- Email: [email protected]
Investigation on wear and damage performance of laser cladding Co-based alloy on single wheel or rail material
Author's Accepted Manuscript
Investigation on wear and damage performance of laser cladding Co-based alloy on single wheel or rail material Huo-ming Guo, Qian Wang, Wen-jian Wang, Jun Guo, Qi-yue Liu, Min-hao Zhu
www.elsevier.com/locate/wear
PII: DOI: Reference:
S0043-1648(15)00174-X http://dx.doi.org/10.1016/j.wear.2015.03.002 WEA101370
To appear in:
Wear
Received date: 9 September 2014 Revised date: 25 February 2015 Accepted date: 3 March 2015 Cite this article as: Huo-ming Guo, Qian Wang, Wen-jian Wang, Jun Guo, Qiyue Liu, Min-hao Zhu, Investigation on wear and damage performance of laser cladding Co-based alloy on single wheel or rail material, Wear, http://dx.doi.org/ 10.1016/j.wear.2015.03.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation on wear and damage performance of laser cladding Co-based alloy on single wheel or rail material Huo-ming Guo
，Qian Wang，Wen-jian Wang ，Jun Guo，Qi-yue Liu，Min-hao Zhu ∗
Tribology Research Institute, State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China Abstract: The aim of this study is to investigate the microstructure and wear behavior of laser cladding Co-based alloy coating on single wheel or rail material (the laser cladding coating is applied only to the wheel or to the rail) using a rolling-sliding wear test apparatus. The coating of laser cladding Co-based alloy consists of dendrite and eutectic. Single-treated wheel or rail specimen undergone the laser cladding treatment effectively decreases rolling friction coefficient and improve wear resistance of both wheel and rail materials. Single-treated wheel or rail roller presents mild wear damage. Furthermore, it is helpful to alleviate the surface damage and plastic flow of wheel and rail rollers. It is concluded that the laser cladding Co-based alloy coating is suitable for single treatment of wheel or rail material. However, further work should be carried out to clarify the rolling contact fatigue mechanism and to characterise the laser cladding Co-based alloy coating. Keywords: Laser cladding; wheel/rail materials; microstructure; rolling wear; surface damage
1. Introduction
With a steady increase in total traffic and loads, the wheel/rail is frequently asked to sustain service conditions more severe than those considered in their original design. A variety of damages happen on wheel/rail system such as the side wear of wheel/rail, rail corrugation, head checks, and so on [1-4]. It is well known that the increase of axle load and traffic volume ∗
Corresponding author. Tel: +86-28-87634304.
E-mail address: [email protected] (Wen-jian Wang).
1
of railway leads to excessive wear of wheel/rail materials and remarkably decreases service life of wheel/rail system [5-7]. Therefore, various methods to improve the wear resistance of wheel/rail materials have been investigated and analyzed [8-11]. In order to improve the wear performance of wheel/rail material, a series of laser surface treatment technologies of wheel/rail material have been used to protect the components from wear and fatigue damage, such as laser quenching, laser melting, and so on [12-13]. The failure of wheel/rail is primarily determined by the surface properties of materials. Therefore, laser cladding wear-resistance material on the top of wheel/rail is one of feasible methods to improve the wear resistance of wheel/rail materials. The advantage of laser cladding is that metals with better mechanical and tribological properties can be welded on top of the original substrate material. This method is potential to be a more cost-effective way of treating rail or repairing some small defects on the wheel/rail surface. Franklin has developed a railhead with an additional surface layer and reported the rolling contact fatigue and wear behavior [14]. Generally speaking, if only wheel or rail material is treated, the difference in hardness of wheel/rail rollers would lead in serious wear and damage of wheel or rail roller without treatment. It should be noted that the improvement of wear resistance of materials resulting from laser quenching markedly decreases wear volume of wheel/rail rollers. On the other hand, single wheel or rail roller that suffered laser quenching treatment obviously aggravate the wear of other friction roller without laser quenching due to the difference in hardness [15]. Therefore, the laser quenching is not suitable for the application of single wheel or rail material. Wang has reported that the laser cladding coating can decrease the wear and damage when both wheel and rail rollers that
2
suffered laser cladding treatment [16]. In this paper, the wear performance of single wheel or rail roller undergone laser cladding is investigated in detail for clarifying anti-wear mechanism of single wheel or rail material treated by laser cladding.
2. Experimental details
The experiments were performed using a rolling-sliding wear test apparatus [17]. Fig.1 shows a schematic of the test apparatus and the geometry of specimens. This machine uses the line contact between twin-discs specimens to simulate the normal load and rolling-sliding behavior at wheel/rail interface. The width of contact area between wheel roller (bottom specimen) and rail roller (top specimen) is 5 mm. The wheel and rail rollers are driven and controlled by a DC motor. The difference of speed of wheel and rail rollers is achieved by means of different gear drive. The top specimen was mounted in a swinging bracket to which load can range from 0 to 2 KN and is applied by a compressed spring. The sliding ratio is 9.45% during the experiment. The speed of wheel roller is 200.0 rpm and the speed of rail roller is 181.1 rpm. The number of cycles of wheel roller is 1.44×105 in the experiment. The normal force in the study is 437 N and the maximal contact pressure of wheel/rail rollers is about 567 MPa. The wheel/rail specimens are cut from real wheel and rail tread. Chemical compositions in weight percentage and mechanical properties of wheel and rail materials resulting from a series of tensile tests (wheel: CL60, rail: U71Mn) are given in Table 1. The substrate material was treated with equipment for the TR-3000 multi-mode crosscurrent CO2 laser. Co-based alloy powders are used to clad and the chemical compositions in weight percentage are given
3
in Table 2. During the laser cladding process, the rectangular spot size is 1×7 mm and the power is 2 kW. The scanning speed is 200 mm/min and the flow rate of alloy powder is about 15 g/min. Each specimen was purposely manufactured undersized to a diameter of 38 mm. The clad layer then restored the specimen to a diameter of 40 mm. The wear experiments were performed in un-lubricated condition and the contact surfaces of wheel/rail were cleaned with alcohol before and after testing. The rollers were weighed before and after each test to monitor wear mass using an electronic balance (TG328A). The friction coefficient is defined as the ratio of friction force to normal force. The friction force and normal force of the interface of wheel/rail rollers are measured and recorded automatically on the computer using a torque sensor and load sensor (Measurement error: ± 5%). The microstructure and wear damage mechanism of single-treated wheel or rail roller were investigated by examining wear scar using optical microscopy (OM) (OLYMPUS BX60M, Japan) and scanning electronic microscopy (SEM) (QUANTA200, FEI, England).
3. Results
3.1 Microstructure and hardness of laser cladding coating Fig.2 (a) shows the microstructure of the laser cladding coatings. A clear metallurgical bond is observed between the coatings and the substrate. Three different regions are presented in the surface layer, i.e. untreated substrate regions, heat affected regions and clad regions. In the deposition of Co-based powder onto the wheel/rail specimen substrate, all cladding fused with the substrate without crack and a desirable coating is observed. Few pores are formed as a result of thermal mismatch between the clad and the substrate and also by trapped gases 4
within the powder during cladding. Microstructures observed are predominantly cellular and columnar dendrites are found within the structure. Columnar dendrites are seen at the boundary between clad and the substrate and cellular dendrites are found in the middle of coatings and around the top surface (Fig.2 (b)), and it is also noted that the grain size is smaller at the top of the clad than in the interior. The variation in microstructure owes to the different growth conditions at the respective regions. This growth direction in laser cladding corresponds to the main heat transfer direction during solidification [18]. Fig.2 (c) shows the microstructure of the dendrite and eutectic. The coating consists of dendrite and eutectic. It can be seen from Fig.2 (d) that some martensite is observed in the heat affected region and the organization is more complicated in the transition bond. The chemical composition (Fe, Co, Cr, Ni and Si) profile along the clad thickness obtained by EDX is described in Fig.3. Although the content of substrate elements (Fe) decreases slightly along the clad, their presence in the whole cross-section is quite homogeneous due to the strong molten pool convection previous to solidification. As measured, the other two elements (Co and Cr) increase slowly and wave slightly at the dendrites and eutectic phase. The transition band between clad composition and substrate composition is fairly narrow, which is approximately 50 µm. Moreover, the diffusion of the substrate material into the clad is negligible at points below 25 µm from interface. Furthermore, the substrate of wheel specimens has an average hardness of 286 HV0.5 and the rail substrate has an average hardness of 305 HV0.5. The hardness of wheel/rail specimens with laser cladding Co-based alloy increases significantly and the coating has an average hardness of 438 HV0.5 on the surface, which increases by 53.0% and 43.4%, respectively.
5
3.2 Friction and wear behaviors Fig.4 displays rolling friction coefficient for the cladding coating and the substrate as a function of the testing cycles. In the case of the untreated wheel/rail, the friction coefficient increases slightly up to 0.71 and maintains at a steady state until the end of the test. However, it is obvious that the rolling friction coefficient decreases markedly and keeps stable after testing for about 2000 cycles when single wheel or rail specimen undergoes laser cladding treatment. In addition, the friction coefficient of single-treated wheel roller is smaller compared to other wheel/rail rollers. The difference in wear debris from wheel/rail contact interface may cause different friction coefficient. This result indicates that the laser cladding treatment can effectively decrease the rolling friction coefficient. Wear rate of the wheel and rail rollers (the ratio of weight loss of wheel or rail roller to rolling distance of respective wheel or rail roller, µg.mm-1) is described in Fig.5. It can be seen that the wear rate of untreated wheel/rail specimens reaches a maximum whereas the wear rate of single treated wheel or rail roller decreases markedly, shown in Fig.5 (a). By comparison, the wear rate of wheel and rail specimens decreases 88.6% and 37.3% when single-treated wheel roller is used, and the wear rate of wheel and rail specimens decreases 20.1% and 79.1% when single rail roller suffers laser cladding treatment. Furthermore, total wear rate of single-treated wheel or rail specimen decreases 69.4% and 42.2% compared to the untreated wheel/rail specimens. In addition, total wear rate of wheel and rail rollers reaches a minimum when single-treated wheel roller is used, shown in Fig.5 (b). Therefore, it is concluded that the laser cladding coating is helpful to improve wear resistance and decrease the wear of wheel/rail system when only wheel or rail material is treated by laser cladding. 6
3.3 Surface damage The observations from Fig.6 (a) show that the wear surface of wheel specimen presents severe spalling and adhesion wear. Furthermore, obvious delaminating damage of rail specimen without treatment is dominant on the wear surface, shown in Fig.6 (b). There are small fatigue micro-cracks in the wear surface of rail roller and the propagation of surface cracks result in the material wearing off from the roller surface and the formation of large fatigue delamination and spalling damage. When single wheel specimen is treated, the surface damage of the wheel roller markedly lightens and presents slight adhesion wear (Fig.7). However, there are surface fatigue micro-cracks in the wear surface of the rail roller without laser cladding coating, shown in Fig.7 (b) and the surface damage presents slight fatigue delamination mechanism. It is evident that the laser cladding coating of the wheel roller alleviates the surface damage of the rail roller without laser treatment. Similarly, when single-treated rail specimen is used (Fig.8), the surface of the rail roller presents very mild adhesion wear and ploughing phenomena. The obvious spalling damage and delamination wear are dominant for the wheel specimen without laser cladding coating. It is observed from cross-sectional plastic deformation of wheel/rail specimens in Fig.9 that there is severe plastic flow along the rolling direction for the wheel/rail rollers without laser treatment. At the same time, the fatigue cracks are easy to initiate on the subsurface of rollers and propagate along the plastic deformation direction, shown in Fig.10 (a). When single wheel or rail specimen is treated by laser cladding, slight plastic flow appears in the cross-section of wheel/rail rollers with laser cladding coating (Fig.9 (b), (c)). Moreover, the friction pair of wheel or rail without cladding coating have obvious plastic flowing layer. But 7
the thickness of deformation is reduced compared to both wheel and rail specimens without laser cladding treatment. That is to say, single-treated wheel or rail specimen is beneficial to enhance the resistance-deformation ability of wheel/rail. Furthermore, no obvious fatigue crack is found on the subsurface and contact surface of wheel or rail rollers undergone laser cladding treatment, shown in Fig.10 (b) and (c). However, the wheel or rail roller without laser cladding coating shows visible fatigue cracks along plastic flowing direction, which is easy to result in the removal of material due to the propagation and connectivity of fatigue cracks. Therefore, it is concluded that the laser cladding coating exhibits good resistance-fatigue crack performance.
4. Discussion
It should be noted that the laser treatment is an effective method for improving the wear resistance of materials. Significant difference in hardness of single wheel or rail material undergone laser quenching would increase total wear of wheel/rail system [15]. That is to say, this laser quenching treatment is not suitable for single treatment of wheel or rail material in the practical application. Single-treated wheel or rail roller has an important effect on the wear and surface damage of wheel/rail, which can decrease the wear rate of wheel and rail rollers during the testing process. Furthermore, the rolling friction coefficient of single-treated wheel or rail roller is smaller than that of the wheel/rail rollers without laser cladding treatment (Fig.4). The result has found that the cladding coating is composed of γ-Co phase and carbide Cr23C6 [16]. Very fine hard phases (Cr23C6) are dispersively distributed in the phases of laser cladding coating, which result in low friction coefficient and high wear resistance [19].
8
Therefore, the laser cladding Co-based alloy coating on surface of wheel or rail specimen effectively improves the wear resistance of wheel/rail system. The grain size of laser cladding Co-based alloy coating becomes smaller compared to the wheel/rail materials without treatment, which increases markedly the strength [20]. These benefit from the effect of rapid condensation of laser cladding. In addition, some alloy elements form the second phase during the cladding process and it effectively improves the hardness of coatings [21-22]. This is helpful to enhance the fatigue resistance performance of laser cladding Co-based alloy coating. Therefore, the wheel and rail rollers undergone laser cladding treatment exhibit excellent fatigue crack resistance and no obvious fatigue crack is found in cross-section of worn wheel/rail specimen (Fig.10). As stated above that cladding Co-based alloy on the surface of single specimen would decrease the wear and damage of wheel or rail material. The materials are difficult to be removed from the wheel/rail specimens due to high hardness with laser cladding during the rolling wear process. Meanwhile, fine hard phases dispersive distribution in Co-based alloy clad layer improves the friction characteristics of wheel/rail interface, which is beneficial for the reduction of wear rate and surface damage of untreated wheel or rail specimen. It is concluded that this laser cladding Co-based alloy coating is suitable to be used to alleviate the wear of wheel/rail materials and prolong wear life of wheel/rail system by means of single treatment of wheel or rail material. Moreover, this laser cladding coating can be used to repair the damaged heavy-haul railway wheel and rail. Moreover, further work should be carried out to clarify the rolling contact fatigue performance and mechanism of laser cladding Co-based alloy coating.
9
5. Conclusions
1. Three different regions (untreated substrate regions, heat affected regions and clad regions) are presented in the cladding layer. The thick columnar and cellar dendrites were observed in the clad regions and the grain size is smaller compared to the substrate. 2. Single-treated wheel or rail specimen undergone the laser cladding effectively decreases the rolling friction coefficient and improve wear resistance of both wheel and rail materials. Compared to the untreated system, the wear rate of the rail/wheel system was reduced by the deposition of the laser clad layer on either the rail or the wheel, with the lowest wear rate being observed for the system with the laser clad wheel material. Moreover, when either the rail or wheel material was clad, the wear rate of the untreated counterbody was less than that observed for the same material when both the rail and wheel material were worn in the untreated condition (i.e. the cladding of one of the bodies did not result in an increase in the wear rate of the untreated counterbody). 3. The untreated wheel or untreated rail specimens present serious wear and surface damage. The wear mechanism of untreated wheel rollers is adhesion wear and the delaminating damage is dominant for untreated rail rollers. Treated wheel or rail roller exhibits mild wear damage and is beneficial to alleviate the wear and plastic flow of wheel/rail materials. 4. It illustrates that the laser cladding Co-based alloy coating is suitable for single treatment of wheel or rail material.
Acknowledgements
10
The work was supported by the National Natural Science Foundation of China (No.51174282),
Sichuan
Province
Science
and
Technology
Support
Program
(No.2014GZ0009-6) and Innovative Research Teams in University (No.IRT1178). The authors are grateful to reviewers for valuable technical advice and kind help in improving the English text of the paper.
References
[1]Q.Y. Liu, B. Zhang, Z.R. Zhou, An experimental study of rail corrugation, Wear 255 (2003) 1121-1126. [2]J.E. Garnham, C.L. Davis, The role of deformed rail microstructure on rolling contact fatigue initiation, Wear 265 (2008) 1363-1372. [3]W.J. Wang, H.M. Guo, X. Du, J. Guo, Q.Y. Liu, M.H. Zhu, Investigation on the damage mechanism and prevention of heavy-haul railway rail, Eng. Fail. Anal. 35 (2013) 206-218. [4]Y. Zhou, S. Wang, T. Wang, Y. Xu, Z. Li, Field and laboratory investigation of the relationship between rail head check and wear in a heavy-haul railway, Wear 315 (2014) 68-77. [5]J. Sandstrom, A. Ekberg, Predicting crack growth and risks of rail breaks due to wheel flat impacts in heavy haul operations, Proc. IMechE, Part F: J. Rail Rapid Transit 223 (J6) (2009) 153-161. [6]Z.L. Li, X. Zhao, C. Esveld, R. Dollevoet, M. Molodova, An investigation into the causes of squats-correlation analysis and numerical modeling, Wear 265 (2008) 1349-1355. [7]C. González-Nicieza, M.I. Álvarez-Fernández, A. Menéndez-Díaz, A.E. Álvarez-Vigil, F.
11
Ariznavarreta-Fernández, Failure analysis of concrete sleepers in heavy haul railway tracks, Eng. Fail. Anal. 15 (2008) 90-117. [8]R. Lewis, R.S. Dwyer-Joyce, Wear at the wheel/rail interface when sanding is used to increase adhesion, Proc. IMechE, Part F: J. Rail Rapid Transit 220 (5) (2006) 29-41. [9]O. Arias-Cuevas, Z.L. Li, R. Lewis, Investigating the lubricity and electrical insulation caused by sanding in dry wheel-rail contacts, Tribology Letters 37 (2010) 623-635. [10]S. Descartes, C. Desrayaud, Y. Berthier, Experimental identification and characterization of the effects of contaminants in the wheel-rail contact, Proc. IMechE, Part F: J. Rail Rapid Transit 222 (1) (2008) 207-216. [11]M. Spiryagin, K.S. Lee, H.H. Yoo, Control system for maximum use of adhesive forces of a railway vehicle in a tractive mode, Mechanical Systems and Signal Processing 22 (2008) 709-720. [12]R.J. DiMelfi, P.G. Sanders, B. Hunter, J.A. Eastman, K.J. Sawley, K.H. Leong, J.M. Kramer, Mitigation of subsurface crack propagation in railroad rails by laser surface modification, Surface and Coatings Technology 106 (1998) 30-43. [13]S. Aldajah, O.O. Ajayi, G.R. Fenske, S. Kumar, Investigation of top of rail lubrication and laser glazing for improved railroad energy efficiency, Journal of Tribology 125 (2003) 643-648. [14]F. Franklin, G.J. Weeda, A. Kapoor, E. Hiensch, Rolling contact fatigue and wear behavior of the infrastar two-material rail, Wear 258 (2005) 1048-1054. [15]W.J. Wang, J. Guo, Q.Y. Liu, M.H. Zhu, Effect of laser quenching on wear and damage of heavy-haul wheel/rail materials, Proc. IMechE, Part J: Journal of Engineering Tribology 228
12
(1) (2014) 114-122. [16]W.J. Wang, J. Hu, J. Guo, Q.Y. Liu, M.H. Zhu, Effect of laser cladding on wear of heavy-haul wheel/rail materials, Wear 311 (2014) 130-136. [17]W.J. Wang, W.J. Jiang, H.Y. Wang, Q.Y. Liu, M.H. Zhu, X.S. Jin, Experimental study on the wear and damage behavior of different wheel/rail materials, Journal of Rail and Rapid Transit, http://pif.sagepub.com/content/early/2014/03/09/0954409714524566,2014. [18]V. Ocelík, I. Furár, J.Th.M. D. Hosson, Microstructure and properties of laser clad coatings studied by orientation imaging microscopy, Acta Materialia 58 (2010) 6763-6772. [19]J.S. Wang, H.X. Lu, Z.W. Yan, G. Li, M.Q. Tang, Z.Q. Feng, Wear-resistance of WC-4Co coating on cast steel by electro-spark deposition, Transactions of Materials and Heat Treatment 36 (1) (2015) 169-172. [20]R. Vilar, R. Colaco, A. Almeida, Laser surface treatment of tool steels, Optical and Quantum Electronic 27 (1995) 1273-1289. [21]J.R. Jiang, L.J. Xue, S.D. Wang, Discrete laser spot transformation hardening of AISIO1 tool steel using pulsed Nd: YAG laser, Surf. Coat. Techno. 205 (21-22) (2011) 5156-5164. [22]C.Y. Cui, J.D. Hu, Y.H. Liu, K. Gao, Z.X. Guo, Formation of nano-crystalline and amorphous on the surface of stainless steel by Nd: YAG pulsed laser irradiation, Appl. Surf. Sci. 254 (21) (2008) 6779-6782.
13
Table caption Table 1 Chemical compositions and mechanical properties of wheel/rail materials (%wt). Table 2 Chemical compositions of the powder (%wt).
14
Table 1
Specimens
C%
Si%
Mn%
P%
S%
σb/MPa
δ5/%
CL60 wheel
0.55~0.65 0.17~0.37 0.50~0.80
≤0.035
≤0.040
910
10
U71Mn rail
0.65~0.76 0.15~0.35 1.10~1.40
≤0.030
≤0.030
880
9
15
Table 2
Element
C
Si
Fe
Cr
Ni
W
Co
Powder
1.1
1.0
1.5
28.5
1.5
4.4
Vol.
16
Highlights 1. The coating of laser cladding Co-based alloy consists of dendrite and eutectic. 2. Single-treated wheel or rail specimen effectively decreases friction coefficient. 3. Single-treated wheel or rail roller improves wear resistance of materials. 4. Single-treated wheel or rail roller presents mild wear damage.
17
Figure caption Fig.1: Schematic of test apparatus and the geometry of specimens (a) rolling-sliding wear test apparatus; (b) the geometry of wheel/rail specimens. Fig.2: Microstructure of the cladding coatings, (a) bottom of coating; (b) middle of coating; (c) high power structure; (d) heat affected region. Fig.3: Composition profile for Co, Cr, Ni, Si and Fe along the clad thickness. Fig.4: Rolling friction coefficient of wheel/rail specimens. Fig.5: Wear rate of wheel/rail specimens, (a) wear rate of wheel/rail specimens; (b) total wear rate. Fig.6: SEM micrographs of worn surface of untreated wheel/untreated rail specimens, (a) untreated wheel specimen; (b) untreated rail specimen. Fig.7: SEM micrographs of worn surface of treated wheel/untreated rail specimens, (a) treated wheel specimen; (b) untreated rail specimen. Fig.8: SEM micrographs of worn surface of untreated wheel/treated rail specimens, (a) untreated wheel specimen; (b) treated rail specimen. Fig.9: Cross-sectional plastic deformation of wheel/rail specimens, (a) untreated wheel/rail specimens; (b) single-treated wheel specimen; (c) single-treated rail specimen. Fig.10: Fatigue damage of wheel/rail specimens, (a) untreated wheel/rail specimens; (b) single-treated wheel specimen; (c) single-treated rail specimen.
18
Fig.1
7
8
9
10
12
11
13
6
Electrical cabinet 5 4
14
15
16
3
Rolling-sliding wear
MMS-2A rolling wear test testapparatus apparatus
2
1
、、、
1- Double speed motor; 2- Triangular belt; 3- Pressure sensor; 4 5 6 9-Gear; 7- Swinging bracket; 8Internal gear; 10- Speed sensor; 11- Bottom specimen; 12- Top specimen; 13- Bolt; 14-Spring; 15-Nut; 16Load sensor
(a)
(b)
19
Fig.2
Top surface of coatings
Substrate Coatings
Columnar dendrites
Cellular dendrites
Transition band
Columnar dendrites
(a)
(b) Coatings Transition band
Dendrites
Martensite Eutectic
(c)
(d)
20
Fig.3
140 Fe 120
Co
Cr
Ni
Si
Transition band
Atom fraction/%
100 Substrate
80
Cladding coating
60 40 20 0 0
90
180 270 Distance/µm
21
360
450
Fig.4
0.90 Untreated wheel/rail
Friction coefficient µ
0.75 0.60
Single-treated rail
0.45 0.30
Single-treated wheel
0.15 0.00 0
0.4 0.8 1.2 1.6 2 Nunber of rolling cycles(×104)
22
2.4
Fig.5
3.5
2.0 Wheel specimen
2.8
Rail specimen
Wear rate/µg.mm-1
Wear rate/µg.mm-1
1.6 1.2 0.8 0.4
2.1 1.4 0.7 0.0
0.0 Untreated
Untreated
Single-treated Single-treated wheel rail
(a)
Single-treated Single-treated wheel rail
(b)
23
Fig.6
(a)
Surface crack
(b)
24
Fig.7
(a)
Surface crack
(b)
25
Fig.8
Surface crack
(a)
(b)
26
Fig.9
Wheel
Rail
Rolling direction
100µm
Rolling direction
100µm
(a) Wheel
Rail
Rolling direction
100µm
Rolling direction
100µm
(b) Wheel
Rail
Rolling direction
100µm
Rolling direction
100µm
(c)
27
Fig.10
Wheel
Rail
Fatigue crack Fatigue crack
(a) Wheel
Rail
Fatigue crack
(b) Wheel
Rail
Fatigue crack
(c)
28
Investigation on wear and damage performance of laser cladding Co-based alloy on single wheel or rail material Huo-ming Guo, Qian Wang, Wen-jian Wang, Jun Guo, Qi-yue Liu, Min-hao Zhu
www.elsevier.com/locate/wear
PII: DOI: Reference:
S0043-1648(15)00174-X http://dx.doi.org/10.1016/j.wear.2015.03.002 WEA101370
To appear in:
Wear
Received date: 9 September 2014 Revised date: 25 February 2015 Accepted date: 3 March 2015 Cite this article as: Huo-ming Guo, Qian Wang, Wen-jian Wang, Jun Guo, Qiyue Liu, Min-hao Zhu, Investigation on wear and damage performance of laser cladding Co-based alloy on single wheel or rail material, Wear, http://dx.doi.org/ 10.1016/j.wear.2015.03.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Investigation on wear and damage performance of laser cladding Co-based alloy on single wheel or rail material Huo-ming Guo
，Qian Wang，Wen-jian Wang ，Jun Guo，Qi-yue Liu，Min-hao Zhu ∗
Tribology Research Institute, State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu 610031, China Abstract: The aim of this study is to investigate the microstructure and wear behavior of laser cladding Co-based alloy coating on single wheel or rail material (the laser cladding coating is applied only to the wheel or to the rail) using a rolling-sliding wear test apparatus. The coating of laser cladding Co-based alloy consists of dendrite and eutectic. Single-treated wheel or rail specimen undergone the laser cladding treatment effectively decreases rolling friction coefficient and improve wear resistance of both wheel and rail materials. Single-treated wheel or rail roller presents mild wear damage. Furthermore, it is helpful to alleviate the surface damage and plastic flow of wheel and rail rollers. It is concluded that the laser cladding Co-based alloy coating is suitable for single treatment of wheel or rail material. However, further work should be carried out to clarify the rolling contact fatigue mechanism and to characterise the laser cladding Co-based alloy coating. Keywords: Laser cladding; wheel/rail materials; microstructure; rolling wear; surface damage
1. Introduction
With a steady increase in total traffic and loads, the wheel/rail is frequently asked to sustain service conditions more severe than those considered in their original design. A variety of damages happen on wheel/rail system such as the side wear of wheel/rail, rail corrugation, head checks, and so on [1-4]. It is well known that the increase of axle load and traffic volume ∗
Corresponding author. Tel: +86-28-87634304.
E-mail address: [email protected] (Wen-jian Wang).
1
of railway leads to excessive wear of wheel/rail materials and remarkably decreases service life of wheel/rail system [5-7]. Therefore, various methods to improve the wear resistance of wheel/rail materials have been investigated and analyzed [8-11]. In order to improve the wear performance of wheel/rail material, a series of laser surface treatment technologies of wheel/rail material have been used to protect the components from wear and fatigue damage, such as laser quenching, laser melting, and so on [12-13]. The failure of wheel/rail is primarily determined by the surface properties of materials. Therefore, laser cladding wear-resistance material on the top of wheel/rail is one of feasible methods to improve the wear resistance of wheel/rail materials. The advantage of laser cladding is that metals with better mechanical and tribological properties can be welded on top of the original substrate material. This method is potential to be a more cost-effective way of treating rail or repairing some small defects on the wheel/rail surface. Franklin has developed a railhead with an additional surface layer and reported the rolling contact fatigue and wear behavior [14]. Generally speaking, if only wheel or rail material is treated, the difference in hardness of wheel/rail rollers would lead in serious wear and damage of wheel or rail roller without treatment. It should be noted that the improvement of wear resistance of materials resulting from laser quenching markedly decreases wear volume of wheel/rail rollers. On the other hand, single wheel or rail roller that suffered laser quenching treatment obviously aggravate the wear of other friction roller without laser quenching due to the difference in hardness [15]. Therefore, the laser quenching is not suitable for the application of single wheel or rail material. Wang has reported that the laser cladding coating can decrease the wear and damage when both wheel and rail rollers that
2
suffered laser cladding treatment [16]. In this paper, the wear performance of single wheel or rail roller undergone laser cladding is investigated in detail for clarifying anti-wear mechanism of single wheel or rail material treated by laser cladding.
2. Experimental details
The experiments were performed using a rolling-sliding wear test apparatus [17]. Fig.1 shows a schematic of the test apparatus and the geometry of specimens. This machine uses the line contact between twin-discs specimens to simulate the normal load and rolling-sliding behavior at wheel/rail interface. The width of contact area between wheel roller (bottom specimen) and rail roller (top specimen) is 5 mm. The wheel and rail rollers are driven and controlled by a DC motor. The difference of speed of wheel and rail rollers is achieved by means of different gear drive. The top specimen was mounted in a swinging bracket to which load can range from 0 to 2 KN and is applied by a compressed spring. The sliding ratio is 9.45% during the experiment. The speed of wheel roller is 200.0 rpm and the speed of rail roller is 181.1 rpm. The number of cycles of wheel roller is 1.44×105 in the experiment. The normal force in the study is 437 N and the maximal contact pressure of wheel/rail rollers is about 567 MPa. The wheel/rail specimens are cut from real wheel and rail tread. Chemical compositions in weight percentage and mechanical properties of wheel and rail materials resulting from a series of tensile tests (wheel: CL60, rail: U71Mn) are given in Table 1. The substrate material was treated with equipment for the TR-3000 multi-mode crosscurrent CO2 laser. Co-based alloy powders are used to clad and the chemical compositions in weight percentage are given
3
in Table 2. During the laser cladding process, the rectangular spot size is 1×7 mm and the power is 2 kW. The scanning speed is 200 mm/min and the flow rate of alloy powder is about 15 g/min. Each specimen was purposely manufactured undersized to a diameter of 38 mm. The clad layer then restored the specimen to a diameter of 40 mm. The wear experiments were performed in un-lubricated condition and the contact surfaces of wheel/rail were cleaned with alcohol before and after testing. The rollers were weighed before and after each test to monitor wear mass using an electronic balance (TG328A). The friction coefficient is defined as the ratio of friction force to normal force. The friction force and normal force of the interface of wheel/rail rollers are measured and recorded automatically on the computer using a torque sensor and load sensor (Measurement error: ± 5%). The microstructure and wear damage mechanism of single-treated wheel or rail roller were investigated by examining wear scar using optical microscopy (OM) (OLYMPUS BX60M, Japan) and scanning electronic microscopy (SEM) (QUANTA200, FEI, England).
3. Results
3.1 Microstructure and hardness of laser cladding coating Fig.2 (a) shows the microstructure of the laser cladding coatings. A clear metallurgical bond is observed between the coatings and the substrate. Three different regions are presented in the surface layer, i.e. untreated substrate regions, heat affected regions and clad regions. In the deposition of Co-based powder onto the wheel/rail specimen substrate, all cladding fused with the substrate without crack and a desirable coating is observed. Few pores are formed as a result of thermal mismatch between the clad and the substrate and also by trapped gases 4
within the powder during cladding. Microstructures observed are predominantly cellular and columnar dendrites are found within the structure. Columnar dendrites are seen at the boundary between clad and the substrate and cellular dendrites are found in the middle of coatings and around the top surface (Fig.2 (b)), and it is also noted that the grain size is smaller at the top of the clad than in the interior. The variation in microstructure owes to the different growth conditions at the respective regions. This growth direction in laser cladding corresponds to the main heat transfer direction during solidification [18]. Fig.2 (c) shows the microstructure of the dendrite and eutectic. The coating consists of dendrite and eutectic. It can be seen from Fig.2 (d) that some martensite is observed in the heat affected region and the organization is more complicated in the transition bond. The chemical composition (Fe, Co, Cr, Ni and Si) profile along the clad thickness obtained by EDX is described in Fig.3. Although the content of substrate elements (Fe) decreases slightly along the clad, their presence in the whole cross-section is quite homogeneous due to the strong molten pool convection previous to solidification. As measured, the other two elements (Co and Cr) increase slowly and wave slightly at the dendrites and eutectic phase. The transition band between clad composition and substrate composition is fairly narrow, which is approximately 50 µm. Moreover, the diffusion of the substrate material into the clad is negligible at points below 25 µm from interface. Furthermore, the substrate of wheel specimens has an average hardness of 286 HV0.5 and the rail substrate has an average hardness of 305 HV0.5. The hardness of wheel/rail specimens with laser cladding Co-based alloy increases significantly and the coating has an average hardness of 438 HV0.5 on the surface, which increases by 53.0% and 43.4%, respectively.
5
3.2 Friction and wear behaviors Fig.4 displays rolling friction coefficient for the cladding coating and the substrate as a function of the testing cycles. In the case of the untreated wheel/rail, the friction coefficient increases slightly up to 0.71 and maintains at a steady state until the end of the test. However, it is obvious that the rolling friction coefficient decreases markedly and keeps stable after testing for about 2000 cycles when single wheel or rail specimen undergoes laser cladding treatment. In addition, the friction coefficient of single-treated wheel roller is smaller compared to other wheel/rail rollers. The difference in wear debris from wheel/rail contact interface may cause different friction coefficient. This result indicates that the laser cladding treatment can effectively decrease the rolling friction coefficient. Wear rate of the wheel and rail rollers (the ratio of weight loss of wheel or rail roller to rolling distance of respective wheel or rail roller, µg.mm-1) is described in Fig.5. It can be seen that the wear rate of untreated wheel/rail specimens reaches a maximum whereas the wear rate of single treated wheel or rail roller decreases markedly, shown in Fig.5 (a). By comparison, the wear rate of wheel and rail specimens decreases 88.6% and 37.3% when single-treated wheel roller is used, and the wear rate of wheel and rail specimens decreases 20.1% and 79.1% when single rail roller suffers laser cladding treatment. Furthermore, total wear rate of single-treated wheel or rail specimen decreases 69.4% and 42.2% compared to the untreated wheel/rail specimens. In addition, total wear rate of wheel and rail rollers reaches a minimum when single-treated wheel roller is used, shown in Fig.5 (b). Therefore, it is concluded that the laser cladding coating is helpful to improve wear resistance and decrease the wear of wheel/rail system when only wheel or rail material is treated by laser cladding. 6
3.3 Surface damage The observations from Fig.6 (a) show that the wear surface of wheel specimen presents severe spalling and adhesion wear. Furthermore, obvious delaminating damage of rail specimen without treatment is dominant on the wear surface, shown in Fig.6 (b). There are small fatigue micro-cracks in the wear surface of rail roller and the propagation of surface cracks result in the material wearing off from the roller surface and the formation of large fatigue delamination and spalling damage. When single wheel specimen is treated, the surface damage of the wheel roller markedly lightens and presents slight adhesion wear (Fig.7). However, there are surface fatigue micro-cracks in the wear surface of the rail roller without laser cladding coating, shown in Fig.7 (b) and the surface damage presents slight fatigue delamination mechanism. It is evident that the laser cladding coating of the wheel roller alleviates the surface damage of the rail roller without laser treatment. Similarly, when single-treated rail specimen is used (Fig.8), the surface of the rail roller presents very mild adhesion wear and ploughing phenomena. The obvious spalling damage and delamination wear are dominant for the wheel specimen without laser cladding coating. It is observed from cross-sectional plastic deformation of wheel/rail specimens in Fig.9 that there is severe plastic flow along the rolling direction for the wheel/rail rollers without laser treatment. At the same time, the fatigue cracks are easy to initiate on the subsurface of rollers and propagate along the plastic deformation direction, shown in Fig.10 (a). When single wheel or rail specimen is treated by laser cladding, slight plastic flow appears in the cross-section of wheel/rail rollers with laser cladding coating (Fig.9 (b), (c)). Moreover, the friction pair of wheel or rail without cladding coating have obvious plastic flowing layer. But 7
the thickness of deformation is reduced compared to both wheel and rail specimens without laser cladding treatment. That is to say, single-treated wheel or rail specimen is beneficial to enhance the resistance-deformation ability of wheel/rail. Furthermore, no obvious fatigue crack is found on the subsurface and contact surface of wheel or rail rollers undergone laser cladding treatment, shown in Fig.10 (b) and (c). However, the wheel or rail roller without laser cladding coating shows visible fatigue cracks along plastic flowing direction, which is easy to result in the removal of material due to the propagation and connectivity of fatigue cracks. Therefore, it is concluded that the laser cladding coating exhibits good resistance-fatigue crack performance.
4. Discussion
It should be noted that the laser treatment is an effective method for improving the wear resistance of materials. Significant difference in hardness of single wheel or rail material undergone laser quenching would increase total wear of wheel/rail system [15]. That is to say, this laser quenching treatment is not suitable for single treatment of wheel or rail material in the practical application. Single-treated wheel or rail roller has an important effect on the wear and surface damage of wheel/rail, which can decrease the wear rate of wheel and rail rollers during the testing process. Furthermore, the rolling friction coefficient of single-treated wheel or rail roller is smaller than that of the wheel/rail rollers without laser cladding treatment (Fig.4). The result has found that the cladding coating is composed of γ-Co phase and carbide Cr23C6 [16]. Very fine hard phases (Cr23C6) are dispersively distributed in the phases of laser cladding coating, which result in low friction coefficient and high wear resistance [19].
8
Therefore, the laser cladding Co-based alloy coating on surface of wheel or rail specimen effectively improves the wear resistance of wheel/rail system. The grain size of laser cladding Co-based alloy coating becomes smaller compared to the wheel/rail materials without treatment, which increases markedly the strength [20]. These benefit from the effect of rapid condensation of laser cladding. In addition, some alloy elements form the second phase during the cladding process and it effectively improves the hardness of coatings [21-22]. This is helpful to enhance the fatigue resistance performance of laser cladding Co-based alloy coating. Therefore, the wheel and rail rollers undergone laser cladding treatment exhibit excellent fatigue crack resistance and no obvious fatigue crack is found in cross-section of worn wheel/rail specimen (Fig.10). As stated above that cladding Co-based alloy on the surface of single specimen would decrease the wear and damage of wheel or rail material. The materials are difficult to be removed from the wheel/rail specimens due to high hardness with laser cladding during the rolling wear process. Meanwhile, fine hard phases dispersive distribution in Co-based alloy clad layer improves the friction characteristics of wheel/rail interface, which is beneficial for the reduction of wear rate and surface damage of untreated wheel or rail specimen. It is concluded that this laser cladding Co-based alloy coating is suitable to be used to alleviate the wear of wheel/rail materials and prolong wear life of wheel/rail system by means of single treatment of wheel or rail material. Moreover, this laser cladding coating can be used to repair the damaged heavy-haul railway wheel and rail. Moreover, further work should be carried out to clarify the rolling contact fatigue performance and mechanism of laser cladding Co-based alloy coating.
9
5. Conclusions
1. Three different regions (untreated substrate regions, heat affected regions and clad regions) are presented in the cladding layer. The thick columnar and cellar dendrites were observed in the clad regions and the grain size is smaller compared to the substrate. 2. Single-treated wheel or rail specimen undergone the laser cladding effectively decreases the rolling friction coefficient and improve wear resistance of both wheel and rail materials. Compared to the untreated system, the wear rate of the rail/wheel system was reduced by the deposition of the laser clad layer on either the rail or the wheel, with the lowest wear rate being observed for the system with the laser clad wheel material. Moreover, when either the rail or wheel material was clad, the wear rate of the untreated counterbody was less than that observed for the same material when both the rail and wheel material were worn in the untreated condition (i.e. the cladding of one of the bodies did not result in an increase in the wear rate of the untreated counterbody). 3. The untreated wheel or untreated rail specimens present serious wear and surface damage. The wear mechanism of untreated wheel rollers is adhesion wear and the delaminating damage is dominant for untreated rail rollers. Treated wheel or rail roller exhibits mild wear damage and is beneficial to alleviate the wear and plastic flow of wheel/rail materials. 4. It illustrates that the laser cladding Co-based alloy coating is suitable for single treatment of wheel or rail material.
Acknowledgements
10
The work was supported by the National Natural Science Foundation of China (No.51174282),
Sichuan
Province
Science
and
Technology
Support
Program
(No.2014GZ0009-6) and Innovative Research Teams in University (No.IRT1178). The authors are grateful to reviewers for valuable technical advice and kind help in improving the English text of the paper.
References
[1]Q.Y. Liu, B. Zhang, Z.R. Zhou, An experimental study of rail corrugation, Wear 255 (2003) 1121-1126. [2]J.E. Garnham, C.L. Davis, The role of deformed rail microstructure on rolling contact fatigue initiation, Wear 265 (2008) 1363-1372. [3]W.J. Wang, H.M. Guo, X. Du, J. Guo, Q.Y. Liu, M.H. Zhu, Investigation on the damage mechanism and prevention of heavy-haul railway rail, Eng. Fail. Anal. 35 (2013) 206-218. [4]Y. Zhou, S. Wang, T. Wang, Y. Xu, Z. Li, Field and laboratory investigation of the relationship between rail head check and wear in a heavy-haul railway, Wear 315 (2014) 68-77. [5]J. Sandstrom, A. Ekberg, Predicting crack growth and risks of rail breaks due to wheel flat impacts in heavy haul operations, Proc. IMechE, Part F: J. Rail Rapid Transit 223 (J6) (2009) 153-161. [6]Z.L. Li, X. Zhao, C. Esveld, R. Dollevoet, M. Molodova, An investigation into the causes of squats-correlation analysis and numerical modeling, Wear 265 (2008) 1349-1355. [7]C. González-Nicieza, M.I. Álvarez-Fernández, A. Menéndez-Díaz, A.E. Álvarez-Vigil, F.
11
Ariznavarreta-Fernández, Failure analysis of concrete sleepers in heavy haul railway tracks, Eng. Fail. Anal. 15 (2008) 90-117. [8]R. Lewis, R.S. Dwyer-Joyce, Wear at the wheel/rail interface when sanding is used to increase adhesion, Proc. IMechE, Part F: J. Rail Rapid Transit 220 (5) (2006) 29-41. [9]O. Arias-Cuevas, Z.L. Li, R. Lewis, Investigating the lubricity and electrical insulation caused by sanding in dry wheel-rail contacts, Tribology Letters 37 (2010) 623-635. [10]S. Descartes, C. Desrayaud, Y. Berthier, Experimental identification and characterization of the effects of contaminants in the wheel-rail contact, Proc. IMechE, Part F: J. Rail Rapid Transit 222 (1) (2008) 207-216. [11]M. Spiryagin, K.S. Lee, H.H. Yoo, Control system for maximum use of adhesive forces of a railway vehicle in a tractive mode, Mechanical Systems and Signal Processing 22 (2008) 709-720. [12]R.J. DiMelfi, P.G. Sanders, B. Hunter, J.A. Eastman, K.J. Sawley, K.H. Leong, J.M. Kramer, Mitigation of subsurface crack propagation in railroad rails by laser surface modification, Surface and Coatings Technology 106 (1998) 30-43. [13]S. Aldajah, O.O. Ajayi, G.R. Fenske, S. Kumar, Investigation of top of rail lubrication and laser glazing for improved railroad energy efficiency, Journal of Tribology 125 (2003) 643-648. [14]F. Franklin, G.J. Weeda, A. Kapoor, E. Hiensch, Rolling contact fatigue and wear behavior of the infrastar two-material rail, Wear 258 (2005) 1048-1054. [15]W.J. Wang, J. Guo, Q.Y. Liu, M.H. Zhu, Effect of laser quenching on wear and damage of heavy-haul wheel/rail materials, Proc. IMechE, Part J: Journal of Engineering Tribology 228
12
(1) (2014) 114-122. [16]W.J. Wang, J. Hu, J. Guo, Q.Y. Liu, M.H. Zhu, Effect of laser cladding on wear of heavy-haul wheel/rail materials, Wear 311 (2014) 130-136. [17]W.J. Wang, W.J. Jiang, H.Y. Wang, Q.Y. Liu, M.H. Zhu, X.S. Jin, Experimental study on the wear and damage behavior of different wheel/rail materials, Journal of Rail and Rapid Transit, http://pif.sagepub.com/content/early/2014/03/09/0954409714524566,2014. [18]V. Ocelík, I. Furár, J.Th.M. D. Hosson, Microstructure and properties of laser clad coatings studied by orientation imaging microscopy, Acta Materialia 58 (2010) 6763-6772. [19]J.S. Wang, H.X. Lu, Z.W. Yan, G. Li, M.Q. Tang, Z.Q. Feng, Wear-resistance of WC-4Co coating on cast steel by electro-spark deposition, Transactions of Materials and Heat Treatment 36 (1) (2015) 169-172. [20]R. Vilar, R. Colaco, A. Almeida, Laser surface treatment of tool steels, Optical and Quantum Electronic 27 (1995) 1273-1289. [21]J.R. Jiang, L.J. Xue, S.D. Wang, Discrete laser spot transformation hardening of AISIO1 tool steel using pulsed Nd: YAG laser, Surf. Coat. Techno. 205 (21-22) (2011) 5156-5164. [22]C.Y. Cui, J.D. Hu, Y.H. Liu, K. Gao, Z.X. Guo, Formation of nano-crystalline and amorphous on the surface of stainless steel by Nd: YAG pulsed laser irradiation, Appl. Surf. Sci. 254 (21) (2008) 6779-6782.
13
Table caption Table 1 Chemical compositions and mechanical properties of wheel/rail materials (%wt). Table 2 Chemical compositions of the powder (%wt).
14
Table 1
Specimens
C%
Si%
Mn%
P%
S%
σb/MPa
δ5/%
CL60 wheel
0.55~0.65 0.17~0.37 0.50~0.80
≤0.035
≤0.040
910
10
U71Mn rail
0.65~0.76 0.15~0.35 1.10~1.40
≤0.030
≤0.030
880
9
15
Table 2
Element
C
Si
Fe
Cr
Ni
W
Co
Powder
1.1
1.0
1.5
28.5
1.5
4.4
Vol.
16
Highlights 1. The coating of laser cladding Co-based alloy consists of dendrite and eutectic. 2. Single-treated wheel or rail specimen effectively decreases friction coefficient. 3. Single-treated wheel or rail roller improves wear resistance of materials. 4. Single-treated wheel or rail roller presents mild wear damage.
17
Figure caption Fig.1: Schematic of test apparatus and the geometry of specimens (a) rolling-sliding wear test apparatus; (b) the geometry of wheel/rail specimens. Fig.2: Microstructure of the cladding coatings, (a) bottom of coating; (b) middle of coating; (c) high power structure; (d) heat affected region. Fig.3: Composition profile for Co, Cr, Ni, Si and Fe along the clad thickness. Fig.4: Rolling friction coefficient of wheel/rail specimens. Fig.5: Wear rate of wheel/rail specimens, (a) wear rate of wheel/rail specimens; (b) total wear rate. Fig.6: SEM micrographs of worn surface of untreated wheel/untreated rail specimens, (a) untreated wheel specimen; (b) untreated rail specimen. Fig.7: SEM micrographs of worn surface of treated wheel/untreated rail specimens, (a) treated wheel specimen; (b) untreated rail specimen. Fig.8: SEM micrographs of worn surface of untreated wheel/treated rail specimens, (a) untreated wheel specimen; (b) treated rail specimen. Fig.9: Cross-sectional plastic deformation of wheel/rail specimens, (a) untreated wheel/rail specimens; (b) single-treated wheel specimen; (c) single-treated rail specimen. Fig.10: Fatigue damage of wheel/rail specimens, (a) untreated wheel/rail specimens; (b) single-treated wheel specimen; (c) single-treated rail specimen.
18
Fig.1
7
8
9
10
12
11
13
6
Electrical cabinet 5 4
14
15
16
3
Rolling-sliding wear
MMS-2A rolling wear test testapparatus apparatus
2
1
、、、
1- Double speed motor; 2- Triangular belt; 3- Pressure sensor; 4 5 6 9-Gear; 7- Swinging bracket; 8Internal gear; 10- Speed sensor; 11- Bottom specimen; 12- Top specimen; 13- Bolt; 14-Spring; 15-Nut; 16Load sensor
(a)
(b)
19
Fig.2
Top surface of coatings
Substrate Coatings
Columnar dendrites
Cellular dendrites
Transition band
Columnar dendrites
(a)
(b) Coatings Transition band
Dendrites
Martensite Eutectic
(c)
(d)
20
Fig.3
140 Fe 120
Co
Cr
Ni
Si
Transition band
Atom fraction/%
100 Substrate
80
Cladding coating
60 40 20 0 0
90
180 270 Distance/µm
21
360
450
Fig.4
0.90 Untreated wheel/rail
Friction coefficient µ
0.75 0.60
Single-treated rail
0.45 0.30
Single-treated wheel
0.15 0.00 0
0.4 0.8 1.2 1.6 2 Nunber of rolling cycles(×104)
22
2.4
Fig.5
3.5
2.0 Wheel specimen
2.8
Rail specimen
Wear rate/µg.mm-1
Wear rate/µg.mm-1
1.6 1.2 0.8 0.4
2.1 1.4 0.7 0.0
0.0 Untreated
Untreated
Single-treated Single-treated wheel rail
(a)
Single-treated Single-treated wheel rail
(b)
23
Fig.6
(a)
Surface crack
(b)
24
Fig.7
(a)
Surface crack
(b)
25
Fig.8
Surface crack
(a)
(b)
26
Fig.9
Wheel
Rail
Rolling direction
100µm
Rolling direction
100µm
(a) Wheel
Rail
Rolling direction
100µm
Rolling direction
100µm
(b) Wheel
Rail
Rolling direction
100µm
Rolling direction
100µm
(c)
27
Fig.10
Wheel
Rail
Fatigue crack Fatigue crack
(a) Wheel
Rail
Fatigue crack
(b) Wheel
Rail
Fatigue crack
(c)
28