Study on Wear and Fatigue Performance of Two Types of High-Speed Railway Wheel Materials at Different Ambient Temperatures

materials

Article Study on Wear and Fatigue Performance of Two Types of High-Speed Railway Wheel Materials at Diﬀerent Ambient Temperatures

Lei MA 1,* , Wenjian WANG 2, Jun GUO 2 and Qiyue LIU 2

1 School of Mechanical Engineering, Xihua University, Chengdu 610039, China 2 Tribology Research Institute, State Key Laboratory of Traction Power, Southwest Jiao Tong University, Chengdu 610031, China; [email protected] (W.W.); [email protected] (J.G.); [email protected] (Q.L.) * Correspondence: [email protected]

Received: 14 January 2020; Accepted: 2 March 2020; Published: 5 March 2020

Abstract: The wear and fatigue behaviors of two newly developed types of high-speed railway wheel materials (named D1 and D2) were studied using the WR-1 wheel/rail rolling–sliding wear simulation device at high temperature (50 C), room temperature (20 C), and low temperature ( 30 C). The ◦ ◦ − ◦ results showed that wear loss, surface hardening, and fatigue damage of the wheel and rail materials at high temperature (50 C) and low temperature ( 30 C) were greater than at room temperature, ◦ − ◦ showing the highest values at low temperature. With high Si and V content reﬁning the pearlite lamellar spacing, D2 presented better resistance to wear and fatigue than D1. Generally, D2 wheel material appears more suitable for high-speed railway wheels.

Keywords: alpine region; high-speed wheel and rail materials; temperature; wear; fatigue damage

1. Introduction Railways play a vital role in the development of rail transportation. The environmental climate has a certain impact on wheel/rail systems exposed to the open air. Especially in China, wheel and rail materials may serve under extreme temperature conditions due to torridity and severe cold. For example, the highest and lowest temperature experienced by the Qinghai–Tibet railway during operation can reach +40 C and 45 C, respectively [1,2]. With the activation of the Haerbin–Dalian ◦ − ◦ railway, the world’s first high-speed railway running through an alpine region, safe railway operation in the alpine region has attracted more and more attention [3]. Wear and fatigue are common damages of wheel and rail materials during railway operation. Wear (especially at the wheel tread and rail ball interface under straight-line running conditions, as studied in this paper and shown in Figure1) will directly lead to the reduction of rail height, thus causing plastic deformation of materials and failure of surface materials [4]. Common wear mechanisms include abrasive wear, adhesive wear, delamination wear [5]. Fatigue derives from the formation of cracks and the detachment of material from the surface due to the repetitive application of alternating forces [6,7], which by first causing defects or surface cracks, may eventually destroy some components [8]. Especially in high-speed railways operating in extremely hot and cold climates, the mechanical properties of temperature-sensitive wheel and rail materials will change at certain extreme temperatures. Furthermore, friction performance and safety can be affected during train operation [9]. In recent years, brittle fractures in steel structures caused by fatigue have occurred from time to time. One of the characteristics of brittle fracture compared with ordinary strength failure is that brittle fracture often occurs in cold winter [10]. In China, 60–80% of rail fracture accidents occur

Materials 2020, 13, 1152; doi:10.3390/ma13051152 www.mdpi.com/journal/materials Materials 2020, 13, 1152 2 of 16 from November to March [11], and in Japan, 83 of the 121 rail failures recorded by a study occurred from November to February [12]. Shank et al. investigated many accidents and found that the change of steel’s notch sensitivity was the main cause of accidents that occurred at extreme temperatures [13]. It was also found that with the gradual decrease of temperature, fractures in railway material will change from the ductile shear fracture mode to the brittle cleavage fracture mode, causing the decline of fracture toughness, impact energy, and plasticity [14–18].

FigureFigure 1.1. ContactContact pointspoints ofof thethe railrail andand wheelwheel inin aa varietyvariety ofof pathspaths [[4].4].

As early as the 1980s, some scholars have studied the material properties of ferritic steel at low temperature by means of mechanical experiments and material preparation methods [19–24]. The results showed that extreme temperatures can cause the degradation of various ferritic steel properties, such as strength and toughness, before the formation of macroscopic cracks, aﬀecting the safety of wheel/rail services [25–27]. Also, Zhu, Lyu et al. [28,29] conducted a study on wheel/rail tribology in a low-temperature environment by using a pin-and-disk testing machine. The experimental results showed that the low-temperature environment had a great impact on the friction performance of the wheel/rail materials. Nevertheless, research on the performance of wheel and rail materials in a low-temperature environment is limited. Therefore, it is very necessary to select wheel and rail materials suitable for cold climates and analyze their wear and fatigue performance under extreme temperature conditions.

2. Materials and Methods The WR-1 rolling–sliding wear apparatus with a temperature-changing device was used to evaluate the wear and rolling contact fatigue (RCF) properties of D1 and D2 wheel rollers with friction pair of U71Mn rail material under different temperature conditions. As shown in Figure2, the apparatus is composed of two rollers which served as a wheel roller (upper specimen) and as a rail roller (lower specimen). The rail roller (4) is driven by a DC motor, while the wheel roller (5) is driven via a pair of gears. A normal force (from 0 to 2000 N) is applied by a compressed spring to the upper specimen. Load sensors (8) are used to measure the tangential friction force and normal force at the wheel/rail interface (measurement error: 5%). A low-temperature environment is created by double ± compressors (C) which make the refrigerant (Freon) to recirculate in the copper tube. A rubber tube (7) and copper cavity (6) connect the WR-1 rolling–sliding wear apparatus (B) to the low-temperature environment, resulting in a low temperature in the copper cavity (6) due to the flow of refrigerant. The wheel and rail rollers are installed in the copper cavity (6). High ambient temperature is reached using an electric heating tape entwined around the copper cavity (6). A digital temperature sensor (3) is fixed into the copper cavity (6) and the temperature is monitored in the cavity in real time. A PLC (Programmable Logic Controller) temperature control system (D) is used to stabilize the temperature in the copper cavity at a set value. The temperature error in the copper cavity is 2 C. During the ± ◦ experiment, the ambient temperature of the wheel and rail rollers are kept stable.

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1. Computer control system of the rolling wear device; 2. Servo motor; 3.Temperature sensor; 4.Rail roller; 5.Wheel roller; 6. Copper cavity; 7. Rubber tube for copper cavity; 8. Load sensor; A. Computer control system of the rolling wear device; B. WR-1 rolling–sliding wear apparatus; C. Low- temperature equipment of the twin-stage compressor unit; D. PLC (programmable logic controller) temperature control system.

Materials 2020, 13, x FORFigureFigure PEER REVIEW 2. Scheme of the WR-1 low-temperature simulation system. 3 of 17

The dimensions of wheel roller (upper specimen) and rail roller (lower specimen are shown in The dimensions of wheel roller (upper specimen) and rail roller (lower specimen are shown in FigureFigure3 a.3a. The The diameter diameter of of wheel wheel and and rail rail rollers rollers was was 40 40 mm. mm. The The wheel wheel and and rail rail rollers rollers were were cut cut from from thethe wheelwheel treadtread andand railheadrailhead (0–30(0–30 mmmm fromfrom thethe surface),surface), respectively,respectively, asas shown shown inin Figure Figure3 b.3b. The The samplessamples werewere moldedmolded intointo 4040 mm-diametermm-diameter rollersrollers throughthroughmachining machiningprocesses processes such such as asturning turningand and µ drilling,drilling, and and all all rollers rollers were were polished polished to surface to surface roughness roughness (Ra) of(Ra) approximately of approximately 0.15 m. 0.15 Before μm. testing, Before eachtesting, roller each was roller thoroughly was thoroughly cleaned by ultrasoniccleaned by cleaning ultrasonic for 15 cleaning min. The for chemical 15 min. compositions The chemical of wheelcompositions and rail of materials wheel and in weight rail materials percentage in weight are given percentage in Table are1. given in Table 1.

(a) (b)

FigureFigure 3. DimensionsDimensions and and sampling sampling position position of wheel of wheel and rail and rollers. rail rollers. (a) Dimensions; (a) Dimensions; (b) sampling (b) samplingposition. position.

Table 1. Chemical composition of the wheel and rail rollers (wt%).

Roller C Si Mn P S Cr Al V D1 wheel 0.52 0.26 0.73 0.0060 <0.002 0.25 0.023 ≤0.005 D2 wheel 0.54 0.68 0.70 0.0047 0.0013 0.055 0.011 0.07 Rail 0.65~0.75 0.1~0.5 0.8~1.3 ≤0.025 ≤0.025 - - -

In the test, the rotational speed of the rail roller was 400 r/min, and the number of cycles was 3.84 × 105. The normal force between the wheel and the rail rollers was about 150 N. The slip ratio was 0.91%. In addition, the experiments were performed at high temperature (50 °C), room temperature (RT: 20 °C), and low temperature (−30 °C). The rollers were ultrasonically cleaned in ethanol, dried, and weighed using an electronic balance (JA4103, measurement accuracy: 0.0001 g) before and after testing. Wear loss of the wheel and rail rollers was determined by calculating the mass loss. Figure 4 shows the sampling positions of the surface and cross sections in the wheel/rail rollers. Three pieces of 1 mm length separated by 120° were taken from each roller. Each section was cut along the rolling direction by wire-cutting processing, mounted in resin, ground with 2000-grit abrasive paper, polished with 0.5 μm diamond, and etched with 4% Nital. The plastic deformation of wheel and rail rollers was characterized using optical microscopy (OM) (OLYMPUS BX60M, Tokyo, Japan). Metallographic microscopy and surface and subsurface damages were inspected using scanning electronic microscopy (SEM) (QUANTA200 FEI, Eindhoven, Netherlands). Metallographic microscopy images of D1 and D2 materials are shown in Figure 5. It can be seen that pearlite composed of cementite and ferrite presents a lamellar structure, and the Pearlite lamellar spacing in D1 is larger than that in D2.

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Table 1. Chemical composition of the wheel and rail rollers (wt%).

Roller C Si Mn P S Cr Al V D1 wheel 0.52 0.26 0.73 0.0060 <0.002 0.25 0.023 0.005 ≤ D2 wheel 0.54 0.68 0.70 0.0047 0.0013 0.055 0.011 0.07 Rail 0.65~0.75 0.1~0.5 0.8~1.3 0.025 0.025 - - - ≤ ≤

In the test, the rotational speed of the rail roller was 400 r/min, and the number of cycles was 3.84 105. The normal force between the wheel and the rail rollers was about 150 N. The slip ratio was × 0.91%. In addition, the experiments were performed at high temperature (50 ◦C), room temperature (RT: 20 C), and low temperature ( 30 C). ◦ − ◦ The rollers were ultrasonically cleaned in ethanol, dried, and weighed using an electronic balance (JA4103, measurement accuracy: 0.0001 g) before and after testing. Wear loss of the wheel and rail rollers was determined by calculating the mass loss. Figure4 shows the sampling positions of the surface and cross sections in the wheel/rail rollers. Three pieces of 1 mm length separated by 120◦ were taken from each roller. Each section was cut along the rolling direction by wire-cutting processing, mounted in resin, ground with 2000-grit abrasive paper, polished with 0.5 µm diamond, and etched with 4% Nital. The plastic deformation of wheel and rail rollers was characterized using optical microscopy (OM) (OLYMPUS BX60M, Tokyo, Japan). Metallographic microscopy and surface and subsurface damages were inspected using scanning electronic microscopy (SEM) (QUANTA200 FEI, Eindhoven, Netherlands). Metallographic microscopy images of D1 and D2 materials are shown in Figure5. It can be seen that pearlite composed of cementite and ferrite presents a lamellar structure, and the Pearlite lamellar spacing in D1 is larger than that in D2. Materials 2020, 13, x FOR PEER REVIEW 4 of 17

Grounding and polishing Sampling Wear scar

Subsurface damage and plastic deformation Surface damage

Materials 2020, 13, x FOR PEER REVIEW 5 of 17 Figure 4.4. SamplingSampling positionspositions of of surface surface and and cross cross sections sections of theof the wheel wheel/rail/rail rollers rollers for examination for examination after wearafter wear testing. testing.

Ferrite

Ferrite 5μm 5μm

(a) (b)

Figure 5. MetallographicMetallographic microscope images of wheel materials. ( aa)) D1; D1; ( (b)) D2. D2.

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3. Results 3. Results

3.1.3.1. Wear Wear Loss FigureFigure 66 shows shows the the wear wear loss loss of of wheel wheel and and rail rail rollers rollers under under di ﬀ differenterent temperatures. temperatures. It is It clear is clear that thatthe wearthe wear loss loss of the of D2the frictionD2 friction pair pair was was slightly slight lowerly lower than than that that of the of the D1 frictionD1 friction pair, pair, and and both both the the wear losses of the D1 and D2 friction pairs were higher at 50 °C and −30 °C than at room wear losses of the D1 and D2 friction pairs were higher at 50 ◦C and 30 ◦C than at room temperature. temperature. The wear loss of D1 at 50 °C was not different from that− at −30 °C, while that of D2 at The wear loss of D1 at 50 ◦C was not diﬀerent from that at 30 ◦C, while that of D2 at 30 ◦C was −30 °C was higher than that at 50 °C. The results showed that− D2 had better wear resistance− than D1, higher than that at 50 ◦C. The results showed that D2 had better wear resistance than D1, especially at especially at 50 °C. For both wheel materials, the wear resistance at RT was the best, followed, in 50 ◦C. For both wheel materials, the wear resistance at RT was the best, followed, in order, by wear order,resistance by wear 50 C resistance and at 30 50 C.°C Atand the at same−30 °C time,. At thethewear same loss time, of the wear rail roller loss wasof the obviously rail roller larger was ◦ − ◦ obviouslythan that oflarger the wheel than roller,that of which the wheel is consistent roller, wh withich plastic is consistent deformation with ofplastic the material deformation occurring of the in materialthe testing occurring process. in the testing process.

0.30 0.30 Rail Rail 0.25 Wheel 0.25 Wheel 0.20 0.20

0.15 0.15 Wear loss/g Wear Wear loss/g 0.10 0.10

0.05 0.05

0.00 0.00 50 Room -30 50 Room -30 Temperature/°C Temperature/°C (a) (b)

Figure 6. 6. WearWear loss loss of of wheel/rail wheel/rail rollers. ( (aa)) D1; D1; ( (bb)) D2. D2.

3.2.3.2. Hardness Hardness andand Plastic Plastic Deformation Deformation HardnessHardness tests were conductedconducted onon thethe wheelwheel andand rail rail samples samples before before and and after after the the experiment. experiment. It Itwas was found found that that the the surface surface hardness hardness of D1 of samples D1 samples was aboutwas about 285 HV 2850.05 HVbefore0.05 before the experiment, the experiment, that of thatD2samples of D2 samples was 300 was HV0.05 300, andHV that0.05, and of the that rail of samples the rail was samples 280 HV was0.05 280(Figure HV70.05). When(Figure comparing 7). When comparingthe surface the hardness surface ofhardness the wheel of the and wheel rail samplesand rail samples before and before after and the after experiment the experiment (Figure (Figure8), the 8),hardness the hardness of the of wheel the wheel and rail and materials rail materials after after the experiment the experiment was was more more than than twice twice that that before before the theexperiment. experiment. The The wheel wheel and and rail samplesrail samples of the of D1 the friction D1 friction pair showed pair showed a significantly a significantly greater greater surface surfacehardness hardness ratio than ratio those than of those the D2 of frictionthe D2 friction pair, indicating pair, indicating that surface that surface hardening hardening of the D1 of frictionthe D1 frictionpair was pair greater was thangreater that than of the that D2 of friction the D2 pair. friction At thepair. same At time,the same the wheeltime, the surface wheel hardening surface hardeningwas greater was than greater that of than the rail,that whichof the rail, caused which a high caused wear a ofhigh the wear rail samples. of the rail With samples. the decrease With the of decreasetemperature, of temperature, the surface hardeningthe surface of hardening D1 decreased, of D1 while decreased, that of thewhile rail that increased. of the rail For increased. the D2 friction For pair, the surface hardening of the wheel and rail samples was greater at 50 C and 30 C than at room the D2 friction pair, the surface hardening of the wheel and rail samples was◦ greater− at◦ 50 °C and −30 temperature; it significantly increased at 30 C. The results showed that the resistance to hardening °C than at room temperature; it significantly− ◦ increased at −30 °C. The results showed that the resistanceof D2 was to stronger hardening than of that D2 ofwas D1; stronger however, than D2 that wheel of D1; material however, was moreD2 wheel significantly material awasffected more by significantlytemperature, affected and surface by temperature, hardening was and the surfac loweste hardening at room temperature. was the lowest at room temperature.

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340 340 320 D1 Rail 320 D2 Rail

0.05 300 0.05 300 280 280 260 260 240 240

Hardness HV 220 Hardness HV 220 200 200 50℃ RT -30℃ 50℃ RT -30℃ Materials 2020, 13, x FOR PEER(a )REVIEW (b) 8 of 17

Figure 7. Surface hardness of wheel and rail samples before the experiment. (a) D1 friction pair; (b) Figure 7. Surface hardness of wheel and rail samples before the experiment. (a) D1 friction pair; (b) D2 D2friction friction pair. pair.

2.6 2.4

2.3 2.5 2.2 2.4 2.1 2.3 Hardness ratio Hardness ratio 2.0 D1 2.2 D2 D1 1.9 D2

2.1 1.8 50 Room -30 50 Room -30 Temperature/°C Temperature/°C (a) (b)

FigureFigure 8. 8.Surface Surface hardnesshardness ratiosratios ofof wheel and rail rail sample sampless of of values values measured measured after after and and before before the theexperiment:.( experiment:.(a) aratios) ratios of of hardness hardness values values of of the the wheel wheel measured measured after after the the experiment experiment and and before before the theexperiment; experiment; (b) ( bratios) ratios of ofhardness hardness values values of ofthe the rail rail me measuredasured after after the the experiment experiment and and before before the theexperiment. experiment.

TheThe plastic plastic deformation deformation morphologies morphologies were were obtained obtained by by metallographic metallographic treatment treatment of of the the wheel wheel andand rail rail samples. samples. In In Figures Figures 9–11,9–11, it it can can be be seen seen that that at different at di ﬀerent temperatures, temperatures, the D1 the and D1 D2 and friction D2 frictionpairs pairsshowed showed the same the same change change trend trend in inplastic plastic de deformationformation thickness, thickness, with thethe greatestgreatest plastic plastic deformationdeformation at at30 −30C, and°C, theand least the plastic least deformationplastic deformation at room temperature. at room temperature. At the same temperature,At the same − ◦ thetemperature, plastic deformation the plastic thickness deformation of D1 thickness was larger of thanD1 was that larger of D2, than which that means of D2, better which deformation means better resistancedeformation of D2 resistance and is consistent of D2 and with is theconsistent results ofwi surfaceth the results hardening. of surface Compared hardening. to the wheelCompared roller, to thethe rail wheel roller roller, showed the strong rail roller resistance showed to deformation, strong resistance and the to greatestdeformation, plastic and deformation the greatest occurred plastic atdeformation30 C, similar occurred to what at observed−30 °C, similar for the to wheel what roller.observed for the wheel roller. − ◦

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D1-Wheel D1-Rail Rolling direction Rolling direction

≈60μm

(a) (b)

D2-Rail Rolling direction

≈70μm

(c) (d) Materials 2020, 13, x FOR PEER REVIEW 10 of 17 Figure 9. Plastic deformation of wheel and rail rollers at 50 °C: (a) D1 wheel; (b) rail of D1 friction pair; Figure 9. Plastic deformation of wheel and rail rollers at 50 ◦C: (a) D1 wheel; (b) rail of D1 friction pair; (c) D2 wheel; (d) rail of D2 friction pair. (c) D2 wheel; (d) rail of D2 friction pair.

D1-Wheel D1-Rail Rolling direction Rolling direction

≈50μm

(a) (b)

D2-Rail

Rolling direction

≈40μm

Figure 10.FigurePlastic 10. Plastic deformation deformation of wheelof wheel and and railrail rollers at at room room temperature: temperature: (a) D1 (a wheel;) D1 wheel; (b) rail (ofb ) rail of D1 friction pair; (c) D2 wheel; (d) rail of D2 friction pair. D1 friction pair; (c) D2 wheel; (d) rail of D2 friction pair.

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D1-Wheel Rolling direction D1-Rail Rolling direction

≈90μm

(a) (b)

D2-Rail Rolling direction

≈80μm

Figure 11. PlasticPlastic deformation deformation of of wheel and rail rollers at −3030 °C:C: ( (aa)) D1 D1 wheel; wheel; ( (b)) rail rail of of D1 D1 friction − ◦ pair; ( c) D2 wheel; (d) rail of D2 friction pair.

3.3. Surface Surface DamageDamage Micrographs ofof thethe worn worn surfaces surfaces of of the the wheel wheel and and rail rollersrail rollers at di ﬀaterent diffe temperaturesrent temperatures are shown are shownin Figures in Figures 12–14. It12–14. can be It seencan be in Figureseen in 12 Figure that at 12 50 that◦C, at peeling 50 °C, andpeeling surface and crack surface damage crack occurreddamage occurredon the surfaces on the ofsurfaces the D1 of friction the D1 pair,friction while pair, for while the D2for frictionthe D2 friction pair, damage pair, damage was mostly was mostly due to duelarge to surface large surface cracks. cracks. The surface The surface damage damage was mild was at mild room at temperature,room temperature, and peeling and peeling was the was main the maindamage damage on the on surface the surface of the D1of the wheel D1 roller,wheel while roller, surface while crackssurface were cracks predominant were predominant on the rail on roller the rail(Figure roller 13 ).(Figure Moreover, 13). Moreover, mixed damage mixed consisting damage ofconsisting ploughing, of ploughing, peeling, and peeling, surface and cracks surface was present cracks wason the present surfaces on ofthe the surfaces D2 friction of the pair D2 at roomfriction temperature. pair at room When temperature. the temperature When decreasedthe temperature to 30 − decreased◦C, the surface to −30 damage °C, theaggravated surface damage (Figure aggravated 14). For the (Figure D1 friction 14). For pair, the surface D1 friction fatigue pair, crack surface was fatiguethe main crack type was of damagethe main on type the of surface damage of on the the wheel surface roller, of the while wheel slight roller, peeling while and slight surface peeling cracks and surfacewere predominant cracks were on predominant the rail surface. on the The rail surface surfac damagee. The surface of the D2damage wheel of roller the D2 was wheel relatively roller mildwas relativelyand was mainlymild and due was to mainly peeling, due while to peeling, a combination while a ofcombination large adhesion of large areas, adhesion spalling, areas, and spalling, surface andcracks surface was observed cracks was on theobserved rail surface. on the With rail the surfac temperaturee. With the declining, temperature the surface declining, damage the gradually surface damageincreased gradually and transformed increased from and surface transformed cracks and from peeling surface to spalling cracks andand adhesion peeling damage.to spalling Overall, and adhesionthe surface damage. damage Overall, of the D2 the wheel surface and damage rail samples of the was D2 morewheel serious and rail than samples that of was the D1more wheel serious and thanrail samples. that of the D1 wheel and rail samples.

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(a)

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FigureFigure 12. 12. MicrographsMicrographs of of the the worn worn surfaces surfaces of of the the wheel wheel and and rail rail rollers rollers at at 50 50 °C.◦C. ( (aa)) D1; D1; ( (bb)) D2. D2.

(a)

(b)

Figure 13. 13. MicrographsMicrographs of of the the worn worn surfaces surfaces of of the the wh wheeleel and and rail rail rollers rollers at at room room temperature. temperature. ( (aa)) D1; D1; (b)) D2. D2.

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(a)

(b)

Figure 14. MicrographsMicrographs of of the worn surfaces of the wheel and rail rollers at −3030 °C,C, ( (aa)) D1; D1; ( (bb)) D2. D2. − ◦ 3.4.3.4. Subsurface Subsurface DamageDamage FiguresFigures 15–1715–17 showshow the the subsurface subsurface damage damage of of th thee D1 D1 and and D2 D2 friction friction pairs pairs at at 50 50 °C,◦C, room room temperature, and 30 C, respectively. At 50 C and 30 C, the fatigue crack length and angle of the temperature, and −30 °C,◦ respectively. At 50 °C◦ and −−30 °C,◦ the fatigue crack length and angle of the D1D1 and and D2 D2 wheel wheel and and rail rail rollers rollers were were larger compar compareded to those at room temperature. It It can can be be seen seen fromfrom Figures 1515 andand 16 16 that that the the subsurface subsurface fatigue fatigue crack crack of of the the D2 D2 wheel wheel roller roller was was relatively relatively short short and anddeep deep compared compared with with that ofthat the of D1 the wheel D1 wheel roller roller at 50 at◦C 50 and °C roomand room temperature. temperature. The long The fatiguelong fatigue crack crackof D1 propagatedof D1 propagated along the along surface, the whilesurface, the while short fatiguethe short crack fatigue of D2 crack extended of D2 slightly extended inward, slightly with a small angle, while at 30 C the subsurface damage of the D1 and D2 wheel rollers was accompanied inward, with a small angle,− ◦ while at −30 °C the subsurface damage of the D1 and D2 wheel rollers wasby subsurface accompanied crack by and subsurface a long internal crack and crack a long in the inte materialrnal crack (Figure in the 17 ).material For the (Figure rail roller, 17). the For main the raildamage roller, at the room main temperature damage at was room a long temperature crack extending was a parallellong crack to theextending surface andparallel a subsurface to the surface crack. Under the conditions of 50 C and 30 C, there was a lamellar material inclusion inside the surface and a subsurface crack. Under◦ the− conditions◦ of 50 °C and −30 °C, there was a lamellar material inclusioncrack of the inside rail rollerthe surface (Figure crack 15b); of the the crack rail roller extended (Figure to the 15b); inside the crack and branched, extendedso to thatthe inside subsurface and branched,crack damage so that increased subsurface compared crack with damage what increased observed atcompared room temperature, with what becoming observed especiallyat room serious at 30 C. The results showed that the subsurface damage of D1 was serious compared to that temperature,− becoming◦ especially serious at −30 °C. The results showed that the subsurface damage ofof D1 D2 was at the serious same temperature.compared to Subsurfacethat of D2 at damage the same was temperature. severely aﬀ ectedSubsurface by low damage temperature was severely and less affectedseverely by by low 50 ◦ C;temperature the slightest and subsurface less severely damage by to50 the°C; two the kindsslightest of wheel subsurface materials damage was observedto the two at kindsroom of temperature. wheel materials was observed at room temperature.

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(a)

(b)

Figure 15. SEMSEM micrographs micrographs showing showing the the types types of of damage damage of of wheel wheel and and rail rail rollers rollers at at 50 50 °C.◦C. ( (aa)) D1; D1; (b)) D2. D2.

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(a)

(b)

FigureFigure 16. 16.SEM SEM micrographs micrographs showing showing the typesthe types of damage of dama of wheelge of andwheel rail and rollers rail at roomrollers temperature. at room (a)temperature. D1; (b) D2. (a) D1; (b) D2.

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(a)

(b)

FigureFigure 17. 17. SEMSEM micrographs micrographs showing showing the the types types of of da damagemage of of wheel wheel and and rail rail rollers rollers at −3030 °C.C. ( (aa)) D1; D1; − ◦ ((bb)) D2. D2.

4. Discussion The ambient temperature has a remarkable inﬂuence on the wear and fatigue properties of D1 and D2 rollers. Our experimental results showed that wear loss, surface hardening, and plastic deformation

Materials 2020, 13, 1152 14 of 16 of D1 and D2 wheel materials were particularly serious at low temperature. Meanwhile, D2 exhibited good wear resistance and hardening resistance compared to D1. Except for the ploughing damage on the surface of the D2 wheel roller at room temperature, surface damage type and degree of the two kinds of wheel materials were basically the same. D2 exhibited a slightly smaller surface damage and fatigue resistance than D1, and the surface and subsurface damage was severe at 30 C. − ◦ Under the condition of 50 C and 30 C, both the fatigue crack angle and the fatigue crack length ◦ − ◦ in the D1 and D2 friction pair increased, compared to those at room temperature. Thus the surface material was more easily damaged at 50 C and 30 C, and the fatigue damage was serious, indicating ◦ − ◦ poor fatigue resistance performance. At the same temperature, the fatigue crack damage generated on the subsurface of the D2 material was slightly less and shorter in length than that on D1, which indicated that the anti-fatigue properties of D2 were slightly better than those of D1. In regard to the composition and alloying elements of the tested materials, the differences in Si and V contents led to different performances of the D1 and D2 wheel materials. In pearlitic steels used for wheel materials, the combination of V and C can form a stable VC structure, which improves the strength of ferrite the material yield strength, and the low-temperature toughness; however, V can also increase the ferrite phase volume fraction and reduce the tensile strength. Meanwhile, the addition of Si refines the pearlite lamellar spacing and reduces the ferrite volume fraction, which significantly increases the yield strength and tensile strength [30]. Generally, Si and V enhance a material’s hardness and tensile strength and refine pearlite lamellar spacing, which improves wear resistance and hinders crack propagation. Therefore, the D2 material, containing 2.6 times as much Si and 14 times as much V as D1, presented increased resistance to wear, hardening, and fatigue. As for the fatigue characteristics of wheel and rail materials, they can be analyzed from a microscopic and kinematic perspective. With the action of Cyclic loading caused branch cracks, and inclusions inside cracks appeared during fatigue crack propagation in D1 and D2 wheel rollers. The subsurface damage of the rail material was more serious than that of the wheel material. Especially at 50 C and 30 C, crack growth, internal inclusions, and subsurface cracks in the rail material were ◦ − ◦ more serious than at room temperature, indicating that the temperature had a significant impact on the damage of the wheel and rail materials. When the material was locally subjected to large contact stress during periodic cycle loading, surface cracks developed forming secondary branches or connections with subsurface cracks, thus extending inside the material. This resulted in further deformation and crushing of the cracks, causing accumulation of material inside the cracks. In some of these cracks, the inclusions accelerated crack propagation to the surface, material fracture under high cyclic load, and surface peeling and spalling. At the same time, the connection of several adjacent surface cracks or of surface and subsurface cracks led to crack growth, material surface peeling, or lamellar material accumulation inside the cracks, which progressed with cyclic loading (Figure 15a). The fatigue cracks generated in the D1 and D2 wheel materials also branched during cyclic loading (Figures 15, 16b and 17).

5. Conclusion (1) Ambient temperature has an obvious inﬂuence on the wear and fatigue properties of D1, D2, and rail materials. At low and high temperatures, wear loss, plastic deformation, and subsurface damage were more severe than at room temperature; they were the greatest at low temperature. while Surface damage aggravated as temperature decreased. In regard to the surface hardness ratio, the D1 friction pair showed slight decline with the temperature decreased, while the D2 friction pair presented the lowest surface hardening at low temperature, slightly better at high temperature, and the mildest at room temperature. (2) With high content of Si and V which reﬁned the pearlite lamellar spacing, the D2 wheel material showed high strength and hardness before testing, which led to a low wear loss and light subsurface damage. Despite the mild surface damage of the D1 wheel material, the resistance to wear and fatigue of the D2 wheel material was better than that of D1. Materials 2020, 13, 1152 15 of 16

(3) Comparing the wear and fatigue properties of the two wheel materials, the D2 wheel material appears more suitable for high-speed railway wheels, and its performance is optimal at room temperature.

Author Contributions: L.M. and W.W. conceived an designed the experiments; L.M. and J.G. performed the experiments; L.M. and Q.L. analyzed the data; J.G. and Q.L. contributed materials and analysis tools; L.M. and W.W. wrote the paper. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the National Natural Science Youth Foundation of China, grant number 51805446; the “Young Scholars Reserve Talents” program of Xihua University. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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