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Carbo-austempered steels with a nanobainitic microstructure in the case are established as an exciting alternative in applications demanding high stability and reliability against wear and fatigue. In this work, the effect of heat treatment conditions, i.e. austenitizing and isothermal transformation temperatures, on the wear behavior under rolling/sliding conditions of a carburized cast steel with a nanobainitic structure in the surface was examined. The results show that wear damage is mainly due to contact fatigue, with a small extent of oxidative wear, abrasion and adhesion. The results were compared with the conventional tempered martensite plus retained austenite microstructure obtained by means of carburizing followed by quenching and tempering. The nanobainitic microstructures show a better wear resistance than the quenched and tempered microstructures. Also, the specific wear rate in carbo-austempered steels was lower as the isothermal heat treatment temperature decreased. Results are explained in terms of the microstructural differences, hardening behavior and plasticity of the materials under study.

This study aims to describe the wear and rolling contact fatigue (RCF) behavior of microalloyed forged railway wheel steel Class D (7NbMo), with microstructures of upper bainite (UB - 350 HV 0.5), lower bainite (LB - 450 HV 0.5), and pearlite (PE - 350 HV 0.5), against 7C steel with a tempered martensite microstructure (660 HV 0.5) in a twin-disc tribometer. The results showed that bainite (LB and UB) exhibited greater wear resistance compared to pearlite (PE). The primary wear mechanism for all three microstructures was the fatigue wear. An analysis of the PE disc revealed continuous (single-layer) RCF cracks, which were relatively large and deep. In contrast, UB and LB cracks were discontinuous (multiple layers), thin, and smaller but more numerous (higher crack density). These characteristics indicated that bainitic microstructures also had greater resistance to RCF than pearlite did. Therefore, the combination of mechanical properties and the morphological arrangement of the microstructure led bainite to develop a shallower depth of deformed layer with a higher capacity for plastic deformation per unit volume. The greater wear and RCF performance of bainite compared to those of pearlite may be associated with how they release the accumulated deformation energy after reaching saturation in relation to volume deformation absorption: regarding bainite, the material releases energy by opening crack surfaces; conversely, regarding pearlite, it is through the detachment of large and wide cracks. [Display omitted] • Bainites exhibit superior wear resistance/RCF compared to pearlite in rolling and sliding. • Surfaces of bainitic discs UB and LB display mild peeling with reduced spacing. • Bainite possesses a greater capacity to absorb plastic deformation per volume. • Tribological performance may relate to the microstructural morphology's deformation energy release. • Bainites release energy through crack surface opening. [ABSTRACT FROM AUTHOR]

A new twin-disc test was performed using discs containing all microstructures of the heat-affected zone (HAZ) of flash-butt welded rails. It was observed that the lower hardness regions (spheroidized cementite in a ferritic matrix) at the HAZ boundary underwent a local deformation, generating valleys and reproducing a common defect type observed in rail welds: double squat-like defect. Longer crack lengths or crack clusters were observed in the HAZ boundary and central weld regions. The cross-section metallography showed that these cracks are directly related to the microstructure of spheroidized cementite at the HAZ edges or pro-eutectoid ferrite at the central region. The decrease in surface roughness before the cracks, the presence of oxide on the bottom crack surface, and superior spheroidized cementite microstructure in the upper crack region suggest that these larger/cluster cracks are predominated by extrusion of the softer material above the harder material (pearlitic/base metal). • Macro-topography analysis revealed increased deformation in the softer HAZ region. • A more significant crack in the weld center was linked to pro-eutectoid ferrite. • Specimens displayed a more noticeable crack precisely within the softer HAZ region. • These larger cracks aligned with deformation valleys and the central weld. [ABSTRACT FROM AUTHOR]

• A ratcheting mechanism-based numerical model was applied to predict damage initiation in twin-disc tests of premium rail steels. • Wear- and RCF-dominated damages are distinguished by the depth of the ratcheting mechanism-based damage initiation locations. • A higher creepage results in a shallower damage initiation location and a shorter damage initiation life. • The initial wear rate increases with a higher creepage for wear-dominated damage. • HE2 rail steel presents the longest damage initiation life and the lowest initial wear rate compared to HE1 and LAHT. This work presents a ratcheting mechanism-based numerical model to study the initiation location and initiation life of rolling contact fatigue (RCF) or wear damage and the initial wear rate of premium rail steels under the laboratory twin-disc test conditions. Twin-disc tests are widely used in the studies of rail steels due to their ability to reproduce critical aspects of full-scale wheel-on-rail contact under controlled conditions and a relatively short test period compared to field tests. This study introduces a computational model to simulate the cyclic rolling contact for three premium rail steels (HE1, HE2, and LAHT) against wheel steel AAR Class-C under twin-disc test conditions. The cyclic rolling contact is achieved by repeatedly moving a non-Hertzian distribution of contact pressure and the calculated longitudinal surface traction upon a segment of the circumferential surface of the lower disc until a stabilized maximum ratcheting strain rate is reached. The RCF or wear damage initiation location is determined by the location showing the stabilized maximum ratcheting strain rate, and the damage initiation life is estimated by applying the ratcheting failure mechanism. Wear and RCF damages are distinguished by the depth of the damage initiation location. Wear damage is dominant when the location is near the surface. Otherwise, the damage will be RCF-dominated. The initial wear rate due to wear-dominated damage is estimated by identifying a critical profile and affected volume of worn material within the ratcheting strain rate field. The predicted initial wear rate is compared with the experimental result of the examined contact pair. This research can assist in a more profound understanding of the experiment results of a twin-disc test and provide a numerical basis for an experimental twin-disc test design. Furthermore, it may provide significant references for choosing rail steels. [ABSTRACT FROM AUTHOR]

An insulated rail joint is a component used to join two abutting rails while keeping them electrically separated from one another. This allows for the construction of track circuits and train detection within signalling systems. Electrical failure of the joints can be caused by plastic flow of the rail steel over the insulating gap, known as lipping. In this paper, this failure mode has been experimentally modelled using twin-disc testing and indicative conclusions have been formed. It has been found in this testing that the thickness of the endpost does not have an effect on the rate of lipping, however, the endpost and rail material do have an effect. An endpost with a higher compressive strength will perform better and tougher/harder rail steel will also improve performance. The application of a laser clad layer of tougher material on the running surface, however, gave the greatest resistance to lipping. [ABSTRACT FROM AUTHOR]

The microstructure evolution on the rolling surface of GCr15 steel subjected to rolling-sliding and pure rolling contact loading was systematically investigated. Experimental results showed that the pearlite structure of the surface layer in the rolling-sliding sample transformed into nanocrystalline α-Fe–C alloy in which cementite underwent severe decomposition while the pearlite lamellae appeared unperturbed in the pure rolling sample. A white etching layer (WEL) was also detected in the surface of the rolling-sliding sample. The WEL formation was found to be due to cyclic shear plastic deformation instead of frictional heating. A surface layer of Fe 3 O 4 was detected in the pure rolling samples. Microhardness depth profiles of the rolling-sliding and pure rolling samples also showed different trends. [ABSTRACT FROM AUTHOR]

Railway transport companies in Brazil and worldwide have become more productive and efficient by increasing the weight transported and using faster locomotives. These actions have caused the premature exchange of wheels and rails due to excessive wear and rapid growth of rolling contact fatigue (RCF) cracks, and both consequences have been minimized by the improvement of pearlitic steels of wheels and rails. However, these pearlitic materials have already shown that there is a limit to the optimization of their mechanical properties. The bainitic microstructure could replace the pearlitic one, however few studies on the bainitic microstructure variants have been carried out, which still makes its behavior in rolling and sliding wear – especially in high carbon microalloyed steels – a matter of debate. Those studies are far from reaching a consensus, and, therefore, an important question arises regarding the materials used in the wheel-rail contact: which microstructure has better tribological and fatigue properties being produced from the same steel, having the same hardness, and being influenced by the same tribological variables: bainite or pearlite? In order to answer this question, twin-disc tests were performed on forged railway wheel microalloyed steel (7NbMo) with pearlitic and bainitic microstructures with the same bulk hardness and on high carbon steel (7C) with tempered martensite. The results revealed that the microstructure had a significant role in wear and RCF performance. The bainitic microalloyed steel (7NbMo–B) showed lower wear rates and greater resistance to RCF than the pearlitic one (7NbMo–P). Similar behavior was observed for their counter-bodies, indicating that the use of bainite in rolling and sliding wear applications would cause the joint benefit of the body and counter-body. • The bainitic microalloyed steel showed lower wear rates and greater resistance to RCF than the pearlitic one. • The best wear performance by the bainitic disc is because of its greater capacity to absorb energy with less volume. • The bainite cracks made a tortuous propagation path due to the interlocking caused by cementite carbides. • The ratchetting was the main mechanism of wear for both microstructures. [ABSTRACT FROM AUTHOR]

Abstract: Rolling contact fatigue and wear damage of the surfaces of tribological components, such as roller bearings, gears, train wheels and rails, can be prevented by surface coating. Laser cladding is a welding process used for coating of surfaces. In this investigation, the laser cladding process and consecutive surface grinding of a twin-disc test specimen were simulated numerically using the finite element (FE) method. The disc was made of the pearlitic UIC grade 900A (R260) material and coated with a Co–Cr alloy. A non-linear isotropic and kinematic hardening model was used to simulate the material characteristics, and annealing material behaviour and phase transformation in the substrate material were also accounted for. Residual stress measurements were carried out for validation of the results from the FE simulations and there was good agreement. In addition, the influence of residual stresses on the safety margin against fatigue failure in a shakedown diagram was examined. It was carried out by comparison of two FE simulations of a twin-disc test: one simulation where the residual stresses from the surface treatment processes were incorporated, and one which was initially free from stresses. The multiaxial fatigue initiation criterion proposed by Dang Van was used in the fatigue evaluation. The results showed that the residual stresses caused by the surface treatment processes reduced the safety margin against fatigue failure in comparison with the initially stress-free FE simulation. [Copyright &y& Elsevier]

Brazilian heavy-haul railway companies have drawn extensive research in the last years for developing solutions that can make the operation in this area more efficient and safer, such as friction management, the use of new mechanical assemblies, and the use of new materials for wheels and rails. Another solution implemented is increasing the weight transported by train wagons; however, the more weight added, the more deterioration caused to wheels and rails, for example, promoting higher maintenance costs and risks to safety. Thus, the key to extending the life cycle of railroad wheels is to reduce the wear and rolling contact fatigue (RCF) in developing new materials and in manufacturing them. Niobium (Nb) and molybdenum (Mo) are usually added to increase the mechanical strength of pearlitic steels by decreasing the interlayer spacing. This way, comparative twin-disc test for wheel material under dry conditions was performed to verify the wear resistance of a commercial railway wheel (7C) and a newly developed class D railway wheel steel (7 M) with Nb and Mo addition. Following the specifications of the Association of American Railroads (AAR) standard, a twin-disc tribometer with automatic control of load and speed was used. The slip ratio was obtained from the difference between the axis rotation. 7 M steel was observed to present lower mass loss compared with 7C steel. The Magnetic Barkhausen noise analysis showed higher residual stress close to the surface for 7 M steel, which correlated with the work-hardened depth. Such results, therefore, indicate that 7 M steel presented better performance than 7C steel regarding the specific characteristics of the tests. To confirm the feasibility of the wheel material for use in service, further twin-disc tests are proposed for both wheel materials (7 M & 7C) against the same rail material. • Microalloyed class D wheel present lower wear than class C wheel. • The ratcheting was the main mechanism of wear for both microstructures. • There was a competition between wear and rolling contact fatigue. • Magnetic Barkhousen noise indicated that class D wheel disc present higher residual stress closer to the surface. [ABSTRACT FROM AUTHOR]