Tribological approach to optimize traction coefficient during running-in period using surface texture

Risk of wheel-climb derailment increases if the traction coefficient in the wheel/rail contact is too high. This has been observed to happen more just after wheel turning. This novel work investigates how the traction coefficient rises during the running-in period, when textured surfaces are used to simulate a freshly turned wheel. Running-in curve of traction coefficient showed a momentary rise and a peak value of traction coefficient was observed to decrease with the increase in magnitude of the wheel surface texture. The change of the subsurface hardness and the microstructure were also dependent on the initial surface texture coincidentally and the work-hardening layer of the textured surface was thicker than that of smooth surface. A mechanism model of the effects of surface texture on traction characteristics during the running-in was presented. The work will allow recommendations of wheel turning to be made to help reduce the problem of wheel-climb derailment

Transitions in rolling-sliding wheel/rail contact condition during running-in

The risk of wheel-climb derailment increases if the traction coefficient in the wheel/rail contact is too high. This has been observed to happen more frequently just after wheel machining. This work investigates how the traction coefficient rises with evolution of the wheel/rail interface during the running-in. Experiments were performed using a full-scale wheel/rail contact rig and an ultrasonic array transducer mounted in the rail. Results were used to determine the stiffness of the contact interface. Contact stiffness appeared to be positively correlated with the traction coefficient. Owing to the conforming of the interface, contact stiffness increases before the traction coefficient rises. The work will allow recommendation of wheel machining to be made to help reduce the problem of wheel-climb derailment

In situ evaluation of contact stiffness in a slip interface with different roughness conditions using ultrasound reflectometry

Understanding the dynamic condition of the interface between a railway wheel and rail is important to reduce the risks and consider the effectiveness of countermeasures for tribological problems. Traditionally the difficulty in obtaining accurate non-destructive interfacial measurements has hindered systematic experimental investigations. Recently, an ultrasound reflectometry technique has been developed as a direct observation method of a rolling–sliding interface; however, the topography dependence under the high contact pressures in a wheel–rail contact has not been clarified. For this reason, a novel in situ measurement of the contact stiffness using ultrasound reflectometry was carried out for three different levels of roughness. A contact pressure equivalent to that in a wheel–rail interface was achieved by using a high-pressure torsion test approach. The dynamic change of contact stiffness with slip was measured using ultrasound and the influence of roughness was investigated. The measured changes were validated using a newly developed numerical simulation, and mechanisms to explain the observed behaviour were proposed in terms of fracture and plastic deformation of the asperity bonds. These findings could help in understanding the traction characteristics for different roughness conditions and also assist in understanding damage mechanisms better, such as wear and rolling contact fatigue

Investigation on viscosity and non-isothermal crystallization behavior of P-bearing steelmaking slags with varying TiO2 content

The viscous flow and crystallization behavior of CaO-SiO2-MgO-Al2O3-FetO-P2O5-TiO2 steelmaking slags have been investigated over a wide range of temperatures under Ar (High purity, >99.999 pct) atmosphere, and the relationship between viscosity and structure was determined. The results indicated that the viscosity of the slags slightly decreased with increasing TiO2 content. The constructed nonisothermal continuous cooling transformation (CCT) diagrams revealed that the addition of TiO2 lowered the crystallization temperature. This can mainly be ascribed to that addition of TiO2 promotes the formation of [TiO6]-octahedra units and, consequently, the formation of MgFe2O4-Mg2TiO4 solid solution. Moreover, the decreasing viscosity has a significant effect on enhancing the diffusion of ion units, such as Ca2+ and [TiO4]-tetrahedra, from bulk melts to the crystal–melt interface. The crystallization of CaTiO3 and CaSiTiO5 was consequently accelerated, which can improve the phosphorus content in P-enriched phase (n2CaO·SiO2-3CaO·P2O5). Finally, the nonisothermal crystallization kinetics was characterized and the activation energy for the primary crystal growth was derived such that the activation energy increases from −265.93 to −185.41 KJ·mol−1 with the addition of TiO2 content, suggesting that TiO2 lowered the tendency for the slags to crystallize

Transition of the friction behaviour and contact stiffness due to repeated high-pressure contact and slip

A rapid increase in the friction coefficient can occur during the running-in between the wheel and rail. Although it has been found that the running-in process depends on the initial topography, the difficulty in obtaining accurate non-destructive interfacial measurements has hindered systematic investigations. In this work, four interfaces, which have different initial topographies, were continuously monitored using ultrasound reflectometry until they became conformal. A contact pressure representative of that in a wheel-rail interface was achieved by using a high-pressure torsion test approach. The transition of contact stiffness and friction coefficient with repeated slip and their relationship were investigated. Based on the experimental results, a mechanistic model for the running-in process of the contact interface was proposed. These findings will help in understanding the running-in process of the wheel-rail interface and assist in managing the wheel and rail appropriately to improve safety. A common insight into the running-in process for metal-to-metal contacts under high contact pressures has also been developed

Tribological aspects to optimize traction coefficient during running-in period using surface texture

Risk of wheel-climb derailment increases if the traction coefficient in the wheel/rail contact is too high. This has been observed to happen more just after wheel turning. This novel work investigates how the traction coefficient rises during the running-in period, when textured surfaces are used to simulate a freshly turned wheel. Running-in curve of traction coefficient showed a momentary rise and a peak value of traction coefficient was observed to decrease with the increase in magnitude of the wheel surface texture. The change of the subsurface hardness and the microstructure were also dependent on the initial surface texture coincidentally and the work-hardening layer of the textured surface was thicker than that of smooth surface. A mechanism model of the effects of surface texture on traction characteristics during the running-in was presented. The work will allow recommendations of wheel turning to be made to help reduce the problem of wheel-climb derailment

Using active ultrasonics to measure wheel-rail contact during a running-in period

Flange climb derailment is most likely to occur during the wheel-rail running-in phase, such as just after the railway vehicle wheels have been re-profiled or during morning rush hour. It is thought that the underlying cause is an increase in traction due to changes in the contact between wheel and rail. However, the mechanism of this increase in traction remains a subject of continuing controversy. Active ultrasonic measurements have been effectively used to measure wheel-rail contact conditions, particularly the contact stiffness. In this study, the ultrasonic method is used to characterise the wheel-rail contact during running-in whilst the surface roughness of the wheel reduces due to cyclic loading against the rail. An array of sensors was mounted behind the wheel flange so as to reflect ultrasound directly from the contact. An initial investigation was carried out to understand the measurement resolution of this ultrasonic array transducer. A static measurement was then performed by traversing the ultrasonic array transducer across the wheel when statically loaded against the rail. Following this, the same array transducer was fixed to the wheel and dynamic measurements were carried out. These measurements were cyclic to investigate the effect on changing surface topography. From this, the relationship between the slip and the contact stiffness was investigated. It was found that an increase of contact stiffness was measurement after slip motion was recognized. Both the static and dynamic measurements showed a difference in the wheel-rail contact conditions. In the case of the rolling condition, the peak position of the contact stiffness in the contact area shifted from the centre of the contact to the exit side. From the cyclic rolling tests, it was observed that the change in surface topography resulted in an increase in interfacial contact stiffness. This phenomenon indicates that there is an increase in real area of contact between the wheel flange and the rail gauge corner during running-in