Pearlitic Rail Steel Research Articles

Microstructure and mechanical properties of the running band in a pearlitic rail steel: Comparison between biaxially deformed steel and field samples

The large deformations occurring in the surface layer of rail heads strongly influence the material behavior. Cracks typically develop from this critical region. However, conventional material testing of this layer is not possible due to the large gradients. A method for obtaining a similar material through predeformation is therefore investigated. Subsequent analyses of the obtained material can improve our understanding of the material behavior close to the running band. Hardness, shear lines and cementite lamella orientation distributions are compared in order to determine whether the obtained material is similar to that of the field samples extracted from rails that have been in service for several years. The predeformation method produces a consistent material, which is representative of the material in the rail field samples that have accumulated large shear strains.

Modeling of kinematic hardening at large biaxial deformations in pearlitic rail steel

Using an Axial-Torsion testing machine, pearlitic R260 steel specimens are twisted until fracture under different axial loads. A well established framework for finite elastoplasticity with kinematic hardening is used to model the deformation of the specimens. In particular, we evaluate the ability of different kinematic hardening laws to predict the observed biaxial load versus displacement response. It is found that the combination of Armstrong–Frederick dynamic recovery and Burlet–Cailletaud radial evanescence saturation is efficient even for the large strains achieved in this study. The results are less conclusive on the appropriateness of replacing the Armstrong–Frederick with an Ohno–Wang type of kinematic hardening law.

Understanding cementite dissolution in pearlitic steels subjected to rolling-sliding contact loading: A combined experimental and theoretical study

Cementite dissolution behavior of pearlitic steels subjected to rolling-sliding contact deformation is comprehensively investigated by combining experimental characterization and phase-field modeling. An elasto-plastic phase-field model, incorporating the elastic strain-induced free energy contribution from first-principles calculations and the plastic counterpart from a rolling-sliding contact finite element model assisted with a plastic strain accumulation model, is originally proposed to simulate the real-time evolution of cementite volume fraction, cementite morphology and carbon distribution for different rolling cycles and contact depths. Upon experimental validations, the proposed model predicts more accurate and realistic results than Sauvage’s model. A three-stage behavior of cementite dissolution is also revealed, which well explains an experimentally observed significant cementite dissolution gradient along the depth direction. Besides, the effect of ferrite/cementite interface thickness and the initial lamellae thickness of cementite on cementite dissolution kinetics is studied. The proposed phase-field model can not only help understand the mechanism of cementite dissolution, but also give new sights into quantitative predictions of the mechanical properties and even the rolling contact fatigue life of pearlitic rail steels in service.

Hydrogen embrittlement behavior of high strength rail steels: A comparison between pearlitic and bainitic microstructures

The present study was attempted to evaluate the hydrogen embrittlement (HE) behavior of two high strength rail steels with pearlitic and bainitic microstructures by using slow strain rate test (SSRT) with notched round bar specimens. The results show that the bainitic rail steel with a mixed microstructure of bainite, tempered martensite and M/A constituents exhibits superior mechanical properties in comparison with the pearlitic rail steel. The results of SSRT revealed that the bainitic rail steel is more susceptible to HE than the pearlitic rail steel, which is ascribed mainly to the higher strength level and the microstructural characteristics of the bainitic rail steel. The attempt of re-tempering treatment of the as-received bainitic rail steel exhibits no notable improvement in the resistance to HE at no expense of strength. It is thus suggested that further efforts concerning obtaining low susceptibility to HE besides obtaining excellent combination of strength and toughness should be conducted to guarantee the safety service of the bainitic steel rails.

Characterization of deformed pearlitic rail steel

Pearlitic steels are commonly used for railway rails because they combine good strength and wear properties. During service, the passage of trains results in a large accumulation of shear strains in the surface layer of the rail, leading to crack initiation. Knowledge of the material properties in this region is therefore important for fatigue life prediction. As the strain is limited to a thin surface layer, very large strain gradients can be found. This makes it very difficult to quantify changes in material behavior. In this study hardness measurements were performed close to the surface using the Knoop hardness test method. The orientation of the pearlitic lamellas was measured to give an overview of the deformed microstructure in the surface of the rail. Microstructural characterization of the material was done by optical microscopy and scanning electron microscopy to evaluate the changes in the microstructure due to the large deformation. A strong gradient can be observed in the top 50 μm of the rail, while deeper into the rail the microstructure of the base material is preserved.

Atom probe study on microstructure change in severely deformed pearlitic steels: application to rail surfaces and drawn wires

Pearlitic steel is used as the material for high tensile steel wires, rails and wheels due to its high work hardening and wear resistance. These properties arise from a layered structure comprising deformable lamellar ferrite and hard lamellar cementite. This paper reviews the microstructural change in heavily drawn pearlitic steels wires and worn surfaces of pearlitic rails using atom probe tomography analysis. The cementite decomposition mechanism was elucidated for heavily drawn pearlitic steel wires. For pearlitic rail steels, atomic scale characterization of worn surfaces and of the white etching layer (WEL) were performed, and a mechanism for the formation of the WEL was proposed. The differences and similarities in microstructure and in the state of the cementite in these severely deformed pearlitic steels are discussed.

Residual stresses generated by repeated local heating events – Modelling of possible mechanisms for crack initiation

The hypothesis of thermal damage mechanisms by short-term local friction heating of rail or wheel steels leading to initiation of cracks, specifically squats (in some works called studs) in rails and crack clusters in wheels has given the inspiration for this study. A Finite Element (FE) model incorporating phase transformation from near pearlitic steel to austenite, and then to martensite was developed to examine developing strength and stress fields. Thermal strains on heating, cooling and phase transformations are naturally included, and the FE model also incorporates shrinkage due to tempering of the martensite during subsequent heat pulses. The material behaviour implemented in the model has been rationalised from experiments done on near pearlitic wheel steels, however the qualitative results are deemed applicable also for local thermal damage on pearlitic rail steels. The material is currently modelled as isotropic, with properties varying with phase, temperature and tempering state. The main contributions of this work is the modelling framework developed, specifically the careful modelling of martensite tempering combined with phase transformation. Different thermal sequences are examined to demonstrate the model´s capability. The computed strength and stress fields are used to discuss possible mechanisms of crack initiation.

Laboratory simulation of martensite formation of white etching layer in rail steel

White etching layer (WEL) is a frequently observed microstructural phenomenon in rail surface, formed during dynamic wheel/rail contact. It is considered as one of the main initiators for rolling contact fatigue cracks. There are several hypotheses for the formation mechanism of WEL. However, due to the complicated wheel/rail contact conditions, none is directly proven. Currently, the most popular hypotheses refer to either formation of martensitic WEL by phase transformations or formation of nanocrystalline ferritic WEL by severe plastic deformation. In this work, WEL formation by martensitic transformation in R260Mn grade pearlitic rail steel was simulated by fast heating and quenching experiments. Microstructural characteristics of the simulated WEL and WEL observed in a field rail specimen were characterized by microhardness, optical microscopy, scanning electron microscopy and electron backscatter diffraction. Microstructures of the two WELs were compared and similarities in morphology were identified. Numerical simulation shows the possible temperature rise up to austenitizing temperatures. Combining comparisons of experimental simulation with observation of WEL in the rail and the thermodynamic calculations, the hypothesis for WEL formation via martensitic transformation is supported.

Phase and microstructural evolution in white etching layer of a pearlitic steel during rolling–sliding friction

To understand the microstructural evolution of a pearlitic rail steel during rolling–sliding contact friction, phase structure and mechanical properties of deformed layers, from top surface to inner matrix, were investigated gradually by scanning electron microscopy, electron probe microanalysis (EPMA), X-ray diffraction, transmission electron microscopy (TEM) and nano-hardness indentation. Focused ion beam method was used to prepare site-specific TEM specimens to study the thickness-dependent deformed microstructure. After the deformation, a white etching layer (WEL), was formed on the top surface, which appeared in white color under an optical microscope. Quantitative EPMA analysis revealed slightly lower carbon concentration, inhomogeneously distributed within the WEL. According to high-resolution TEM observation and quantitative electron diffraction analysis, it was found that the WEL was indeed composed of uniform nano-grains of martensite and ferrite, where no cementite and austenite were found. All the results supported that the WEL was formed by severe plastic deformation, and the hardness value of WEL reached 12GPa by nano-hardness indentation.

Kinetics of damage accumulation in pearlitic rail steels under contact fatigue

The kinetics of damage accumulation in specimens of rail steels with the pearlitic structure manufactured by various companies has been studied in contact fatigue tests. The parameters of damage to the steels, such as the number, dimensions, and area of defects, the angular coefficients of cumulative size statistical distributions of the defects, as well as the velocity of propagation and the attenuation coefficient of ultrasonic waves, have been assessed. New criteria have been proposed to characterize the evolution of damage under contact fatigue. A linear interrelation between the number of surface defects and the number of subsurface defects in the zone of contact has been established, which can serve as a basis for continuously monitoring the condition rail steels and for assessing their current and remaining service lives.

Fatigue and fracture behavior of pearlitic Grade 900A steel used in railway applications

In overloaded railway networks, rail surface defects developed under rolling contact fatigue (RCF) if left untreated, may propagate to the bulk of the rail and become critical for its structural integrity. To avoid critical situations, the fatigue and damage tolerance behavior of railway steels is of high importance. In this experimental study, the fatigue and fracture performance of a pearlitic Grade 900A rail steel, which is used in the railway network of Attiko Metro in Athens has been investigated. The rail material is an annealed steel with pearlitic microstructure, combining high strength with a high strain hardening rate and excellent high cycle fatigue behavior. On the other hand, the material exhibits brittle fracture characteristics with inferior fracture toughness and fatigue crack growth resistance compared to other pearlitic rail steels. The mechanical performance suggests that the material resists crack initiation, but for application purposes maintenance schedules should be able to identify fatigue cracks at early stages to avoid propagation of cracks to critical lengths compromising structural integrity.

EBSD study of early-damaging phenomena in wheel–rail model test

In the early-damage regime of rails subjected to rolling contact fatigue, the material undergoes severe plastic deformation forming a modified sub-surface microstructure. This zone close to the surface determines subsequent fatigue properties of the whole component and can differ considerably from an initial microstructure of the same material. Therefore, a fundamental understanding of early damage phenomena (<4000 cycles) is the focus of this work by comparing exemplary as a rolled standard R260 steel with a surface finished R260 pearlitic steel regarding their response to the rolling stresses by plastic deformation. Thus, electron backscatter diffraction technique based results were used to describe the material modification and set up a model for early-damage response of a pearlitic rail steel. Thereby, increased plastic deformation was found in the decarburized zone already after 100 cycles while the surface finished R260 remains almost undeformed. After 4000 cycles, both specimens have undergone high plastic deformation with work hardening concentrated directly below the surface for the decarburized steel and up to higher depths in the surface finished one.

An efficient computational approach to evaluate the ratcheting performance of rail steels under cyclic rolling contact in service

A comprehensive study was carried out to numerically evaluate the ratcheting performance of three high strength pearlitic rail steels under different wheel–rail cyclic rolling contact conditions, i.e. free rolling, partial slip, and full slip conditions, different friction coefficients and different axle loads. The wheel–rail cyclic rolling contact was simulated by repeatedly passing a distributed contact pressure and a distributed tangential traction on the rail surface. This study combined the non-Hertzian contact pressure from finite element analysis with the longitudinal tangential traction from Carter’s theory to simulate the wheel–rail cyclic rolling contact problems. A cyclic plasticity material model considering the non-proportionally loading effect developed recently by the authors was applied to simulate the ratcheting behaviour of rail steels. The ratcheting performance of the rail steels was evaluated by the crack initiation life which was determined from the stabilized ratcheting strain rate and the ductility limit of the rail materials. The numerical results indicate that the crack initiation life decreases with the increase of the normalized tangential traction, the friction coefficient and the axle load for all three rail steels. Among the three rail steels, the hypereutectoid rail steel grade with a lower carbon content provides the best ratcheting performance under higher axle loads such as those used railway transport of mineral products in Australia. Furthermore, the numerical results obtained in this study are in reasonable agreement with the in-service performance of the three rail steels. This indicates that the developed approach has the capacity to evaluate the ratcheting performance of other rail steels under service loading conditions. The outcomes can provide useful information to the development and application of rail steels and the development of effective rail maintenance strategies in order to mitigate rail degradation.

Fatigue crack growth rate of two pearlitic rail steels

A study of fatigue crack growth rate was conducted in two pearlitic rail steels, namely R260 and R370CrHT. Two crack plane orientations with respect to the rolling direction were tested and separate experiments were performed to cover a wide variation in fatigue crack growth, from the fatigue threshold (circa 10−9m/cycle) to rapid fracture conditions. The fracture surfaces were examined and some correlations between microstructural features such as interlamellar spacing and mechanical properties were made.

Sliding wear and low cycle fatigue properties of new carbide free bainitic rail steel

A new carbide free bainitic rail steel was prepared, whose comprehensive mechanical properties are equal or superior to current premium pearlitic and bainitic rail steels. The new bainitic rail steel possesses better low cycle fatigue properties and approximate resistance to wear compared with current pearlitic rail steel. The carbon enriched film-like austenite between the ferrite of the new bainitic rail steel can delay crack initiation and propagation in fatigue processes, resulting in a relatively high low cycle fatigue life (about two times) compared to the pearlitic rail steel. Finally, a wear model during sliding wear and deformation model during the low cycle fatigue of pearlitic and bainitic rail steels were established. As a result, the bainitic rail steel with a relatively reasonable combination of wear and fatigue properties compared to pearlitic rail steel is obtained, in which the initiation and propagation of cracks may be partially or entirely removed during the wear process.

Reciprocating Sliding Wear Behavior of Newly Developed Bainitic Steels

The present work discusses the mechanical properties and wear behavior of newly developed bainitic rail steels with nominal composition of 0.71 pct C, 0.35 pct Si, 1.15 pct Mn, 0.59 pct Cr, 0.40 pct Cu, and 0.20 pct Ni (all in wt pct)). Isothermal transformation has been carried out at different time and temperatures for obtaining different bainitic morphologies. Linearly reciprocating sliding wear behavior of the steels has been studied and compared with that of the conventional pearlitic rail steel. Considerable improvement in mechanical properties of the bainitic steels has been noticed. The hardness of the bainitic steels increases with decrease in isothermal transformation temperature. It leads to enhancement of wear resistance of the bainitic steels compared to the conventional rail steel. Finally, it clearly draws correlation between mechanical properties, wear resistance, and microstructural variation of a series of bainitic rail steels.

Rail grade dependent damage behaviour – Characteristics and damage formation hypothesis

This paper investigates the damage behaviour of several pearlitic rail steels with special focus on three specific damage mechanisms – wear, plastic flow and rolling contact fatigue (RCF, head checks in particular). Multiple tests were conducted on a full scale test-rig at voestalpine Schienen GmbH under dry contact conditions. The obtained results were compared with data from selected track tests. The test-rig is capable of producing realistic contact conditions that allow the formation of wear and RCF defects in very short time periods within 100,000 wheel passes. The test-rig results showed the improved wear and RCF resistance of high strength steels clearly. Additionally, a system wear analysis was conducted to outline the interaction of these high strength rail steels with a standard wheel steel. Although the trends are consistent on the rig and in track, the absolute values concerning wear and RCF differ due to some specific differences between track and test-rig conditions. Finally ideas are postulated that explain on one hand the test-rig specific wear behaviour of the rail grades and on the other hand the formation of periodic, rail grade dependent crack spacing of the defect type head checks.

The effect of anisotropy on crack propagation in pearlitic rail steel

One of the main sources of damage in railway components is the large plastic deformations that accumulate in the surface layer under rolling contact loading. Large irreversible deformations in components made of pearlitic steel induce anisotropy in mechanical properties of the material in the surface layer. In the present work the influence of the anisotropic layer on propagation of cracks in rail head is investigated. Based on the concept of material forces, a computational framework for simulation of propagation of planar cracks is formulated where the propagation rate is linked to a crack-driving force. An anisotropic fracture surface model is employed to capture the effect of changes in the resistance against crack propagation in different directions and depths in the surface layer. Results of simulations for cases with different characteristics in the surface layer show that the anisotropic layer has a substantial influence on the crack path.

Microstructural Evaluation of Welded Pearlitic Rail Steel

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.

Subsurface microstructural analysis and mechanical properties of pearlitic rail steels in service

Ductile materials commonly exhibit plastic deformation at and near the contact surface and their flow behaviour at large strain has a clear effect on wear resistance. These materials almost always fail while under high levels of compression, but behaviour under these conditions cannot be investigated by standard materials tests at atmospheric pressure. In this work, the characteristics of the near-surface region of one new and four used rails are examined using samples from rails which have been in service in the UK. The results are presented as stress–strain response curves from the materials under high hydrostatic compression conditions, which are typical of rail–wheel contacts. In addition, shear strain and hardness variation with depth below the worn rail surface are presented, along with micrographs of the sites examined. A thin white surface layer, frequently called white etching layer (WEL), with high hardness (up to 1040HV) was observed on the surface of one of the used rails. These results have application in the modelling of rail failure by wear and in modelling rolling contact fatigue crack initiation, and can therefore be used to improve rail maintenance planning and risk assessments.