Contact Fatigue Resistance Research Articles

Effect of Processing Techniques on Microstructure and Mechanical Properties of Carbide-free Bainitic Rail Steels

Rail manufacture processing techniques, such as roller straightening, tempering and air-forced quenching, have shown to make a great influence on the microstructure and mechanical properties of rail steels. In this study, five carbide-free bainitic rail steels with same chemical composition were developed from the above rail processing routes in an industrial production line. In general, the carbide-free bainitic steels consisted of bainitic ferrite (BF) plates with distinct morphologies of retained austenite (RA) and minor occurrence of martensite in some conditions. Electron microscopy confirmed the orientation relationship between BF and RA was in line with the Nishiyama-Wassermann (N-W) orientation relationship, i.e., {111}γ//{110}α,<112>γ//<110>α. The roller straightening and tempering processes could facilitate the transformation of unstable RA. The small volume fraction of stable RA contributed to an obvious improvement of yield strength, hardness and impact toughness, but at the cost of ductility and strain hardening ability. In addition, the in-line heat treatment resulted in the refinement of bainitic ferrite plates, leading to an increase in the tensile strength of carbide-free bainitic steels. In summary, there was a clear positive relationship between the rail processing techniques and the mechanical properties of the carbide-free bainitic steels. This study can benefit the development of new bainitic steels with a combination of wear and rolling contact fatigue resistance by appropriate selection of processing techniques.

Effect of Gas Nitriding on Interface Adhesion and Surface Damage of CL60 Railway Wheels under Rolling Contact Conditions

The influence of surface gas nitriding on wheel/rail rolling contact fatigue and wear behavior of CL60 wheel was studied on a new rolling contact fatigue/wear tester (JD-DRCF/M). The failure mechanisms of the wheel/rail surface after the gas nitriding and without gas nitriding on the wheel surface were compared and analyzed. The results show that the wheel with gas nitriding could form a dense and hard white bright layer which was approximately 25 μm thick and a diffusion layer which was approximately 70 μm thick on the wheel surface. Thus, the gas nitriding on the railway wheel not only significantly improved the wear resistance on the surface of the wheel, but also effectively reduced the wear of the rail; the results show that the material loss reduced by 58.05% and 10.77%, respectively. After the wheel surface was subjected to gas nitriding, the adhesive coefficient between the wheel/rail was reduced by 11.7% in dry conditions, and was reduced by 18.4% in water media, but even so, the wheel with gas nitriding still could keep a satisfactory adhesive coefficient between the wheel/rail systems, which can prevent the occurrence of phenomena such as wheel-slip. In short, the gas nitriding on the wheel surface can effectively reduce the wear, and improve the rolling contact fatigue resistance of the wheel/rail system. This study enlarges the application field of gas nitriding and provides a new method for the surface protection of railway wheels in heavy-duty transportation.

Role of Nb in the failure of dual-phase steel in heterogeneous welds

The use of Cr-Ni austenitic steel interlayers is an effective solution for heterogeneous welds, especially in combination with high-carbon steels. High contact-fatigue resistance and resistance to impact loads are required in the case of railway crossing applications. The failure of Nb-alloyed cast Cr-Ni austenitic steel used as an insert in the flash butt resistance weld of Hadfield steel with carbon steel is the subject of the presented analyses. The identification of the failure mechanism is based on light and scanning electron microscopy and energy dispersive X-ray analyses, including electron backscattered analyses. The initiation of hot-crack defects is detected in the heat affected zone (HAZ). In contrast to the usual conditions for the formation of hot cracks in the HAZ, cracks are found solely outside the superheating zone. The decisive influence of Nb-based intermetallics formed at the phase interface of δ-ferrite and the austenitic matrix is indicated. The eutectics formation between niobium carbonitrides and the austenitic matrix is assumed to be responsible for the hot-cracking susceptibility of austenitic steel. The particular combination of the temperature gradient together with the chemical gradient leading to a localization of the critical zone is documented. The paper describes the influence of carbon redistribution at the weld interface on the morphology of delta-ferrite in different layers of the heat-affected zone. Induced structural differences are associated with Nb-particle formation. Fractographic analyses show the formation of low-melting phases in the critical layer of the HAZ. The influence of secondary phases, especially Nb-eutectic, on the degradation of austenitic steel plasticity is documented.

Study on the wear and damage behaviors of hypereutectoid rail steel in low temperature environment

In this study, the wear and damage behaviors of three heavy haul rail steels (including one hypereutectoid rail steel and two eutectoid rail steels) were investigated at room (around 20 °C) and low (−40 °C) temperatures using a rolling-sliding wear test machine. The results indicated that, compared with eutectoid steels, the hypereutectoid rail steel had a better wear resistance and a better rolling contact fatigue (RCF) resistance at the room temperature, and better wear resistance but worse RCF resistance at the low temperature. Compared with the room temperature, at the low temperature both the hypereutectoid and eutectoid rails had better wear resistance, the RCF resistance of eutectoid steels was improved but the RCF resistance of hypereutectoid rail was reduced. At the room temperature, the worn surfaces of both wheel and rail materials were dominated by surface cracks. On the contrary, at the low temperature, the worn surfaces of wheel and rail materials were dominated by third body layers (TBLs) and oxidized spots, respectively.

Effect of spherical and ballast dents on rolling contact fatigue of rail materials

Surface dents on railroad rails may be caused, for example, by the drop of freight goods or the impact of ballast rocks and may have a deep effect on the rail service life. To better understand this problem, this investigation aims at exploring the rolling contact fatigue behavior of the rail materials (as, for example, PD3 steel) after being impacted by two kinds of hard bodies: a simple spherical steel ball and a more realistic ballast rock. The results show that the residual tensile stress, which is distributed on the shoulders of the dent, decreases the rolling contact fatigue (RCF) resistance of the material around the dent. Moreover, the unbalanced wear behavior between the two shoulders of the dent makes the situation even worse on the side of the dent where the tangential force acts firstly. Finally, due to the higher martensite hardness with respect to the matrix material, the crack propagates faster around the dent in presence of white etching layer on the bottom of the dent itself.

Cyclic contact fatigue resistance of ceramics for monolithic and multilayer dental restorations

ObjectiveTo evaluate the cyclic contact fatigue resistance and failure mode of ceramics for monolithic and multilayer restorations. MethodsCeramic structures (10 mm × 1.8 mm) were fabricated as follows (n = 28): (1) CAD-on- trilayer structure composed of Y-TZP (yttria stabilized tetragonal zirconia polycrystal- IPS e.max ZirCAD) infrastructure, fusion glass–ceramic (IPS e.max CAD Crystall/Connect) and lithium disilicate-based glass–ceramic (IPS e.max CAD); (2) ZFC- bilayer structure composed of Y-TZP infrastructure veneered by a fluorapatite glass–ceramic (IPS e.max Ceram); (3) LDC- monolithic lithium-disilicate glass–ceramic (IPS e.max CAD); and (4) YZW- monolithic Y-TZP (Zenostar Zr Translucent). All ceramics structures were bonded to a dentin analog substrate (G10). Specimens were submitted to cyclic contact fatigue test in a pneumatic cycling machine with 80 N load and 2 Hz frequency in distilled water at 37 °C. Test was interrupted after 104, 105, 5 × 105 and 106 cycles and the presence or absence of failure was recorded. Fatigue data were analyzed using Kaplan–Meier (log rank) and Holm–Sidak tests (α = 0.05). The relationship between the type of crack leading to failure and the experimental group was analyzed using chi-square test (α = 0.05). ResultsThere was no statistical difference between CAD-on and YZW groups (p = 0.516), which presented the highest survival rates after cyclic loading, followed by ZFC and LDC groups (p < 0.01). There was a significant relationship between type of crack and experimental group (p < 0.001). LDC specimens showed the greatest frequency of radial cracks, while cone cracks were more prevalent for ZFC and CAD-on specimens. SignificanceMonolithic Y-TZP (YZW) showed similar fatigue resistance to CAD-on multilayer specimens, but different failure mode. Monolithic lithium disilicate glass–ceramic (LDC) and Y-TZP conventionally veneered by glass–ceramic (ZFC) showed lower survival time under fatigue.

Assessment of modern rolling stock pulling drive gear reliability

This paper presents an issue of rolling stock pulling drive gear (PDG) reliability assessment in operation, considering the gear and pitch wheel wear. Among main damages attributive to gear and pitch wheel reject are cheeping of teeth, peeling, plastic deformation. It is noted that the PDG reliability assessment shall be reasonably carried out upon a significant probability of their fail-safe operation that is determined by the rolling contact fatigue resistance. The provided mathematical study design on the PDG reliability assessment of ER-2 electrical multiple train and the corresponding mathematical model PH.=f(Δ1, Δ2) is developed. The auxiliary curve is added to the reasonable selection of gear-pitch wheel pair. The provided materials allow to reasonably select gear-pitch wheel pairs obtaining the maximum pulling drive gear fail-safe operation probability values that result in the reliability increase. Recommendations on the further application of the suggested approach during repair of pulling drive gears of modern rolling stock are provided.

Effects of carbon content and hardness on rolling contact fatigue resistance in heavily loaded pearlitic rail steels

To reproduce the rolling contact fatigue (RCF) damage of rail steel used in heavy haul railways and to clarify its possible mechanism, the RCF characteristics of pearlitic steel with different carbon content and initial hardness values are evaluated using a two-disk-type machine. The results indicate that the number of spalls and the crack depth decrease with increasing carbon content of pearlitic steel even when the initial hardness is almost the same level. Furthermore, an increase in the carbon content increases the hardness of the rolling contact surface, resulting in the suppression of the plastic flow development. Results also indicate that the crack inclination angle varies depending on the carbon content. According to these results, a possible mechanism for the suppression of fatigue crack propagation in higher carbon pearlitic steel is proposed on the basis of the fracture mechanics approach.

Friction modifiers effects on tribological behaviour of bainitic rail steels

Bainitic steels are gaining popularity for application in the railway switches and crossing, thanks to their better rolling contact fatigue (RCF) and wear resistance. The rail degradation caused by RCF and wear could be also reduced by the use of friction modifiers (FM) through their ability to reduce the lateral load during wheel passage. This paper presents the microstructure and the mechanical properties of newly designed bainitic steel grades; B1400 + and Cr–B potential candidates for switches and turnouts railroad industry. It investigates their friction and wear performances using the HOrizontal twin DIsk Machine. The tribological behavior is evaluated under dry and lubricated conditions in combination with three commercial friction modifiers. Results show a better wear resistance of Cr–B steel in case of abrasive mechanism that dominates the dry experiments. This work defines the efficiency of FM as a low and stable friction coefficient concomitant to a low wear rate. It appears that a good tribological performance is untimely linked to the chemical composition of the friction modifier. In case of lubricated contact, cross-sectioning of wear scare confirms the generation of an interfacial layers. These layers have an impact on the wear mechanism and debris detachment susceptibilities of bainitic steels.

Enhancement of rolling contact fatigue performance of ferrite–pearlite steel with laser shock peening

The laser shock peening effect and its influence on rolling contact fatigue of ferrite–pearlite steel were investigated experimentally. Microhardness on the steel surface is increased after laser shock treatment. The depth of surface layer affected by laser shock hardening is above 1 mm. The maximum residual compressive stress generated on the specimen surface is around 1 GPa. The twin-disk test showed that laser shock-treated steel disk presents better rolling contact fatigue resistance than untreated one. Laser shock-induced strain hardening and compressive residual stress alter crack inclination angle and propagation behavior, which results in a longer fatigue life.

Development of and Perspective on High-Performance Nanostructured Bainitic Bearing Steel

Abstract Bearings are the most important component of nearly all mechanical equipment, as they guarantee the steady running of the equipment, which is especially important for high-end equipment such as high-speed trains and shield tunneling machines. Requirements regarding the quality of bearings are increasing with the rapid development in technology. A country’s bearings manufacturing level directly reflects the level of that country’s steel metallurgy and machinery manufacturing. The performance of the bearing steel is the critical factor that determines the quality of a bearing. The development of new bearing steel with higher performance is the ambition of material researchers and the expectation of the manufacturing industry. Many famous bearing manufacturing enterprises are competing to develop the new generation of bearing steel. Nanostructured bainitic bearing steel (NBBS), which is a newly developed bearing steel, not only possesses high strength and toughness, but also exhibits excellent wear resistance and rolling contact fatigue (RCF) resistance. In recent years, relevant achievements in NBBS in China have led to significant progress in this field. NBBS was first used in China to manufacture large bearings for wind turbines and heavy-duty bearings, with excellent performance. As a result, NBBS and its corresponding heat-treatment process have been included in the national and industry standards for the first time. The bearing industry considers the exploitation of NBBS to be epoch-making, and has termed this kind of bearing as the second generation of bainitic bearing. In this paper, the development of NBBS is reviewed in detail, including its advantages and disadvantages. Further research directions for NBBS are also proposed.

Scuffing and rolling contact fatigue resistance of discrete laser spot hardened austempered ductile iron

This study determined the influence of discrete laser spot hardening on the scuffing wear and rolling contact fatigue resistance of Cu-Ni austempered ductile iron (ADI) specimens. The frequency of the single laser pulses were such so as to produce patterns of separated spots, adjacent spots or overlapping spots.Lubricated sliding wear tests were carried out to determine the behaviour of ADI under starved lubrication conditions, usually occurring during the start-up of gears. ADI specimens hardened with adjacent and separated laser spots displayed higher resistance to scuffing than the surfaces treated with overlapping laser spots and the control samples (those tested in the as-austempered condition). Results obtained also show that laser hardening increased the average rolling contact fatigue life over the as-austempered ductile iron specimens. This improved performance was attributed to the higher volume of martensite formed and hence higher surface hardness, and also the residual compressive stresses induced in the laser-treated surfaces. This benefit outweighed the negative effect of the slight roughness induced by the laser treatment.

EFFECT OF LASER QUENCHING ON MICROSTRUCTURE AND PROPERTIES OF THE SURFACE OF TRACK MATERIALS

With the rapid development of rail transit, the wear problem of track has become a serious challenge. Laser quenching is a new surface modification process, and it is expected to improve the service life of the track. Laser quenching was carried on the surface of rails by using 6[Formula: see text]kW fiber laser. The microstructure, hardness, wear resistance, and contact fatigue resistance (CFR) of the phase transformation hardening layer were studied by optical microscope (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), microhardness tester, and wear tester. The results show that the microstructure of the hardened layer is composed of fine martensite and a small fraction of retained austenite. The microstructure of substrate is pearlite and ferrite. The hardness of rails quenched by the laser increased nearly two times than that of the un-treated track materials. When the laser power is 1000[Formula: see text]W, the depth of hardened layer reaches 0.8[Formula: see text]mm, and the wear resistance of hardened layer increases 167% than that of the substrate. The number, length, and depth of fatigue cracks decreased on the cross-section of worn rails after laser quenching, which indicates that laser quenching can improve the CFR of rails.

Influence of the axial loads of rolling stock on the contact-fatigue life of rails

Analysis of operational data for defective and highly defective rails showed that up to 25 % is the contact-fatigue defects. In connection with the development of heavy haul traffic on the Russian railways, it is relevant to determine the influence of cars with increased axial loads of 25 and 27 tf on the contact fatigue life of rails. The solution of this problem is set forth in this article. The Brown-Miller model of multi-axial fatigue was used in the calculation. This model is integrated into the Fatigue software system, which is tied to the Marc calculation system through Pat-ran. Since under operating conditions the wheel moves (rolls) along the rail on meandering trajectory, in computer modeling weight coefficients were taken into account that characterize the percentage of wheels in the cross-sectional areas of the rail. Calculations of contact fatigue life took into account the variability of vertical loads from the impact on the track of trains formed from innovative open cars with axial loads of 23.5, 25 and 27 tf under operating conditions, loaded with real loading blocks. According to the analysis of calculated data with an increase in axial loads from 23.5 to 25 tf, it is necessary to expect a decrease in the service life of rails in contact fatigue resistance by 19 %, with a further increase in axle loads of up to 27 tf per 32 %. Considering that the share of freight cars with axial loads of 25 tf does not exceed 15...20 %, then on the routes of its use the service life of rails should be expected to decrease by 3...4 %. The method proposed by the authors for predicting the contact fatigue life of rails with increasing axial loads is advisable to improve in part of the experimental determination of the fatigue and strength characteristics of rail steel from the degree of hardening of the rolling surface, its probabilistic properties and the use of the integral distribution law for vertical forces, taking into account the structure of the freight traffic passing through the section. The work was carried out according to the RFBR project 17-20 01088.

Surface Hardening Vs. Surface Embrittlement in Carburizing of Porous Steels

Abstract Carburizing increases the contact fatigue resistance of sintered steels, but the surface hardening may result the formation of surface brittle cracks due to the combined effect of high hardness and porosity. The effect of carburizing on the embrittlement of the case of a 7.3 g/cm3 1.5%Mo - 0.25%C sintered steel was studied. The phenomenon was analyzed theoretically and verified by experiments. The resistance of the carburized steel to surface brittle cracking increases with the load bearing surface and the decrease of the maximum pore size, of the surface microhardness and the friction coefficient. The theoretical analysis was implemented in a design procedure for parts subject to contact stresses.

Microstructural evolution and performance change of a carburized nanostructured bainitic bearing steel during rolling contact fatigue process

The nanostructured bainitic (NB) bearing steel exhibits excellent rolling contact fatigue (RCF) resistance, which is critical to the service of bearing. However, the detailed microstructural evolution of the steel during the RCF process is still unclear. In this paper, a carburized NB bearing steel was studied and a detailed process investigation on microstructural evolution of the steel during the RCF progress was carried out. Results show that the nano-sized bainitic ferrite is gradually refined, and the undissolved carbides gradually flake away and some of them remain in solution during the RCF process. It is interesting to find that there are three stages for the martensitic transformation of retained austenite. During the first stage of 0–0.48 × 107 cycles, nearly all of the block-like retained austenite in the top surface transforms into martensite rapidly, and during the second stage of about 0.48–1.92 × 107 cycles, nearly no retained austenite further transforms into martensite, but when the fatigue exceeds the 1.92 × 107 cycles, the film-like retained austenite gradually transforms into martensite at a slow rate. The transformed martensite not only increases the surface hardness but further brings additional residual compressive stress in surface layer. It is analyzed from the microstructure characterization along the longitudinal section of the fatigue specimen that the fatigue crakes are initiated at subsurface of specimen, which propagate up to the surface and finally result in fatigue flake.

Study on the Induction Quenching Process and Fatigue Testing for Cr-Mo Steel Ball Screw

Due to the deficiencies such as shallow hardened layer and short fatigue life of the ball screws treated by the traditional induction quenching process, a new process has been developed. The new induction quenching process has four-turn induction coil. The power of the this process is 150-160kw, feeding speed 270mm/min and the spraying cooling water distance is 35mm. It has been successfully used on the Cr-Mo steel ball screw with a diameter of 80 mm. The microstructure and hardness distribution of the induction hardened layers were investigated by a metallographic microscope and a micro-hardness apparatus. The fatigue life of the ball screw was measured through a constant stress accelerated fatigue test. The degree of fatigue wear on the raceway of the ball screw was analyzed by a field emission scanning electron microscopy (FE-SEM). The experimental results showed that the microstructure and hardness distribution of the hardened layer of the Cr-Mo steel ball screw could satisfy the design requirements of contact fatigue resistance. The fatigue life was longer than the theoretical service life and there was no obvious fatigue wear on the raceway after the fatigue testing. It shows that the safety and reliability of the Cr-Mo steel ball screw fabricated through the new induction quenching process could be guaranteed in practical application.

Rolling contact fatigue resistance of austempered ductile iron processed at various austempering holding times

The rolling contact fatigue resistance of austempered ductile iron (ADI) produced at various holding times was evaluated by using a twin-disk rolling configuration rig. The resulting fatigue life of ADI samples was compared with that of conventional quenched and tempered (Q&T) ductile iron samples with the same chemical composition. It was found that the macro hardness of ADI samples decreased and the fatigue life became shorter for longer holding time. For similar macro hardness, ADI samples had higher fatigue resistance as compared with Q&T ductile iron samples. Optical microscopy and scanning electron microscopy indicated that the graphite nodules acted as crack nucleation sites for the ductile iron. The cracks propagated towards either the surface or adjacent graphite nodules until the formation of pits or spalls occurred. The excellent fatigue resistance of ADI samples could be attributed to the increase of surface hardness which was caused by the strain induced transformation of retained austenite into martensite. By using X-ray diffraction, the difference of retained austenite present in the wear track and off the wear track ranged from 4.42% to 26.15%.

Case study: The effect of running distance on the microstructure and properties of railroad axle bearings

As a key component, axle bearings play an important role in transmission systems. Motivated by the rapid growth of railway network and the significance of bearing safety, we report here how different operating duration may alter the macroscopic and microscopic mechanical property of high speed train axle bearing's outer ring made of GCr15 steel, a high-carbon chromium bearing steel known for its superb contact fatigue resistance, good dimensional stability and excellent corrosion resistance. We characterized the structural and mechanical properties of the load carrying zone at operating duration of new, 1.2 Mkm and 2.4 Mkm bearing. In contact to the as-received ones, the outer ring samples show clear grain refinement, exhibit inhomogeneous hardness distribution and show a decrease of about 42% for the tensile elongation after 2.4 Mkm operation.

Shot peening of austempered ductile iron gears

ABSTRACTThis study investigates the suitability of shot peening as a surface treatment for austempered ductile iron (ADI) gears to lengthen their service life. Several studies have shown that shot peening prolongs the bending fatigue life of components. However, from the few works conducted on the tribological behaviour of ADI, it is not clear whether an improvement exists by shot peening. Hence, this work was conducted to study the contact fatigue resistance of shot peened ADI gears. Tests on as-ADI and shot peened ADI gears, as well as carburised steel gears were carried out. The results showed that the carburised steel gears exhibited superior performance and withstood the longest number of cycles before failure. The shot peened gears survived the lowest number of cycles. The poor performance after shot peening was attributed to an increase in the surface roughness induced by the peening process. In fact, the negative impact resulting from the roughened surface is greater than the benefits prompted by the induced compressive residual stresses, higher surface hardness and stress-induced austenite to martensite transformation.