Railway Axles Research Articles

Using a bonded solid film lubricant to prevent fretting wear in the axle journal bearings of rail vehicles

Fretting degrades the inboard inner ring and the backing ring contact surfaces of axle journal roller bearings used in railway applications. This study attempts to prevent fretting wear between them as well as fretting wear particles from entering the axle bearing interior without introducing new bearing components. It has been reported that coating metallic surfaces with bonded solid film lubricants improves lubricity and wear resistance while reducing fretting wear. To evaluate the impact on fretting wear prevention, a full-size railway axle bearing that was coated a backing ring side face with a bonded solid film lubricant was rotated. The findings demonstrate that applying the bonded solid film lubricant to the backing ring side face does not prevent the axle bearings from fretting. The surface temperature probably exceeds the heat-resistant temperature of the solid film lubricant because of the fretting friction.

STUDY OF THE INFLUENCE OF THE CHEMICAL COMPOSITION OF RAILWAY AXLE STEEL ON THE UNIFORMITY OF THE FINAL MICROSTRUCTURE

The relevance of the work. For modern railway transport, increasing the strength and operational reliability of rolling stock parts, in particular railway axles, is an urgent task. It is known that the inhomogeneity of the distribution of chemical elements in the structure of structural grade carbon steels is formed mainly during their crystallization and may subsequently affect the final structural state and set of properties. Purpose. Study of the influence of the chemical composition of steels for railway axles on the uniformity of the structure. Methodology. Experimental ingots of different chemical compositions with different amounts and ratios of the main chemical elements within the range of OS and EA1N were produced in laboratory conditions. The grain structure was studied on microsections after etching with nital. Chemical inhomogeneity (“traces” of the dendritic structure) − the distribution of chemical elements in the microstructure of cast samples was detected by etching in a hot solution of sodium picrate. Metallographic analysis was performed on a light (optical) microscope “Axiovert 200 M MAT” manufactured by “Carl Zeiss”. The results. The effect of changing the chemical composition of carbon steel, which is intended for the manufacture of railway axles, on the peculiarities of the formation of ferrite-pearlite heterogeneity and grain size is determined. It is shown that in samples of steels corresponding to the EA1N brand with a lower carbon content, the distribution of phases is uneven. The mechanism of formation of such a structure is explained from the point of view of two theories. In samples of steels with an average carbon content, which correspond to steel of the OS grade, the distribution of the pearlite and ferrite phases is more uniform, the relationship between the size and the degree of darkening of the pearlite areas with the order of the former dendritic branches has been established. Based on the results of the quantitative analysis of the final grain structure, it was established that OS steel with an average carbon content at different Mn/Si ratios has a more uniform grain structure. EA1N steel with a lower carbon content has a pronounced grain size, with an increase in the Mn/Si ratio, the largest maximum of the dependence of the distribution shifts to a larger grain size (smaller number). Practical results. Increasing the homogeneity of the final microstructure contributes to increasing the plasticity and viscosity of steel, which directly affects the reliability and durability of railway axles.

Crack detection in railway axles using continuous monitoring and periodic non-destructive inspection: experimental investigation on a freight wagon using a full-scale roller rig

This paper presents an experimental investigation aimed at assessing the possible use of axle-box acceleration measurements to identify the presence of a transverse crack in railway axles, detecting components of axle vibration occurring at frequencies that are integer multiples of the axle’s frequency of revolution (NxRev components). Experiments were carried out on an instrumented freight wagon equipped with two pre-cracked axles. Cracks were further propagated using a full-scale roller rig in order to reproduce conditions similar to the service ones, while ensuring safety. The size of the cracks during the tests was estimated by a dedicated non-destructive testing procedure based on magnetic particle testing and phased array ultrasonic testing, specifically designed to be simple and ergonomic so as to be suited for the use on the axles installed on the wagon. The results of the experiments confirm the feasibility of detecting a cracked axle by tracking the difference of the 3xRev components of the horizontal axle-box acceleration measured at the two sides of the axle, provided that the running speed of the wagon is at least 70 km/h. The minimum size of a detectable crack is in the order of 11% of the total cross section of the axle, which means that approximately 50% of the size that would lead to the final unstable phase of crack propagation preceding the failure of the axle.

Fatigue behavior assessment of heavy-duty freight railway axles under different heat treatments

The safety of heavy-duty freight railway axles is threatened by defects, including inclusions, corrosion pits, fretting cracks and foreign object impact damages. Accordingly, surface strengthening techniques are applied to reduce the detrimental effect of defects on the structural integrity of railway axles. This work studied the effectiveness of heat treatment, in which the specimens machined from a heavy-duty freight railway axle were induction hardened under three types of processing parameters and tempered. The fatigue strengths of specimens with electrical discharge machined (EDM) notches and impact craters on their surfaces were tested. The fatigue behavior of induction-hardened (IH) specimens was compared with that of as-received (AR) specimens and fully-quenched (FH) specimens. The fatigue limit of smooth IH specimens is close to that of FH specimens while almost twice as much as that of AR specimens. Inclusions were found on the crack initiation sites on the surface or subsurface of AR and FH specimens. For IH specimens, the crack initiation sites moved to the zone between the case and the core. Fatigue strengths of IH and FH specimens decline with increasing EDM notch size but remain higher than AR specimens’. The influence of impact damage on fatigue properties depends on damage size and heat treatment process.

Gradient residual strain determination of surface impacted railway S38C axles by neutron Bragg-edge transmission imaging

Non-destructive and quantitative mapping of gradient residual strain distribution in surface-hardened railway S38C axles could provide a positive reference for determining service lifetime and maintenance strategy. To tackle this concern, time-of-flight neutron Bragg-edge transmission imaging was employed by real axle samples with and without impacted crater. A novel and simple procedure to formulate the residual strain field was also developed in this work, with the transmission batch code in Appendix A. By mapping the global two- dimensional residual strains, it can be verified that the residual strains into the axle are uniformly distributed in the hoop direction. Subsequently, it was revealed that the axial and hoop residual strains, respectively in the cylinder and the long strip samples prepared from a real S38C hollow axle, indicated a gradient evolution distribution with a depth of ∼ 8 mm, covering a range of −5500 ∼ 1000 με for axial strains and −6500 ∼ 1000 με for hoop strains. More importantly, the maximum compressive lattice strain of the cylinder sample was increased by 15.61 %, and 22.35 % at the impacting speeds of 100, and 125 m/s, respectively; and that of the long strip sample increased by 29.17 %, and 43.70 %, respectively. It can thus be concluded that lattice strains have redistributed around the impact crater, demonstrating the local alteration of the residual strain field. These new findings suggest the localized variation in residual strains should be taken into account while evaluating the service damage evolution of railway axles, especially those affected by high-speed impacts during operation.

Studies on energy consumption and other important machining characteristics in sustainable turning of EA1N railway axle steel

The present research focuses on comprehensively evaluating energy consumption and other vital machining characteristics during the turning process, aiming to optimize efficiency while minimizing environmental impact. The experimental data is collected through a series of machining tests on EA1N railway axle steel under dry, minimum quantity lubrication (MQL), and cryogenic cooling conditions. Under these cutting conditions, the machinability criteria (energy consumption, tool wear, surface quality, chip morphology) of train wheel axle steel were tried to be improved. As a result, cryogenic cooling at constant cutting speed gave 40% and 53% better results in terms of energy consumption than MQL and dry environment, respectively. When the same situation was examined in terms of tool wear and surface quality, 10–18% and 8–14% gave better results, respectively. In other words, it is worthy to mention that the research findings not only benefit the manufacturing industry by optimizing resource utilization but also align with global efforts to promote environmentally conscious practices in the engineering and transportation sectors.

Capacity assessment of railway axle journal bearing addressing a localized rolling contact fatigue failure mechanism

Abstract A framework for assessing the capacity of railway axle journal bearings is presented after elaborating on the locale rolling contact fatigue (RCF) failure mechanism. Three phases are conducted, including the determination of bearing service loads, finite element analysis (FEA) on the localized RCF stresses of bearing components, and the life-related capacity assessment on the bearings and bearing components. Results prove that wheel-track (WT) contact forces inspected by an instrumented wheel set can be transferred into bearing service loads. Two-step FEA from the global wheel set to the local bearings is used to obtain localized RCF stresses at dangerous locations around the bearing contact regions. After determining the locations, a life-related capacity assessment is performed by evaluating equivalent RCF stresses and lives at the locations to determine the weakest bearing component. Typical practice is given in agreement with FEA. This method deals with the FEA of bearing structures, material cyclic stress-strain curves, and probabilistic RCF life curves. Therefore, this work greatly enhances the development of rail-bearing materials, innovative structures, and capacity assessment.

Study of CNC skew rolling of hollow rail axles with a mandrel

The manuscript presents a new manufacturing technology for the production of hollow railway axle forgings. The manufacturing technology analyzed is based on three-roll skew rolling (TRSR) using a computer numerically controlled (CNC) rolling mill. The study focused on comparing the rolling capability of hollow products without and with a mandrel calibrating the hole of the forging. The influence of tube billet size on the rolling process was also analyzed. FE analysis and experimental studies were carried out. An analysis of the accuracy of the internal hole of the forging depending on the adopted parameters of the rolling process was carried out. Based on numerical simulations, the state of strain and the flow pattern of the material and temperature distribution during rolling were determined. The force parameters of the rolling process of hollow forgings were also analyzed. Based on the research, a two-stage rolling technology for rolling hollow railway axle forgings was proposed. The results obtained indicate the suitability of using a calibrating mandrel to improve hole accuracy in hollow forgings rolled from a tubular billet. The gap c between the bore diameter of the forging and the diameter of the mandrel was measured. Increasing the tubular billet dimensions from Ø42.4 × 10 mm to Ø48.3 × 12.5 mm reduced the gap c by 49.8%. Rolling the billet Ø51 × 14.2 mm in two passes compared to rolling in one pass reduced the gap c by 45.5%.

Strain-Based Assessment to Evaluate Damage Caused by Deep Rolling.

The positive effects of deep rolling on fatigue strength-reduced surface roughness, work hardening and compressive residual stress-in the near-surface region are achieved by controlled high plasticisation of the treated material. However, excessive and/or repeated plasticising poses a risk of damage to the machined component. This paper investigates the damage caused by deep rolling of a railway axle. Two sections of the axle are experimentally deep rolled repeatedly at different feed rates until damage is detected. For comparative analysis, these experiments are numerically analysed and the damage is assessed using the strain-based damage calculation. The results are compared and a damage sum of ~120% is evaluated for both tests, thus developing a reliable and conservative assessment method. The single deep rolling treatment at a feed rate of 0.25 mm causes damage of 6.1%, and at a feed rate of 0.5 mm, damage of 4.7%. The developed and experimentally validated evaluation method allows for investigating the limits of applicability of different deep rolling parameters. The influence of the deep rolling force and feed rate and a proposed optimisation with multiple deep rolling with reduced deep rolling forces are investigated.

Integration of the Ashby Technique and Pahl-Beitz Quantitative Ranking for Railway Axle Material Selection

Integration of the Ashby Technique and Pahl-Beitz Quantitative Ranking for Railway Axle Material Selection

METHODOLOGICAL APPROACH TO THE DETERMINATION OF THE EFFECTIVE COMPONENT COMPOSITION OF CARBON STEELS FOR THE RESPONSIBLE APPOINTMENT

Of particular importance in the metallurgical industry is the issue of optimizing the chemical composition of steel in order to ensure the required level of properties. However, a significant number of modern complex approaches are based on the application of information technologies. The purpose of the work is to study the influence of the chemical composition of carbon steels on the formation of their microstructure and hardness. As part of the research, experimental ingots of carbon steels of various compositions were melted, which were then subjected to hot plastic deformation (HPD) followed by heat treatment (HPD+HT). The research material was laboratory steels, which are comparable in chemical composition to steels for railway axles according to the state standard, European and American standard. During the research, it was found that the chemical composition affects the number of structural components and the hardness of the studied steels in different states, such as cast, after HPD and HPD +TO. It was found that for a certain chemical composition, in which the value of the integral parameter of the state of charge (ZY) is less than 0.20 e, the hardness of steel in the state after HPD exceeds the hardness of steel in the state after HPD +TO.

An acoustic emission identification model for train axle fatigue cracks based on deep belief network

Railway axles are safety-critical components of the railroad rolling stock and the consequences of possible in-service failures can have a huge impact. Axle fatigue cracks are relatively common defects during train operation, but how to intelligently identify axle fatigue cracks in running trains is still a great challenge. In order to identify axle fatigue cracks more intelligently, the problem that needs to be solved is how to overcome the manual extraction of features by manual experience as well as shallow networks. Therefore, in this paper, an acoustic emission signal identification method based on deep belief networks (DBNs) for axle fatigue cracks is proposed. In this method, a DBN model is constructed. The axle fatigue crack acoustic emission signal data were obtained by our designed acquisition experimental setup, and these data were used to verify the accuracy of the constructed DBN network model identification. The experimental results show that the method of identification of axle fatigue cracks based on DBN, compared with the traditional fault diagnosis method, eliminates the operations of data feature extraction, feature screening, feature fusion, etc and makes complete use of all the information contained in the fault data. The method can not only identify fatigue crack signals but also has a high identification rate of fatigue cracks at different stages. In the axle fatigue crack acoustic emission identification field, it can be seen that the proposed method in this paper will be a promising approach.

A proposed tacho-less order tracking technique suitable for railway vehicle condition monitoring applications

To perform a condition monitoring for rotating machines with variable running speed, order tracking is considered as an effective technique suitable for their non-stationary vibration signals. A tachometer signal is necessary to perform a computed order tracking (COT) technique; however, in some applications it's not possible to add tachometer hardware due to its complexity or shortage of power. To improve the applicability of order tracking techniques, some tacho-less order tracking techniques have been introduced in the last few years. These techniques are working off-line by inspecting the vibration signals in time-frequency domain to extract the machine rotating speed. In this paper, a new tacho-less technique suitable for continuous structural health monitoring applications of railway vehicles, is proposed. In the presented approach, a coarse angular speed of the railway axle is estimated firstly using estimation train speed. Then this coarse angular speed is used to improve the applicability of the traditional tacho-less techniques to be able to work online. The train speed is estimated from two acceleration signals obtained from vertical vibration of axle-boxes from same side of train. The sequential properties of the two signals are used to estimate the delay time of one signal with respect to the other one by means of cross-correlation function. By knowing the horizontal distance between the two sensors collected the vibration signals, the train speed can be estimated. This modification of tacho-less technique improves the effectiveness and robustness of the traditional maxima tracking strategy to be performed automatically. Thereafter, signal processing technique based on the computed order tracking as well as averaging technique can be applied to extract the useful information from noisy and non-stationary signals. This technique has been proposed in a framework of a project investigating crack detection using vibration measurements, therefore, vibration signals of cracked axle have been processed using tacho-based method and using the proposed tacho-less technique and a good agreement has been found between the two techniques.

Gradient residual stress and fatigue life prediction of induction hardened carbon steel S38C axles: Experiment and simulation

Gradient distribution of triaxial residual stresses to a depth of several millimeters is retained in middle carbon steel S38C axles after high-frequency induction hardening, which has become a critical concern for fatigue structural integrity. To address this, the axial, hoop, and radial gradient residual strains inside the axles were measured for the first time by advanced neutron diffraction. The SIGINI Fortran subroutine was then adopted to reconstruct the global initial residual stress field from the measured data. Experimental and simulation results show that residual stresses of about −520 MPa (axial), −710 MPa (hoop), and −40 MPa (radial) residual stress were retained below the axle surface. Subsequently, the fatigue crack propagation behavior of S38C axles was numerically investigated in the framework of fracture mechanics. The calculated results clearly show that the compressive residual stresses at a depth of 0–3 mm from the axle surface lead to a low crack growth driving force, and that fatigue cracks do not propagate as long as the crack depth is less than 3.7 mm for hollow S38C axles. These results further indicate that the maximum defect size allowed in routine inspections is acceptable from a safety and economic point of view. Accurate measurement and characterization of the global gradient residual stress field through experiments and simulations can provide an important reference for optimizing the mileage intervals of nondestructive testing (NDT) of surface defects in these surface-strengthened railway axles.

Application of local fatigue strength approach to assess and optimise the impact of deep rolling on the fatigue performance of railway axles

Deep rolling is a frequently used post-treatment process to increase the service life of railway axles. Through the process application, the surface roughness is reduced, the surface is work-hardened and compressive residual stresses are introduced in the near surface region in a controlled manner. These three effects have a positive impact on service life, and in the investigated case of railway axles, the introduced residual stress state constitutes the most significant potential. The compressive residual stresses introduced near the surface, which are responsible for the positive lifetime effect, are inevitably balanced by compensating tensile residual stresses below the surface. Therefore, it is generally challenging to estimate the actual potential of the process application properly. Classical design methods consider only the condition at the surface and are thus not suitable for a surface layer-based analysis. In the present publication, the concept of local fatigue strength is applied for the fatigue strength assessment of deep rolled railway axles. The locally permissible fatigue strength at each surface-layer depth of the railway axle is determined as a function of local properties and compared with the locally occurring load. This allows a proper fatigue design in depth and, at the same time, the estimation of the critical point at which the potential of crack initiation and thus failure occurs. Based on the methodology presented, it is possible to determine the fatigue benefit achieved by deep rolling railway axles considering the influence of various deep rolling parameters, and to further systematically optimise the significant process parameters. An increase in fatigue strength of 4.7 % can be proven by deep rolling the railway axle with reference parameters compared with the not deep rolled state. By applying the proposed optimisation, fatigue strength can be further increased up to 10.4 %.

Effect of interference magnitude on fretting wear and fatigue strength of scaled press-fitted railway axles

Railway axles represent a typical example of interference fit structures that are vulnerable to fretting fatigue induced by rotational bending loads. This study investigated the influence of interference magnitude on the fretting wear and fatigue of scaled press-fitted railway axles. In this study, the fretting fatigue strength was tested at three interference magnitudes within the standard range, and the fretting wear and fretting cracks were analyzed. The results indicate a trend where the fretting fatigue strength initially rises and then declines with increasing interference magnitude. 3D finite element models, with and without considering the fretting wear, were developed to investigate the variation in contact parameters in the press-fitted region, and the Smith-Watson-Topper multiaxial fatigue model was employed to evaluate the fretting fatigue behavior. The results demonstrate that the competition between contact pressure and axial slip produces relatively slight fretting wear when the interference magnitude takes the middle value, thereby causing a minor stress concentration and ultimately leading to higher fretting fatigue strength.

Residual Stresses and the Microstructure of Modeled Laser-Hardened Railway Axle Seats under Fatigue

Railway wheels are usually attached to axles by press-fitting; therefore, the mechanical processes taking place during operation can result in failure, with fatal consequences for the axle seats. This manuscript describes the effect of laser hardening on the residual stress state, microstructural parameters (lattice defects—dislocations, crystallites, microstrains, etc.), and mechanical properties of laser-hardened EA1N steel railway axles under fatigue life conditions. Differences were found between ground, single-track, and multi-track hardened surfaces. Tensile residual stresses, low dislocation densities and hardnesses, and different microstructures (tempered cubic martensite) were found at the overlapped tracks and at the boundary of the heat-affected zone and bulk surface compared with the hardened zone. As a result, the surface treatment of axle seats by laser hardening improved the fatigue failure resistance compared with untreated seats. Optimal properties of the integrity of the axle seat surface were achieved, including fatigue resistance, which seems to be positively influenced mainly by sufficient hardness and the appropriate microstructure. The influence of the other investigated parameters was not evident, and was reduced by the presence of fretting corrosion and press-fitting.

Numerical parameter sensitivity analysis of residual stresses induced by deep rolling for a 34CrNiMo6 steel railway axle

To optimise the benefits of the deep-rolling process in the service life context of treated components, the process application must be investigated. In addition to the reduction in surface roughness and near-surface material strengthening, compressive residual stresses are introduced, which are primarily responsible for the increase in service life for components, especially in the case of high-strength steel materials. A numerical parameter sensitivity analysis is performed in order to investigate the introduced residual stresses in detail. For this purpose, a validated deep-rolling simulation model is used, which replicates the deep rolling of a railway axle made of the high-strength steel material 34CrNiMo6. The model is based on an elastic-plastic Chaboche material model parameterised on uniaxial tensile and LCF test results and validated with residual stress measurements. Using this model as a basis, the effect of the main process parameters deep-rolling force, feed rate, friction coefficient, number of overruns, tool geometry, and shaft geometry on the resulting residual stress state are investigated. The results reveal that the deep-rolling force has the most significant influence on the introduced residual stress state and should therefore be highlighted. In the case of applying a deep-rolling force of more than 10 kN, maximum compressive residual stresses of around − 1000 MPa are introduced, and a strong saturating behaviour is shown. Maximum compensating tensile residual stresses of + 100 MPa occur below the surface. The main influence of the deep-rolling force is the effective depth achieved, which is determined by the depth of the zero crossing. This varies from 1 mm with an applied force of 2 kN to more than 3.5 mm with 20 kN. Furthermore, the results are analysed to conclude suggestions for the process’s applicability, and a proposal for an optimised deep-rolling treatment is presented. There multiple deep rolling with decreased deep-rolling forces is used to achieve a comparably optimised residual stress state. In summary, with the presented results, a contribution to a deeper understanding of the deep-rolling process can be achieved; the influence of the most important process parameters on the residual stress in-depth profiles is established; an optimisation proposal is presented; and correlations are found. Thus, the base work for further fatigue strength assessments and the optimisation of the deep-rolling process regarding the increase of service is laid.

Numerical prediction of fretting fatigue crack growth in scaled railway axles considering fretting wear evolution

To predict the fretting fatigue crack growth (FFCG) life of railway axles, a series of interrupted fatigue experiments were conducted on scaled railway axles. Subsequently, the evolutions of fretting wear and fatigue cracks in the press-fitted region were analyzed. Based on the test results, finite element models incorporating fretting wear evolution were established, and the FFCG was investigated using the maximum tangential stress criterion, cyclic resistance curve, and the modified NASGRO equation. The analysis revealed that the fretting wear evolution leads to stress redistribution at the press-fitted region, thereby promoting FFCG. When considering fretting wear evolution, the equivalent stress intensity factor (SIF) range of the crack remains above the threshold value throughout the short crack stage. However, neglecting fretting wear evolution results in the SIF range being below the threshold value for cracks shallower than 0.30 mm. This implies that considering fretting wear evolution enables life prediction throughout the short crack stage. As the crack length increases, the influence of fretting wear evolution on crack growth gradually diminishes. By accounting for fretting wear, a more accurate stress distribution in the press-fitted region can be obtained, leading to a more precise and conservative prediction of crack growth life.

Corrosion fatigue behavior and life prediction of railway axle EA4T alloy steel with artificial indentation

The combined effects of aggressive media and artificial surface indentation on the fatigue behaviours were investigated to evaluate the defect tolerance of high-speed hollow railway EA4T steel axles. Corrosive rotating-bending fatigue tests were performed on both smooth and notched specimens to assess their fatigue strengths and failure mechanisms. Based on the corrosion testing campaign, which mainly included examinations of the microstructure, residual stress, and fatigue crack resistance, material deformation underneath the bottom of indentations triggered corrosion fatigue cracks. The results also showed that the corrosive media was responsible for the decrease in fatigue strength, and notched specimens were more sensitive to 3.5 wt% NaCl solution than to air. The fracture surface indicated that cracks were mainly initiated from the notch of specimens in air, whereas cracks were formed from multiple sources in specimens in the corrosive medium. Finally, a three-parameter Kitagawa-Takahashi diagram was established to correlate the threshold stress range (Δσth) and critical defect size (a) with the corrosion fatigue life (N) in terms of the El-Haddad model, thus providing a scientific reference for the service safe region identification of high-speed railway axles subjected to corrosive environments.