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Rail grinding is a widely-used method to eliminate surface damages on railway tracks. The grinding chips are closely related to the grinding parameters and the grinding quality. Therefore, the investigation on the characteristics and the formation mechanism of chips from rail grinding is beneficial for the optimization of rail grinding process. In the present study, the grinding chips from the real rail grinding in the field and from the rail grinding experiments in the laboratory were analyzed in terms of their morphologies, microstructural evolution and chemical characteristics. The results showed that flowing, knife, slice and spherical chips were generated during rail grinding. The formation mechanism of flowing, knife and slice chips was plastic deformation by forces from abrasive grains with oxidation in air by grinding heat. Primary and secondary shear zones and central zones were generated. In the primary shear zone, cracks were generated in knife and slice chips. In the secondary shear zone, obvious plastic deformation was formed in knife and slice chips and a significant white etching layer (WEL) was generated in knife chips. The formation mechanism of spherical chips was melting of small powdery grinding debris by the high temperature and the subsequent solidification, accompanied with severe oxidization in air. The oxides produced in chips consisted of FeO, Fe3O4, Fe2O3 and AlFe2O4. The grinding quality could be predicted through observing the grinding debris. The larger percentage of flowing chips represented the better grinding quality (smaller surface roughness, slighter surface burn and thinner WEL on ground rail), which could be the goal for the grinding parameters choice.

In this paper, Fe-based and Co-based alloy powders were chosen to perform laser cladding on wheel materials through conventional laser cladding (CLC) and ultra-high-speed laser cladding (UHSLC) processes, respectively. The microstructures, element distribution, phase composition and hardness of the Fe-based alloy and Co-based alloy coating layers using the CLC and UHSLC processes were compared and analysed. The results show that the CLC and UHSLC alloy coatings were dense and free of defects such as pores and cracks. Compared with the CLC alloy coating, the grain size of the UHSLC alloy coating was smaller, the coating composition was close to the powder design composition, and the distribution of Cr within and between the grains was more uniform. The Fe-based coating was mainly composed of (Fe, Ni) and Cr7C3, and the Co-based coating was mainly composed of γ-Co and Cr23C6. It was found that the cooling rate of the CLC alloy coating was smaller than that of the USHLC, and the hardness of the CLC alloy coating was less than that of the USHLC. The average hardness of the UHSLC Fe-based and Co-based alloy coatings was 709 HV and 525 HV, respectively. The average hardness of the CLC Fe-based and Co-based alloy coatings was 615 HV and 493 HV, respectively. The rolling friction and wear tests were carried out with the CLC-treated and UHSLC-treated wheel specimens on the GPM-30 rolling contact fatigue testing machine. The results showed that the wear rate of the UHSLC alloy coating on the wheel specimens was significantly lower than that of the CLC alloy coating on the wheel specimens. The wear rates of the UHSLC Fe-based and Co-based alloy coatings on the wheel specimens were reduced by 40.7% and 73.8%, respectively. It was demonstrated that the wear resistance of the USHLC alloy coatings was better than those of the CLC alloy coatings. The CLC alloy coating exhibited more severe fatigue damage with small cracks. Furthermore, the damage of the UHSLC alloy coating was relatively minor, with slight spalling. The Co-based alloy coating exhibited superior wear properties with the same laser cladding process.

This study presents a numerical model of the grinding force during rail grinding that considers the influence of the swing angle of the grinding stone on the position of the grinding area. The numerical model of movement process and material removal of single abrasive grain at different swing angles are established. The surface characteristics of the grinding stones, such as protrusion heights and density of grains, are measured to establish the model of grinding force in the whole grinding area. The results indicate that the grinding forces increase with the grinding pressure and forward speed. In contrast, the grinding forces decrease with the increasing rotational speed and grinding stone granularity.

The fatigue damage of an on-site rail welding joint was analysed. The fatigue cracks were longer and deeper on the welding joint than on the normal rail material. The influence of two laser strengthening techniques, laser quenching (LQ) and laser shock peening (LSP), on the anti-wear and anti-fatigue properties of the welding joint were explored through rolling contact fatigue (RCF) tests. The LQ process significantly improved the anti-wear property of the rail welding joint, but the anti-fatigue property got worse. The wear rate of the LQ treated welding joint was decreased by 91.6–92.4%, however, the average crack angle (23–25°) and the average crack length (62–74 μm) on the LQ treated welding joint were larger than those on the untreated rail welding joint (13–15° and 37–59 μm, respectively). The LSP process improved both the anti-wear and anti-fatigue properties of the welding joint. The wear rate of the LSP treated welding joint was decreased by 5.1–10.1%. Meanwhile, the density of cracks (0.9–4.2 mm−1), the average crack angle (10–13°), and the average crack length (16–26 μm) on the LSP treated welding joint were smaller than those on the untreated and LQ treated welding joints.

The influence of grinding pressure on the removal behaviours of rail material was investigated using a rail grinding simulation testing apparatus. Grinding tests were performed at three types of section radius of rail head. The results indicated that with the increase in the grinding pressure, the grinding efficiency was obviously improved and the grinding amount and chip thickness were increased. However, the surface quality became worse and residual stresses were increased. Furthermore, surface burn occurred when the grinding pressure was larger than 1600 N at R13, 1800 N at R80, and 2000 N at R300, respectively. The grinding debris was composed of curled chips, lump chips, and spherical debris. In addition, the wear mechanism of abrasive grains presented macro fracture.

In railway network, friction is an important factor to consider in terms of the service behaviors of wheel-rail system. The objective of this study was to investigate the effect of a solid friction modifier (FM) in a railway environment. This was achieved by studying the friction, wear, and rolling contact fatigue (RCF) damage on the wheel-rail materials at different slip ratios. The results showed that when a solid FM was applied, the friction coefficient decreased. After the solid FM was separated from the wheel-rail interface, the friction coefficient gradually increased to its original level. With the application of the solid FM, the wear rates of the wheel-rail decreased. In addition, the thickness and hardness of the plastic deformation layers of the wheel-rail materials were reduced. The worn surfaces of the wheel-rail were dominated by pits and RCF cracks. Without the FM, RCF cracks ranged from 84 to 120 mu m, and subsurface cracks were generated. However, with the FM, RCF cracks ranged from 17 to 97 mu m and no subsurface cracks were generated. These findings indicate possible methods of improving the performance of railway rolling stock by managing friction, and reducing wear and permanent RCF damage affecting both the wheels and rails. Tření je jedním z nejdůležitějších faktorů kolejové dopravy. Cílem této studie bylo vyšetřit vliv tuhého modifikátorů tření na tření, opotřebení a únavové poškození kol a kolejnic pro různé hodnoty skluzu. Výsledky ukazují, že aplikace tuhého modifikátoru tření je doprovázena poklesem součinitele tření. Po ukončení aplikace byl pozorován opětovný nárůst součinitele tření na jeho původní hodnotu. Testy opotřebení ukázaly, že aplikace tuhého modifikátoru tření vede k celkové redukci opotřebení. Současně bylo zjištěno, že tloušťka a tvrdost vrstvy plasticky deformované vrstvy byla nižší než v případě testů za suchých podmínek (bez modifikátoru tření). Povrchová analýzy ukázaly, že dominantním poškozením povrchů byly důlky a únavové trhliny. V případě suchých testů byla délka těchto trhlin od 84 to 120 µm a současně byly nalezeny také podpovrchové trhliny. V případě testů s tuhým modifikátorem tření byla délka únavových trhlin v rozmezí od 17 do 97 µm a tělesa byla bez podpovrchových trhlin. Tyto výsledky ukazují, že modifikace tření pomocí tuhých modifikátorů tření může vést ke snížení opotřebení a únavového poškození kol a kolejnic, což povede k celkovému zvýšení efektivnosti a výkonosti kolejové dopravy.

Surface damages on rail became severe with the increase in the axle load and speed. Laser cladding was used to repair the local damages on rail surfaces to ensure the service performance and prolong the life of rails. In the present study, five types of cladding materials (304, 314, 2Cr13, 316L, and 434L) were clad at a small part (trapezoidal shape) of the rail disc to simulate the defected rail in the field. Vickers hardness tester, optical microscopy (OM), scanning electron microscopy (SEM), and electron-dispersive X-ray spectroscopy (EDS) were employed to investigate the service properties of the repaired rail discs. Results indicated that all these five clad rail discs had lower wear-rates than the unclad rail disc. The wear-rates decreased with the increase in the initial hardness of the clad layer. Cracks with large angles mainly propagated along the boundary at the front edge, while at the rear edge, the crack morphology was closely related to the hardness of clad layer. 434L would induce the minimum wear loss and the modest rolling contact fatigue (RCF) of the repaired rail disc, which could be the potential cladding material for repairing the damaged rail.

Usually, rail materials are exactly affected by the erosion of windblown sand in the desert environment. For this reason, the influence of impact angle, particle velocity, and particle size on the erosion wear behavior of the U75V heat-treated rail steel, a material frequently employed in Chinese railways, were studied in this work. The results showed that, with increasing impact angle, the erosion rate increased between 15 deg and 45 deg, decreased between 45 deg and 75 deg, and then increased again between 75 deg and 90 deg. The highest erosion rate occurred at about 45 deg. When the particle velocity increased, the erosion rate increased approximately in a quadratic way. As the sand particle size increased, the erosion rate presented a decreasing trend. During the initial stage of erosion, shear craters, indentation craters, and ploughing craters were the main surface damage features. The shear craters predominated at the impact angle of 45 deg whereas the indentation craters predominated at 90 deg. During the steady-state of erosion, the rail damage was mainly composed of craters, platelets, and cracks. Both the length and depth of craters increased almost linearly with increasing particle velocity, whereas the increased rate of length was significantly higher than that of depth. The length and depth of craters increased with increasing particle size at 90 deg, whereas only the length increased with increasing particle size at 45 deg. The microstructure evolution and the formation mechanism of platelet at low impact angles were different from those at high impact angles. Platelet formation was the main erosion wear mechanism.

Low adhesion problem is one of the major problems for railways all over the world because this phenomenon can occur anytime and anywhere. To investigate when poor adhesion conditions can be expected in real operation, a ball-on-disc tribometer with a climate chamber was employed to simulate rolling-sliding contact under various environmental conditions. Clean and contaminated discs with leaf extract were used to simulate different surface conditions. Results indicate that contact operating under rolling-sliding conditions is more prone to the occurrence of low adhesion than found by others for pure sliding contact. Very low adhesion (0.05) were identified for contaminated and oxidized specimens operating under humid and wet conditions. For clean surfaces, low adhesion (0.15) were found under dew conditions. Fenomén nízké adheze představuje celosvětový problém v oblasti kolejové dopravy. Důvodem je zejména to, že nízká adheze může nastat kdekoliv a kdykoliv. Cílem této studie je získat analytické závislosti, které bude možné využít pro predikci nízké adheze v reálném provozu. Pro tento účel byl využit tribometr v konfiguraci ball-on-disc, který je vybaven klimatickou komorou umožňující simulovat různé environmentální podmínky v kontaktu. Při experimentech byly využity dva typy disků a to disk čistý a disk kontaminovaný listovým extraktem. Výsledky ukazují, že kontakt v konfiguraci ball-on-disc je obecně mnohem více náchylný na výskyt nízké adheze, než bylo doposud publikováno pro kontakt provozovaný za podmínek čistého skluzu. Pro kontaminované a zoxidované povrchy byla při vysoké hladině vlhkosti a při kontaminaci vodou pozorována velmi nízká adheze (0.05). V případě čistého disku (povrchu) byla pozorována pouze nízká adheze (0.15), a to za podmínek odpovídající rosnému bodu.

This study aims to deeply understand the effect of contact stress and slip ratio on wear performances of bainitic rail steels. The results showed that the wear loss increased as the contact stress and slip ratio increased. Based on the surface damage morphology and microstructural analyses, it revealed that the rolling contact fatigue wear mechanism played a significant role under the low slip ratio, but the dominant wear mechanism transferred to the abrasive wear at the high slip ratio. Meanwhile, the bainitic steel specifically presented worse wear resistance under the abrasive wear mode. Compared with the influence of a slip ratio, the increase in contact stress led to severer plastic flows and contributed to the propagation of cracks. In addition, the contact stress and slip ratio had the opposite effect on the friction coefficient, that is, the friction coefficient of bainitic steels behaved the inverse proportion with the contact stress, but positive proportion with the slip ratio. At last, the increase in slip ratio had more significant effect on the reduction of retained austenite (RA) than the enlargement of contact stress due to the fact that the RA would probably be removed before the martensitic transformation occurred under the abrasive wear mechanism.

Rolling contact fatigue (RCF) damages often occur, sometimes even leading to shelling on locomotive wheel treads. In this work, the RCF damage behaviors of two locomotive wheel materials with different molybdenum (Mo) contents were studied, and the influence of depth of wheel material was explored as well. The result indicates that with the increase in the Mo content from 0.01 wt.% (wheel 1, i.e., a standard wheel) to 0.04 wt.% (wheel 2, i.e., an improved wheel), the proeutectoid ferrite content and the interlamellar spacing of pearlite decreased, the depth and length of the RCF cracks increased and the average RCF live of locomotive wheel steel improved by 34.06%. With the increase in the depth of material, the proeutectoid ferrite content and the interlamellar spacing of pearlite increased, the depth of RCF cracks increased, the length of RCF cracks of wheel 1 increased and then decreased whereas that of wheel 2 decreased, the RCF live showed a decrease trend for wheel 1, while the RCF life increased and then decreased for wheel 2. The processes of shelling can be divided into three patterns: cracks propagating back to the surface, crack connection and fragments of surface materials.

Under tribological conditions in aqueous medium, the contact of materials does involve some degradations of materials. Especially friction under small reciprocal displacement, i.e., fretting corrosion, is occurring, this topic has been highlighted since the 80&prime, s regarding hip implants. Hip prosthesis is assembled from three parts: femoral stem, neck and head. Fretting corrosion or friction corrosion between metallic parts first involves some degradation of the oxides layers. This step is governed by mechanics and it is related to some few minutes. Afterwards the corrosion occurrs enhanced by mechanical degradation. As well focused some oxides and some metallic ions are related to biocompatibility issues. Some strategies are available in order to avoid metal against metal friction and/or fretting. Some hard coatings and some smooth coatings were investigated. The first one is diamond-like carbon (DLC), and the second is a polyetheretherketone (PEEK), polymeric one. The investigations were focused on fretting corrosion solicitations of Ti-6Al-4V vs. Ti-6Al-4V + coating. DLC as a coating delays the corrosion degradation. The PEEK coating does not promote any corrosion degradation of the metallic counter part and more generally any wear.

The wear and rolling contact fatigue of wheel-rail materials were investigated through varying wheel/rail hardness ratios (Hw/Hr) and creepages. The results indicated that with the Hw/Hr increasing from 0.927 to 1.218, the wheel wear rate reduced significantly in the case of Hw/Hr = 1.218, the rail wear rate showed an increasing trend. Both the wheel and rail wear rates increased as the creepage enlarged. The synergistic results of Hw/Hr and creepage caused a transition of the wear and damage mechanisms on the wheel-rail steels. Besides, the fatigue damage of ER7 and CL60 wheel materials was dominated by slender multi-layer cracks, while the fatigue cracks were short and contained lots of interlayer broken materials on C-class wheel steel.

The wear and rolling contact fatigue (RCF) performance of eutectoid and hypereutectoid rail steels were studied. Subsequently, the microstructure evolutions of subsurface materials of these two rail steels after testing were comparatively investigated. The results indicated that, compared with the eutectoid rail, the hypereutectoid rail had lower wear loss, but longer RCF crack lengths. After the rolling-sliding testing, the original equiaxed and undeformed coarse grains of rails in the matrix were transformed into nano-grains rich in high angle grain boundaries (HAGBs). The microstructure evolution of rail steels under rolling-sliding contact could be explained by two mechanisms. Firstly, the initial deformation and the breaking of the lamellar structure of pearlite grains occurred through the movement of dislocations. Secondly, the formation of the nanostructure grains happened through continuous dynamic recrystallization (cDRX). Compared with the eutectoid rail, the hypereutectoid rail had finer interlamellar spacing and higher contents of low angle grain boundaries (LAGBs) in the matrix. After the rolling-sliding testing, the hypereutectoid rail had more severe LAGBs-to-HAGBs transitions and a more severe cDRX process than the eutectoid rail, which might be the reason for the longer RCF crack lengths it exhibited.

Laser cladding could be applied on railway wheels to improve their service lives. Fe-based alloy powder is widely used as the laser cladding material due to its good self-fluxing ability and low price. However, the friction coefficients and wear rates of Fe-based alloy laser claddings are high. WS2 is used as the additive for common laser cladding treatment, while it has not been used for claddings on railway wheels. Understanding the influence of WS2 concentration on the mechanical and tribological properties of Fe-based alloy laser claddings on railway wheels has significance for extending the application of laser treatment on railway transit. Therefore, laser claddings of Fe-based alloy powders with different contents of WS2 powder (0, 2%, 4%, 6%, 8%) were produced on the railway CL60 wheel material. Rolling-sliding tests were conducted on a twin-disc tribometer using the uncladded and cladded wheel samples against an uncladded U71Mn rail sample. The results indicated that the microstructure of laser claddings was composed of dendritic and eutectic phases. With the increase in the WS2 content from 0 to 6%, the size of dendritic phases was decreased from around 20 μm to less than 10 μm. With 8% WS2, the size of dendritic phases was increased to around 10–20 μm. Laser claddings of Fe-based alloy powders with WS2 included (Fe, Ni) and (Fe, Si) solid solutions, hard carbide (Cr7C3), Ni–Cr–Fe, and CrS phases. The hardness was increased to 730–820 HV0.5 and residual stresses were changed to be compressive state. Rolling contact fatigue (RCF) cracks preferentially initiated and developed along the boundary between dendritic and eutectic phases. The smallest wear rate and shortest RCF cracks occurred on the cladding from powders with 6% WS2. Thus, the optimum content of WS2 in the Fe-based alloy powder was 6% for laser cladding on the railway wheel material.

The purpose of this study was to investigate the relationship between the microstructure and the wear characteristic of rail materials. Different rail materials with different austenite grain sizes (40–135 µm), pearlite colony sizes (5.4–10.6 µm) and pearlite interlamellar spacings (0.16–0.33 µm) were obtained through heat treatment. Rolling-sliding wear tests were performed. With the increases in the microstructure size (i.e., pearlite interlamellar spacing, pearlite colony size and austenite grain size), the wear rates of rail material were increased. The pearlite colony size has the most significant influence on wear resistance, followed by the pearlite interlamellar spacing and the austenite grain size. With the increase in the microstructure size, the lengths of rolling contact fatigue cracks were decreased but the crack angles were increased.

The wheel-rail contact is an open system contact, which is subjected to various environmental conditions, such as temperature, humidity, water, and even leaves. All these environmental factors influence wheel-rail wear. Classical wheel-rail wear has been extensively studied under dry and clean conditions previously. However, with changes in environmental conditions, the wear rate and wear mechanism can change. This paper reviews recent contributions to wheel-rail wear with a special focus on the influence of environmental conditions. The main part includes the basics of wheel-rail wear, experimental methodology, wear and rolling contact fatigue (RCF), and some measures to counter these degradation mechanisms.

WS2-C is produced from a hydrothermal reaction, in which WS2 nano-sheets are coated with carbon, using glucose as the carbon source. In order to investigate the tribological properties of WS2-C as a lubricant additive, WS2-C was modified by surfactant Span80, and friction tests were carried out on an MRS-10A four-ball friction and wear tester. The results show that Span80 can promote the dispersibility of WS2-C effectively in base oil. Adding an appropriate concentration of WS2-C can improve the anti-wear and anti-friction performance of the base oil. The friction coefficient reached its lowest point upon adding 0.1 wt % WS2-C, reduced by 16.7% compared to the base oil. Meanwhile, the wear scar diameter reached its minimum with 0.15 wt % WS2, decreasing by 26.45%. Moreover, at this concentration, the depth and width of the groove and the surface roughness on the wear scar achieved their minimum. It is concluded that WS2-C dispersed in oil could enter friction pairs to avoid their direct contact, thereby effectively reducing friction and wear. At the same time, WS2-C reacts with the friction matrix material to form a protective film, composed of C, Fe2O3, FeSO4, WO3, and WS2, repairing the worn surface.

The influence of diamond-like carbon (DLC) coating positions—coated flat, coated cylinder, and self-mated coated surface tribopairs—on the fretting behaviors of Ti-6Al-4V were investigated using a ...

A tribofilm was formed on the Ti–6Al–4V surface when sliding against a DLC coating under fretting condition in air. The tribofilm is mainly composed of the oxidized wear product of Ti–6Al–4V alloy. The mechanical properties of the tribofilm and the Ti–6Al–4V alloy were characterized using nano-indentation and micro-pillar compression techniques. From nano-indentation testing, the hardness of tribofilm (10 ± 0.4 GPa) was 2.6 times higher than that of Ti–6Al–4V alloy (3.9 ± 1.5 GPa). The reduced Young's modulus of tribofilm (170 ± 3 GPa) was around 1.3 times higher than that of Ti–6Al–4V alloy (130 ± 30 GPa). From micro-pillar compression testing, the compression modulus of tribofilm (around 160 GPa) was 2.5–3.3 times higher than that of Ti–6Al–4V alloy (47–62 GPa). The yield strength of tribofilm (around 6.4 GPa) measured by micro-pillar compression, was 6.7–7.1 times higher than that of Ti–6Al–4V alloy (0.90–0.95 GPa). When measuring the hardness on the tribofilm, micro-pillar compression tests could obtain more accurate values than the nano-indentation tests.

Low friction between DLC coating and Ti 6Al 4V alloy was investigated under fretting conditions on a fretting-wear testing machine. The results indicated that, during the beginning period, the Ti 6Al 4V surface was damaged as a consequence of adhesion (and abrasion), leading to high friction coefficients of around 0.5. With the test ongoing, a tribofilm was formed on the rubbed Ti 6Al 4V surface. This tribofilm was derived from the wear product of Ti 6Al 4V alloy with oxidization. Its nano-hardness and reduced elastic modulus were greater than the Ti 6Al 4V matrix. Meanwhile, structural transformation occurred on the rubbed DLC surface. The tribofilm and transformed carbonaceous layer prevented the Ti 6Al 4V alloy and the DLC surface from direct contact and led to low friction coefficients (below 0.2).

Rail grinding is widely used as a technique to both reprofile the railhead in case of wear and to remove damage. However, grinding may lead to surface burning and the formation of a white etching layer (WEL). Taking into account the rail head position, the study established an analytical thermal model based on a non-uniformly distributed heat source to predict the temperature field during grinding. The grinding temperature during a rail grinding experiment is measured through thermocouples to validate the model. In addition, the rail material response in terms of surface burn and white etching layer is analyzed in detail. Results indicate that with a grinding temperature of around 400 °C, a WEL starts to appear on the rail surface. Retained austenite is found on the ground rail surfaces, indicating the existence of martensite, resulting from the coupling effect of thermal stress and mechanical stress. A diagram is developed to describe the relationship between the grinding temperature, surface burn, and WEL.

Defects such as dents on the rail surface might induce severe rolling contact fatigue (RCF) and then threaten the running safety of railway vehicles. This study aims to explore the initiation and propagation of RCF cracks around dents with different sizes through observing both surfaces and cross-sections of the defective rail. The experiments are conducted on a wheel/rail twin-disc machine. Results indicate that the microstructure evolution process around the dent can be divided into three main stages, namely crack initiation, crack propagation and dent removal. Dents in different sizes act very similarly in each stage, including the position, the crack growth path and the final appearance after the dent has been fully worn off. However, dents in different sizes cause different lifetimes of crack initiation and different lasting times of dents at each stage. For the lager dent, cracks initiate later but propagate for a longer time period, so the dent size plays an important role in the RCF evolution process.

The objective of this study is to investigate the wear and rolling contact fatigue (RCF) damage of wheel and rail materials under alternating temperature conditions. Two series of rolling-sliding tests were performed: (1) at 20 °C for 75,000 cycles, and then continued at −40 °C for 10,000, 30,000, and 75,000 cycles, respectively; (2) at −40 °C for 75,000 cycles, and then continued at 20 °C for 10,000, 30,000, and 75,000 cycles, respectively. The results indicated that the decrease in the temperature would alleviate the wheel wear due to the formation of wear debris layer. Both the rising and dropping of the environmental temperature during the tests could lead to the increase in the rail wear. Besides, the decrease in the temperature could increase the plastic deformation and work hardening of wheel and rail discs. In addition, the crack initiation was correlated with the behaviour of plastic flow on the wheel. At 20 °C, long single cracks initiated and propagated along the highly deformed ferrite boundaries. At −40 °C, white-etching layer (WEL) was observed only on the wheel surface, which was mainly attributed to the severe plastic deformation. Then, the refined ferrites and WELs were the main crack initiation sources on the wheel.