- Email: [email protected]
Investigation on the rolling wear and damage properties of laser discrete quenched rail material with different quenching shapes and patterns
Surface & Coatings Technology 378 (2019) 124991
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Investigation on the rolling wear and damage properties of laser discrete quenched rail material with different quenching shapes and patterns
T
H.H. Dinga,b, C.R. Sub, W.J. Wanga,b, , Z.B. Caib, D.Z. Wangc, J. Guoa, Q.Y. Liua,b, Z.R. Zhoua,b ⁎
a
Tribology Research Institute, State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, China c Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China b
ARTICLE INFO
ABSTRACT
Keywords: Laser discrete quenching Rail material Wear Damage Fatigue crack
The influence of laser discrete quenching (LDQ) on the rolling wear and damage properties of the U75V rail material was investigated. Different quenching shapes (including spot, strip, and grid shapes) and different quenching patterns (including different diameters of quenching spots, distances between adjacent spots, and directions of quenching strips) were studied. The results indicated that LDQ created martensite regions with an average hardness of around 927 HV0.5 on the rail surface. LDQ improved the wear resistance of rail material. On the untreated rail roller and in the untreated zones on treated rail rollers, cracks propagated with small angles of around 4–17° and small depth of around 11–22 μm. In the quenching regions, cracks propagated with large angles of around 58–80° and large depth of around 73–263 μm. The wear resistance and fatigue resistance of treated rail rollers were improved with the increase in the diameter of quenching spots and the decrease in the distance between adjacent spots. The wear resistance and fatigue resistance of treated rail with quenching grid were better than treated rail with quenching strips.
1. Introduction With the rapid development of high-speed and heavy-haul railways, railway transportation is becoming increasingly important for the modern life. As the service time increases, the wear and damage of wheel and rail materials become more serious. Wear and rolling contact fatigue (RCF) are considered to be two of the main failure mechanisms for wheels and rails [1–4]. These surface defects could decrease the service life and threaten the running safety of railway systems [5–7]. Therefore, attempts should be made to improve the wear resistance, alleviate the surface damage, and then extend the service life of wheel/ rail. Laser quenching (LQ) is a promising technology that has already been applied in various industries including automotive, power generation, and general engineering [8–12]. It is an effective way to improve surface properties that involves scanning the material surface with a high energy laser beam, and then, martensitic phase transformation occurs due to rapid self-cooling and solidification [13–19]. But when the scanning area is greater than the laser spot, multiple overlapping regions for laser hardening trajectories are indispensable. The major problem in this case is the formation of soft zones caused by back-tempering among overlapped laser tracks, which may limit wear
⁎
resistance of components [20]. In order to solve this problem, laser discrete quenching (LDQ) has been proposed, which creates isolated laser hardening spots distributed in a certain pattern on a component surface [21]. LDQ has been applied into wheel and rail treatment in some previous works [22–25]. Zeng et al. [23] compared the wear resistance of untreated and LDQ treated wheel steel and found that the wear rate of the LDQ treated wheel steel was about 0.3 times that of untreated wheel steel. Zheng et al. [24] investigated the influence of the size of quenching spot on the wear and damage properties of rail material. With the increase in the size of quenching spot, the wear resistance of rail was improved but the fatigue resistance got worse. Cao et al. [25] found that the wear resistance and surface damage of the wheel and rail materials deteriorated with the increase in the LDQ spacing (i.e. the distance between two adjacent quenching spots). Furthermore, the LDQ (spot-like quenching) has been practically applied in the field on rails in Wuhan Railway Bureau of China, and the service life of the treated rails was improved by 2.2–3.5 times [24]. Reviewing previous works, researchers mainly focused on the spotlike quenching zones. However, very limited information is available on the influence of the shape of quenching zone on the wear and damage of wheel and rail materials. Thus, the objective of the present study is to
Corresponding author at: Tribology Research Institute, State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China. E-mail address: [email protected] (W.J. Wang).
https://doi.org/10.1016/j.surfcoat.2019.124991 Received 5 August 2019; Received in revised form 2 September 2019; Accepted 12 September 2019 Available online 18 November 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
discussed in detail. 2. Experimental details 2.1. Experimental machine and samples Wear tests were carried out under the dry condition using a rolling–sliding wear testing machine (MJP-30A, China) [25]. The schematic outline of the machine is shown in Fig. 1. The wheel roller (lower specimen) and rail roller (upper specimen) were powered and controlled by two servo motors, respectively. The normal force was applied between wheel and rail rollers. The wheel and rail rollers were cut from the C-class wheel and the U75V rail, which are typical wheel and rail materials used in China. The sampling positions and sizes of rollers are shown in Fig. 2. The diameter of wheel and rail rollers is 60 mm and the contact width is 10 mm. The chemical compositions of wheel and rail materials are given in Table 1. The rail rollers were LDQ treated using a fiber laser (YLR-6000, IPG) as shown in Fig. 3. It was a high-power single-mode ytterbium fiber laser. The wavelength of the output laser was 1.07 μm and the maximum power of the laser could reach 50 kW. In order to investigate the effect of quenching shapes and patterns on the wear and damage behaviors of rail material, rail rollers were treated with different quenching shapes (spot, stripe, and grid) and different quenching patterns, as shown in Fig. 4. For the spot-shaped quenching (Fig. 3a–f), four different diameters of spots (diameter Φ = 3, 4, 6, 8 mm, respectively, as shown in Fig. 3a–d) and three different quenching spacings (distance between two adjacent spots D = 0, 1, 2 mm, respectively, as shown in Fig. 3e, a and f) were studied. During the quenching process, different powers of the fiber laser (around 700, 820, 1450, 2000 W, respectively) were used to achieve different diameters of quenching spots (around 3, 4, 6, 8 mm, respectively). The laser radiation lasted for around 0.7 s for each quenching spot. For the stripshaped quenching (Fig. 3g–i), three different distribution directions of strips (parallel, vertical or oblique to the rolling direction of the rail roller) were studied. During the quenching process, the power of the fiber laser was around 650 W and the scanning speed was around 400 mm/min. After quenching, the width of the quenching strips was around 2 mm and the interval between two strips was also around 2 mm. For the grid-shaped quenching (Fig. 4j), both the parallel and vertical stripes were quenched on the rail roller surface. Table 2 shows the quenching parameters and the dimensions of the quenching zones. It is clear that some rail rollers surfaces (e.g., Fig. 4g and j) became rougher after laser treatment. This is probably because the surfaces were melt locally during the laser scanning [26]. Thus, all the treated rail rollers were polished to be smooth before wear tests. The surfaces of treated rollers were composed of laser-treated zones (spots, strips or grid) and untreated zones (surfaces between laser-treated zones), as shown in Fig. 4. The wheel rollers were untreated.
Fig. 1. Schematic outline of the test rig.
Fig. 2. Sampling positions and sizes of wheel and rail rollers. Table 1 Chemical compositions of wheel and rail materials (wt%). Material
C
Si
Mn
P
S
C-class wheel U75V rail
0.67–0.77 0.71–0.80
0.15–1.00 0.50–0.80
0.60–0.90 0.70–1.50
0.030 ≤0.030
0.005–0.040 ≤0.030
Fig. 3. The schematic of the laser discrete quenching.
investigate the tribological performances of the LDQ treated rails with different quenching shapes and patterns for achieving a better result. Firstly, the U75V rail material was treated by LDQ. Different quenching shapes (spot, stripe, and grid) and different quenching patterns were studied. For the spot-shaped quenching, four different diameters of spots and three different quenching spacings (i.e. distances between two adjacent spots) were adopted. For the strip-shaped quenching, three different distribution directions of strips (parallel, vertical or oblique to the rolling direction of the wheel/rail interface) were studied. For the grid-shaped quenching, both the parallel and vertical stripes were quenched on the rail surface. Secondly, wear tests were conducted using the LDQ treated rail specimens with different quenching shapes and patterns. Based on the comparison of the wear rates and damage behaviors of rail specimens, the optimization of the quenching shapes and patterns could be determined for the application of LDQ to rails, which could improve the service life of the rails in the field. Furthermore, the damage mechanism of quenching region was
2.2. Experimental conditions All tests were carried out under dry conditions at room temperature. The normal force between wheel/rail rollers was 5110 N, leading to the maximum contact pressure of around 1100 MPa, which simulated the axle load of 14 t in the field according to the Hertzian simulation rule [23]. The rotational speed of the lower roller (wheel roller) was 500 r/ min, and the slip ratio of wheel/rail rollers was 0.75% (slip ratio means the difference in speed between wheel/rail rollers). The number of cycles of the lower roller was 80,000. All wear tests were performed twice. The wheel/rail rollers were ultrasonically cleaned in ethanol, dried, and weighed using an electronic balance (JA4103, accuracy: 0.0001 g) before and after testing. The wear rate (μg/m) of a roller was calculated by mass loss (μg) per rolling distance (m). The hardness of treated and 2
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 4. Different quenching shapes and patterns on rail rollers: (a) spot-shape with diameter Φ = 3 mm and LDQ spacing D = 1 mm; (b) spot-shape with Φ = 4 mm and D = 1 mm; (c) spot-shape with Φ = 6 mm and D = 1 mm; (d) spot-shape with Φ = 8 mm and D = 1 mm; (e) spot-shape with Φ = 3 mm and D = 0 mm; (f) spotshape Φ = 3 mm and D = 2 mm; (g) strip-shape with strips parallel to the rolling direction; (h) strip-shape with strips vertical to the rolling direction; (i) strip-shape with strips oblique to the rolling direction; (j) grid-shape.
untreated rollers was measured using a Vickers hardness instrument (MVK-H21, Japan). The cross sections of rollers were ground to 2000 grit, polished to 0.5 μm diamond, etched with 5% natal (i.e., 5 ml nitric acid + 95 ml ethanol). The wear scars and cross sections were observed using an optical microscopy (OM) (OLYMPUS BX60 M, Japan) and a scanning electronic microscopy (SEM) (JSM-7001F, Japan).
3. Results 3.1. Microstructures and hardness of laser quenched rail Fig. 5 shows SEM micrographs of a cross section of a laser quenched rail. The cross section consisted of two different regions, i.e. quenching 3
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Table 2 Quenching parameters and corresponding dimensions of the quenching zones. Dimensions of quenching zones
1 2 3 4 5
Quenching parameters
Shape
Diameter/mm
Width/mm
Maximum depth/mm
Laser power/W
Lasting time/s
Scanning speed/mm/min
Spot Spot Spot Spot Strip
≈3 ≈4 ≈6 ≈8 –
– – – – ≈2
≈0.38 ≈0.50 ≈0.94 ≈1.11 ≈0.63
700 820 1450 2000 650
0.7 0.7 0.7 0.7 –
– – – – 400
Fig. 5. SEM micrographs of (a) the cross section of laser quenched rail, (b) the quenching region, (c) the interface between quenching region and substrate region, and (d) the substrate region.
region and substrate region. The quenching region was crescentshaped. It is clear from Fig. 5b that the quenching region was mainly composed of martensite owing to the rapid self-cooling and high solidification rates during laser treatment [13,27]. The substrate region presented a typical microstructure of pearlite rail material. Fig. 5c indicates the intimately bonded interface between the substrate region and the quenching region. Fig. 6a shows the surface hardness of rail material before and after laser quenching. Before laser quenching, the surface hardness of rail was around 329 HV0.5. After laser quenching, the surface hardness of the laser-treated zone was increased by 2.8 times to around 927 HV0.5. The improved hardness was mainly attributed to the martensite structure produced during laser quenching [23]. For the untreated zone between laser-treated zones, the surface hardness was similar to the rail hardness before laser quenching. Fig. 6b shows the hardness measured on the cross section of laser quenched rail from the surface to the depth.
In the quenching region, the hardness was high and stable. The hardness value was similar to that obtained on the surface of laser-treated zone. With the increase in depth, the hardness showed a sharp decrease at the boundary between the quenching region and the substrate region. In the substrate region, the hardness value was similar to that of the rail material before quenching. 3.2. Wear rates The wear rates of wheel and rail rollers are shown in Fig. 7. For the test with untreated rollers, the wear rate of rail roller was around 39 μg/m and the wear rate of wheel roller was around 31 μg/m. For tests with laser treated rail rollers, the wear rates of rail rollers were decreased, which means that LDQ could improve the wear resistance of rail material. However, the wear rates of the untreated counterbodies (i.e. untreated wheel rollers) were increased.
4
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 6. (a) Surface hardness of rail before and after laser quenching, and (b) hardness on the cross section of the laser quenched rail.
For the spot-shaped quenching (Fig. 6a and b), the quenching spot diameter and the LDQ spacing had influence on the wear rates of wheel/rail rollers. With the increase in the diameter of quenching spots (Fig. 7a), the wear rate of treated rail rollers showed a decrease trend, the wear rate of the wheel roller had no obvious change, and the total wear rate of wheel/rail rollers was decreased. With the increase in the LDQ spacing (Fig. 7b), the wear rate of treated rail roller showed an increase trend, the wear rate of the wheel roller showed a decrease trend, and the total wear rate of wheel/rail rollers showed no obvious change. For the strip-shaped quenching (Fig. 7c), the distribution direction of quenching strips had influence on the wear rates of wheel/rail rollers. The wear rate of treated rail roller with quenching strips parallel to the rolling direction was larger than vertical or oblique to the rolling direction. The wear rate of treated rail roller with quenching strips vertical to the rolling direction was similar to oblique to the rolling direction. Concerning the wheel wear rate and the total wear rate, they showed a decrease trend for tests with quenching strips parallel, vertical or oblique to the rolling direction, respectively. For the grid-shaped quenching (Fig. 7c), both the rail wear rate and the wheel wear rate were smaller than those for spot-shaped and stripshaped quenching. The wear rate of the grid quenched rail roller was around 11 μg/m (0.28 times that of untreated rail roller) and the wear rate of the wheel roller was around 36 μg/m.
Fig. 7. Wear rates of wheel/rail rollers: (a) spot-shaped quenching with different spot diameters; (b) spot-shaped quenching with different LDQ spacings; (c) strip-shaped and grid-shaped quenching.
3.3. Surface damage behaviors The worn surfaces of wheel and rail rollers were observed after wear testing. Fig. 8 shows the worn surfaces of the untreated wheel/untreated rail rollers. The surface damage of untreated wheel and untreated rail rollers was mild and similar. The worn surfaces were dominated by pitting and spalling.
5
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 8. Worn surfaces of (a) untreated wheel roller and (b) untreated rail roller.
Fig. 9 shows the worn surfaces of the laser treated rail rollers with different quenching shapes and patterns. For the rail rollers treated with quenching spots (Fig. 8a–f), obvious cracks were observed at the boundary between treated and untreated zones and in the treated zone near the boundary. Furthermore, with the increase in the diameter of quenching spot (Fig. 8a–d), the cracks were alleviated. For example, cracks observed on the treated rail roller with diameter Φ = 8 mm (Fig. 9d) were milder than observed on the rollers with smaller quenching spots (Fig. 8b and c). With the increase in the LDQ spacing (Fig. 8e, a and f, respectively), the cracks became more serious. For example, cracks observed on the treated rail roller with the LDQ spacing D = 0 mm (Fig. 9e) were less and milder than observed on the rollers with larger distances (Fig. 8a and f). For the rail rollers treated with quenching strips (Fig. 8g–i), cracks were also observed in the treated zone. The cracks on the treated rail rollers with vertical and oblique quenching strips (Fig. 8h and i) were more obvious than those on the rail roller with parallel quenching strips (Fig. 9g). For the rail roller treated with quenching grid (Fig. 9j), the cracks were less and milder than most of the treated rollers with quenching spots and quenching strips. In addition, grooves were generated in the rolling direction in the untreated zones, as shown in Fig. 8a, f and g. This is because the untreated zone is softer than the treated zone. The softer untreated zone led to the larger wear loss and then the groove on the rail roller. Fig. 10 shows the representative worn surfaces of the wheel rollers which rolled against the treated rail rollers. The surfaces of wheel rollers were dominated by spalling. Furthermore, a ridge was observed on the wheel roller (Fig. 10b) which was in correspondence with the groove generated on the rail roller (Fig. 9f). Fig. 11 shows the plastic deformation observed on the cross sections of rail rollers after testing. For the untreated rail roller (Fig. 11a), the plastic deformation layer reached around 160 μm. For the treated rail rollers, the plastic deformation of the quenching region was negligible, and the plastic deformation of the substrate region was obviously decreased. For example, on the treated rail roller with quenching strips vertical to the rolling direction (Fig. 11b), the thickness of the plastic deformation layer of the substrate region was around 45 μm.
Fig. 12 shows the cracks on the untreated wheel/untreated rail rollers. Cracks were shallow and the angle of cracks was small. This kind of cracks was easy to be worn off during the rolling/sliding process. Fig. 13 shows the cracks on the cross sections of laser treated rail rollers with different quenching shapes and patterns. Obvious fatigue cracks were observed in the quenching regions and the angles of cracks were large even reaching around 90° in many cases. Furthermore, cracks tended to generate at the edge of the quenching region, this was mainly due to the small thickness at the edge. The quenching shapes and patterns had significant influence on the fatigue damage of quenching regions. For the rail rollers treated with quenching spots (Fig. 12a–f), the fatigue damage was influence by the diameter of quenching spots and the LDQ spacing. With the increase in the diameter of quenching spots (Fig. 12a–d), the fatigue damage was alleviated. Specifically, with large spot diameters (Φ = 4, 6, and 8 mm, as shown in Fig. 12b–d), the fatigue damage was relatively mild. Only one or two fatigue cracks were generated at the edge of the quenching region. The fatigue cracks stopped near the boundary between quenching and substrate regions. For the rail roller with the small quenching spot diameter (Φ = 3 mm, as shown in Fig. 12a), the fatigue damage was more serious. More cracks were generated in the quenching region. The growth direction of the crack was changed at the bottom of the quenching region to be along the boundary. Furthermore, branch cracks were developed in the quenching region (Fig. 13a). With the increase in the LDQ spacing (Fig. 12e, a and f, respectively), the fatigue damage of quenching regions became more serious. Specifically, for the treated rail roller with the LDQ spacing D = 0 mm (Fig. 13e), only a single small fatigue crack was observed in the quenching region. For the rollers with larger spacings (D = 1 and 2 mm, as shown in Fig. 12a and f), more fatigue cracks were generated. Furthermore, for the roller with the largest spacing of 2 mm (Fig. 13f), severe spalling occurred in the quenching region due to the connection of the fatigue cracks. In summary, for the spot-shaped quenching, rail rollers treated with large quenching spots (Φ = 8 mm, Fig. 13d) or with small DLQ spacing (D = 0 mm, Fig. 13e) exhibited better anti-fatigue properties. For rail rollers treated with quenching strips (Fig. 12g–i), two or more fatigue cracks were generated at the edge of the quenching region. For the rail roller with quenching strips vertical to the rolling direction (Fig. 13h), spalling also occurred in the quenching region. Fig. 13j shows the cross section of the rail roller treated with quenching grid. Only small cracks were observed in the quenching region. The fatigue damage of the quenching grid (Fig. 13j) was milder than that of the
3.4. Fatigue cracks on cross sections In order to further understand the damage behaviors of the wheel and rail materials, cracks were observed on the cross sections of rollers.
6
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 9. Worn surfaces of laser treated rail rollers with different quenching shapes and patterns: (a) spot-shape with diameter Φ = 3 mm and LDQ spacing D = 1 mm; (b) spot-shape with Φ = 4 mm and D = 1 mm; (c) spot-shape with Φ = 6 mm and D = 1 mm; (d) spot-shape with Φ = 8 mm and D = 1 mm; (e) spot-shape with Φ = 3 mm and D = 0 mm; (f) spot-shape Φ = 3 mm and D = 2 mm; (g) strip-shape with strips parallel to the rolling direction; (h) strip-shape with strips vertical to the rolling direction; (i) strip-shape with strips oblique to the rolling direction; (j) grid-shape.
7
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 10. Representative worn surfaces of wheel rollers which rolled against the treated rail rollers: (a) wheel roller against the rail roller with quenching spot of Φ = 8 mm and D = 1 mm; (b) wheel roller against the rail roller with quenching spot of Φ = 3 mm and D = 2 mm.
Fig. 11. Plastic deformation of (a) the untreated rail roller and (b) the treated rail roller with quenching strips vertical to the rolling direction.
Fig. 12. Cracks on longitudinal cross sections of (a) untreated wheel roller and (b) untreated rail roller.
quenching strips (Fig. 12g–i). In order to evaluate the fatigue damage of rail rollers, the statistical data of cracks (including the crack angle and crack depth observed though OM) in the quenching and substrate regions are shown in Table 3. On the untreated rail roller, the crack angle and the crack depth were small. The average crack angle was around 8° and the average crack depth was around 13 μm. On the treated rail rollers, the crack angle and the crack depth were also small in the substrate regions. The average crack angle was in the range of around 4–17° and the average crack depth was in the range of around 11–22 μm.
However, the crack angle and the crack depth were significantly large in the quenching regions. The average crack angle reached around 58–80° and the average crack depth reached around 73–263 μm. In order to further understand the damage behavior of laser quenched rail roller, the crack tips near the boundary between quenching and substrate regions were observed using SEM, as shown in Fig. 14. In general, the crack propagation resistance is influenced by the properties of localized material at the crack tip. Cracks propagate when the accumulated energy exceeds the limit of the resistance [28]. For the treated rail roller whose fatigue damage was mild (e.g. quenching spot
8
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
with Φ = 8 mm and D = 1 mm, as shown in Fig. 14a), the crack stopped inside the quenching region near the boundary. For the treated rail roller whose fatigue damage was relatively severe (e.g. quenching spot with Φ = 3 mm and D = 1 mm, as shown in Fig. 14b), the growth direction of crack was changed in the quenching region to be parallel to the boundary. The crack did not develop into the substrate region. For the treated rail roller whose fatigue damage was more severe (e.g. quenching strip vertical to rolling direction, as shown in Fig. 13c and d), the crack propagated into the substrate region. Branch cracks were developed and the direction of cracks was changed. The branch cracks then grew along the boundary in the substrate region (Fig. 14d).
4. Discussion 4.1. Influence of LDQ on wear of rail material For the untreated rail roller, the wear rate was large than treated rail rollers (Fig. 7). According to the wear process of rail material [29], plastic strain accumulated at the surface layer under the rolling/sliding condition (as shown in Fig. 11a). With the accumulation of plastic strain, cracks initiated and propagated along the plastic deformation flow at a shallow depth (Fig. 12b). After a certain number of cycles, the material above the crack fractured, leading to the wear loss of material
Fig. 13. Cracks on cross sections of laser treated rail rollers with different quenching shapes and patterns: (a) spot-shape with diameter Φ = 3 mm and LDQ spacing D = 1 mm; (b) spot-shape with Φ = 4 mm and D = 1 mm; (c) spot-shape with Φ = 6 mm and D = 1 mm; (d) spot-shape with Φ = 8 mm and D = 1 mm; (e) spotshape with Φ = 3 mm and D = 0 mm; (f) spot-shape Φ = 3 mm and D = 2 mm; (g) strip-shape with strips parallel to the rolling direction; (h) strip-shape with strips vertical to the rolling direction; (i) strip-shape with strips oblique to the rolling direction; (j) grid-shape.
9
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 13. (continued)
10
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 13. (continued) Table 3 Statistical results of fatigue cracks on rail rollers. Rail roller
Untreated Quenching Quenching Quenching Quenching Quenching Quenching Quenching Quenching Quenching Quenching
Average crack angle/°
spot, Φ = 3 mm, D = 1 spot, Φ = 4 mm, D = 1 spot, Φ = 6 mm, D = 1 spot, Φ = 8 mm, D = 1 spot, Φ = 3 mm, D = 0 spot, Φ = 3 mm, D = 2 strip, parallel to rolling strip, vertical to rolling strip, oblique to rolling grid
mm mm mm mm mm mm
Average crack depth/μm
Quenching region
Substrate region
Quenching region
Substrate region
– 71 77 72 71 61 58 63 62 80 66
8 ± 3 4 ± 1 17 ± 8 12 ± 8 11 ± 5 – 10 ± 4 5 ± 2 9 ± 6 10 ± 5 –
– 224 ± 99 206 ± 57 263 ± 91 196 ± 93 73 ± 37 196 ± 54 195 ± 100 176 ± 57 206 ± 76 154 ± 145
13 11 20 13 22 – 17 15 11 16 –
± ± ± ± ± ± ± ± ± ±
19 11 17 15 30 17 30 22 8 5
± ± ± ± ±
4 5 13 3 5
± ± ± ±
7 6 7 14
Fig. 14. SEM micrographs of crack tips: (a) quenching spot with Φ = 8 mm and D = 1 mm; (b) quenching spot with Φ = 3 mm and D = 1 mm; (c) and (d) quenching strip vertical to rolling direction. 11
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
rollers. Concerning the untreated rail roller, the pearlite material underwent severe plastic deformation under the rolling/sliding condition (Fig. 16a). Dislocations were generated, slid and piled up, forming empty space, which possessed high energy [31]. The releasing of energy at the surface led to the initiation of fatigue cracks. The cracks propagated with small angles in the shallow surface layer, as shown in Fig. 12b. These shallow cracks were prone to be removed by wear (i.e. delamination) of the surface layer. Concerning the treated rail rollers, the cracks on the untreated surface were similar to those on the untreated rollers, i.e. cracks were propagated with small angles in the shallow surface layer. However, the fatigue cracks in the quenching regions were totally different with those on untreated rollers (Fig. 16). The hardness of the quenching region (i.e. martensite) was high, thus the plastic deformation was hard to occur at the surface of the quenched layer. During the rolling/sliding process, when the regional stress on the surface exceeded the strength limit of the martensite layer, fatigue cracks would initiate on the surface. Furthermore, the cracks tended to initiate at the edge of the quenching region. This is because the edge of quenching layer had smaller thickness and thus relatively lower strength than the middle part of the quenching region. Under repeated loads, cracks propagated into the quenched layer with large angles (e.g. Fig. 13b). As discussed above, quenching shapes and patterns had influence on the anti-fatigue properties of the quenching layer. The fatigue damage was alleviated with the increase in the diameter of quenching spots and the decrease in the LDQ spacing (Figs. 9 and 13). Meanwhile, the gridshaped quenching presented better anti-fatigue property than stripshaped and spot-shaped quenching. Previous researches had a dispute over the law of crack propagation inside martensite layer formed on the rail. Clayton and Allery [32] supposed that the cracks rarely penetrated the boundary between the martensite layer and the matrix, and expanded inside the matrix along the boundary. However, other researches [31] found that cracks could penetrate the boundary and propagate in the matrix. In the present study, the law of crack propagation varied depending on the quenching shapes and patterns. Fig. 15b–d show the different forms of fatigue damage on the quenched rail rollers. For the quenched rail roller with relatively good anti-fatigue property (e.g. quenching spot with Φ = 8 mm and D = 1 mm, as shown in Figs. 13d and 14a), the fatigue cracks stopped inside the martensite layer, and did not penetrate the boundary, as shown in Fig. 16a. For the quenched rail roller with relatively poor anti-fatigue property (e.g. quenching spot with Φ = 3 mm and D = 1 mm, as shown in Figs. 13a and 14b), the direction of cracks turned to be parallel to the boundary, and branch cracks were developed in the quenching region. Fatigue cracks still did not penetrate the boundary. Meanwhile, the convergence of cracks in the quenching region caused the spalling of quenched material, as shown in Fig. 16b. For the quenched rail roller with the worst anti-fatigue property (e.g. quenching strip vertical to rolling direction, as shown in Fig. 13c, d), the fatigue cracks penetrated the boundary and then branch cracks were generated in the matrix. After that the branch cracks propagated along the boundary in the matrix. Meanwhile severe spalling occurred in the quenching layer, as shown in Fig. 16c. In summary, the quenching shapes and patterns had significant influence on the anti-wear and anti-fatigue behaviors. For the spot-shaped quenching, both the wear resistance and the fatigue resistance were improved with the increase in the diameter of quenching spots and the decrease in the LDQ spacing. For the strip-shaped quenching and the grid-shaped quenching, the quenching grid presented better wear resistance and fatigue resistance than quenching strips. Therefore, the quenching shapes and patterns should be taken into consideration for the application of LDQ in the field. Among the specimens in this study, the spot-shaped quenching with the largest diameter Φ = 8 mm, the
Fig. 15. Relationship between wear rate and quenching surface ratio.
[29]. Thus, the wear resistance of untreated rail was dominated by the resistance to plastic deformation. Concerning the treated rail rollers, the wear rates were smaller (Fig. 7). In the treated zone, martensite with high hardness was produced by LDQ, which improved the resistance to plastic deformation, and consequently to wear. This point can also be verified by the topography of the treated rail roller with strips parallel to the rolling direction (as shown in Fig. 9g). The groove in the untreated zone and the ridge in the treated zone indicate that the wear resistance of quenched zone (i.e. martensite) was superior to the untreated rail material (i.e. pearlite). Furthermore, the wear resistance of untreated zone between treated zones could also be improved. According to [30], during the plastic deformation of mixed microstructures, the strength of the soft phase could be enhanced by surrounding hard phases. In the present study, the soft untreated zone was surrounded by hard treated zones. The plastic deformation of the substrate region between quenching regions (Fig. 11b) was much shallower than the untreated roller (Fig. 11a). Thus the wear of the untreated zone between treated zones might be milder than the untreated roller. This could also be verified by the Fig. 9f that groove was generated between two lines of quenching spots but no grooves were generated in the untreated zones between two treated zones in the rolling direction, which indicated that the presence of the harder treated zones could suppress the wear of the softer substrate material between them. As discussed above, LDQ could improve the wear resistance of rail rollers. Furthermore, quenching shapes and patterns had influence on the improvement of wear resistance. The wear resistance was further improved with the increase in the diameter of quenching spots (Fig. 7a) and the decrease in the LDQ spacing (Fig. 7b). Meanwhile, the gridshaped quenching presented better wear resistance than strip-shaped and spot-shaped quenching. In the present study, a parameter –– quenching area ratio (i.e. the ratio of quenching areas to the total surface area on the roller surface) –– was calculated. Fig. 15 presents the relationship between the wear rate and the quenching area ratio. With the increase in the quenching area ratio, the wear rate showed a decrease trend, i.e. the wear resistance was improved with the quenching area ratio. The best wear resistance was obtained on the treated roller with quenching grid. 4.2. Influence of LDQ on fatigue damage of rail material The damage mechanisms of the untreated rail and the LDQ-treated rail were different. Fig. 16 shows the different damage forms of rail
12
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 16. Illustration of damage forms of untreated and treated rails: (a) untreated rail; (b) treated rail with good anti-fatigue property; (c) treated rail with poor antifatigue property; and (d) treated rail with the worst anti-fatigue property.
spot-shaped quenching with the smallest spacing D = 0 mm, and the grid-shaped quenching were superior to other cases. In addition, the fatigue cracks of the quenching regions are severer than the untreated rail material. So, long term tests (> 110,000 cycles) were conducted to see the fatigue damage of the treated samples (quenching spot with Φ = 8 mm and D = 1 mm and the quenching grid) as well as the untreated rail sample. Furthermore, to accelerate the development of cracks, these tests were conducted under the wet condition. The feed rate of water to the wheel/rail interface was 1.6 l/ min. Fig. 17 shows the OM photos after testing. Severe spalling was
observed on the untreated rail roller. On the treated rail rollers, the spalling was milder and pits could be observed. Moreover, further study should be performed in the future to verify by longer term tests whether the cracks of the quenched rail will develop into severe spalling and failure. 5. Conclusions The influence of LDQ on the rolling wear and damage properties of rail material was investigated. Different quenching shapes and patterns
13
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 17. Surface damages of rail rollers under wet condition with large cycles: (a) untreated rail roller after 4,180,000 cycles; (b) treated rail roller with quenching spots of Φ = 8 mm and D = 1 mm after 1,100,000 cycles; (c) treated rail roller with quenching grid after 1,230,000 cycles.
were explored. Conclusions can be drawn as following:
Acknowledgements
1. LDQ created quenching regions with an average hardness of around 927 HV0.5 on the surface layer of the rail material. The quenching regions were mainly composed of martensite. 2. LDQ improved the wear resistance of rail material. The wear resistance was further improved with the increase in the diameter of quenching spots and the decrease in the LDQ spacing. The wear resistance of treated rail with quenching grid was better than treated rails with quenching strips or quenching spots. The wear rate of treated rail roller with quenching grid was 0.28 times that of the untreated rail roller. 3. On the untreated rail roller and in the untreated zones on treated rail rollers, cracks propagated with small angles of around 4–17° and small depth of around 11–22 μm. In the quenching regions, cracks propagated with large angles of around 58–80° and large depth of around 73–263 μm. 4. The fatigue resistance of treated rail was improved with the increase in the diameter of quenching spots and the decrease in the LDQ spacing. The fatigue resistance of treated rail with quenching grid was better than treated rail with quenching strips.
The work was supported by Sichuan Science and Technology Program (No.2019YFG0289), National Key Research and Develop Program (No.2016YFB1102601) and Sichuan province University Scientific Research Innovation Team Project (No.18TD0005). References [1] Y. Zhu, Y. Yang, X. Mu, W. Wang, Z. Yao, H. Yang, Study on wear and RCF performance of repaired damage railway wheels: assessing laser cladding to repair local defects on wheels, Wear 430–431 (2019) 126–136. [2] G. Li, Z.Y. Hong, Q.Z. Yan, The influence of microstructure on the rolling contact fatigue of steel for high-speed-train wheel, Wear 342–343 (2015) 349–355. [3] J.X. Wang, X. Chen, X.G. Li, Y.J. Wu, Influence of heavy haul railway curve parameters on rail wear, Eng. Fail. Anal. 57 (2015) 511–520. [4] W.J. Wang, S.R. Lewis, R. Lewis, A. Beagles, C.G. Hem, Q.Y. Liu, The role of slip ratio in rolling contact fatigue of rail materials under wet conditions, Wear 376–377 (2017) 1892–1900. [5] A. Ekberg, B. Akesson, E. Kabo, Wheel/rail rolling contact fatigue–probe, predict, prevent, Wear 314 (2014) 2–12. [6] M. Steenbergen, Rolling contact fatigue: spalling versus transverse fracture of rails, Wear 380–381 (2017) 96–105. [7] Y.B. Huang, L.B. Shi, X.J. Zhao, Z.B. Cai, Q.Y. Liu, W.J. Wang, On the formation and
14
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
[8] [9]
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
damage mechanism of rolling contact fatigue surface cracks of wheel/rail under the dry condition, Wear 400–401 (2018) 62–73. Q.B. Liu, H. Liu, Experimental study of the laser quenching of 40CrNiMoA steel, J. Mater. Process. Technol. 88 (1999) 77–82. A.K. Nath, S. Sarkar, Laser transformation hardening of steel, Advances in Laser Materials Processing, Second edition, Woodhead Publishing Series in Welding and Other Joining Technologies, 2018, pp. 257–298, , https://doi.org/10.1016/B978-008-101252-9.00011-X. S.M. Shariff, T.K. Pal, G. Padmanabham, S.V. Joshi, Sliding wear behaviour of laser surface modified pearlitic rail steel, Surf. Eng. 26 (2010) 199–208. S.M. Shariff, T.K. Pal, G. Padmanabham, S.V. Joshi, Influence of chemical composition and prior microstructure on diode laser hardening of railroad steels, Surf. Coat. Technol. 228 (2013) 14–26. X.J. Chen, Q. Yan, Q. Ma, Influence of the laser pre-quenched substrate on an electroplated chromium coating/steel substrate, Appl. Surf. Sci. 405 (2017) 273–279. C. Soriano, J. Leunda, J. Lambarri, V. Garcia Navas, C. Sanz, Effect of laser surface hardening on the microstructure, hardness and residual stresses of austempered ductile iron grades, Appl. Surf. Sci. 257 (2011) 7101–7106. D.J. Kong, C.Y. Xie, Effect of laser quenching on fatigue properties and fracture morphologies of boronized layer on Cr12MoV steel, Int. J. Fatigue 80 (2015) 391–396. Z.K. Chen, Q.H. Zhu, J. Wang, X. Yun, B. He, J.S. Luo, Behaviors of 40Cr steel treated by laser quenching on impact abrasive wear, Opt. Laser Technol. 103 (2018) 118–125. L. Orazi, A. Fortunato, G. Cuccolini, G. Tani, An efficient model for laser surface hardening of hypo-eutectoid steels, Appl. Surf. Sci. 256 (2010) 1913–1919. M. Moradi, M.K. Moghadam, High power diode laser surface hardening of AISI 4130; statistical modelling and optimization, Opt. Laser Technol. 111 (2019) 554–570. M. Moradi, M.M. Fallah, S.J. Nasab, Experimental study of surface hardening of AISI 420 martensitic stainless steel using high power diode laser, Trans. Indian Inst. Metals 71 (2018) 2043–2050. M. Moradi, D. Ghorbani, M.K. Moghadam, M. Kazazi, F. Rouzbahani, S. Karazi, Nd:YAG laser hardening of AISI 410 stainless steel: microstructural evaluation, mechanical properties, and corrosion behavior, J. Alloys Compd. 795 (2019) 213–222.
[20] W. Su, T. Zhou, P. Zhang, H. Zhou, H. Li, Effect of distribution of striated laser hardening tracks on dry sliding wear resistance of biomimetic surface, Opt. Laser Technol. 98 (2018) 281–290. [21] J.R. Jiang, L.J. Xue, S.D. Wang, Discrete laser spot transformation hardening of AISI O1 tool steel using pulsed Nd:YAG laser, Surf. Coat. Technol. 205 (2011) 5156–5164. [22] Z.Y. Li, X.H. Xing, M.J. Yang, B. Yang, Z.Y. Ren, H.Q. Lin, Investigation on rolling sliding wear behavior of wheel steel by laser dispersed treatment, Wear 314 (2014) 236–240. [23] D.F. Zeng, L.T. Lu, Z.Y. Li, J.W. Zhang, X.S. Jin, M.H. Zhu, Influence of laser dispersed treatment on rolling contact wear and fatigue behavior of railway wheel steel, Mater. Des. 54 (2014) 137–143. [24] Y.L. Zheng, Q.W. Hu, C.Y. Li, D.Z. Wang, L. Meng, J.G. Luo, J.P. Wang, X.Y. Zeng, A novel laser surface compositing by selective laser quenching to enhance railway service life, Tribol. Int. 106 (2017) 46–54. [25] X. Cao, L.B. Shi, Z.B. Cai, Q.Y. Liu, Z.R. Zhou, W.J. Wang, Investigation on the microstructure and damage characteristics of wheel and rail materials subject to laser dispersed quenching, Appl. Surf. Sci. 450 (2018) 468–483. [26] M. Moradi, M.K. Moghadam, M. Kazazi, Improved laser surface hardening of AISI 4130 low alloy steel with electrophoretically deposited carbon coating, Optik 178 (2019) 614–622. [27] C.R. Su, L.B. Shi, W.J. Wang, D.Z. Wang, Z.B. Cai, Q.Y. Liu, Z.R. Zhou, Investigation on the rolling wear and damage properties of laser dispersed quenched rail materials treated with different ratios, Tribol. Int. 135 (2019) 488–499. [28] H.C. Shi, L.B. Shi, H.H. Ding, W.J. Wang, W.J. Jiang, J. Guo, Q.Y. Liu, Influence of laser strengthening techniques on anti-wear and anti-fatigue properties of rail welding joint, Eng. Fail. Anal. 101 (2019) 72–85. [29] H.H. Ding, C.G. He, L. Ma, J. Guo, Q.Y. Liu, W.J. Wang, Wear mapping and transitions in wheel and rail materials under different contact pressure and sliding velocity conditions, Wear 352-353 (2016) 1–8. [30] C.H. Young, H.K.D.H. Bhadeshia, Strength of mixtures of bainite and martensite, Mater. Sci. Technol. 10 (1994) 209–214. [31] Y.D. Chen, X.J. Zhao, P.T. Liu, R. Pan, R.M. Ren, Influences of local laser quenching on wear performance of D1 wheel steel, Wear 414–415 (2018) 243–250. [32] P. Clayton, M.B.P. Allery, Metallurgical aspects of surface damage problems in rails, Can. Metall. Q. 21 (1982) 31–46.
15
Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Investigation on the rolling wear and damage properties of laser discrete quenched rail material with different quenching shapes and patterns
T
H.H. Dinga,b, C.R. Sub, W.J. Wanga,b, , Z.B. Caib, D.Z. Wangc, J. Guoa, Q.Y. Liua,b, Z.R. Zhoua,b ⁎
a
Tribology Research Institute, State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China School of Mechanical Engineering, Southwest Jiaotong University, Chengdu 610031, China c Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, China b
ARTICLE INFO
ABSTRACT
Keywords: Laser discrete quenching Rail material Wear Damage Fatigue crack
The influence of laser discrete quenching (LDQ) on the rolling wear and damage properties of the U75V rail material was investigated. Different quenching shapes (including spot, strip, and grid shapes) and different quenching patterns (including different diameters of quenching spots, distances between adjacent spots, and directions of quenching strips) were studied. The results indicated that LDQ created martensite regions with an average hardness of around 927 HV0.5 on the rail surface. LDQ improved the wear resistance of rail material. On the untreated rail roller and in the untreated zones on treated rail rollers, cracks propagated with small angles of around 4–17° and small depth of around 11–22 μm. In the quenching regions, cracks propagated with large angles of around 58–80° and large depth of around 73–263 μm. The wear resistance and fatigue resistance of treated rail rollers were improved with the increase in the diameter of quenching spots and the decrease in the distance between adjacent spots. The wear resistance and fatigue resistance of treated rail with quenching grid were better than treated rail with quenching strips.
1. Introduction With the rapid development of high-speed and heavy-haul railways, railway transportation is becoming increasingly important for the modern life. As the service time increases, the wear and damage of wheel and rail materials become more serious. Wear and rolling contact fatigue (RCF) are considered to be two of the main failure mechanisms for wheels and rails [1–4]. These surface defects could decrease the service life and threaten the running safety of railway systems [5–7]. Therefore, attempts should be made to improve the wear resistance, alleviate the surface damage, and then extend the service life of wheel/ rail. Laser quenching (LQ) is a promising technology that has already been applied in various industries including automotive, power generation, and general engineering [8–12]. It is an effective way to improve surface properties that involves scanning the material surface with a high energy laser beam, and then, martensitic phase transformation occurs due to rapid self-cooling and solidification [13–19]. But when the scanning area is greater than the laser spot, multiple overlapping regions for laser hardening trajectories are indispensable. The major problem in this case is the formation of soft zones caused by back-tempering among overlapped laser tracks, which may limit wear
⁎
resistance of components [20]. In order to solve this problem, laser discrete quenching (LDQ) has been proposed, which creates isolated laser hardening spots distributed in a certain pattern on a component surface [21]. LDQ has been applied into wheel and rail treatment in some previous works [22–25]. Zeng et al. [23] compared the wear resistance of untreated and LDQ treated wheel steel and found that the wear rate of the LDQ treated wheel steel was about 0.3 times that of untreated wheel steel. Zheng et al. [24] investigated the influence of the size of quenching spot on the wear and damage properties of rail material. With the increase in the size of quenching spot, the wear resistance of rail was improved but the fatigue resistance got worse. Cao et al. [25] found that the wear resistance and surface damage of the wheel and rail materials deteriorated with the increase in the LDQ spacing (i.e. the distance between two adjacent quenching spots). Furthermore, the LDQ (spot-like quenching) has been practically applied in the field on rails in Wuhan Railway Bureau of China, and the service life of the treated rails was improved by 2.2–3.5 times [24]. Reviewing previous works, researchers mainly focused on the spotlike quenching zones. However, very limited information is available on the influence of the shape of quenching zone on the wear and damage of wheel and rail materials. Thus, the objective of the present study is to
Corresponding author at: Tribology Research Institute, State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China. E-mail address: [email protected] (W.J. Wang).
https://doi.org/10.1016/j.surfcoat.2019.124991 Received 5 August 2019; Received in revised form 2 September 2019; Accepted 12 September 2019 Available online 18 November 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
discussed in detail. 2. Experimental details 2.1. Experimental machine and samples Wear tests were carried out under the dry condition using a rolling–sliding wear testing machine (MJP-30A, China) [25]. The schematic outline of the machine is shown in Fig. 1. The wheel roller (lower specimen) and rail roller (upper specimen) were powered and controlled by two servo motors, respectively. The normal force was applied between wheel and rail rollers. The wheel and rail rollers were cut from the C-class wheel and the U75V rail, which are typical wheel and rail materials used in China. The sampling positions and sizes of rollers are shown in Fig. 2. The diameter of wheel and rail rollers is 60 mm and the contact width is 10 mm. The chemical compositions of wheel and rail materials are given in Table 1. The rail rollers were LDQ treated using a fiber laser (YLR-6000, IPG) as shown in Fig. 3. It was a high-power single-mode ytterbium fiber laser. The wavelength of the output laser was 1.07 μm and the maximum power of the laser could reach 50 kW. In order to investigate the effect of quenching shapes and patterns on the wear and damage behaviors of rail material, rail rollers were treated with different quenching shapes (spot, stripe, and grid) and different quenching patterns, as shown in Fig. 4. For the spot-shaped quenching (Fig. 3a–f), four different diameters of spots (diameter Φ = 3, 4, 6, 8 mm, respectively, as shown in Fig. 3a–d) and three different quenching spacings (distance between two adjacent spots D = 0, 1, 2 mm, respectively, as shown in Fig. 3e, a and f) were studied. During the quenching process, different powers of the fiber laser (around 700, 820, 1450, 2000 W, respectively) were used to achieve different diameters of quenching spots (around 3, 4, 6, 8 mm, respectively). The laser radiation lasted for around 0.7 s for each quenching spot. For the stripshaped quenching (Fig. 3g–i), three different distribution directions of strips (parallel, vertical or oblique to the rolling direction of the rail roller) were studied. During the quenching process, the power of the fiber laser was around 650 W and the scanning speed was around 400 mm/min. After quenching, the width of the quenching strips was around 2 mm and the interval between two strips was also around 2 mm. For the grid-shaped quenching (Fig. 4j), both the parallel and vertical stripes were quenched on the rail roller surface. Table 2 shows the quenching parameters and the dimensions of the quenching zones. It is clear that some rail rollers surfaces (e.g., Fig. 4g and j) became rougher after laser treatment. This is probably because the surfaces were melt locally during the laser scanning [26]. Thus, all the treated rail rollers were polished to be smooth before wear tests. The surfaces of treated rollers were composed of laser-treated zones (spots, strips or grid) and untreated zones (surfaces between laser-treated zones), as shown in Fig. 4. The wheel rollers were untreated.
Fig. 1. Schematic outline of the test rig.
Fig. 2. Sampling positions and sizes of wheel and rail rollers. Table 1 Chemical compositions of wheel and rail materials (wt%). Material
C
Si
Mn
P
S
C-class wheel U75V rail
0.67–0.77 0.71–0.80
0.15–1.00 0.50–0.80
0.60–0.90 0.70–1.50
0.030 ≤0.030
0.005–0.040 ≤0.030
Fig. 3. The schematic of the laser discrete quenching.
investigate the tribological performances of the LDQ treated rails with different quenching shapes and patterns for achieving a better result. Firstly, the U75V rail material was treated by LDQ. Different quenching shapes (spot, stripe, and grid) and different quenching patterns were studied. For the spot-shaped quenching, four different diameters of spots and three different quenching spacings (i.e. distances between two adjacent spots) were adopted. For the strip-shaped quenching, three different distribution directions of strips (parallel, vertical or oblique to the rolling direction of the wheel/rail interface) were studied. For the grid-shaped quenching, both the parallel and vertical stripes were quenched on the rail surface. Secondly, wear tests were conducted using the LDQ treated rail specimens with different quenching shapes and patterns. Based on the comparison of the wear rates and damage behaviors of rail specimens, the optimization of the quenching shapes and patterns could be determined for the application of LDQ to rails, which could improve the service life of the rails in the field. Furthermore, the damage mechanism of quenching region was
2.2. Experimental conditions All tests were carried out under dry conditions at room temperature. The normal force between wheel/rail rollers was 5110 N, leading to the maximum contact pressure of around 1100 MPa, which simulated the axle load of 14 t in the field according to the Hertzian simulation rule [23]. The rotational speed of the lower roller (wheel roller) was 500 r/ min, and the slip ratio of wheel/rail rollers was 0.75% (slip ratio means the difference in speed between wheel/rail rollers). The number of cycles of the lower roller was 80,000. All wear tests were performed twice. The wheel/rail rollers were ultrasonically cleaned in ethanol, dried, and weighed using an electronic balance (JA4103, accuracy: 0.0001 g) before and after testing. The wear rate (μg/m) of a roller was calculated by mass loss (μg) per rolling distance (m). The hardness of treated and 2
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 4. Different quenching shapes and patterns on rail rollers: (a) spot-shape with diameter Φ = 3 mm and LDQ spacing D = 1 mm; (b) spot-shape with Φ = 4 mm and D = 1 mm; (c) spot-shape with Φ = 6 mm and D = 1 mm; (d) spot-shape with Φ = 8 mm and D = 1 mm; (e) spot-shape with Φ = 3 mm and D = 0 mm; (f) spotshape Φ = 3 mm and D = 2 mm; (g) strip-shape with strips parallel to the rolling direction; (h) strip-shape with strips vertical to the rolling direction; (i) strip-shape with strips oblique to the rolling direction; (j) grid-shape.
untreated rollers was measured using a Vickers hardness instrument (MVK-H21, Japan). The cross sections of rollers were ground to 2000 grit, polished to 0.5 μm diamond, etched with 5% natal (i.e., 5 ml nitric acid + 95 ml ethanol). The wear scars and cross sections were observed using an optical microscopy (OM) (OLYMPUS BX60 M, Japan) and a scanning electronic microscopy (SEM) (JSM-7001F, Japan).
3. Results 3.1. Microstructures and hardness of laser quenched rail Fig. 5 shows SEM micrographs of a cross section of a laser quenched rail. The cross section consisted of two different regions, i.e. quenching 3
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Table 2 Quenching parameters and corresponding dimensions of the quenching zones. Dimensions of quenching zones
1 2 3 4 5
Quenching parameters
Shape
Diameter/mm
Width/mm
Maximum depth/mm
Laser power/W
Lasting time/s
Scanning speed/mm/min
Spot Spot Spot Spot Strip
≈3 ≈4 ≈6 ≈8 –
– – – – ≈2
≈0.38 ≈0.50 ≈0.94 ≈1.11 ≈0.63
700 820 1450 2000 650
0.7 0.7 0.7 0.7 –
– – – – 400
Fig. 5. SEM micrographs of (a) the cross section of laser quenched rail, (b) the quenching region, (c) the interface between quenching region and substrate region, and (d) the substrate region.
region and substrate region. The quenching region was crescentshaped. It is clear from Fig. 5b that the quenching region was mainly composed of martensite owing to the rapid self-cooling and high solidification rates during laser treatment [13,27]. The substrate region presented a typical microstructure of pearlite rail material. Fig. 5c indicates the intimately bonded interface between the substrate region and the quenching region. Fig. 6a shows the surface hardness of rail material before and after laser quenching. Before laser quenching, the surface hardness of rail was around 329 HV0.5. After laser quenching, the surface hardness of the laser-treated zone was increased by 2.8 times to around 927 HV0.5. The improved hardness was mainly attributed to the martensite structure produced during laser quenching [23]. For the untreated zone between laser-treated zones, the surface hardness was similar to the rail hardness before laser quenching. Fig. 6b shows the hardness measured on the cross section of laser quenched rail from the surface to the depth.
In the quenching region, the hardness was high and stable. The hardness value was similar to that obtained on the surface of laser-treated zone. With the increase in depth, the hardness showed a sharp decrease at the boundary between the quenching region and the substrate region. In the substrate region, the hardness value was similar to that of the rail material before quenching. 3.2. Wear rates The wear rates of wheel and rail rollers are shown in Fig. 7. For the test with untreated rollers, the wear rate of rail roller was around 39 μg/m and the wear rate of wheel roller was around 31 μg/m. For tests with laser treated rail rollers, the wear rates of rail rollers were decreased, which means that LDQ could improve the wear resistance of rail material. However, the wear rates of the untreated counterbodies (i.e. untreated wheel rollers) were increased.
4
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 6. (a) Surface hardness of rail before and after laser quenching, and (b) hardness on the cross section of the laser quenched rail.
For the spot-shaped quenching (Fig. 6a and b), the quenching spot diameter and the LDQ spacing had influence on the wear rates of wheel/rail rollers. With the increase in the diameter of quenching spots (Fig. 7a), the wear rate of treated rail rollers showed a decrease trend, the wear rate of the wheel roller had no obvious change, and the total wear rate of wheel/rail rollers was decreased. With the increase in the LDQ spacing (Fig. 7b), the wear rate of treated rail roller showed an increase trend, the wear rate of the wheel roller showed a decrease trend, and the total wear rate of wheel/rail rollers showed no obvious change. For the strip-shaped quenching (Fig. 7c), the distribution direction of quenching strips had influence on the wear rates of wheel/rail rollers. The wear rate of treated rail roller with quenching strips parallel to the rolling direction was larger than vertical or oblique to the rolling direction. The wear rate of treated rail roller with quenching strips vertical to the rolling direction was similar to oblique to the rolling direction. Concerning the wheel wear rate and the total wear rate, they showed a decrease trend for tests with quenching strips parallel, vertical or oblique to the rolling direction, respectively. For the grid-shaped quenching (Fig. 7c), both the rail wear rate and the wheel wear rate were smaller than those for spot-shaped and stripshaped quenching. The wear rate of the grid quenched rail roller was around 11 μg/m (0.28 times that of untreated rail roller) and the wear rate of the wheel roller was around 36 μg/m.
Fig. 7. Wear rates of wheel/rail rollers: (a) spot-shaped quenching with different spot diameters; (b) spot-shaped quenching with different LDQ spacings; (c) strip-shaped and grid-shaped quenching.
3.3. Surface damage behaviors The worn surfaces of wheel and rail rollers were observed after wear testing. Fig. 8 shows the worn surfaces of the untreated wheel/untreated rail rollers. The surface damage of untreated wheel and untreated rail rollers was mild and similar. The worn surfaces were dominated by pitting and spalling.
5
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 8. Worn surfaces of (a) untreated wheel roller and (b) untreated rail roller.
Fig. 9 shows the worn surfaces of the laser treated rail rollers with different quenching shapes and patterns. For the rail rollers treated with quenching spots (Fig. 8a–f), obvious cracks were observed at the boundary between treated and untreated zones and in the treated zone near the boundary. Furthermore, with the increase in the diameter of quenching spot (Fig. 8a–d), the cracks were alleviated. For example, cracks observed on the treated rail roller with diameter Φ = 8 mm (Fig. 9d) were milder than observed on the rollers with smaller quenching spots (Fig. 8b and c). With the increase in the LDQ spacing (Fig. 8e, a and f, respectively), the cracks became more serious. For example, cracks observed on the treated rail roller with the LDQ spacing D = 0 mm (Fig. 9e) were less and milder than observed on the rollers with larger distances (Fig. 8a and f). For the rail rollers treated with quenching strips (Fig. 8g–i), cracks were also observed in the treated zone. The cracks on the treated rail rollers with vertical and oblique quenching strips (Fig. 8h and i) were more obvious than those on the rail roller with parallel quenching strips (Fig. 9g). For the rail roller treated with quenching grid (Fig. 9j), the cracks were less and milder than most of the treated rollers with quenching spots and quenching strips. In addition, grooves were generated in the rolling direction in the untreated zones, as shown in Fig. 8a, f and g. This is because the untreated zone is softer than the treated zone. The softer untreated zone led to the larger wear loss and then the groove on the rail roller. Fig. 10 shows the representative worn surfaces of the wheel rollers which rolled against the treated rail rollers. The surfaces of wheel rollers were dominated by spalling. Furthermore, a ridge was observed on the wheel roller (Fig. 10b) which was in correspondence with the groove generated on the rail roller (Fig. 9f). Fig. 11 shows the plastic deformation observed on the cross sections of rail rollers after testing. For the untreated rail roller (Fig. 11a), the plastic deformation layer reached around 160 μm. For the treated rail rollers, the plastic deformation of the quenching region was negligible, and the plastic deformation of the substrate region was obviously decreased. For example, on the treated rail roller with quenching strips vertical to the rolling direction (Fig. 11b), the thickness of the plastic deformation layer of the substrate region was around 45 μm.
Fig. 12 shows the cracks on the untreated wheel/untreated rail rollers. Cracks were shallow and the angle of cracks was small. This kind of cracks was easy to be worn off during the rolling/sliding process. Fig. 13 shows the cracks on the cross sections of laser treated rail rollers with different quenching shapes and patterns. Obvious fatigue cracks were observed in the quenching regions and the angles of cracks were large even reaching around 90° in many cases. Furthermore, cracks tended to generate at the edge of the quenching region, this was mainly due to the small thickness at the edge. The quenching shapes and patterns had significant influence on the fatigue damage of quenching regions. For the rail rollers treated with quenching spots (Fig. 12a–f), the fatigue damage was influence by the diameter of quenching spots and the LDQ spacing. With the increase in the diameter of quenching spots (Fig. 12a–d), the fatigue damage was alleviated. Specifically, with large spot diameters (Φ = 4, 6, and 8 mm, as shown in Fig. 12b–d), the fatigue damage was relatively mild. Only one or two fatigue cracks were generated at the edge of the quenching region. The fatigue cracks stopped near the boundary between quenching and substrate regions. For the rail roller with the small quenching spot diameter (Φ = 3 mm, as shown in Fig. 12a), the fatigue damage was more serious. More cracks were generated in the quenching region. The growth direction of the crack was changed at the bottom of the quenching region to be along the boundary. Furthermore, branch cracks were developed in the quenching region (Fig. 13a). With the increase in the LDQ spacing (Fig. 12e, a and f, respectively), the fatigue damage of quenching regions became more serious. Specifically, for the treated rail roller with the LDQ spacing D = 0 mm (Fig. 13e), only a single small fatigue crack was observed in the quenching region. For the rollers with larger spacings (D = 1 and 2 mm, as shown in Fig. 12a and f), more fatigue cracks were generated. Furthermore, for the roller with the largest spacing of 2 mm (Fig. 13f), severe spalling occurred in the quenching region due to the connection of the fatigue cracks. In summary, for the spot-shaped quenching, rail rollers treated with large quenching spots (Φ = 8 mm, Fig. 13d) or with small DLQ spacing (D = 0 mm, Fig. 13e) exhibited better anti-fatigue properties. For rail rollers treated with quenching strips (Fig. 12g–i), two or more fatigue cracks were generated at the edge of the quenching region. For the rail roller with quenching strips vertical to the rolling direction (Fig. 13h), spalling also occurred in the quenching region. Fig. 13j shows the cross section of the rail roller treated with quenching grid. Only small cracks were observed in the quenching region. The fatigue damage of the quenching grid (Fig. 13j) was milder than that of the
3.4. Fatigue cracks on cross sections In order to further understand the damage behaviors of the wheel and rail materials, cracks were observed on the cross sections of rollers.
6
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 9. Worn surfaces of laser treated rail rollers with different quenching shapes and patterns: (a) spot-shape with diameter Φ = 3 mm and LDQ spacing D = 1 mm; (b) spot-shape with Φ = 4 mm and D = 1 mm; (c) spot-shape with Φ = 6 mm and D = 1 mm; (d) spot-shape with Φ = 8 mm and D = 1 mm; (e) spot-shape with Φ = 3 mm and D = 0 mm; (f) spot-shape Φ = 3 mm and D = 2 mm; (g) strip-shape with strips parallel to the rolling direction; (h) strip-shape with strips vertical to the rolling direction; (i) strip-shape with strips oblique to the rolling direction; (j) grid-shape.
7
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 10. Representative worn surfaces of wheel rollers which rolled against the treated rail rollers: (a) wheel roller against the rail roller with quenching spot of Φ = 8 mm and D = 1 mm; (b) wheel roller against the rail roller with quenching spot of Φ = 3 mm and D = 2 mm.
Fig. 11. Plastic deformation of (a) the untreated rail roller and (b) the treated rail roller with quenching strips vertical to the rolling direction.
Fig. 12. Cracks on longitudinal cross sections of (a) untreated wheel roller and (b) untreated rail roller.
quenching strips (Fig. 12g–i). In order to evaluate the fatigue damage of rail rollers, the statistical data of cracks (including the crack angle and crack depth observed though OM) in the quenching and substrate regions are shown in Table 3. On the untreated rail roller, the crack angle and the crack depth were small. The average crack angle was around 8° and the average crack depth was around 13 μm. On the treated rail rollers, the crack angle and the crack depth were also small in the substrate regions. The average crack angle was in the range of around 4–17° and the average crack depth was in the range of around 11–22 μm.
However, the crack angle and the crack depth were significantly large in the quenching regions. The average crack angle reached around 58–80° and the average crack depth reached around 73–263 μm. In order to further understand the damage behavior of laser quenched rail roller, the crack tips near the boundary between quenching and substrate regions were observed using SEM, as shown in Fig. 14. In general, the crack propagation resistance is influenced by the properties of localized material at the crack tip. Cracks propagate when the accumulated energy exceeds the limit of the resistance [28]. For the treated rail roller whose fatigue damage was mild (e.g. quenching spot
8
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
with Φ = 8 mm and D = 1 mm, as shown in Fig. 14a), the crack stopped inside the quenching region near the boundary. For the treated rail roller whose fatigue damage was relatively severe (e.g. quenching spot with Φ = 3 mm and D = 1 mm, as shown in Fig. 14b), the growth direction of crack was changed in the quenching region to be parallel to the boundary. The crack did not develop into the substrate region. For the treated rail roller whose fatigue damage was more severe (e.g. quenching strip vertical to rolling direction, as shown in Fig. 13c and d), the crack propagated into the substrate region. Branch cracks were developed and the direction of cracks was changed. The branch cracks then grew along the boundary in the substrate region (Fig. 14d).
4. Discussion 4.1. Influence of LDQ on wear of rail material For the untreated rail roller, the wear rate was large than treated rail rollers (Fig. 7). According to the wear process of rail material [29], plastic strain accumulated at the surface layer under the rolling/sliding condition (as shown in Fig. 11a). With the accumulation of plastic strain, cracks initiated and propagated along the plastic deformation flow at a shallow depth (Fig. 12b). After a certain number of cycles, the material above the crack fractured, leading to the wear loss of material
Fig. 13. Cracks on cross sections of laser treated rail rollers with different quenching shapes and patterns: (a) spot-shape with diameter Φ = 3 mm and LDQ spacing D = 1 mm; (b) spot-shape with Φ = 4 mm and D = 1 mm; (c) spot-shape with Φ = 6 mm and D = 1 mm; (d) spot-shape with Φ = 8 mm and D = 1 mm; (e) spotshape with Φ = 3 mm and D = 0 mm; (f) spot-shape Φ = 3 mm and D = 2 mm; (g) strip-shape with strips parallel to the rolling direction; (h) strip-shape with strips vertical to the rolling direction; (i) strip-shape with strips oblique to the rolling direction; (j) grid-shape.
9
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 13. (continued)
10
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 13. (continued) Table 3 Statistical results of fatigue cracks on rail rollers. Rail roller
Untreated Quenching Quenching Quenching Quenching Quenching Quenching Quenching Quenching Quenching Quenching
Average crack angle/°
spot, Φ = 3 mm, D = 1 spot, Φ = 4 mm, D = 1 spot, Φ = 6 mm, D = 1 spot, Φ = 8 mm, D = 1 spot, Φ = 3 mm, D = 0 spot, Φ = 3 mm, D = 2 strip, parallel to rolling strip, vertical to rolling strip, oblique to rolling grid
mm mm mm mm mm mm
Average crack depth/μm
Quenching region
Substrate region
Quenching region
Substrate region
– 71 77 72 71 61 58 63 62 80 66
8 ± 3 4 ± 1 17 ± 8 12 ± 8 11 ± 5 – 10 ± 4 5 ± 2 9 ± 6 10 ± 5 –
– 224 ± 99 206 ± 57 263 ± 91 196 ± 93 73 ± 37 196 ± 54 195 ± 100 176 ± 57 206 ± 76 154 ± 145
13 11 20 13 22 – 17 15 11 16 –
± ± ± ± ± ± ± ± ± ±
19 11 17 15 30 17 30 22 8 5
± ± ± ± ±
4 5 13 3 5
± ± ± ±
7 6 7 14
Fig. 14. SEM micrographs of crack tips: (a) quenching spot with Φ = 8 mm and D = 1 mm; (b) quenching spot with Φ = 3 mm and D = 1 mm; (c) and (d) quenching strip vertical to rolling direction. 11
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
rollers. Concerning the untreated rail roller, the pearlite material underwent severe plastic deformation under the rolling/sliding condition (Fig. 16a). Dislocations were generated, slid and piled up, forming empty space, which possessed high energy [31]. The releasing of energy at the surface led to the initiation of fatigue cracks. The cracks propagated with small angles in the shallow surface layer, as shown in Fig. 12b. These shallow cracks were prone to be removed by wear (i.e. delamination) of the surface layer. Concerning the treated rail rollers, the cracks on the untreated surface were similar to those on the untreated rollers, i.e. cracks were propagated with small angles in the shallow surface layer. However, the fatigue cracks in the quenching regions were totally different with those on untreated rollers (Fig. 16). The hardness of the quenching region (i.e. martensite) was high, thus the plastic deformation was hard to occur at the surface of the quenched layer. During the rolling/sliding process, when the regional stress on the surface exceeded the strength limit of the martensite layer, fatigue cracks would initiate on the surface. Furthermore, the cracks tended to initiate at the edge of the quenching region. This is because the edge of quenching layer had smaller thickness and thus relatively lower strength than the middle part of the quenching region. Under repeated loads, cracks propagated into the quenched layer with large angles (e.g. Fig. 13b). As discussed above, quenching shapes and patterns had influence on the anti-fatigue properties of the quenching layer. The fatigue damage was alleviated with the increase in the diameter of quenching spots and the decrease in the LDQ spacing (Figs. 9 and 13). Meanwhile, the gridshaped quenching presented better anti-fatigue property than stripshaped and spot-shaped quenching. Previous researches had a dispute over the law of crack propagation inside martensite layer formed on the rail. Clayton and Allery [32] supposed that the cracks rarely penetrated the boundary between the martensite layer and the matrix, and expanded inside the matrix along the boundary. However, other researches [31] found that cracks could penetrate the boundary and propagate in the matrix. In the present study, the law of crack propagation varied depending on the quenching shapes and patterns. Fig. 15b–d show the different forms of fatigue damage on the quenched rail rollers. For the quenched rail roller with relatively good anti-fatigue property (e.g. quenching spot with Φ = 8 mm and D = 1 mm, as shown in Figs. 13d and 14a), the fatigue cracks stopped inside the martensite layer, and did not penetrate the boundary, as shown in Fig. 16a. For the quenched rail roller with relatively poor anti-fatigue property (e.g. quenching spot with Φ = 3 mm and D = 1 mm, as shown in Figs. 13a and 14b), the direction of cracks turned to be parallel to the boundary, and branch cracks were developed in the quenching region. Fatigue cracks still did not penetrate the boundary. Meanwhile, the convergence of cracks in the quenching region caused the spalling of quenched material, as shown in Fig. 16b. For the quenched rail roller with the worst anti-fatigue property (e.g. quenching strip vertical to rolling direction, as shown in Fig. 13c, d), the fatigue cracks penetrated the boundary and then branch cracks were generated in the matrix. After that the branch cracks propagated along the boundary in the matrix. Meanwhile severe spalling occurred in the quenching layer, as shown in Fig. 16c. In summary, the quenching shapes and patterns had significant influence on the anti-wear and anti-fatigue behaviors. For the spot-shaped quenching, both the wear resistance and the fatigue resistance were improved with the increase in the diameter of quenching spots and the decrease in the LDQ spacing. For the strip-shaped quenching and the grid-shaped quenching, the quenching grid presented better wear resistance and fatigue resistance than quenching strips. Therefore, the quenching shapes and patterns should be taken into consideration for the application of LDQ in the field. Among the specimens in this study, the spot-shaped quenching with the largest diameter Φ = 8 mm, the
Fig. 15. Relationship between wear rate and quenching surface ratio.
[29]. Thus, the wear resistance of untreated rail was dominated by the resistance to plastic deformation. Concerning the treated rail rollers, the wear rates were smaller (Fig. 7). In the treated zone, martensite with high hardness was produced by LDQ, which improved the resistance to plastic deformation, and consequently to wear. This point can also be verified by the topography of the treated rail roller with strips parallel to the rolling direction (as shown in Fig. 9g). The groove in the untreated zone and the ridge in the treated zone indicate that the wear resistance of quenched zone (i.e. martensite) was superior to the untreated rail material (i.e. pearlite). Furthermore, the wear resistance of untreated zone between treated zones could also be improved. According to [30], during the plastic deformation of mixed microstructures, the strength of the soft phase could be enhanced by surrounding hard phases. In the present study, the soft untreated zone was surrounded by hard treated zones. The plastic deformation of the substrate region between quenching regions (Fig. 11b) was much shallower than the untreated roller (Fig. 11a). Thus the wear of the untreated zone between treated zones might be milder than the untreated roller. This could also be verified by the Fig. 9f that groove was generated between two lines of quenching spots but no grooves were generated in the untreated zones between two treated zones in the rolling direction, which indicated that the presence of the harder treated zones could suppress the wear of the softer substrate material between them. As discussed above, LDQ could improve the wear resistance of rail rollers. Furthermore, quenching shapes and patterns had influence on the improvement of wear resistance. The wear resistance was further improved with the increase in the diameter of quenching spots (Fig. 7a) and the decrease in the LDQ spacing (Fig. 7b). Meanwhile, the gridshaped quenching presented better wear resistance than strip-shaped and spot-shaped quenching. In the present study, a parameter –– quenching area ratio (i.e. the ratio of quenching areas to the total surface area on the roller surface) –– was calculated. Fig. 15 presents the relationship between the wear rate and the quenching area ratio. With the increase in the quenching area ratio, the wear rate showed a decrease trend, i.e. the wear resistance was improved with the quenching area ratio. The best wear resistance was obtained on the treated roller with quenching grid. 4.2. Influence of LDQ on fatigue damage of rail material The damage mechanisms of the untreated rail and the LDQ-treated rail were different. Fig. 16 shows the different damage forms of rail
12
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 16. Illustration of damage forms of untreated and treated rails: (a) untreated rail; (b) treated rail with good anti-fatigue property; (c) treated rail with poor antifatigue property; and (d) treated rail with the worst anti-fatigue property.
spot-shaped quenching with the smallest spacing D = 0 mm, and the grid-shaped quenching were superior to other cases. In addition, the fatigue cracks of the quenching regions are severer than the untreated rail material. So, long term tests (> 110,000 cycles) were conducted to see the fatigue damage of the treated samples (quenching spot with Φ = 8 mm and D = 1 mm and the quenching grid) as well as the untreated rail sample. Furthermore, to accelerate the development of cracks, these tests were conducted under the wet condition. The feed rate of water to the wheel/rail interface was 1.6 l/ min. Fig. 17 shows the OM photos after testing. Severe spalling was
observed on the untreated rail roller. On the treated rail rollers, the spalling was milder and pits could be observed. Moreover, further study should be performed in the future to verify by longer term tests whether the cracks of the quenched rail will develop into severe spalling and failure. 5. Conclusions The influence of LDQ on the rolling wear and damage properties of rail material was investigated. Different quenching shapes and patterns
13
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
Fig. 17. Surface damages of rail rollers under wet condition with large cycles: (a) untreated rail roller after 4,180,000 cycles; (b) treated rail roller with quenching spots of Φ = 8 mm and D = 1 mm after 1,100,000 cycles; (c) treated rail roller with quenching grid after 1,230,000 cycles.
were explored. Conclusions can be drawn as following:
Acknowledgements
1. LDQ created quenching regions with an average hardness of around 927 HV0.5 on the surface layer of the rail material. The quenching regions were mainly composed of martensite. 2. LDQ improved the wear resistance of rail material. The wear resistance was further improved with the increase in the diameter of quenching spots and the decrease in the LDQ spacing. The wear resistance of treated rail with quenching grid was better than treated rails with quenching strips or quenching spots. The wear rate of treated rail roller with quenching grid was 0.28 times that of the untreated rail roller. 3. On the untreated rail roller and in the untreated zones on treated rail rollers, cracks propagated with small angles of around 4–17° and small depth of around 11–22 μm. In the quenching regions, cracks propagated with large angles of around 58–80° and large depth of around 73–263 μm. 4. The fatigue resistance of treated rail was improved with the increase in the diameter of quenching spots and the decrease in the LDQ spacing. The fatigue resistance of treated rail with quenching grid was better than treated rail with quenching strips.
The work was supported by Sichuan Science and Technology Program (No.2019YFG0289), National Key Research and Develop Program (No.2016YFB1102601) and Sichuan province University Scientific Research Innovation Team Project (No.18TD0005). References [1] Y. Zhu, Y. Yang, X. Mu, W. Wang, Z. Yao, H. Yang, Study on wear and RCF performance of repaired damage railway wheels: assessing laser cladding to repair local defects on wheels, Wear 430–431 (2019) 126–136. [2] G. Li, Z.Y. Hong, Q.Z. Yan, The influence of microstructure on the rolling contact fatigue of steel for high-speed-train wheel, Wear 342–343 (2015) 349–355. [3] J.X. Wang, X. Chen, X.G. Li, Y.J. Wu, Influence of heavy haul railway curve parameters on rail wear, Eng. Fail. Anal. 57 (2015) 511–520. [4] W.J. Wang, S.R. Lewis, R. Lewis, A. Beagles, C.G. Hem, Q.Y. Liu, The role of slip ratio in rolling contact fatigue of rail materials under wet conditions, Wear 376–377 (2017) 1892–1900. [5] A. Ekberg, B. Akesson, E. Kabo, Wheel/rail rolling contact fatigue–probe, predict, prevent, Wear 314 (2014) 2–12. [6] M. Steenbergen, Rolling contact fatigue: spalling versus transverse fracture of rails, Wear 380–381 (2017) 96–105. [7] Y.B. Huang, L.B. Shi, X.J. Zhao, Z.B. Cai, Q.Y. Liu, W.J. Wang, On the formation and
14
Surface & Coatings Technology 378 (2019) 124991
H.H. Ding, et al.
[8] [9]
[10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
damage mechanism of rolling contact fatigue surface cracks of wheel/rail under the dry condition, Wear 400–401 (2018) 62–73. Q.B. Liu, H. Liu, Experimental study of the laser quenching of 40CrNiMoA steel, J. Mater. Process. Technol. 88 (1999) 77–82. A.K. Nath, S. Sarkar, Laser transformation hardening of steel, Advances in Laser Materials Processing, Second edition, Woodhead Publishing Series in Welding and Other Joining Technologies, 2018, pp. 257–298, , https://doi.org/10.1016/B978-008-101252-9.00011-X. S.M. Shariff, T.K. Pal, G. Padmanabham, S.V. Joshi, Sliding wear behaviour of laser surface modified pearlitic rail steel, Surf. Eng. 26 (2010) 199–208. S.M. Shariff, T.K. Pal, G. Padmanabham, S.V. Joshi, Influence of chemical composition and prior microstructure on diode laser hardening of railroad steels, Surf. Coat. Technol. 228 (2013) 14–26. X.J. Chen, Q. Yan, Q. Ma, Influence of the laser pre-quenched substrate on an electroplated chromium coating/steel substrate, Appl. Surf. Sci. 405 (2017) 273–279. C. Soriano, J. Leunda, J. Lambarri, V. Garcia Navas, C. Sanz, Effect of laser surface hardening on the microstructure, hardness and residual stresses of austempered ductile iron grades, Appl. Surf. Sci. 257 (2011) 7101–7106. D.J. Kong, C.Y. Xie, Effect of laser quenching on fatigue properties and fracture morphologies of boronized layer on Cr12MoV steel, Int. J. Fatigue 80 (2015) 391–396. Z.K. Chen, Q.H. Zhu, J. Wang, X. Yun, B. He, J.S. Luo, Behaviors of 40Cr steel treated by laser quenching on impact abrasive wear, Opt. Laser Technol. 103 (2018) 118–125. L. Orazi, A. Fortunato, G. Cuccolini, G. Tani, An efficient model for laser surface hardening of hypo-eutectoid steels, Appl. Surf. Sci. 256 (2010) 1913–1919. M. Moradi, M.K. Moghadam, High power diode laser surface hardening of AISI 4130; statistical modelling and optimization, Opt. Laser Technol. 111 (2019) 554–570. M. Moradi, M.M. Fallah, S.J. Nasab, Experimental study of surface hardening of AISI 420 martensitic stainless steel using high power diode laser, Trans. Indian Inst. Metals 71 (2018) 2043–2050. M. Moradi, D. Ghorbani, M.K. Moghadam, M. Kazazi, F. Rouzbahani, S. Karazi, Nd:YAG laser hardening of AISI 410 stainless steel: microstructural evaluation, mechanical properties, and corrosion behavior, J. Alloys Compd. 795 (2019) 213–222.
[20] W. Su, T. Zhou, P. Zhang, H. Zhou, H. Li, Effect of distribution of striated laser hardening tracks on dry sliding wear resistance of biomimetic surface, Opt. Laser Technol. 98 (2018) 281–290. [21] J.R. Jiang, L.J. Xue, S.D. Wang, Discrete laser spot transformation hardening of AISI O1 tool steel using pulsed Nd:YAG laser, Surf. Coat. Technol. 205 (2011) 5156–5164. [22] Z.Y. Li, X.H. Xing, M.J. Yang, B. Yang, Z.Y. Ren, H.Q. Lin, Investigation on rolling sliding wear behavior of wheel steel by laser dispersed treatment, Wear 314 (2014) 236–240. [23] D.F. Zeng, L.T. Lu, Z.Y. Li, J.W. Zhang, X.S. Jin, M.H. Zhu, Influence of laser dispersed treatment on rolling contact wear and fatigue behavior of railway wheel steel, Mater. Des. 54 (2014) 137–143. [24] Y.L. Zheng, Q.W. Hu, C.Y. Li, D.Z. Wang, L. Meng, J.G. Luo, J.P. Wang, X.Y. Zeng, A novel laser surface compositing by selective laser quenching to enhance railway service life, Tribol. Int. 106 (2017) 46–54. [25] X. Cao, L.B. Shi, Z.B. Cai, Q.Y. Liu, Z.R. Zhou, W.J. Wang, Investigation on the microstructure and damage characteristics of wheel and rail materials subject to laser dispersed quenching, Appl. Surf. Sci. 450 (2018) 468–483. [26] M. Moradi, M.K. Moghadam, M. Kazazi, Improved laser surface hardening of AISI 4130 low alloy steel with electrophoretically deposited carbon coating, Optik 178 (2019) 614–622. [27] C.R. Su, L.B. Shi, W.J. Wang, D.Z. Wang, Z.B. Cai, Q.Y. Liu, Z.R. Zhou, Investigation on the rolling wear and damage properties of laser dispersed quenched rail materials treated with different ratios, Tribol. Int. 135 (2019) 488–499. [28] H.C. Shi, L.B. Shi, H.H. Ding, W.J. Wang, W.J. Jiang, J. Guo, Q.Y. Liu, Influence of laser strengthening techniques on anti-wear and anti-fatigue properties of rail welding joint, Eng. Fail. Anal. 101 (2019) 72–85. [29] H.H. Ding, C.G. He, L. Ma, J. Guo, Q.Y. Liu, W.J. Wang, Wear mapping and transitions in wheel and rail materials under different contact pressure and sliding velocity conditions, Wear 352-353 (2016) 1–8. [30] C.H. Young, H.K.D.H. Bhadeshia, Strength of mixtures of bainite and martensite, Mater. Sci. Technol. 10 (1994) 209–214. [31] Y.D. Chen, X.J. Zhao, P.T. Liu, R. Pan, R.M. Ren, Influences of local laser quenching on wear performance of D1 wheel steel, Wear 414–415 (2018) 243–250. [32] P. Clayton, M.B.P. Allery, Metallurgical aspects of surface damage problems in rails, Can. Metall. Q. 21 (1982) 31–46.
15