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An investigation into the microstructure and tribological properties of rail materials with plasma selective quenching
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An investigation into the microstructure and tribological properties of rail materials with plasma selective quenching Jingmang Xu a, b, *, Kai Wang a, b, Ronghe Zhang a, b, Qiang Guo a, b, Ping Wang a, b, Rong Chen a, b, Dongfang Zeng c, Fuhai Li b, Jun Guo c, Lu Li d a
MOE Key Laboratory of High-speed Railway Engineering, Southwest Jiaotong University, Chengdu, 610031, China School of Civil Engineering, Southwest Jiaotong University, Chengdu, 610031, China State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu, 610031, China d Sichuan Jinhong Plasma Technology Co. LTD, Chengdu, 610000, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Plasma selective quenching Microstructure Tribological properties Contact damage
Plasma selective quenching (PSQ) forms dispersed hardening microscopic structures on the rail material surface by the action of energetic plasma. In this study, the microstructure and tribological properties of PSQ treated rail materials are investigated by the rolling fatigue test and metallographic analysis. The effect of plasma parameters on the surface quality and microstructure is analyzed, and the optimum diameter of PSQ region is 5.0 mm. The microhardness in the PSQ region significantly increased and the tribological properties of rail materials improved. Meanwhile, severe spalling occurs on the surface in the PSQ region. Based on the field application, the wear resistance of the switch rails is enhanced by approximately 3 times that of untreated rail materials.
1. Introduction Plasma selective quenching (PSQ) forms dispersed hardening microscopic structures on the rail material surface, which can improve the hardness and wear resistance of the rail surface. The principle is that high-energy plasma quickly heats the rail surface to a point above its austenitizing temperature, and then the rail will cool down quickly due to its good thermal conductivity, generating a hardened structure. Part of this hardened structure consists of quenched martensite. The laminar plasma surface hardening technique uses a high-efficiency, high-energy, long-life and numerically controlled fascicular ultra-high-temperature heat source (the temperature can be set, and accurately repeated, to between 500 and 15,000 � C). As a high-quality ultra-high-temperature long-beam heat source characterized by its good scalability, intelligent control and modularity, it represents a revolutionary substitute for existing ion heat sources such as turbulence. Since laminar plasma surface hardening is inexpensive and easy to carry out by means of a large-scale, automated production process, it is a very suitable technique for hardening the surface of rail materials, which can improve the wear and rolling contact fatigue resistance and the service life of rails. With the increase of axle weight and running speed of railway ve hicles, the contact damage of wheels and rails become more serious
[1–5], which affects the safe operation of railways. At present, the common used rail surface strengthening methods are: cyclic-accelerated cooling of the head of pearlite rails [6,7], laser cladding [8–15], laser quenching [16–20], laser melting [21,22] and plasma quenching [23, 24]. However, it can’t provide controlled uniform heating of the mate rial surface for the original turbulent plasma strengthening method, limiting the development of plasma strengthening. The above problem has been solved by laminar plasma enhancement as a controllable and effective reinforcement method [25]. Studies have shown that after plasma strengthening the hardness of the material is increased by 2–3 times [23], and the wear resistance and service life are increased by more than 10 times [26]. Metallographic analysis showed that the plasma-enhanced inner core region forms a relatively ordered and non-uniform martensite structure [27,28], while the boundary region is in a chaotic state, very disordered and irregular. Since the internal phase transformation process of the material is unclear during the quenching process [29,30], there is no uniform conclusion on the choice of pro cessing parameters in the strengthening process. It has been determined that the choice of different plasma quenching treatment parameters is related to the material’s thermal expansion coefficient and material composition [31]. With the increase of heating power, gas flow rate and treatment rate, the depth of the heat affected zone increases. As the
* Corresponding author. MOE Key Laboratory of High-speed Railway Engineering, Southwest Jiaotong University, Chengdu, 610031, China. E-mail address: [email protected] (J. Xu). https://doi.org/10.1016/j.triboint.2019.106032 Received 27 July 2019; Received in revised form 21 October 2019; Accepted 21 October 2019 Available online 22 October 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Jingmang Xu, Tribology International, https://doi.org/10.1016/j.triboint.2019.106032
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Table 1 The composition of U75V rail material. Material composition
C
Si
Mn
V
P
S
Percentage (%)
0.71–0.8
0.5–0.8
0.7–1.05
0.04–0.12
�0.03
�0.03
Fig. 2. Influence of irradiation time on quenching size of U75V rail material.
Fig. 1. Influence of irradiation time on quenched rail material of U75V.
in quenching width, thickness and structural morphology. Fig. 1 shows the microstructure of a quenched zone irradiated for 0.10s–0.35s and measured under an optical microscope of U75V rail material. Since the matrix fails to be heated to the austenitizing temperature for only 0.10s of irradiation, no quenched zone is observed under the optical micro scope. A clearly visible quenched zone is observed after irradiation lasting 0.15s–0.35s. The detailed sizes of quenched zones for different times of irradiation are showed in Fig. 2. With the increase of irradiated time, the martensite strengthening zone appears with a significantly increased depth and a moderately increased width, leading to a decline in the width-depth ratio of the quenched zone. The width and thickness increase markedly at 0.15s–0.30s, and slightly at 0.30s–0.35s. The study of quenching test shows that with the increase of irradiated time, the quenching depth-width ratio decreases as the quenching depth increases more markedly than the width. Moreover, when the treatment time is extended from 0.30s to 0.35s, the size increases more slowly. The influence of irradiation time on the structure of quenched zones on the surface of U75V rail material is showed in Fig. 3. With the irra diation time extended from 0.15s to 0.25s, the thickness of the martensite strips increases slightly, while that of the martensite struc ture increases more markedly. The carbon content in martensite rises as time increases, turning the martensite into high-carbon acicular martensite. Although acicular martensite is harder, it is also very brittle and cannot be safely used in rails. However, lath martensite has good strength, ductility and toughness. Therefore, the irradiation time does not need to be too long for a lath-shaped martensitic structure to be created. In fact, according to experimental results, the irradiation time does not need to exceed 0.25s. An analysis of the quenched zone sizes shows that the irradiation time should ideally range from 0.15s to 0.25s. In addition to irradiation time, other technical parameters such as generator power, mixed airflow, argon ratio and spraying distance, have an appreciable impact on hardening quality as well. Fig. 4 shows the impact of different parameters on quenching depth. The quenching depth basically shows a linear relationship with power: quenching depth increases with power (4(a)). The quenching depth increases with the rise in mixed airflow until it reaches a peak at 8.5 L/min, before gradually beginning to decrease (4(b)). The analysis shows that this phenomenon may be related to the burning loss of the rail surface. As airflow in creases, heat transfer on the rail surface speeds up, and the steel in the
processing speed decreases, the hardness of hardened surface increases; while the heating power and gas flow rate change, the hardness hardly changes [32–34]. Therefore, different plasma quenching treatment methods and parameters are often used for different materials. The common treatment method is to perform single or multiple beam rein forcement on the surface of the substrate. Since the processing area of the wheel/rail is large and the size of the plasma-enhanced heating zone is small, pluralities of overlapping regions are inevitable. The overlap region causes a tempering effect and leads to the formation of soft re gions, which will generate residual tensile stress and accelerate the initiation and propagation of fatigue cracks in the overlap region [35–39]. In order to avoid the occurrence of overlapping regions, the rail materials are strengthened with plasma selective quenching. Experi ments and field applications show that the proposed strengthening method can effectively improve the hardness and damage resistance of the material. In this study, the microstructure and tribological properties of rail materials with plasma selective quenching are studied. The micro structure and metallographic composition of the treated rail materials is analyzed. To investigate the tribological properties of the treated rail materials, the rolling contact fatigue (RCF) tests are performed to analyze the wear resistance and damage of the treated material in comparison with untreated materials, concluding that the laminar plasma surface hardening technique can improve the wear resistance of rails. The results of the study can provide support for the application of the laminar plasma surface hardening technique to rail materials. 2. Plasma selective quenching For the laminar plasma quenching of rail materials in this study, the laminar plasma generator is applied to conduct the dispersed point-like quenching areas on the rail material surface in order to achieve a finer hardening effect. The influence of technical parameters on the quenching quality of U75V rail material is investigated to obtain an optimal range of parameters, including irradiation time, power of generator, rate of mixed airflow and spraying distance. The composition of U75V rail material is listed in Table 1. For the rail materials irradiated with the laminar plasma generator for different periods of time, there are some obvious changes appeared 2
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Fig. 3. Influence of irradiation time on microstructure of quenched zones on the U75V rail surface.
Fig. 4. Change in quenching depth under various technical conditions.
Fig. 5. Surface topography of quenched zones under different conditions.
quenching center melts. However, the airflow also blows apart the melted liquid steel, creating a pit in the quenching center and lowering the quenching depth. The depth decreases as the argon ratio goes up, indicating that the plasma temperature is inversely proportional to argon ratio (4(c)). The quenching depth decreases as the spraying dis tance increases, while a spraying distance of less than 14 mm causes ablation to form a pit. A spraying distance above 18 mm accelerates the decrease in depth, which changes slightly from 14 to 18 mm (4(d)). The corroded and cleaned surface topography of quenched zones treated under different technical conditions is showed in Fig. 5. The quenched zones do not ablate as power changes (see Fig. 5(a)). The
quenching center dot is enlarged slightly as power increases. More specifically, the power reaches a maximum value at the center dot, which forms a grain, while the microscopy does not display any cracks in the dot corroded by muriatic acid. The quenching center dot is enlarged as airflow increases, gradually inducing ablation on the surface (Fig. 5 (b)). The dot forms a grain with airflow at 8 L/min, slightly ablated at 8.5 L/min and 9 L/min, and obviously ablated at 9.5 L/min. Microscopy does not show any cracks after corrosion in muriatic acid. The quenching center dot gradually diminishes as the argon ratio rises, and the surface ablation is gradually alleviated. The surface is slightly ab lated at an argon ratio of 20% and 25%, and the center dot becomes 3
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Fig. 6. GPM rolling contact fatigue tester.
larger at 30%, forming a grain. Microscopy does not show any cracks after corrosion in muriatic acid. The surface is obviously ablated at a spraying distance of 12 mm when the generator power is set to 22 kW; the surface is also obviously ablated at a spraying distance of 14 mm when the generator power is set to 23 kW; the surface is not obviously ablated at a spraying distance of 16–20 mm when the generator power is set to 24 kW (5(d)). Microscopy does not show any cracks after corrosion in muriatic acid. An analysis of the above test results shows that, factors such as the appearance of quenched zones, quenching depth, width and metallur gical structure are taken into consideration, an excellent laminar plasma surface quenching effect can be achieved under the following condi tions: irradiation time of 0.15s–0.25s, power of 23–24 kW, airflow of 8.0–8.5 L/min, argon ratio of 35–40%, and spraying distance of 15–17 mm, to protect the quenching surface from becoming ablated and granulated. Fig. 7. The size of wheel-rail specimen.
3. Experimental details 3.1. Experimental procedure
Table 2 Rail specimens featuring different diameters and duty ratios of quenched zones.
The experimental procedure for rail material with plasma selective quenching consists of following parts: ①microstructure analysis of treated rail material, ②rolling contact damage resistance test, ③contact damage and morphology after rolling contact test. The experimental procedure is carried out as follows: To analyze the microstructure and tribological properties of rail materials with plasma selective quenching, the microstructure of the treated rail material is observed with the optical microscope, scanning electron microscope and transmission electron microscope, and metal lographic structure is analyzed with X-ray diffraction and the micro hardness distribution is measured. Then, the rolling contact damage resistance test is performed on the rolling contact fatigue tester, and the tribological properties of treated rail materials with different quenching conditions are studied to obtain the optimum diameter of PSQ region. Finally, the contact damage and morphology of the treated rail materials after rolling contact test is analyzed, including material wear, rolling contact fatigue, surface morphology and profile structure.
Rail sample no.
1
2
3
4
5
6
Diameter (mm) Duty Ratio (%)
/ 0
5 15
5 30
5 45
3 30
6.25 30
is 20 mm; the lateral radius of the wheel specimen is set to 30 mm and the lateral radius of the rail specimen is set to be infinite. The upper specimen consists of wheel material and the lower specimen of rail material, specifically U75V. In order to simulate an actual wheel-rail contact state, the test parameters are obtained as follows based on the Hertz contact theory: The maximum contact stress at the center of the contact patch is 1800 MPa, the wheel specimen speed is 400r/min, the rail specimen speed is 398r/min, and the rotational slip ratio is 0.5%, the amount of rotation is 200,000, the testing time is 500min. The size of different quenched zones and the area ratio (duty ratio) of the quenched zones on the rail surface may exert an effect on the wear and contact fatigue resistance of the rails. For instance, if the quenched zone is too large, or its area ratio too high, cracks may easily be generated to reduce fatigue resistance. However, if the quenched zone is too small, or its area ratio too low, the wear resistance may not be significantly improved. Therefore, the diameter size and surface ratio of a quenched zone should be taken as major factors influencing the rolling contact damage resistance of rails. The parameters of test specimen are listed in Table 2. Specimen 1# is a contrast specimen made of an un treated material; Specimen 2#, 3# and 4# are specimens featuring different ratios of quenched zone to total surface area, namely different duty ratios; Specimen 3#, 5# and 6# are specimens featuring different quenching diameters. The rail and wheel specimens are shown in Fig. 8.
3.2. Rolling contact damage resistance test The rolling contact damage resistance test is performed using a GPM rolling contact fatigue tester (Fig. 6). The test is conducted according to the Hertz simulation procedure, also known as the Hertz contact theory. This theory ensures that the average contact stress between wheel-rail specimens can be faithfully measured in the laboratory, and that the ratio of major-to minor-axis length of the contact ellipse can be accu rately calculated as well. The wheel-rail specimen size is set as shown in Fig. 7: the diameter of wheel and rail specimens is 60 mm and the width 4
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4. Microstructure and tribological properties 4.1. Microstructure and microhardness The microstructure of the rail material treated by plasma selective quenching is observed with the optical microscope, scanning electron microscope and transmission electron microscope. The microstructure of different parts on the profile section of the treated specimen includes the quenched zone, transition zone and primary structure, as showed in Fig. 9. The whole quenched zone looks like a spherical crown (Fig. 9 (a)). The hardened structure in Fig. 9 (b) shows the noticeable characteristics of acicular martensite. As shown in Fig. 9(c), the primary structure is composed of laminar pearlite and a small amount of grain boundary allotriomorphic ferrite. The lath-acicular martensite at a certain width exists in the quenched zone with a specific angle based on the trans mission electron microscopy. For the martensite clusters, the laths are not completely parallel to each other, though there is a small angle between them; a single martensite lath is about 150–300 nm wide, and it is difficult to see other structures in the visible range. During the process of plasma surface hardening, the original structure in the quenched zone is austenitized at high temperature and then generates martensitic
Fig. 8. Test specimens of rail and wheel materials.
There is a conspicuous, round spot-like quenched zone on the rail specimen treated with the laminar plasma surface hardening technique, with black surface oxidation visible around the quenched zone; this can be washed away using muriatic acid. The specimen surface is smooth rather than granulated.
Fig. 9. Microstructure of the rail specimen treated by plasma selective quenching. 5
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The front side of the quenched zone is sampled first, and its metal lographic composition analyzed using X-ray diffraction. The results of the XRD analysis are showed in Fig. 10. The three peaks inosculate well with α-Fe after XRD. For the supersaturated solid solution of carbon atoms in α-Fe, the X-ray diffraction peak of martensite in steel is very approximate to α-Fe. According to the micro-structure observations and XRD analysis, the martensite in the quenched zone is fully transformed, while a small amount of pearlite is not turned into martensite in the transition zone due to the high heating speed and short time, leaving the transition structure as a mixture of martensite and austenite. The microhardness of the rail materials treated by plasma selective quenching is measured in the material surface and depth direction, as shown in Fig. 11. Fig. 11 (a) shows the microhardness distribution of the quenching surface, from the quenching center to the primary structure in the lateral direction; Fig. 11 (b) shows the microhardness distribution of the quenching center in the depth direction. It can be seen that the surface microhardness of the rail material is significantly improved with plasma selective quenching, and that the microhardness of the quenched zone is significantly higher than that of the primary structure. The microhardness of the primary structure is 300HV~400HV, and the microhardness of the quenched zone goes up to 850HV~950HV, char acterized by a high homogeneity; the microhardness suddenly changes at the interface, along with the microstructure.
Fig. 10. XRD analysis of the laminar plasma-quenched zone on U75V rail surface.
4.2. Tribological properties
transformation due to the self-quenching and rapid cooling. For the regular macroscopic interface between the quenched zone and primary structure (Fig. 9 (a)), it can be seen that the quenched structure has stuck into the primary structure to form the jagged interface with the transi tion zone only about 18 μm thick (Fig. 9 (e), (f)). The pearlite in the primary structure near the interface is heated to a very high temperature within a short time and fails to be fully tempered, leading to an insig nificant change in its structural morphology (Fig. 9 (d)).
Based on rolling contact damage resistance test, the material removal and structural morphology for 6 sets of rail and wheel specimens are analyzed and compared. The influence of duty ratio and diameters of quenched zones on the material removal of the rail specimens is studied and showed in Fig. 12. The duty ratio and diameters of quenched zones have a great impact on the material removal of the rail specimens. The surface ratio of the quenched zones equals 0, 15%, 30% and 45%
Fig. 11. Microhardness of the treated rail materials: (a) the material surface; (b) depth direction.
Fig. 12. Influence on the material removal of rail specimens (a) duty ratios (b) diameters of the quenched zones. 6
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Fig. 13. Surface structure morphology: (a)–(b) untreated material, (c)–(f) hardened material.
respectively, the material removal of the hardened rail specimen is lower than that of the contrast specimen. In addition, the material removal of the rail specimen decreases as the duty ratio of the quenched zones increases. The diameter of the quenched zones equals 3 mm, 5 mm and 6.25 mm respectively, the material removal of the hardened rail specimen is also lower than that of the contrast specimen. The material removal of the rail specimen first decreases and then increases as the diameter of the quenched zones increases. The material removal reaches
a minimum of 0.094 g when the diameter equals 5 mm, 45% lower than that of the contrast specimen. A quenching diameter of 5 mm is the proposed value for actual rail hardening. Based on the analysis of the material removal on all specimens, the plasma selective quenching can significantly improve the wear resistance of the rail materials. The structural morphology of the worn-out surface and cross-section of the rail specimens is observed after testing. By comparing the wornout surface of both untreated and hardened materials, it can be seen that both the surface of the untreated material (Fig. 13 (b)) and the inner surface of the quenched zone (Fig. 13 (d)) remain smooth. However, there are a few pits and delaminations on the surface of the untreated material. Furthermore, the direction of delamination is consistent with the rolling direction, while there is no delamination present on the surface of the hardened material. There are no conspicuous cracks on the surface of the untreated material (Fig. 13 (a)), while there are some cracks at the edge of the hardened material (Fig. 13 (e), (f)). The cracks exist at the rear end of the quenched zone relative to the rolling direction specifically (Fig. 13 (c)), and propagate to the front end along the edge of the quenched zone. The cracks form in the edge of the quenched zone and propagate to the inside of the hardened zone, but not to the primary materials. It is concluded that the plasma selective quenching can pre vent delamination from occurring inside the quenched zone, and the
Fig. 14. Longitudinal profile structure of the standard material. 7
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Fig. 15. Longitudinal profile structure of the hardened material.
cracks are formed and extend within the quenched zone and have no impact on the primary materials. In order to observe the inner structure of the rail specimens after testing, it is cut open along the wear scar center. Meanwhile, the quenched zone and primary structure are selected as observation sam ples by polishing the zones, and then corroding them in 5% nital and washing them in alcohol. The profile structure of the worn-out rail specimens is observed under an optical microscope, revealing the lon gitudinal profile morphology of the untreated and hardened materials under different amplification factors, as shown in Fig. 14 and Fig. 15. The untreated material is composed of pearlite and ferrite (Fig. 14 (a)), and a conspicuous crescent structure forms on the quenched zone, clearly different from the primary material (Fig. 15 (a)). The primary material outside the quenched zone is also composed of pearlite and ferrite (Fig. 15 (b)), but in the crescent quenched zone a uniform, compact martensitic structure can be observed (Fig. 15 (c)). There are cracks at the rear end of the crescent quenched zone in the rolling di rection of the specimen (Fig. 15 (e)), but merely propagate within the quenched zone and don’t extend to the primary material. The untreated material has obvious plastic fluidity (Fig. 14 (b)), and the plastic deformation layer is relatively thick, at about 80 μm. In the process of rolling test, the wheel-rail surface materials move out of
position under the action of cyclic contact stress and tangential friction force. The pearlite and ferrite are compressed and elongated, plastically deformed. Moreover, the direction of plastic deformation is consistent with the direction of the tangential force. With the increase of the dis tance to the contact surface, the plastic fluidity will be higher. There is no obvious plastic deformation in the quenched zone (Fig. 15 (c)), and without spalling pit present on the surface of the specimen (Fig. 15 (d)). A high-hardness martensitic structure with high yield strength is generated after plasma selective quenching. The martensitic structure contains tiny crystal grains and the plastic deformation resistance en hances with the increase of the ratio of the grain boundary to crystals. Moreover, martensitic transformation occurs in the form of a coherent shear. A huge array of micro-defects including dislocation and twins appears in the crystal grains, which could hinder the removal of material and effectively suppress plastic flow and material spalling. A slight plastic deformation occurs on the surface of the primary material be tween the quenched zones of the wheel-rail specimens, and the thickness of the plastic deformation is significantly less than that of the plastic deformation layer in the untreated wheel-rail specimens. Furthermore, as the distance to the quenched zones is closer, the plastic deformation layer will be thinner (Fig. 15 (d)). The analysis of tribological properties shows that the plasma selective quenching can significantly prevent 8
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the service duration of the hardened switch rail is three times as long as that of the untreated switch rail, the wear loss of the hardened section of the hardened switch rail is lower than that of the untreated switch rail. However, the wear loss of the unhardened sections on hardened switch rail is higher due to the longer service time. According to the service conditions of the two switch rails, the plasma selective quenching can significantly improve the rolling contact damage resistance of switch rails. 6. Conclusion Fig. 16. Field application of the switch rail treated with plasma selec tive quenching.
With respect to the application of the plasma selective quenching to rail treatment, this paper conducts a wear and contact fatigue test on a laminar plasma-hardened rail material and analyzes the microstructure and microhardness distribution of the hardened rail material. The wear loss and damage degree of the rail material after testing is studied. Finally, based on field applications, the wear resistance and surface damage of the laminar plasma-hardened and untreated switch rails is compared and analyzed. The research conclusions are as follows:
plastic deformation from occurring on the surface of the rail material. 5. Field application and verification The plasma selective quenching has been initially applied to switch rail surface treatment on the No.50-6 symmetrical turnout switch rails at Chengdu North Marshalling Station. Untreated switch rails needed to be replaced every 6 months, but the hardened switch rails have been used for about 9 months so far, and are still in good condition, prolonging the service life by 3 times. An obvious spotted quenched zone can be observed on the rail sur face (Fig. 16(b)). Compared with the primary structure, the quenched zone is outwardly smooth and free of bulges. Moreover, homogeneous light bands can be observed on the surface. The overall wear resistance of the switch rail improves and the material removal of the quenched zone and primary structure is the same, which guarantees the whole worn switch rail surface smooth. The untreated switch rails in service are severely delaminated and spalled (Fig. 16 (a)), while the hardened switch rail are only slightly damaged. Similar to the wear and contact fatigue test results, cracks occur along the hardened section after longterm service. More specifically, the cracks appear in the edge of the quenched zones and propagate within the quenched zones (Fig. 16 (b)). In addition, the quenched zones is slightly spalled, but both size and range of spalling pits are much less than that of untreated sections, effectively improving the rolling contact damage resistance of switch rail. In order to compare and analyze the wear resistance of the hardened and untreated switch rails, the profile measurement for two switch rails at the same operating conditions is conducted and the comparison of wear loss is showed in Fig. 17. The hardened switch rail has been in operation for about 9 months, and the quenched zone ranges from the point of the switch rail to the section with the width of 50 mm; the untreated switch rail has been in operation for about 3 months under the same operating conditions. The side wear and vertical wear of the two switch rails on selected sections are compared. Under the condition that
(1) For the rail specimen treated with the plasma selective quench ing, microstructure observations and XDR analyses can confirm that the quenched zone is of an evenly and compactly distributed martensitic structure. There is no obvious transition zone in terms of micro-structure between the quenched zone and the primary structure. The hardness of the quenched zone is significantly higher than that of the primary structure, and it changes abruptly at the interface, rapidly decreasing from 900 HV to 400 HV. (2) The wear resistance of the laminar plasma-hardened specimen is significantly higher than that of the untreated specimen. The changes in wear loss under different hardening conditions are compared and analyzed. The duty ratio and diameter of quenched zones have a great influence on the wear loss of the rail specimen. The wear loss of the hardened specimen decreases by 45% compared with the untreated rail specimen. (3) The structural morphology of the rail specimen is observed after rolling test. There are some corrosion pits and delamination visible on the surface of the standard material. While, there is no delamination visible on the surface of the hardened material. The profile structure of the quenched zone of the hardened specimen is obviously crescent-shaped. Observations of surface structure show that the cracks only propagate within the quenched zone and do not extend into the primary structure. (4) The plasma selective quenching has been applied to switch rail treatment for field application. The technique can significantly improve the rolling contact damage resistance of the switch rail and greatly prolong its service life.
Fig. 17. Comparison of wear loss between the hardened and untreated switch rail. 9
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Declaration of competing interest
[16] Su C, Shi L, Wang W, Wang D, Cai Z, Liu Q, et al. Investigation on the rolling wear and damage properties of laser dispersed quenched rail materials treated with different ratios. Tribol Int 2019;135:488–99. [17] Aldajah S, Ajayi OO, Fenske GR, Kumar S. Investigation of top of rail lubrication and laser glazing for improved railroad energy efficiency. J Tribol Trans ASME 2003;125:643–8. [18] Wang W, Guo J, Liu Q, Zhu M. Effect of laser quenching on wear and damage of heavy-haul wheel/rail materials. Proc Inst Mech Eng J J Eng Tribol 2014;228: 114–22. [19] Shariff S, Pal T, Padmanabham G, Joshi S. Sliding wear behaviour of laser surface modified pearlitic rail steel. Surf Eng 2010;26:199–208. [20] Cao X, Shi L, Cai Z, Liu Q, Zhou Z, Wang W. Investigation on the microstructure and damage characteristics of wheel and rail materials subject to laser dispersed quenching. Appl Surf Sci 2018;450:468–83. [21] Sing SL, Wiria FE, Yeong WY. Selective laser melting of lattice structures: a statistical approach to manufacturability and mechanical behavior. Robot Comput Integr Manuf 2018;49:170–80. [22] Hu Z, Zhu H, Zhang C, Zhang H, Qi T, Zeng X. Contact angle evolution during selective laser melting. Mater Des 2018;139:304–13. [23] Pan W, Meng X, Li G, Fei Q, Wu C. Feasibility of laminar plasma-jet hardening of cast iron surface. Surf Coat Technol 2005;197:345–50. [24] Tanaka Y, Sakuta T. Investigation on plasma-quenching efficiency of various gases using the inductively coupled thermal plasma technique: effect of various gas injection on Ar thermal ICP. J Phys D Appl Phys 2002;35:2149. [25] Petrov S, Saakov A. Technology and equipment for plasma surface hardening of heavy-duty parts. Mater Manuf Process 2002;17:363–78. [26] Korotkov V. Wear resistance of plasma-hardened materials. J Frict Wear 2011;32: 17–22. [27] Tyuftyaev A, Mordynskii V, Spector N, Frolova M. Improvement of friction surface wear resistance by plasma treatment. Chem Petrol Eng 2017;53:406–11. [28] Kanaev A, Bogomolov A, Sarsembaeva T. Improving the wear resistance of wheelpair rims by plasma quenching. Steel Transl 2012;42:544–7. [29] Rahman F, Kharlamov Y, Chattha J. The influence of surface treatment and microstructural parameters on the rail/wheel tribological behaviours. In: ASME 2005 international design engineering technical conferences and computers and information in engineering conference. American Society of Mechanical Engineers Digital Collection; 2008. p. 2161–71. [30] Anijdan SM, Sediako D, Yue S. Optimization of flow stress in cool deformed Nbmicroalloyed steel by combining strain induced transformation of retained austenite, cooling rate and heat treatment. Acta Mater 2012;60:1221–9. [31] Korotkov VA. Wear resistance of carbon steel with different types of hardening. J Frict Wear 2015;36:149–52. [32] Isakaev EK, Sinkevich OA, Spector NO, Tyuftyaev AS, Tazikova TyF, Merkulov VV. Thermal fluxes in a generator of low temperature plasma with a divergent channel of the outlet electrode. High Temp 2011;49:797–802. [33] Yang Y, Guo J, Yan M, Zhu Y, Zhang Y, Wang Y. Improvement of wear resistance for carburised steel by Ti depositing and plasma nitriding. Surf Eng 2018;34:132–8. [34] Grigoriev S, Fedorov S. Modification of the structure and properties of high-speed steel by combined vacuum-plasma treatment. Met Sci Heat Treat 2012;54:8–12. [35] Yao C, Xu B, Huang J, Zhang P, Wu Y. Study on the softening in overlapping zone by laser-overlapping scanning surface hardening for carbon and alloyed steel. Opt Lasers Eng 2010;48:20–6. [36] Lian G, Yao M, Zhang Y, Chen C. Analysis and prediction on geometric characteristics of multi-track overlapping laser cladding. Int J Adv Manuf Technol 2018;97:2397–407. [37] Moradi M, Arabi H, Nasab SJ, Benyounis KY. A comparative study of laser surface hardening of AISI 410 and 420 martensitic stainless steels by using diode laser. Opt Laser Technol 2019;111:347–57. [38] Xiang Y, Yu D, Cao X, Liu Y, Yao J. Effects of thermal plasma surface hardening on wear and damage properties of rail steel. Proc Inst Mech Eng J J Eng Tribol 2018; 232:787–96. [39] Yang L. Plasma surface hardening of ASSAB 760 steel specimens with Taguchi optimisation of the processing parameters. J Mater Process Technol 2001;113: 521–6.
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The present work has been supported by The National Key Research and Development Program of China (2016YFB1102600) and the Na tional Natural Science Foundation of China (51608459, 51778542, 51978586 and U1734207) and Fundamental Research Funds for the Central Universities (2682018CX01) and Key Research and Develop ment Projects of Science and Technology Program for Sichuan Province (2019YFG0283) and Open fund of key laboratory of heavy haul railway engineering structure of Ministry of Education (2018JZZ02). References [1] Nejad RM, Shariati M, Farhangdoost K. Effect of wear on rolling contact fatigue crack growth in rails. Tribol Int 2016;94:118–25. [2] Wang P, Ma XC, Xu JM, Wang J, Chen R. Numerical investigation on effect of the relative motion of stock/switch rails on the load transfer distribution along the switch panel in high-speed railway turnout. Vehicle Syst Dyn 2018;57(2):226–46. [3] Huang Y, Shi L, Zhao X, Cai Z, Liu Q, Wang W. On the formation and damage mechanism of rolling contact fatigue surface cracks of wheel/rail under the dry condition. Wear 2018;400:62–73. [4] Wang WJ, Fu ZK, Guo J, Zhang YQ, Liu QY, Zhu MH. Investigation on wear resistance and fatigue damage of laser cladding coating on wheel and rail materials under the oil lubrication condition. Tribol Trans 2016;59:810–7. [5] Anijdan SM, Sabzi M. The effect of heat treatment process parameters on mechanical properties, precipitation, fatigue life, and fracture mode of an austenitic Mn hadfield steel. J Mater Eng Perform 2018;27:5246–53. [6] Ma M, Wang Z, Zeng X. A comparison on metallurgical behaviors of 316L stainless steel by selective laser melting and laser cladding deposition. Mater Sci Eng AStruct 2017;685:265–73. [7] Lei J, Shi C, Zhou S, Gu Z, Zhang L-C. Enhanced corrosion and wear resistance properties of carbon fiber reinforced Ni-based composite coating by laser cladding. Surf Coat Technol 2018;334:274–85. [8] Liu J, Yu H, Chen C, Weng F, Dai J. Research and development status of laser cladding on magnesium alloys: a review. Opt Laser Eng 2017;93:195–210. [9] Fu Z, Ding H, Wang W, Liu Q, Guo J, Zhu M. Investigation on microstructure and wear characteristic of laser cladding Fe-based alloy on wheel/rail materials. Wear 2015;330:592–9. [10] Guo HM, Wang Q, Wang WJ, Guo J, Liu QY, Zhu MH. Investigation on wear and damage performance of laser cladding Co-based alloy on single wheel or rail material. Wear 2015;328:329–37. [11] Wang W, Fu Z, Cao X, Guo J, Liu Q, Zhu M. The role of lanthanum oxide on wear and contact fatigue damage resistance of laser cladding Fe-based alloy coating under oil lubrication condition. Tribol Int 2016;94:470–8. [12] Wang W, Hu J, Guo J, Liu Q, Zhu M. Effect of laser cladding on wear and damage behaviors of heavy-haul wheel/rail materials. Wear 2014;311:130–6. [13] Lewis S, Lewis R, Fletcher D. Assessment of laser cladding as an option for repairing/enhancing rails. Wear 2015;330:581–91. [14] Lewis S, Fretwell-Smith S, Goodwin P, Smith L, Lewis R, Aslam M, et al. Improving rail wear and RCF performance using laser cladding. Wear 2016;366:268–78. [15] Lu P, Lewis S, Fretwell-Smith S, Engelberg D, Fletcher D, Lewis R. Laser cladding of rail; the effects of depositing material on lower rail grades. Wear 2019;438: 203045.
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Tribology International journal homepage: http://www.elsevier.com/locate/triboint
An investigation into the microstructure and tribological properties of rail materials with plasma selective quenching Jingmang Xu a, b, *, Kai Wang a, b, Ronghe Zhang a, b, Qiang Guo a, b, Ping Wang a, b, Rong Chen a, b, Dongfang Zeng c, Fuhai Li b, Jun Guo c, Lu Li d a
MOE Key Laboratory of High-speed Railway Engineering, Southwest Jiaotong University, Chengdu, 610031, China School of Civil Engineering, Southwest Jiaotong University, Chengdu, 610031, China State Key Laboratory of Traction Power, Southwest Jiaotong University, Chengdu, 610031, China d Sichuan Jinhong Plasma Technology Co. LTD, Chengdu, 610000, China b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Plasma selective quenching Microstructure Tribological properties Contact damage
Plasma selective quenching (PSQ) forms dispersed hardening microscopic structures on the rail material surface by the action of energetic plasma. In this study, the microstructure and tribological properties of PSQ treated rail materials are investigated by the rolling fatigue test and metallographic analysis. The effect of plasma parameters on the surface quality and microstructure is analyzed, and the optimum diameter of PSQ region is 5.0 mm. The microhardness in the PSQ region significantly increased and the tribological properties of rail materials improved. Meanwhile, severe spalling occurs on the surface in the PSQ region. Based on the field application, the wear resistance of the switch rails is enhanced by approximately 3 times that of untreated rail materials.
1. Introduction Plasma selective quenching (PSQ) forms dispersed hardening microscopic structures on the rail material surface, which can improve the hardness and wear resistance of the rail surface. The principle is that high-energy plasma quickly heats the rail surface to a point above its austenitizing temperature, and then the rail will cool down quickly due to its good thermal conductivity, generating a hardened structure. Part of this hardened structure consists of quenched martensite. The laminar plasma surface hardening technique uses a high-efficiency, high-energy, long-life and numerically controlled fascicular ultra-high-temperature heat source (the temperature can be set, and accurately repeated, to between 500 and 15,000 � C). As a high-quality ultra-high-temperature long-beam heat source characterized by its good scalability, intelligent control and modularity, it represents a revolutionary substitute for existing ion heat sources such as turbulence. Since laminar plasma surface hardening is inexpensive and easy to carry out by means of a large-scale, automated production process, it is a very suitable technique for hardening the surface of rail materials, which can improve the wear and rolling contact fatigue resistance and the service life of rails. With the increase of axle weight and running speed of railway ve hicles, the contact damage of wheels and rails become more serious
[1–5], which affects the safe operation of railways. At present, the common used rail surface strengthening methods are: cyclic-accelerated cooling of the head of pearlite rails [6,7], laser cladding [8–15], laser quenching [16–20], laser melting [21,22] and plasma quenching [23, 24]. However, it can’t provide controlled uniform heating of the mate rial surface for the original turbulent plasma strengthening method, limiting the development of plasma strengthening. The above problem has been solved by laminar plasma enhancement as a controllable and effective reinforcement method [25]. Studies have shown that after plasma strengthening the hardness of the material is increased by 2–3 times [23], and the wear resistance and service life are increased by more than 10 times [26]. Metallographic analysis showed that the plasma-enhanced inner core region forms a relatively ordered and non-uniform martensite structure [27,28], while the boundary region is in a chaotic state, very disordered and irregular. Since the internal phase transformation process of the material is unclear during the quenching process [29,30], there is no uniform conclusion on the choice of pro cessing parameters in the strengthening process. It has been determined that the choice of different plasma quenching treatment parameters is related to the material’s thermal expansion coefficient and material composition [31]. With the increase of heating power, gas flow rate and treatment rate, the depth of the heat affected zone increases. As the
* Corresponding author. MOE Key Laboratory of High-speed Railway Engineering, Southwest Jiaotong University, Chengdu, 610031, China. E-mail address: [email protected] (J. Xu). https://doi.org/10.1016/j.triboint.2019.106032 Received 27 July 2019; Received in revised form 21 October 2019; Accepted 21 October 2019 Available online 22 October 2019 0301-679X/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Jingmang Xu, Tribology International, https://doi.org/10.1016/j.triboint.2019.106032
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Table 1 The composition of U75V rail material. Material composition
C
Si
Mn
V
P
S
Percentage (%)
0.71–0.8
0.5–0.8
0.7–1.05
0.04–0.12
�0.03
�0.03
Fig. 2. Influence of irradiation time on quenching size of U75V rail material.
Fig. 1. Influence of irradiation time on quenched rail material of U75V.
in quenching width, thickness and structural morphology. Fig. 1 shows the microstructure of a quenched zone irradiated for 0.10s–0.35s and measured under an optical microscope of U75V rail material. Since the matrix fails to be heated to the austenitizing temperature for only 0.10s of irradiation, no quenched zone is observed under the optical micro scope. A clearly visible quenched zone is observed after irradiation lasting 0.15s–0.35s. The detailed sizes of quenched zones for different times of irradiation are showed in Fig. 2. With the increase of irradiated time, the martensite strengthening zone appears with a significantly increased depth and a moderately increased width, leading to a decline in the width-depth ratio of the quenched zone. The width and thickness increase markedly at 0.15s–0.30s, and slightly at 0.30s–0.35s. The study of quenching test shows that with the increase of irradiated time, the quenching depth-width ratio decreases as the quenching depth increases more markedly than the width. Moreover, when the treatment time is extended from 0.30s to 0.35s, the size increases more slowly. The influence of irradiation time on the structure of quenched zones on the surface of U75V rail material is showed in Fig. 3. With the irra diation time extended from 0.15s to 0.25s, the thickness of the martensite strips increases slightly, while that of the martensite struc ture increases more markedly. The carbon content in martensite rises as time increases, turning the martensite into high-carbon acicular martensite. Although acicular martensite is harder, it is also very brittle and cannot be safely used in rails. However, lath martensite has good strength, ductility and toughness. Therefore, the irradiation time does not need to be too long for a lath-shaped martensitic structure to be created. In fact, according to experimental results, the irradiation time does not need to exceed 0.25s. An analysis of the quenched zone sizes shows that the irradiation time should ideally range from 0.15s to 0.25s. In addition to irradiation time, other technical parameters such as generator power, mixed airflow, argon ratio and spraying distance, have an appreciable impact on hardening quality as well. Fig. 4 shows the impact of different parameters on quenching depth. The quenching depth basically shows a linear relationship with power: quenching depth increases with power (4(a)). The quenching depth increases with the rise in mixed airflow until it reaches a peak at 8.5 L/min, before gradually beginning to decrease (4(b)). The analysis shows that this phenomenon may be related to the burning loss of the rail surface. As airflow in creases, heat transfer on the rail surface speeds up, and the steel in the
processing speed decreases, the hardness of hardened surface increases; while the heating power and gas flow rate change, the hardness hardly changes [32–34]. Therefore, different plasma quenching treatment methods and parameters are often used for different materials. The common treatment method is to perform single or multiple beam rein forcement on the surface of the substrate. Since the processing area of the wheel/rail is large and the size of the plasma-enhanced heating zone is small, pluralities of overlapping regions are inevitable. The overlap region causes a tempering effect and leads to the formation of soft re gions, which will generate residual tensile stress and accelerate the initiation and propagation of fatigue cracks in the overlap region [35–39]. In order to avoid the occurrence of overlapping regions, the rail materials are strengthened with plasma selective quenching. Experi ments and field applications show that the proposed strengthening method can effectively improve the hardness and damage resistance of the material. In this study, the microstructure and tribological properties of rail materials with plasma selective quenching are studied. The micro structure and metallographic composition of the treated rail materials is analyzed. To investigate the tribological properties of the treated rail materials, the rolling contact fatigue (RCF) tests are performed to analyze the wear resistance and damage of the treated material in comparison with untreated materials, concluding that the laminar plasma surface hardening technique can improve the wear resistance of rails. The results of the study can provide support for the application of the laminar plasma surface hardening technique to rail materials. 2. Plasma selective quenching For the laminar plasma quenching of rail materials in this study, the laminar plasma generator is applied to conduct the dispersed point-like quenching areas on the rail material surface in order to achieve a finer hardening effect. The influence of technical parameters on the quenching quality of U75V rail material is investigated to obtain an optimal range of parameters, including irradiation time, power of generator, rate of mixed airflow and spraying distance. The composition of U75V rail material is listed in Table 1. For the rail materials irradiated with the laminar plasma generator for different periods of time, there are some obvious changes appeared 2
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Fig. 3. Influence of irradiation time on microstructure of quenched zones on the U75V rail surface.
Fig. 4. Change in quenching depth under various technical conditions.
Fig. 5. Surface topography of quenched zones under different conditions.
quenching center melts. However, the airflow also blows apart the melted liquid steel, creating a pit in the quenching center and lowering the quenching depth. The depth decreases as the argon ratio goes up, indicating that the plasma temperature is inversely proportional to argon ratio (4(c)). The quenching depth decreases as the spraying dis tance increases, while a spraying distance of less than 14 mm causes ablation to form a pit. A spraying distance above 18 mm accelerates the decrease in depth, which changes slightly from 14 to 18 mm (4(d)). The corroded and cleaned surface topography of quenched zones treated under different technical conditions is showed in Fig. 5. The quenched zones do not ablate as power changes (see Fig. 5(a)). The
quenching center dot is enlarged slightly as power increases. More specifically, the power reaches a maximum value at the center dot, which forms a grain, while the microscopy does not display any cracks in the dot corroded by muriatic acid. The quenching center dot is enlarged as airflow increases, gradually inducing ablation on the surface (Fig. 5 (b)). The dot forms a grain with airflow at 8 L/min, slightly ablated at 8.5 L/min and 9 L/min, and obviously ablated at 9.5 L/min. Microscopy does not show any cracks after corrosion in muriatic acid. The quenching center dot gradually diminishes as the argon ratio rises, and the surface ablation is gradually alleviated. The surface is slightly ab lated at an argon ratio of 20% and 25%, and the center dot becomes 3
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Fig. 6. GPM rolling contact fatigue tester.
larger at 30%, forming a grain. Microscopy does not show any cracks after corrosion in muriatic acid. The surface is obviously ablated at a spraying distance of 12 mm when the generator power is set to 22 kW; the surface is also obviously ablated at a spraying distance of 14 mm when the generator power is set to 23 kW; the surface is not obviously ablated at a spraying distance of 16–20 mm when the generator power is set to 24 kW (5(d)). Microscopy does not show any cracks after corrosion in muriatic acid. An analysis of the above test results shows that, factors such as the appearance of quenched zones, quenching depth, width and metallur gical structure are taken into consideration, an excellent laminar plasma surface quenching effect can be achieved under the following condi tions: irradiation time of 0.15s–0.25s, power of 23–24 kW, airflow of 8.0–8.5 L/min, argon ratio of 35–40%, and spraying distance of 15–17 mm, to protect the quenching surface from becoming ablated and granulated. Fig. 7. The size of wheel-rail specimen.
3. Experimental details 3.1. Experimental procedure
Table 2 Rail specimens featuring different diameters and duty ratios of quenched zones.
The experimental procedure for rail material with plasma selective quenching consists of following parts: ①microstructure analysis of treated rail material, ②rolling contact damage resistance test, ③contact damage and morphology after rolling contact test. The experimental procedure is carried out as follows: To analyze the microstructure and tribological properties of rail materials with plasma selective quenching, the microstructure of the treated rail material is observed with the optical microscope, scanning electron microscope and transmission electron microscope, and metal lographic structure is analyzed with X-ray diffraction and the micro hardness distribution is measured. Then, the rolling contact damage resistance test is performed on the rolling contact fatigue tester, and the tribological properties of treated rail materials with different quenching conditions are studied to obtain the optimum diameter of PSQ region. Finally, the contact damage and morphology of the treated rail materials after rolling contact test is analyzed, including material wear, rolling contact fatigue, surface morphology and profile structure.
Rail sample no.
1
2
3
4
5
6
Diameter (mm) Duty Ratio (%)
/ 0
5 15
5 30
5 45
3 30
6.25 30
is 20 mm; the lateral radius of the wheel specimen is set to 30 mm and the lateral radius of the rail specimen is set to be infinite. The upper specimen consists of wheel material and the lower specimen of rail material, specifically U75V. In order to simulate an actual wheel-rail contact state, the test parameters are obtained as follows based on the Hertz contact theory: The maximum contact stress at the center of the contact patch is 1800 MPa, the wheel specimen speed is 400r/min, the rail specimen speed is 398r/min, and the rotational slip ratio is 0.5%, the amount of rotation is 200,000, the testing time is 500min. The size of different quenched zones and the area ratio (duty ratio) of the quenched zones on the rail surface may exert an effect on the wear and contact fatigue resistance of the rails. For instance, if the quenched zone is too large, or its area ratio too high, cracks may easily be generated to reduce fatigue resistance. However, if the quenched zone is too small, or its area ratio too low, the wear resistance may not be significantly improved. Therefore, the diameter size and surface ratio of a quenched zone should be taken as major factors influencing the rolling contact damage resistance of rails. The parameters of test specimen are listed in Table 2. Specimen 1# is a contrast specimen made of an un treated material; Specimen 2#, 3# and 4# are specimens featuring different ratios of quenched zone to total surface area, namely different duty ratios; Specimen 3#, 5# and 6# are specimens featuring different quenching diameters. The rail and wheel specimens are shown in Fig. 8.
3.2. Rolling contact damage resistance test The rolling contact damage resistance test is performed using a GPM rolling contact fatigue tester (Fig. 6). The test is conducted according to the Hertz simulation procedure, also known as the Hertz contact theory. This theory ensures that the average contact stress between wheel-rail specimens can be faithfully measured in the laboratory, and that the ratio of major-to minor-axis length of the contact ellipse can be accu rately calculated as well. The wheel-rail specimen size is set as shown in Fig. 7: the diameter of wheel and rail specimens is 60 mm and the width 4
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4. Microstructure and tribological properties 4.1. Microstructure and microhardness The microstructure of the rail material treated by plasma selective quenching is observed with the optical microscope, scanning electron microscope and transmission electron microscope. The microstructure of different parts on the profile section of the treated specimen includes the quenched zone, transition zone and primary structure, as showed in Fig. 9. The whole quenched zone looks like a spherical crown (Fig. 9 (a)). The hardened structure in Fig. 9 (b) shows the noticeable characteristics of acicular martensite. As shown in Fig. 9(c), the primary structure is composed of laminar pearlite and a small amount of grain boundary allotriomorphic ferrite. The lath-acicular martensite at a certain width exists in the quenched zone with a specific angle based on the trans mission electron microscopy. For the martensite clusters, the laths are not completely parallel to each other, though there is a small angle between them; a single martensite lath is about 150–300 nm wide, and it is difficult to see other structures in the visible range. During the process of plasma surface hardening, the original structure in the quenched zone is austenitized at high temperature and then generates martensitic
Fig. 8. Test specimens of rail and wheel materials.
There is a conspicuous, round spot-like quenched zone on the rail specimen treated with the laminar plasma surface hardening technique, with black surface oxidation visible around the quenched zone; this can be washed away using muriatic acid. The specimen surface is smooth rather than granulated.
Fig. 9. Microstructure of the rail specimen treated by plasma selective quenching. 5
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The front side of the quenched zone is sampled first, and its metal lographic composition analyzed using X-ray diffraction. The results of the XRD analysis are showed in Fig. 10. The three peaks inosculate well with α-Fe after XRD. For the supersaturated solid solution of carbon atoms in α-Fe, the X-ray diffraction peak of martensite in steel is very approximate to α-Fe. According to the micro-structure observations and XRD analysis, the martensite in the quenched zone is fully transformed, while a small amount of pearlite is not turned into martensite in the transition zone due to the high heating speed and short time, leaving the transition structure as a mixture of martensite and austenite. The microhardness of the rail materials treated by plasma selective quenching is measured in the material surface and depth direction, as shown in Fig. 11. Fig. 11 (a) shows the microhardness distribution of the quenching surface, from the quenching center to the primary structure in the lateral direction; Fig. 11 (b) shows the microhardness distribution of the quenching center in the depth direction. It can be seen that the surface microhardness of the rail material is significantly improved with plasma selective quenching, and that the microhardness of the quenched zone is significantly higher than that of the primary structure. The microhardness of the primary structure is 300HV~400HV, and the microhardness of the quenched zone goes up to 850HV~950HV, char acterized by a high homogeneity; the microhardness suddenly changes at the interface, along with the microstructure.
Fig. 10. XRD analysis of the laminar plasma-quenched zone on U75V rail surface.
4.2. Tribological properties
transformation due to the self-quenching and rapid cooling. For the regular macroscopic interface between the quenched zone and primary structure (Fig. 9 (a)), it can be seen that the quenched structure has stuck into the primary structure to form the jagged interface with the transi tion zone only about 18 μm thick (Fig. 9 (e), (f)). The pearlite in the primary structure near the interface is heated to a very high temperature within a short time and fails to be fully tempered, leading to an insig nificant change in its structural morphology (Fig. 9 (d)).
Based on rolling contact damage resistance test, the material removal and structural morphology for 6 sets of rail and wheel specimens are analyzed and compared. The influence of duty ratio and diameters of quenched zones on the material removal of the rail specimens is studied and showed in Fig. 12. The duty ratio and diameters of quenched zones have a great impact on the material removal of the rail specimens. The surface ratio of the quenched zones equals 0, 15%, 30% and 45%
Fig. 11. Microhardness of the treated rail materials: (a) the material surface; (b) depth direction.
Fig. 12. Influence on the material removal of rail specimens (a) duty ratios (b) diameters of the quenched zones. 6
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Fig. 13. Surface structure morphology: (a)–(b) untreated material, (c)–(f) hardened material.
respectively, the material removal of the hardened rail specimen is lower than that of the contrast specimen. In addition, the material removal of the rail specimen decreases as the duty ratio of the quenched zones increases. The diameter of the quenched zones equals 3 mm, 5 mm and 6.25 mm respectively, the material removal of the hardened rail specimen is also lower than that of the contrast specimen. The material removal of the rail specimen first decreases and then increases as the diameter of the quenched zones increases. The material removal reaches
a minimum of 0.094 g when the diameter equals 5 mm, 45% lower than that of the contrast specimen. A quenching diameter of 5 mm is the proposed value for actual rail hardening. Based on the analysis of the material removal on all specimens, the plasma selective quenching can significantly improve the wear resistance of the rail materials. The structural morphology of the worn-out surface and cross-section of the rail specimens is observed after testing. By comparing the wornout surface of both untreated and hardened materials, it can be seen that both the surface of the untreated material (Fig. 13 (b)) and the inner surface of the quenched zone (Fig. 13 (d)) remain smooth. However, there are a few pits and delaminations on the surface of the untreated material. Furthermore, the direction of delamination is consistent with the rolling direction, while there is no delamination present on the surface of the hardened material. There are no conspicuous cracks on the surface of the untreated material (Fig. 13 (a)), while there are some cracks at the edge of the hardened material (Fig. 13 (e), (f)). The cracks exist at the rear end of the quenched zone relative to the rolling direction specifically (Fig. 13 (c)), and propagate to the front end along the edge of the quenched zone. The cracks form in the edge of the quenched zone and propagate to the inside of the hardened zone, but not to the primary materials. It is concluded that the plasma selective quenching can pre vent delamination from occurring inside the quenched zone, and the
Fig. 14. Longitudinal profile structure of the standard material. 7
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Fig. 15. Longitudinal profile structure of the hardened material.
cracks are formed and extend within the quenched zone and have no impact on the primary materials. In order to observe the inner structure of the rail specimens after testing, it is cut open along the wear scar center. Meanwhile, the quenched zone and primary structure are selected as observation sam ples by polishing the zones, and then corroding them in 5% nital and washing them in alcohol. The profile structure of the worn-out rail specimens is observed under an optical microscope, revealing the lon gitudinal profile morphology of the untreated and hardened materials under different amplification factors, as shown in Fig. 14 and Fig. 15. The untreated material is composed of pearlite and ferrite (Fig. 14 (a)), and a conspicuous crescent structure forms on the quenched zone, clearly different from the primary material (Fig. 15 (a)). The primary material outside the quenched zone is also composed of pearlite and ferrite (Fig. 15 (b)), but in the crescent quenched zone a uniform, compact martensitic structure can be observed (Fig. 15 (c)). There are cracks at the rear end of the crescent quenched zone in the rolling di rection of the specimen (Fig. 15 (e)), but merely propagate within the quenched zone and don’t extend to the primary material. The untreated material has obvious plastic fluidity (Fig. 14 (b)), and the plastic deformation layer is relatively thick, at about 80 μm. In the process of rolling test, the wheel-rail surface materials move out of
position under the action of cyclic contact stress and tangential friction force. The pearlite and ferrite are compressed and elongated, plastically deformed. Moreover, the direction of plastic deformation is consistent with the direction of the tangential force. With the increase of the dis tance to the contact surface, the plastic fluidity will be higher. There is no obvious plastic deformation in the quenched zone (Fig. 15 (c)), and without spalling pit present on the surface of the specimen (Fig. 15 (d)). A high-hardness martensitic structure with high yield strength is generated after plasma selective quenching. The martensitic structure contains tiny crystal grains and the plastic deformation resistance en hances with the increase of the ratio of the grain boundary to crystals. Moreover, martensitic transformation occurs in the form of a coherent shear. A huge array of micro-defects including dislocation and twins appears in the crystal grains, which could hinder the removal of material and effectively suppress plastic flow and material spalling. A slight plastic deformation occurs on the surface of the primary material be tween the quenched zones of the wheel-rail specimens, and the thickness of the plastic deformation is significantly less than that of the plastic deformation layer in the untreated wheel-rail specimens. Furthermore, as the distance to the quenched zones is closer, the plastic deformation layer will be thinner (Fig. 15 (d)). The analysis of tribological properties shows that the plasma selective quenching can significantly prevent 8
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the service duration of the hardened switch rail is three times as long as that of the untreated switch rail, the wear loss of the hardened section of the hardened switch rail is lower than that of the untreated switch rail. However, the wear loss of the unhardened sections on hardened switch rail is higher due to the longer service time. According to the service conditions of the two switch rails, the plasma selective quenching can significantly improve the rolling contact damage resistance of switch rails. 6. Conclusion Fig. 16. Field application of the switch rail treated with plasma selec tive quenching.
With respect to the application of the plasma selective quenching to rail treatment, this paper conducts a wear and contact fatigue test on a laminar plasma-hardened rail material and analyzes the microstructure and microhardness distribution of the hardened rail material. The wear loss and damage degree of the rail material after testing is studied. Finally, based on field applications, the wear resistance and surface damage of the laminar plasma-hardened and untreated switch rails is compared and analyzed. The research conclusions are as follows:
plastic deformation from occurring on the surface of the rail material. 5. Field application and verification The plasma selective quenching has been initially applied to switch rail surface treatment on the No.50-6 symmetrical turnout switch rails at Chengdu North Marshalling Station. Untreated switch rails needed to be replaced every 6 months, but the hardened switch rails have been used for about 9 months so far, and are still in good condition, prolonging the service life by 3 times. An obvious spotted quenched zone can be observed on the rail sur face (Fig. 16(b)). Compared with the primary structure, the quenched zone is outwardly smooth and free of bulges. Moreover, homogeneous light bands can be observed on the surface. The overall wear resistance of the switch rail improves and the material removal of the quenched zone and primary structure is the same, which guarantees the whole worn switch rail surface smooth. The untreated switch rails in service are severely delaminated and spalled (Fig. 16 (a)), while the hardened switch rail are only slightly damaged. Similar to the wear and contact fatigue test results, cracks occur along the hardened section after longterm service. More specifically, the cracks appear in the edge of the quenched zones and propagate within the quenched zones (Fig. 16 (b)). In addition, the quenched zones is slightly spalled, but both size and range of spalling pits are much less than that of untreated sections, effectively improving the rolling contact damage resistance of switch rail. In order to compare and analyze the wear resistance of the hardened and untreated switch rails, the profile measurement for two switch rails at the same operating conditions is conducted and the comparison of wear loss is showed in Fig. 17. The hardened switch rail has been in operation for about 9 months, and the quenched zone ranges from the point of the switch rail to the section with the width of 50 mm; the untreated switch rail has been in operation for about 3 months under the same operating conditions. The side wear and vertical wear of the two switch rails on selected sections are compared. Under the condition that
(1) For the rail specimen treated with the plasma selective quench ing, microstructure observations and XDR analyses can confirm that the quenched zone is of an evenly and compactly distributed martensitic structure. There is no obvious transition zone in terms of micro-structure between the quenched zone and the primary structure. The hardness of the quenched zone is significantly higher than that of the primary structure, and it changes abruptly at the interface, rapidly decreasing from 900 HV to 400 HV. (2) The wear resistance of the laminar plasma-hardened specimen is significantly higher than that of the untreated specimen. The changes in wear loss under different hardening conditions are compared and analyzed. The duty ratio and diameter of quenched zones have a great influence on the wear loss of the rail specimen. The wear loss of the hardened specimen decreases by 45% compared with the untreated rail specimen. (3) The structural morphology of the rail specimen is observed after rolling test. There are some corrosion pits and delamination visible on the surface of the standard material. While, there is no delamination visible on the surface of the hardened material. The profile structure of the quenched zone of the hardened specimen is obviously crescent-shaped. Observations of surface structure show that the cracks only propagate within the quenched zone and do not extend into the primary structure. (4) The plasma selective quenching has been applied to switch rail treatment for field application. The technique can significantly improve the rolling contact damage resistance of the switch rail and greatly prolong its service life.
Fig. 17. Comparison of wear loss between the hardened and untreated switch rail. 9
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Declaration of competing interest
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