John H. Beynon

Deterioration of rolling contact fatigue life of pearlitic rail steel due to dry-wet rolling-sliding line contact

This study is aimed at the deterioration of rolling contact fatigue (RCF) life of pearlitic rail steel, under rolling-sliding conditions, where the wet phase of the test is preceded by different numbers of dry cycles. It is shown that initial dry cycles above a critical number causes sudden and significant deterioration in RCF life. This effect has been explained using the argument of plastic strain accumulation (ratchetting) in the surface layer during the dry phase when the coefficient of friction is above 0.25. A strong correlation was found between the degree of ratchetting and the deterioration in RCF life. An empirical relationship to estimate this deterioration was concluded.

The steady state wear behaviour of pearlitic rail steel under dry rolling-sliding contact conditions

Abstract The present study is aimed at studying the onset of steady state wear behaviour of pearlitic rail steel. Wheel-rail contact is simulated by a rolling-sliding line contact. The results show that steady state wear rate prevails after a certain number of rolling-sliding cycles. The effect of strain hardening and uni-directional plastic strain accumulation on the wear behaviour has also been studied. It has been shown that the start of the steady state wear rate coincides with the cessation of plastic strain accumulation and additional strain hardening. The ratchetting failure mechanism has been employed to explain this coincidence.

Prediction of fatigue crack initiation for rolling contact fatigue

In finite element (FE) simulations of a twin disc test of a wheel/rail contact, fatigue crack initiation criteria for elastic shakedown, plastic shakedown and ratchetting material responses were evaluated for a pearlitic rail steel BS11 normal grade. The Chaboche material model for nonlinear isotropic and kinematic hardening was used in the FE simulations. The ratchetting material response results were compared with a constitutive ratchetting model, and there was good agreement with respect to the number of cycles to crack initiation and shear strain distribution below the contact surface. In addition, angles for critical planes for crack initiation were calculated for both plastic shakedown and ratchetting material responses. Results from simulations with the ratchetting model at constant contact pressures and varying friction coefficient showed asymptotic values of the friction coefficient at which crack initiation due to ratchetting will not occur.

Development of a machine for closely controlled rolling contact fatigue and wear testing

In order to gain detailed insight into the processes taking place during rolling contact fatigue (RCF) and rolling contact wear, the test machine used must be closely controllable and offer comprehensive data collection facilities. This paper describes a machine that has been developed to offer these facilities over a wide range of test conditions, and that has the facility for the early detection of cracks by an eddy current method. An existing twin-disk contact simulation machine has been comprehensively updated to enhance the range of tests possible and, most importantly, to offer close control over very short numbers of machine cycles, allowing the very early stages of the development of fatigue cracks and wear damage to be examined. Results of tests to examine the capabilities of the rebuilt machine and a study of the repeatability of results under two particular test conditions are presented.

Dry rolling-sliding wear of bainitic and pearlitic steels

Abstract The rolling-sliding dry-wear behaviour of a series of bainitic steels and a pearlitic rail steel has been investigated over a wide range of contact stress and creepage conditions. A wear-testing machine designed and built at Leicester — LEROS — allows very high contact stresses to be combined with high creepages under well-controlled conditions. Despite better standard mechanical properties, the wear resistance of the lower carbon bainitic steels was inferior to that of the pearlitic steel. A bainitic steel of the same carbon content as the pearlitic steel wore a little less, but at considerable expense to the counter-material in the wear couple. The wear resistance of the bainitic steels depends on the volume fractions of hard phases, carbide and martensite-austenite phase, for rolling-sliding as well as other types of dry-wear loading. Pearlite performs exceptionally well under rolling-sliding conditions since the lamellar microstructure is modified so as to present a greater area fraction of hard carbide plates at the wear surface, a fraction in excess of the bulk volume fraction of carbide. Recommendations are made for the application of these steels in rolling-sliding and other dry-wear circumstances.

Rolling contact fatigue of three pearlitic rail steels

Abstract The effects of varying contact pressure and creepage on the early stages of rolling contact fatigue in three pearlitic rail steels have been investigated in the laboratory under water lubricated conditions. In the test machine used, a non-contacting eddy current probe detected very small fatigue cracks and was used to stop tests before spalling could destroy the fatigue initiation sites in order that the metallography of crack initiation could be undertaken. Crack morphologies similar to those found in wheel-rail contact have been observed. Except at the highest creepages and contact stress used, there was little evidence that manganese sulphide inclusions contributed to crack initiation. In contrast, hard oxide inclusions did contribute to initiation. The head-hardened grade eutectoid steel had the best resistance to rolling contact fatigue. The naturally hard eutectoid steel was the next best, whilst the lowest strength steel gave the worst performance. Crack networks were formed in the specimens tested at nominal contact stresses of 1200 MPa and 1500 MPa. Isolated cracks were produced in the specimens tested at 1800 MPa.

Tribology of hot metal forming

Many aspects of the tribology of hot metal forming are poorly understood. This leads to difficulties when modelling the forming operations to develop process improvements. It also handicaps the development of new tool materials, since the operating conditions with which they will have to cope are not fully described. Several key issues in friction, heat transfer, lubrication, wear and fatigue are discussed. This is an area of tribology with many opportunities for research. It is concluded that progress will be made through a combination of techniques, including laboratory simulation, computer modelling and industrial trials.

Microstructure and wear resistance of pearlitic rail steels

Abstract Despite competition from bainitic and martensitic steels, pearlitic microstructures remain dominant for railway track. Techniques developed over recent years have progressively refined the interlamellar spacing to produce harder, more wear-resistant pearlitic steels. This study aims to explain the mechanisms for the wear performance by observing how the microstructure adapts to the wear loading. Four pearlitic rail steels, with similar chemical compositions but with different interlamellar spacings, have been examined. Wear tests have been performed under both pure sliding and rolling-sliding conditions, the latter designed to simulate track conditions. The worn surfaces and the plastically deformed subsurface regions have been examined by optical metallography and scanning electron microscopy. It was observed that the plastic deformation produced considerable fracturing and realignment of the hard cementite lamellae. The softer ferrite matrix was severely deformed, allowing a reduction in the interlamellar spacing on approaching the worn surface. The effect of these realignments on the surface was to present an increased area fraction of hard cementite lamellae on planes parallel to the surface. Thinner cementite lamellae, associated with low interlamellar spacings, were easier to bend before fracturing. It is believed that shear ductility plays an important role in the period of time that any particular volume of material remains at the surface before becoming a loose particle.

The early detection of rolling-sliding contact fatigue cracks

Abstract Many previous laboratory studies of the rolling-sliding contact fatigue behaviour of rail steels have been performed on twin-disc wear machines. Usually the discs have been run until spalling occurs due to extensive rolling contact fatigue (RCF) damage; hence metallurgical factors affecting crack initiation and propagation have been difficult to assess. A new wear machine is described, capable of testing large, standard cylindrical discs at high contact stresses with accurate control at low creepages. An eddy current method has been used for the detection of the initial propagation of cracks during an RCF test. The first significant crack or cracks are accurately located by a scanning, high resolution, differential eddy current probe. The method is being used to examine the relationship between rolling-sliding contact fatigue, contact stress, creepage, microstructure and surface events for a range of pearlitic rail steels.

The effect of rolling direction reversal on the wear rate and wear mechanism of pearlitic rail steel

Abstract The effect of different single and multiple rolling direction reversal (RDR) regimes on wear rate and mechanism is studied in this paper. Changes in structure deformation morphology and accumulated plastic strain are also analysed. Evidence that unidirectional rolling sliding contact can result in directional mechanical properties of the deformed layer is given. Results obtained under the test conditions used show that RDR has a beneficial effect on the wear rate of pearlitic rail steel. Multiple short RDR resulted in the lowest wear rate, less than half the unidirectional value.