Pearlitic Rail Steel Research Articles

Effect of Preheating Temperature on the Mechanical and Fracture Properties of Welded Pearlitic Rail Steels

The effect of preheating temperature on the mechanical and fracture behavior, hardness, and the microstructure of slot welded pearlitic rail steel were studied. Railhead sections with slots were preheated to 200℃, 300℃, 350℃ and 400℃ before gas metal arc filling to simulate defects repair. Another sample, welded at room temperature (RT) with no preheat, was studied in comparison. The parent rail steel has ultimate strength, yield strength and strain to failure of 1146 MPa, 717 MPa and 9.3%, respectively. Optimum values of these properties for the welded rail steels were found to be 1023 MPa, 655 MPa and 4.7%, respectively, for the 200℃ preheat temperature. On this basis, the optimum weld efficiency was found to be 89.2%. The average apparent fracture toughness KI for the parent rail was 127 MPa.m0.5, while that for the optimum welded joint (200℃ preheat) was 116.5 MPa.m0.5. In addition, the average hardness values of the weld, fusion zone, and heat affected zone (HAZ) were 313.5, 332 and 313.6 HB, respectively, while that for parent rail steel was about 360 HB. Dominance of bainite and acicular ferrite phase in the weld microstructure was observed at 200℃ preheat.

Modelling the Effect of Steady State Wheel Temperature on Rail Wear

A moving train subjects a rail cross section to many wheel passes and heat generation at the interface from bulk friction and microslip. Frictional heating at the interface causes the temperature of the contacting rail surface and wheel surface to rise. After the train has passed, the rail temperature drops to ambient till the next train arrives. Wheels, on the other hand, pick up the frictional heat input continuously and hence their steady state temperature could be much higher than that of the rail. A hot wheel provides an additional temperature rise at the rail surface. The accompanying thermal stresses and thermal softening may enhance the rail wear rate. This has been investigated in the present study. A ratchetting failure-based computer simulation has been employed to assess the wear rate of a pearlitic rail steel. It is found that the thermal stress effect on wear rate is only the modest, but thermal softening can enhance the wear rate by up to an order of magnitude for the conditions considered.

Investigation of rolling contact crack initiation in bainitic and pearlitic rail steels

ABSTRACTThe stress–strain history and the crack initiation lives of bainitic and head‐hardened pearlitic rail steels were determined under rolling contact loading by implementing the semi‐analytical Jiang–Sehitoglu rolling contact model that incorporates both ratchetting and multiaxial fatigue damage. The calculations revealed that the bainitic steel withstands higher loads than the pearlitic steel at low shear tractions, however; both materials behave in an increasingly similar manner as the shear tractions increase. Furthermore, maximum damage occurs in both steels when ratchetting and fatigue damage coincide on the surface. In addition to shedding light on the rolling contact fatigue (RCF) performance of bainitic and pearlitic rail steels, the current work also establishes a methodology for the realistic prediction of crack initiation under RCF.

Study on Corrosion Behavior of Pearlitic Rail Steel

The present study is an attempt to find out the effect of environmental corrosion on the commonly used pearlitic rail steel. The cases of corrosion of rail under Indian environment have been studied. It has been found that in marine environment both the yield strength and tensile strength decreases with increasing corrosion rate. In acidic environment yield strength increased with increasing corrosion rate, although the tensile strength decreases with increasing corrosion rate.

Effect of Large Shear Deformations on the Fracture Behavior of a Fully Pearlitic Steel

High-pressure torsion (HPT) has been used for investigating the influence of predeformation on the fracture toughness of a fully pearlitic rail steel. The use of HPT enables one to investigate changes in fracture toughness as a function of predeformation over a wide range of strain while simultaneously studying the influence of the crack plane orientation on the fracture toughness. With increasing prestrain, besides a strong increase in hardness, a pronounced anisotropy in the fracture toughness was found. Both the increase in hardness and the anisotropic fracture behavior can be attributed to the shear deformation process leading to an anisotropic composite structure on the nanometer scale.

Very early stage rolling contact fatigue crack growth in pearlitic rail steels

Abstract This paper reports on metallurgical characterisation of small cracks in pearlitic steels, using observations from both twin disc tests and rails removed from service, plus methods to determine three-dimensional (3D) aspects of such cracks. Tests to varying fractions of rolling contact fatigue (RCF) life have been assessed, based on twin disc tests where “average 100% RCF life” has been defined by eddy current detection of cracks between 200 and 800 μm in depth. Contact surface features and microstructural alterations have been mapped against near-surface strain and compared with worn rail examinations. Initial crack growth along favourably aligned and strained, prior-austenite (PA) grain boundaries, particularly if the boundary contains pro-eutectoid (PE) ferrite, was found to be restricted or halted at the PA grain limit or where the boundary altered to be less-favourably aligned. For samples with up to 55% RCF life, initial crack propagation length was still less than the average PA grain size. 3D aspects of this early growth and the deformed PA grain shapes were found to be linked to ratcheting flakes and flake cracks on test disc and rail contact surfaces. The results have shown how initial RCF crack propagation is very microstructurally influenced.

Mechanical and fracture analysis of welded pearlitic rail steels

Mechanical and fracture analysis of welded pearlitic rail steels

Sliding wear behaviour of laser surface modified pearlitic rail steel

Improving wear resistance of rails has a direct impact on the performance of rail–wheel system in railroad technology. Enhancement of sliding wear resistance at curved track, where factors such as adhesion, high slip ratios and contact fatigue act at contact patch of rail–wheel system, is particularly desirable. In the present investigation, influence of laser surface modification on sliding wear performance of a pearlitic rail steel (used in Indian railways) under two different conditions, namely, laser hardening (without any melting involved) and laser melting (with thin surface layer melting), has been studied under laboratory conditions. Before sliding wear testing, the effect of laser scanning speed on the treated layer depth has been optimised, utilising a 9 kW CO2 laser system. Sliding wear tests were carried out using a pin-on-disc device, with laser treated and untreated pearlitic rail steel discs and sliding pins made of wheel steel material, tungsten carbide (WC) and high speed steel (HSS). The tests were performed under normal prototypic loads and unlubricated conditions. Microhardness in the laser melted layer was in the range of 830–900 HV as against 890–1070 HV in the hardened layer, and was found to depend on the laser scanning speed. Sliding wear resistance of both hardened and melted layers was found to be significantly improved compared to untreated rail steel. The coefficient of friction was also marginally reduced in the laser surface melted layers.

Overview of Pearlitic Rail Steel: Accelerated Cooling, Quenching, Microstructure, and Mechanical Properties

Abstract Railway networks form an integral part of the infrastructure development of a developing country with ever-increasing passenger and freight volume. Increase in train speed, pay load, reliability, and safety are the major thrust areas for railways requiring more stringent mechanical properties such as wear, deformation resistance, and fatigue life from the railway steel. Steel chemistry control and thermomechanical processing significantly affect final properties and performance of the railway steel. For example, for a given steel composition, a number of stable or metastable microstructures can be obtained by controlling heat treatment operations. Conventional rail steels primarily contain nearly eutectoid pearlitic microstructure, which is dependent on the criticality of the application. An overview of the physical metallurgy principles involved during the manufacturing of rail steel will be provided here. The primary focus of this review is thermal processing including quenching and accelerated cooling of the rail steel. In addition, other important aspects relating to design and production of rail steel are discussed, including: impact of steel chemistry on the phase diagram, effect of thermomechanical processing on microstructure, and influence of microstructure or residual stress on mechanical properties.

Severe Plastic Deformation of Steels

In order to demonstrate the structural changes in steels during severe plastic deformation (SPD), Armco iron, a ferritic steel P800 and a pearlitic rail steel S900A are subjected to a high pressure torsion (HPT) deformation process. The microstructural evolution, the changes in microhardness and the flow stress behaviour are investigated. Depending on the main process parameters like the deformation temperature and the applied strain, the impact of alloying on the evolving microstructure is studied in detail. The results clearly show that pure metals and single phase alloys follow a relatively uniform deformation behaviour. With increasing strain the structural size decreases ending in saturation at an equivalent strain between 10 and 30. In the case of the pearlitic rail steel a continuous growth in shear stress without saturation is investigated in the range of the applied strains. The microstructures of the examined materials Armco iron and P800 contain an ultra fine grain structure whereas for alloy S900A the initially randomly oriented lamellae are heavily bent and broken into small fragments. Possible reasons for the observed differences in the deformation behaviour are discussed.

Severe Plastic Deformation of a Bainitic Rail Steel

Severe Plastic Deformation (SPD) is known to be an effective method of producing nanocrystalline materials, for instance by HPT and ECAP. These techniques are also capable of reproducing microstructures which arise naturally when high pressure and friction is involved, for example in wheel-rail contact problems. The resulting deformation layers build the origin point for fatigue cracks. For that reason the knowledge of the mechanical properties of these deformation layers are of vital importance. In the framework of this study a baintic rail steel quality was deformed by High Pressure Torsion up to distinctive equivalent strains at a nominal pressure of 6 GPa up to a final equivalent strain of 16. Afterwards the evolution of the resulting microstructure was investigated by Scanning Electron Microscopy, by microhardness measurements and X-ray diffraction. The bainitic structure showed a strong alignment and fragmentation into the shear direction with increasing strain, which was accompanied by an increase in hardness as well. X-ray diffraction measurements showed that the amount of retained austenite decreases dramatically after small amounts of strain, which indicates that retained austenite cannot be stabilized by high pressures. Torque measurements during deformation showed after strong hardening at the beginning, a saturation behaviour for higher strains, whereas for instance pearlitic rail steel qualities show further hardening.

Predicting the Effect of Grinding Corrugated Rail Surface on the Wear Behavior of Pearlitic Rail Steel

Rail grinding has become an increasingly permanent way maintenance practice to tackle rail corrugation, as well as extending the rolling contact fatigue life of rails. However, and as far as material loss is concerned, such a grinding is considered as an artificial wear process added to the natural wear. The work presented in this article investigates the quantitative effect of grinding the whole deformed rail surface layer on the overall wear process of the running surface of pearlitic rail steel. Results show that if wear behavior is known as an empirical model which can predict the effect of grinding that could be obtained.

Changes in the mechanical properties of a pearlitic steel due to large shear deformation

The fully pearlitic rail steel R260 was deformed by equal channel angular pressing at room temperature. Changes in the microstructure as well as changes in mechanical properties were determined. With increasing predeformation significant changes of the tensile strength, the fatigue crack propagation rate and the fracture toughness occurred. Due to the alignment of the cementite lamellae, also a markedly anisotropy in all these properties evolved.

Fatigue Damage Tolerance of Bainitic and Pearlitic Rail Steels

The microstructure-properties relationships, fatigue crack growth, and fracture surface morphology of J6 bainitic and premium pearlitic rail steels are studied. Specimens are cut from the middle of each railhead along the longitudinal direction of the rail using electrical discharge machining. Fatigue crack propagation tests are conducted under load control conditions using a servo hydraulic material testing system. A simple form of the fatigue power law is used to rank the fatigue crack growth kinetics of the two materials. The results show that bainitic steel has superior fatigue damage tolerance as represented by the fatigue lifetime, fatigue crack propagation kinetics, and fatigue fracture surface morphological features. Fracture surface analysis of fatigue failed specimens revealed the various mechanisms by which bainitic steel acquired its superior resistance to fatigue crack growth. Bainitic rail steel displayed more ductile fracture features such as tearing, and extensive ridge formation during the stable crack propagation process than pearlitic steel. These features are responsible for the crack deceleration and indicate a considerably high energy consuming process associated with the crack propagation of bainitic steel. Pulled-up pearlitic lamella, limited microcracks, and microvoid coalescence are found on the fracture surface of pearlitic rail steel during the stable crack process. The unstable crack propagation region of bainitic steel exhibits both large and small dimples indicative of high resistance to material separation. On the other hand, cleavage and intergranular separation are associated with the unstable crack region in pearlitic steel.

Steels: processing, structure, and performance

In this Second Edition, new information and references are integrated into chapters. Emphasis is still on processing, alloying, microstructure, deformation, fracture and properties of major steel types ranging from low-carbon sheet steels, pearlitic rail and wire steels, to quench and tempered medium- and high-carbon martensitic steels. Microstructural aspects of steelmaking, hardenability, tempering, surface hardening, and embrittlement phenomena are updated, and chapters on stainless and tool steels remain in the Second edition. The work is intended to be tutorial and is an essential state-of-the-art reference for anyone that makes, uses, studies and designs with steel.

Influential microstructural changes on rolling contact fatigue crack initiation in pearlitic rail steels

Rail life is controlled by the balance between wear and fatigue damage due to in service loading. To model and optimise rail life, knowledge of the fatigue crack initiation mechanism is required. This paper reports the effect of in service loading on microstructural changes in the subsurface layer of pearlitic rail steels and observations of early stage (10–50 μm length) fatigue crack formation. Micro and nanohardness measurements are reported, along with microstructural observations, showing differential work hardening in the proeutectoid ferrite and pearlite phases. It is proposed that the differential straining results in ductility exhaustion in the proeutectoid ferrite and therefore fatigue crack initiation and initial growth in the proeutectoid ferrite phase. Observations of short (<50 μm) cracks in rails taken out of service containing significant amounts of proeutectoid ferrite (≈20%) confirm the proposed mechanism.

Wear of conventional pearlitic and improved bainitic rail steels

A major objective in the development of new rail materials is improving their wear performance. Historically, the main method to accomplish this goal was to manufacture rail steels with higher hardness. However, there is a limit to the hardness that can be achieved with conventional pearlitic rail steels. As a result, there has been an increased focus on improving rail steel hardness with alternative microstructures, such as bainitic microstructure, which indeed can achieve higher hardness than conventional pearlitic rail steels. Unexpectedly, the newly developed harder bainitic rails exhibited worst wear performance than conventional pearlitic rails when tested in actual rail track trials. In the first part of this paper, we obtained bainitic and pearlitic rail steel samples that were tested in an actual wheel/rail track. Using these samples, detailed micro-Vickers and Rockwell C hardness measurements at different length scales were conducted to investigate the cause of their wear behavior, namely the fact that softer pearlitic steels exhibited better wear performance than the harder bainitic steels. It was found that the initially softer pearlitic rail steel was work hardened more than the initially harder bainitic rail steel as in-service stresses accumulated on the rails, and thus the better wear performance. The second part of the paper describes pure sliding laboratory experiments that were performed using both pearlitic and bainitic samples. These simpler laboratory experiments, confirmed that indeed pearlitic steel work hardens more with tribological contact testing and exhibits less wear compared to J6 bainitic steel, supporting the rail track findings. It is therefore important to consider the in-service work microhardening behavior of rail steels as the initial hardness cannot reliably predict rail wear and rail life.

Mechanical and fracture behavior of bainitic rail steel

Microstructural analysis of J6 bainitic rail steel was performed and the results compared with those for the microstructure of premium pearlitic rail steel. The mechanical properties, fracture behavior and fracture toughness of the J6 bainitic steel were evaluated and compared with those for a premium pearlitic rail steel. The microstructural origin of the strength and toughness of the bainitic steel was identified. It was found that bainitic steel has superior mechanical properties and fracture toughness in comparison with those of pearlitic rail steel. The failure mechanisms of bainitic steel were also identified.

Fracture behavior of bainitic and pearlitic rail steel webs

The failure of railway rail occurs mainly due to processes such as: detail fracture, shelling, and transverse fissures [1]. Plastic deformation of the railhead due to the heavy rolling loads transmitted through the wheel/rail contact surface is the main reason for the formation of detail fracture [2]. Such a failure process is visible, and catastrophic rail failure can be prevented by regular examination of the top surface of the railhead. The fracture behavior of railheads has been studied for a long time and some significant results have been obtained [3–8]. The study of the failure of rail webs is less reported. In webs, defect accumulation may occur under cyclic service loadings. Once the crack size reaches the sub-critical dimension, piping propagation along the vertical direction of the web can be accelerated, while the cyclic vertical tension loads can cause horizontal fissures of the web. Considering the complexity of the failure modes related to the cyclic loadings of rail tracks, it is necessary to study the fracture behavior of rail webs under a simplified loading profile first. Recently an improved bainitic steel for heavy load application has been studied [9–12]. It has some significant benefits over the existing pearlitic rail steels. Bainitic steels derive their strength from ultra-fine structures with a lot of dislocations which are harmless but confer high strength [13]. In contrast, pearlitic steels obtain their strength from the fine grains of pearlite. However, there is a limit to the production of very fine grains in the manufacturing and post-heat treatment processes. In the present work, the fracture behavior of specimens cut from bainitic and pearlitic steel rail webs was studied. An empirical model of mode I stress intensity factor was used to evaluate the plane strain fracture toughness of the two materials. Rail web specimens from the two materials, with simulated cracks emanating from a trough thickness circular defect, were prepared by electric discharge machining to simulate horizontal fissure of rail webs. Static tensile tests were performed under displacement control to study the overloading behavior and obtain the residual strength for the plane strain fracture toughness calculation based on this geometry. The rails used in the present work were new (have not been in service) and were supplied by the Transporta-

Computer simulation of strain accumulation and hardening for pearlitic rail steel undergoing repeated contact

This paper presents a validated model of plastic strain accumulation in railway rail steel under repeated wheel-rail contact. Such contacts subject the rails to severe stresses, taking the material local to the contact beyond yield, and leading to the incremental accumulation of plastic deformation (ratheting) as wheels pass. This process is at the root of several rail wear and rolling contact fatigue crack growth mechanisms. Existing plasticity models are inadequate for modelling the strain accumulation taking place in this material, which is under high hydrostatic compression (of the order of 1 GPa) and is severely anisotropic. The model described here is based on a ratcheting law derived from small-scale twin-disc rolling-sliding contact experiments and simulates tens of thousands of ratcheting cycles and the corresponding strain hardening in a few minutes on a personal computer. Results indicate that, to model these processes successfully, and to represent correctly the high levels of ductility seen in rail steels under compressive load, stress-strain data generated under high hydrostatic compression are required.