in size due to the amount of deformation and the crack driving force [50]. Branching cracks arose at the base of the main crack and propagate along the ferrite line parallel at an angle of deviation. Additionally, the materials above the cracked prevented the internal cracked materials from contacting the counter-frictional substrate, but they were easily crushed and broken under cyclic load (Figure 10(c1)).

Fatigue Cracks Damage
When there is an inhomogeneous plastic deformation locally on the material surface, the ratcheting effect occurs as the plastic deformation accumulates, eventually leading to crack sprouting [45][46][47]. As can be seen above, as the running speed increases, the thickness of the GS layer on the surface of material and the degree of accumulation of plastic deformation gradually increased [48]. Figure 10 shows the fatigue cracking of the specimens at different running speeds. When the accumulation of plastic deformation reaches its limit, fatigue cracks start to appear on the surface or subsurface of material, forming a variety of cracks, such as main cracks, multilayer cracks, subsurface cracks, and branching cracks, and propagating along the matrix in the direction of plastic deformation [49]. However, these cracks do not always increase in size due to the amount of deformation and the crack driving force [50]. Branching cracks arose at the base of the main crack and propagate along the ferrite line parallel at an angle of deviation. Additionally, the materials above the cracked prevented the internal cracked materials from contacting the counter-frictional substrate, but they were easily crushed and broken under cyclic load ( Figure  10(c1)).

Discussion
With the running speed increasing from 80 km/h to 150 km/h, the thickness of the GS layer (TGS) increases from 42 to 110 μm and the hardening rate from 1.24 to 1.35, as shown in Figure 11. To provide insight into the wear-resistance of rolling-sliding wear of the novel BW presented in this study, alongside some representative wheel steel reported in previous studies (Table 5) [15,25,51,52]. The specific wear rate of the test steel was 1.01 (80 km/h), 0.88 (80 km/h) and 0.85 (80 km/h), which was one of the lowest compared to the pearlitic wheel ( Figure 12). Additionally, opposite to the pearlitic wheel [15], the wear rate of the novel bainite wheel declines with the increase in running speed. It is well known that wear loss is a crucial parameter measuring the wear-resistance properties of wheel/rail materials [20,22,25].

Discussion
With the running speed increasing from 80 km/h to 150 km/h, the thickness of the GS layer (T GS ) increases from 42 to 110 µm and the hardening rate from 1.24 to 1.35, as shown in Figure 11. To provide insight into the wear-resistance of rolling-sliding wear of the novel BW presented in this study, alongside some representative wheel steel reported in previous studies (Table 5) [15,25,51,52]. The specific wear rate of the test steel was 1.01 (80 km/h), 0.88 (80 km/h) and 0.85 (80 km/h), which was one of the lowest compared to the pearlitic wheel ( Figure 12). Additionally, opposite to the pearlitic wheel [15], the wear rate of the novel bainite wheel declines with the increase in running speed. It is well known that wear loss is a crucial parameter measuring the wear-resistance properties of wheel/rail materials [20,22,25]. fined due to the SPD. The refinement of bainitic is the essential reason for the strengthening of microhardness, according to Equation (7). As the running speed increased, the shear stress [55] and strain of the GS layer were enhanced. The higher thickness and hardening were produced on the GS layer under higher running speed (Figure 11), which leads to the improvement of the wear resistance of the novel bainitic wheel steel. The information of specific wear rates could provide a guide in choosing bainitic wheel steel for the railway.     A relationship between the crack propagation depth and the crack propagation angle with the running speed is shown in Figure 13. The crack propagation angle and depth were at a minimum of ~11.4° and ~1.1 μm, respectively at 80 km/h, where the end of the crack appeared to buckle upwards. The crack was relatively small and was easy to spall. As the speed increases to 120 km/h, the crack propagation angle and crack propagation depth increase to ~13.1° and ~1.5 μm. At 150 km/h, the crack propagation angle and depth of crack propagation were at a maximum of ~16.8° and ~4.2 μm, respectively, where wear and fatigue damage were both particularly severe. If further expansion occurs, a fatigue fracture will occur, leading to the peeling of the wheel surface material. Thus, as the speed increases, the RCF cracks propagate along the line of plastic deformation, which leads to a gradual increase in the crack propagation depth and the crack propagation angle. When subjected to continued alternating stresses, the cracks propagated into the interior of the material, resulting in severe fatigue damage to the wheel.
Normally, the wear has connected with the RCF damage. In the rolling-sliding wear tests, the wear was indicated by the mass losses of specimens, while the RCF damage was characterized by the cracks in the surface layer. In addition, the fracture of fatigue cracks Previous studies have shown that the grain refinement due to the plastic deformation can improve the mechanical properties of materials according to the Hall-Petch strengthening mechanism [53]. In this study, the strengthening hardness of the gradient strain layer can be analyzed according to the classic Hall-Petch relation [54]: where σ is the yield strength, σ 0 is material constant, K is the Hall-Petch coefficient, and d is the grain size of prior austenite. With the formation of the gradient strain layer of wheel specimens, the grade of plastic deformation of bainite decreased gradually from surface to matrix. The grain was refined due to the SPD. The refinement of bainitic is the essential reason for the strengthening of microhardness, according to Equation (7). As the running speed increased, the shear stress [55] and strain of the GS layer were enhanced. The higher thickness and hardening were produced on the GS layer under higher running speed (Figure 11), which leads to the improvement of the wear resistance of the novel bainitic wheel steel. The information of specific wear rates could provide a guide in choosing bainitic wheel steel for the railway.
A relationship between the crack propagation depth and the crack propagation angle with the running speed is shown in Figure 13. The crack propagation angle and depth were at a minimum of~11.4 • and~1.1 µm, respectively at 80 km/h, where the end of the crack appeared to buckle upwards. The crack was relatively small and was easy to spall. As the speed increases to 120 km/h, the crack propagation angle and crack propagation depth increase to~13.1 • and~1.5 µm. At 150 km/h, the crack propagation angle and depth of crack propagation were at a maximum of~16.8 • and~4.2 µm, respectively, where wear and fatigue damage were both particularly severe. If further expansion occurs, a fatigue fracture will occur, leading to the peeling of the wheel surface material. Thus, as the speed increases, the RCF cracks propagate along the line of plastic deformation, which leads to a gradual increase in the crack propagation depth and the crack propagation angle. When subjected to continued alternating stresses, the cracks propagated into the interior of the material, resulting in severe fatigue damage to the wheel.

Conclusions
In this work, a novel bainitic wheel steel with a superior rolling-sliding wear-resistance was designed and prepared. The following main conclusions can be drawn: (1) The novel bainitic wheel steel consists of carbide-free bainite and film-like retained austenite and exhibited outstanding mechanical properties with a high yield strength of 950 MPa and a hardness of 415 HV, which were superior to those of most of the reported typical pearlitic wheel steel. (2) During the process of wear, the surface damage of the wheel was mainly adhesive wear and fatigue damage, and the gradient strain layer (GS layer) was formed on the wheel surface. With the increasing running speed, the fatigue cracks on wheel specimens were more serious and adhesive wear lightens, and the surface morphology of the wheel turned from pitting pits to peeling. (3) As the running speed increased, the shear stress and strain of the GS layer were enhanced. The higher thickness and hardening were produced on the GS layer under higher running speed, which led to the improvement of the wear resistance of the novel bainitic wheel steel. This is the main reason for the wear rate of the bainite wheel decreasing with increasing running speed. (4) The novel bainitic wheel steel exhibited an excellent wear-resistance after rollingsliding wear, which was much better than that of most of the reported representative pearlitic wheel steel. Thus, the novel bainitic wheel is a very promising wheel material for heavy-haul railway applications. Normally, the wear has connected with the RCF damage. In the rolling-sliding wear tests, the wear was indicated by the mass losses of specimens, while the RCF damage was characterized by the cracks in the surface layer. In addition, the fracture of fatigue cracks would lead to the formation of wear debris, which could eventually result in material loss. Peeling due to fatigue is one of the forms of wear [56]. Thus, RCF damage can lead to wear. In a rolling-sliding wear test at a low running speed, the main wear mechanism was adhesion wear on the surface of the wheel with a small number of pits. As the speed increases, the density of cracks gradually decreases, but the length increases. The fatigue damage increases as the speed increases and adhesion wear cannot effectively eliminate cracks [57].

Conclusions
In this work, a novel bainitic wheel steel with a superior rolling-sliding wear-resistance was designed and prepared. The following main conclusions can be drawn: (1) The novel bainitic wheel steel consists of carbide-free bainite and film-like retained austenite and exhibited outstanding mechanical properties with a high yield strength of 950 MPa and a hardness of 415 HV, which were superior to those of most of the reported typical pearlitic wheel steel. (2) During the process of wear, the surface damage of the wheel was mainly adhesive wear and fatigue damage, and the gradient strain layer (GS layer) was formed on the wheel surface. With the increasing running speed, the fatigue cracks on wheel specimens were more serious and adhesive wear lightens, and the surface morphology of the wheel turned from pitting pits to peeling. (3) As the running speed increased, the shear stress and strain of the GS layer were enhanced. The higher thickness and hardening were produced on the GS layer under higher running speed, which led to the improvement of the wear resistance of the novel bainitic wheel steel. This is the main reason for the wear rate of the bainite wheel decreasing with increasing running speed. (4) The novel bainitic wheel steel exhibited an excellent wear-resistance after rollingsliding wear, which was much better than that of most of the reported representative pearlitic wheel steel. Thus, the novel bainitic wheel is a very promising wheel material for heavy-haul railway applications.

Data Availability Statement:
All data that support the findings of this study are included within the article.