Effect of Processing Techniques on Microstructure and Mechanical Properties of Carbide-free Bainitic Rail Steels (original) (raw)

Effect of Processing Techniques on Microstructure and Mechanical Properties of Carbide-free Bainitic Rail Steels

J.P. Liu a,e{ }^{\mathrm{a}, \mathrm{e}}, Y.Q. Li a{ }^{\mathrm{a}}, J.Y Jin b{ }^{\mathrm{b}}, Y.H. Zhang a{ }^{\mathrm{a}}, F.S. Liu a{ }^{\mathrm{a}}, R. SUc\mathrm{SU}^{\mathrm{c}}, Balaji Narayanaswamy d{ }^{\mathrm{d}}, Q.Y. Zhou a{ }^{\mathrm{a}}
a{ }^{a} Metals and Chemistry Research Institute, China Academy of Railway Sciences, Beijing 100081, P.R. China
b { }^{\text {b }} Technical Center, Anshan Iron & Steel Group Corporation, Liaoning 114009, P.R. China
c { }^{\text {c }} School of Materials Science and Engineering, Hebei University of Science and Technology, Shijiazhuang 050018, P.R. China
d{ }^{d} School of Engineering, University of Waikato, Hamilton, New Zealand

A R T I C L E I N F O

Keywords:
Carbide-free bainitic rail
steels
Roller straightening
Tempering
In-line heat treatment
Retained austenite

A B STR A C T

Rail manufacture processing techniques, such as roller straightening, tempering and air-forced quenching, have shown to make a great influence on the microstructure and mechanical properties of rail steels. In this study, five carbide-free bainitic rail steels with same chemical composition were developed from the above rail processing routes in an industrial production line. In general, the carbide-free bainitic steels consisted of bainitic ferrite (BF) plates with distinct morphologies of retained austenite (RA) and minor occurrence of martensite in some conditions. Electron microscopy confirmed the orientation relationship between BF and RA was in line with the Nishiyama-Wassermann (N-W) orientation relationship, i.e., (111)7//(110)30<112>3//<110>40(111)_{7} / /(110)_{30}<112>_{3} / /<110>_{40}. The roller straightening and tempering processes could facilitate the transformation of unstable RA. The small volume fraction of stable RA contributed to an obvious improvement of yield strength, hardness and impact toughness, but at the cost of ductility and strain hardening ability. In addition, the in-line heat treatment resulted in the refinement of bainitic ferrite plates, leading to an increase in the tensile strength of carbide-free bainitic steels. In summary, there was a clear positive relationship between the rail processing techniques and the mechanical properties of the carbide-free bainitic steels. This study can benefit the development of new bainitic steels with a combination of wear and rolling contact fatigue resistance by appropriate selection of processing techniques.

1. Introduction

Modern railway systems are subjected to continuous use, facing the challenge of high speed and large axle loads. Pearlite steel has been widely used as a rail material across the world. The tensile strength of a pearlitic steel can be enhanced up to 1300 MPa by heat treatment and/ or adding alloying elements [1-4]. However, the low toughness ( ∼10\sim 10 20 J in Charpy V-notch impact test) of pearlite steel could increase the occurrence of rolling contact fatigue (RCF) defects leading to significant track maintenance and replacement costs [5]. In recent years, a newly designed carbide-free bainitic steel consisting of bainitic ferrite (BF) and retained austenite (RA) constituents have gained significant attention in rail industries. These steels possess enhanced fracture toughness and superior RCF resistance making them a potential candidate for more severe rail service conditions [6-9]. The characteristics of the microstructural constituents (i.e. BF & RA) are largely responsible for the unique combination of high strength and toughness in carbide- free bainitic steels making them largely suitable for tribological conditions in railway applications.

Thereby, the relationship between the microstructure and field application performances of the bainitic steel has attracted much attention among researchers. Hui et al. [10] attempted to evaluate the hydrogen embrittlement behavior of two types of high-strength rail steel with pearlitic and bainitic microstructures. Xiu et al. [11] studied the effect of cyclic stress loading on the microstructural evolution and tensile properties of the medium-carbon super bainitic steel. The retained austenite (RA) (film-like or blocky morphology) has been proved to play a significant role in the field service performances, such as the initiation and propagation of RCF defects [12-15]. Several researchers studied the positive impact of microstructure-mechanical property relationships on the wear resistance on both bainitic and pearlitic steels [16-19]. Despite several investigations on the microstructure-property relationship of carbide free bainitic steels, there still remains a knowledge gap on the possible ways of obtaining bainitic microstructures

[1]

1. a { }^{\text {a }} Corresponding author at: No. 2 Daliushu Road, Haidian District, China Academy of Railway Sciences, Beijing 100081, P.R. China E-mail address: edisonadd881210@gmail.com (J.P. Liu). ↩︎

with superior mechanical properties for rail applications. By careful selection of chemical composition and appropriate rail production techniques, bainitic microstructures with differential properties can be obtained.

Therefore, three primary rail manufacturing methods (tempering, roller straightening and in-line heat treatment) were studied to understand their effect on the formation of microstructure and mechanical properties of rail steels.

Tempering treatment is usually done to relieve residual stresses and improve the toughness of microstructures. The tempering temperature and the holding time of this process determine the resultant microstructural constituents and their corresponding properties [5,20-22]. However there is very little research on effect of tempering treatment on the carbide-free bainitic steels. Meanwhile, considering the production efficiency and cost, some tempering parameters proposed from laboratory investigation were too ideal to realize in reality, such as long holding time. Therefore, it becomes necessary to optimize the tempering parameters on carbide-free bainitic rail steels that could be employed in an industry specific environment.

Roller straightening is one of the key rail production stages, as it ensures the geometric quality in terms of straightness, flatness. It is a metal forming technique to reduce internal stresses and minimize surface defects. Significant breakthroughs have been made on controlling the residual stresses on rail foot by analyzing the distribution of stresses from the roller straightening processes [23-26]. However, there is insufficient data on understanding the effect of roller strengthening process on the mechanical properties of the rail steels. So this study aims to include this processing technique in the quest for producing carbide free bainitic steels.

In-line heat treatment is widely used in the production of headhardened pearlitic rails with excellent resistance to wear and RCF damage [4,27]. The in-line heat treatment not only increases the tensile strength but also enhances the ductility and toughness of pearlitic rails. Considering the favorable outcomes of this process, the current study would encompass this technique in the production of carbide free bainitic rails.

In the present work, five carbide-free bainitic rail steels produced through different processing stages from an industrial production line. The effect of these processing techniques, i.e., roller straightening, tempering and in-line heat treatment, on the bainitic microstructure and mechanical properties were studied. The findings from this study would contribute to a better application of rail processing techniques in future.

2. Materials and experiment details

2.1. Testing materials

The chemical composition of the carbide free bainitic steel employed in the current study has been shown in Table 1. Five different carbide-free bainitic rail steels in terms of microstructural characteristics were produced using from different processing routes (Fig. 1).

The hot rolling (HR) state of rail steel is numbered as 1#1 \# specimen. Then, the following process of roller straightening (RS) was applied to the 1#1 \# rail steel, and the resultant rail steel is defined as 2#2 \# specimen (HR + RS). The process of RS was applied by the combined horizontalvertical straightening from Danieli of Italy. There were nine horizontal and seven vertical rollers. When the tempering (T) process (heating at

Table 1
Chemical composition (wt. %) of the studied bainitic rail steel.

Minor quantities of other elements exist.

593 K for 2 hours) was performed to 2#2 \# specimen, the new rail steel was named as 3#3 \# specimen (HR + RS + T). In the case 4#4 \# specimen, a hot rolled rail steel was subjected to an in-line heat treatment, i.e., quenching (Q) with forced air, followed by roller strengthening ( Q+\mathrm{Q}+ RS). With a subsequent tempering process to 4#4 \# specimen, the 5#5 \# rail steel (Q+RS+T)(\mathrm{Q}+\mathrm{RS}+\mathrm{T}) was produced.

2.2. Mechanical experiments

In accordance with GB/T228.1-2010, tensile specimens of ∼10 mm\sim 10 \mathrm{~mm} diameter and 50 mm gauge length were machined from the rail head parallel to the rolling direction. Two sets of uniaxial tensile tests were conducted using a MTS-CMT5305 universal testing machine for each group at room temperature. The bulk hardness was measured by IntronRockwell 574 hardness tester on the real rail head with elimination of the decarbonized zone. Four sets of Charpy u-notch specimens ( 10×1010 \times 10 ×50 mm3\times 50 \mathrm{~mm}^{3} ) were used to measure the impact toughness at room temperature by a MTS-ZBC3302-A impact tester.

2.3. Microstructural characterization

The microstructures of five types of bainitic steels were characterized by the optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD). SEM and TEM investigations were performed using FEI Scios and FEI Tecnai G2 F20 analytical instruments, respectively… Furthermore, EBSD measurements were undertaken using FEI Scios facility equipped with a field emission gun (FEG) and HKL Channel 5 detector to study the crystallographic features of the bainitic rail steels… The samples were prepared according to the standard preparation method and electrolytically polished with 7%7 \% perchloric acid and 93%93 \% ethanol for 1 min . EBSD scans were carried out at an accelerating voltage of 20 kV , working distance of about 9.8 mm , and step size of 150 nm .

Meanwhile, the X-ray diffraction (XRD) measurements were conducted using Bruker D8 Advance X-ray diffractometer to measure the volume fraction of RA in each bainitic steel. Co-K α\alpha radiation was selected as the incident X-ray beam. The information on diffracted X-ray ranging from 30∘30^{\circ} to 115∘115^{\circ} was collected by the Lynxeye XE detector with a step size of 0.02∘0.02^{\circ}. The volume fraction of RA was calculated based on the integrated intensities of the diffraction peaks (200)n,(211)n,(200)n(200)_{n},(211)_{n},(200)_{n}, (220)3(220)_{3} and (311)n(311)_{n}. The relevant estimation method and equations can be found elsewhere [12,21].

3. Results

3.1. Microstructures of testing materials

Due to the microstructural similarities of the carbide-free bainitic steels, rail steel specimens 3#3 \# (HR + RS + T) and 5#(Q+RS+T)5 \#(\mathrm{Q}+\mathrm{RS}+\mathrm{T}) were analyzed for their microstructural features using SEM observations (Fig. 2). From Figs. 2a and 2b, it can be seen that the film RA of 3#3 \# bainitic steel locates between the subunits of a given sheaf of bainite ferrite (BF), and the coarser blocky RA usually falls between different sheaves of BF. Compared with Fig. 2a and Fig. 2c, the length of BF plate in 5#5 \# is shorter than that in 3#3 \#, indicating smaller grain size of 5#5 \# due to the quenching process. In Fig. 2d, it can be seen that the sheaves of BF plates in 5#5 \# are more packed than 3#3 \#, leading to a thinner thickness of BF plate in 5#5 \# bainitic steel.

For further analysis on the characteristics of BF and RA, TEM observations were conducted on 3#3 \# and 5#5 \# bainitic steels (Fig. 3). The bright field images (Figs. 3a-b, d-e), of 3#3 \# and 5#5 \# bainitic steels revealed BF plates, films of RA, blocky M/A (M-martensite and A- austenite) islands and martensite regions with twinning morphology. One can also observe the presence of nano-carbides being precipitated within the BF plates (Fig. 3e). Due to the rapid cooling rate and wide

Fig. 1. Schematic diagram of manufacture process of bainitic rail steels.

Fig. 2. SEM micrographs of two types of bainitic steels, (a) and (b) for 3#3 \# bainitic steel (HR + RS + T); © and (d) for 5#5 \# bainitic steel ( Q+RS+T\mathrm{Q}+\mathrm{RS}+\mathrm{T} ).
range of phase transformation temperatures, small amount of lowcarbon martensite plates could also be formed. However, the very little difference between the morphological features of a martensite and BF plate, made it harder to distinguish and clearly identify them (Fig. 3).

Two regions of selected area electron diffraction (SASD) patterns confirm the presence of blocky and film RA (Figs. 3c and f). The orientations of BF and film RA are clearly indexed as follows {111}v//\{111\}_{\mathrm{v}} / / {110}m<112>v//<110>m\{110\}_{\mathrm{m}}<112>_{v} / /<110>_{m} (Fig. 3f). It can be seen that there is more amount of blocky RA in the 3#3 \# than in 5#5 \# bainitic steel.

Based on these in-depth microstructural observations (i.e. SEM and TEM), the thickness of BF plate for each bainitic steel was estimated (Table 2). It can be seen that the roller straightening process has decreased the thickness of BF plates due to plastic deformation. Also, the quenching process can also assist in reducing the thickness of BF plate by increasing the cooling rate. On the other hand, the tempering process at relatively low temperature ( 593 K ) seems to have a limited effect on the refinement of bainitic microstructure.

EBSD analysis and their orientation images demonstrated the presence of BF plate and film RA in #3 specimen (Fig. 4). The volume fraction of RA in EBSD characterization (Fig. 4b) was ∼5.62%\sim 5.62 \%.

Meanwhile, the inverse pole figures (Fig. 5) have revealed a fiber texture in the BF of 3#3 \# specimen, showing that (001)n(001)_{n} tends to be parallel to the rolling direction (Fig. 5a). As for the RA, Fig. 5b demonstrates a preferred orientation that {110}v\{110\}_{\mathrm{v}} is parallels to the rolling direction.

Generally, there are two kinds of relationship used to describe the orientation between BF and austenite, i.e., Kurdjumov-Sachs (K-S) and Nishiyama-Wasserman (N-W) relationship. The K-S relationship can be represented as: {111}v//{110}m<110>v//<111>m<112>v\{111\}_{\mathrm{v}} / /\{110\}_{\mathrm{m}}<110>_{v} / /<111>_{m}<112>_{v} //<112>m/ /<112>_{m}. The N-W relationship can be expressed as: {111}v//\{111\}_{\mathrm{v}} / / {110}m<110>v//<001>m<112>v//<110>m\{110\}_{\mathrm{m}}<110>_{v} / /<001>_{m}<112>_{v} / /<110>_{m}. It was reported that the orientation relationship of the lower bainite and austenite were in accordance with K-S relationship, whereas the upper bainite/austenite orientation relationship deviated slightly from the K-S relationship to the N-W relationship [28-29]. However, it is still unclear for the carbide-free bainite steels. In our current study, it can be observed that <110>v//<001>m//<110>_{v} / /<001>_{m} / / rolling direction relationship indicates that the #3 carbide-free bainitic steel is likely to be in line with the N-W orientation relationship.

3.2. Volume fraction of retained austenite in different specimens

XRD patterns of five carbide free bainitic steels obtained through rail processing techniques were analyzed (Fig. 6a). Based on the fitting principle, the integrated intensities of (200)m,(211)m,(200)v,(220)r(200)_{m},(211)_{m},(200)_{v},(220)_{r} and {311}r\{311\}_{\mathrm{r}} diffraction peaks [12,21] were calculated to estimate the volume fraction of RA(Table 3). Moreover, the uniaxial tensile tests have been conducted (with a process of loading to 3%3 \% strain and then unloading) to evaluate the stability of RA in the carbide-free bainitic steels. The cross-sectional samples from gauge area were cut for XRD characterization. A comparative study on the XRD spectra of #1 specimen was undertaken before and after tensile test (Fig. 6b). The volume fraction and the stability factor (θ)(\theta) of RA after the application of 3%3 \% tensile strain were calculated for all the five steels (Table 3). Here, the stability factor θ\theta is defined as the percentage of untransformed RA in the initial RA. Amongst the five rail steels, 1#1 \# displayed the most unstable RA (θ∼74.2%)(\theta \sim 74.2 \%) without RS. With the process of RS, a high percentage of the unstable RA had been mechanically transformed to martensite, so the rest of RA in 2#2 \# sample remained a close stability factor compared with other four types of bainitic steels ( 93.3%∼93.3 \% \sim 95.3%)95.3 \%). Besides, it should also be noted that the stability of RA could be enhanced by tempering process.

Fig. 3. TEM micrographs of two types of steels, (a) and (b) corresponding to bright field imaging of 3#3 \#; (d) and (e) corresponding to bright field imaging of 5#5 \# bainitic steel; © and (f) corresponding to the SAED patterns of 3#3 \# and 5#5 \# bainitic steels, respectively.

Table 2
Thickness of BF plates in five bainitic steels.

HR=\mathrm{HR}= Hot Rolling, RS=\mathrm{RS}= Roller Straightening, T=\mathrm{T}= Tempering, Q=\mathrm{Q}= Quenching

3.3. Mechanical properties of testing materials

The common mechanical properties (i.e., bulk hardness, impact toughness and tensile strength) of the five rail steels were measured in this investigation. Engineering stress-strain (SS) curves for all rail specimens were plotted and parameters from the uniaxial tensile experiments were tabulated. Table 4 summarizes six typical parameters from SS curves, i.e., yield strength ( Rpo,2\mathrm{R}_{\mathrm{po}, 2} ), ultimate tensile strength ( Rm\mathrm{R}_{\mathrm{m}} ), elongation (A), reduction of area (Z), yield-tensile ratio and strain hardening exponent (n). The strain hardening exponent (n) was
calculated from the plastic deformation section of true SS curves. The true stress-strain curves converted from Fig. 7 were applied to fit by Eq. (1), which is the logarithmic form of Hollomon equation (σ=kεα)\left(\sigma=\mathrm{k} \varepsilon^{\alpha}\right) [30]:
ln⁡σ=ln⁡k+nln⁡ε(1)\ln \sigma=\ln \mathrm{k}+\mathrm{n} \ln \varepsilon(1)
where k is the strength coefficient, σ\sigma is the true stress and ε\varepsilon is the true strain.

In general, the bulk hardness and impact toughness of rail steels increased with the decrease of the thickness of BF plate, except for 2#2 \# rail specimen (Tables 2&42 \& 4 ).

The five carbide-free bainitic steels demonstrated distinct mechanical properties. It can be seen that the bainitic steels (i.e. 1#,2#1 \#, 2 \# and 3#3 \# ) obtained without the quenching technique displayed an almost similar tensile strength ( ∼1300MPa\sim 1300 \mathrm{MPa} ). On the other hand, the bainitic rail steels of 4#4 \# and 5#5 \# produced through in-line quenching exhibited a higher tensile strength than hot rolled rail steels. It reveals that the cooling rate after hot rolling could play a vital role in enhancing the tensile strength of rail steel. It is also interesting to see that five kinds of bainitic steels obtain different yield strength indicating that the yield strength can be affected by more multiple factors. The strain hardening

Fig. 4. EBSD orientation images of #3: a) showing the phase distribution, and b) the orientation distribution of retained austenite.

Fig. 5. Inverse pole figures of (a) bainitic ferrite (BF) and (b) retained austenite (RA).
exponent decreases with the increase of yield-tensile ratio.

4. Discussion

4.1. Effect of roller straightening on bainitic microstructure and mechanical properties

The roller straightening (RS) process is an important rail forming technique to ensure a high geometric quality of straightness and flatness in rail steels. During the RS process, the volume fraction of RA was reduced from 12.44%12.44 \% to 10.6%10.6 \% indicating that the irreversible stress induced martensitic transformation has occurred in rail steels specimens #1 and #2 (Table 3). The blocky RA morphology is mechanically unstable leading to quick transformation to martensite than the film RA [9,31]. Moreover, due to a large plastic deformation occurring on rail head during the RS process, the microstructure of bainitic steel were more refined based on the analyses of BF plate thickness in Table 2.

Due to the unstable RA existing in 1# bainitic steel, the stress induced martensite (SIM) could be formed at a lower stress and thus contributed to extra phase transformation strain, leading to a lower yield strength ( 746 MPa ) (Fig. 8a) and greater elongation ( 18%) (Fig. 8b). With the reduction of the instability of RA and the plate thickness of BF by RS process, the yield strength and hardness were improved in 2# bainitic steel, but compromising ductility (Fig. 8b).

4.2. Effect of tempering on bainitic microstructure and mechanical properties

Tempering has significant effect on the ultimate mechanical properties of steel, and is known to relieve the residual stress and stabilize

Fig. 6. XRD patterns of five kinds of steels, (a) before tensile tests, (b) the comparison of 1# bainitic steel before and after 3% tensile strain.

Table 4
Mechanical properties of five kinds of bainitic steels.

Fig. 7. Engineering stress-strain curves of carbide free bainitic steels.

4.3. Effect of in-line heat treatment on bainitic microstructure and mechanical properties

The in-line heat treatment technique has been widely used in the industry. The effect of in-line treatment (i.e. quenching) was analyzed by comparing the rail steel samples 3#3 \# and 5#5 \# (Table 3). In general, it was found that the volume fraction of RA was reduced after quenching process. Besides, quenching can also lead to the precipitation of nanocarbides within the BF plates (Fig. 3e) and the refinement of bainitic ferritic plates. An in-depth TEM analysis has revealed that appearance of blocky RA was substantially less in 5#5 \# specimen than in the rail sample 3#3 \# (Figs. 3b & d). This indicates that quenching could favor the martensitic transformation of unstable blocky RA. Based on the calibration of SAED patterns of the film RA embed between BF plates (Fig. 3f), the orientation of BF and film RA in 5#5 \# bainitic steel confirms the N-W orientation relationship [28-29], i.e., (111)γ//(111)_{\gamma} / / (110)ns<112>γ//<110>n(110)_{n s}<112>_{\gamma} / /<110>_{n}. Furthermore, the inverse pole figures of BF and film RA in 3#3 \# bainitic steel (Fig. 5), also obeys the N-W orientation relationship, i.e., <110>γ//<001>n<110>_{\gamma} / /<001>_{n}. Therefore, the
orientation of BF and film RA in bainitic steel is in alliance with the NW orientation relationship without the influence of quenching.

It is important to note that the quenching process not only increased the yield strength but also the tensile strength (Fig. 8a). This clearly indicates that the tensile strength is largely dependent on the cooling rate which attributes to the microstructural refinement, i.e. finer bainitic ferrite. Nevertheless, the yield strength could be improved by one of the production techniques (i.e. RS, tempering and quenching) which eventually affects the volume fraction and the stability of RA in carbidefree bainitic steels.

It is interesting to observe that the curves of strain hardening exponent and elongation as a function of volume fraction of RA exhibited a similar trend (Fig. 8b). This proves the positive effect of RA on ductility and strain hardening ability in carbide-free bainitic steels due to the transformation-induced plasticity (TRIP) effect. However, the same cannot be extended on the yield strength (Fig. 8a) and impact toughness (Table 4) of carbide-free bainitic steels. The strain hardening ability of bainitic steel is considered to be lower than that of pearlitic steel [32], which could result in a disadvantage in wear resistance. On the other hand, the toughness of bainitic steel is related to the RCF resistance. Therefore, the wear resistance and the RCF resistance probably show a counteracting relationship in the carbide-free bainitic steel, and it is necessary to adjust processing techniques properly to obtain a better combination of wear resistance and RCF resistance.

5. Conclusions

The mechanical properties and microstructures of five kinds of carbide-free bainitic rail steels with same chemical composition but different processing routes have been investigated through SEM, TEM, EBSD, XRD and uniaxial tensile measurements. Following conclusions have been summarized as follows:

1. Based on the observations of SEM, TEM and EBSD, three distinct metallurgical phases appeared in the microstructures of carbide-free bainitic rail steels (i.e. bainite ferrite (BF), film retained austenite (RA), blocky RA and twinning martensite). The orientation relationship between BF and RA was in line with the Nishiyama-Wassermann (N-W) orientation relationship, i.e., (111)γ//(110)ns<112>γ//<110>n(111)_{\gamma} / /(110)_{n s}<112>_{\gamma} / /<110>_{n}.

Fig. 8. The relationship between volume fraction of RA and mechanical properties of carbide-free bainitic steels; (a) strength, (b) strain hardening exponent and elongation.

1. The roller straightening process contributed to the transformation of mechanically unstable RA into martensite and grain refinement in the microstructure of rail head.
2. The tempering process could have facilitated the TRIP effect and the process of stabilizing RA in the carbide-free bainitic steel. An obvious enhancement of yield strength and impact toughness from tempering indicates their susceptibility to the volume fraction and the stability of RA.
3. The in-line heat treatment process has reduced the volume fraction of RA and further refining the bainitic ferrite plates resulting in an enhanced yield strength. The rapid cooling rate could have led to the precipitation of nano-carbides, but has a limited effect on the change of orientation relationship between BF and RA.
4. In summary, a limited volume fraction of RA with good mechanical stability can largely improve the mechanical properties (i.e. yield strength, hardness and impact toughness) in carbide-free bainitic steels. Therefore, it is necessary to optimize the volume fraction of RA and grain size by roller straightening, tempering and in-line heat treatment to design a new kind of carbide-free bainitic rail with a better combination of wear resistance and RCF resistance.

Additional information

Competing financial interests: The authors declare no competing financial interests.

Declaration of Competing Interest

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

This study was supported by the China Academy of Railway Sciences within the major issues of the fund No. 2017YJ083. We would like to thank all these financial supports for conducting this research.

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