Author: Site Editor Publish Time: 2024-02-22 Origin: Site
According to the relevant data, many railway rolling stock around the world are plagued by wheel tread stripping during operation. Tread stripping of this tread abnormal wear, in the railway industry in many countries around the world is a serious problem and the situation is getting more and more serious. Wheel tread abnormal wear, not only increases the operation and maintenance costs but, to a certain extent will even directly affect the safety of the vehicle.
Railway wheel tread stripping problems can be divided into three categories: contact fatigue stripping, braking stripping, and scuffing stripping. Braking stripping only occurs under tread braking conditions, the reason is that poor braking conditions lead to thermal cracks on the tread surface caused by abrasion stripping in the tread braking, non-tread braking conditions may occur, the reason is that the sliding or rolling between the wheel and rail leads to the wheel tread surface produced by the martensite caused by the stripping of the two types of problems can be mitigated from the improvement of the vehicle braking and use of the working conditions; this paper mainly from the perspective of This paper explores and analyses the contact fatigue stripping phenomenon of the tread surface from the material point of view.
The main mode of operation of the wheelset is to do a rolling-like movement on the rails (actually creep and slide). Wheel through a very small wheel-rail contact area of the vehicle load transferred to the rail, usually makes the local load exceed the elastic limit of the wheel or rail material, wheel-rail contact surface in the contact compressive stress after repeated long-term action, it will cause the contact surface due to fatigue damage to the local area of the small pieces of metal stripping, this fatigue damage phenomenon is called contact fatigue. Contact fatigue and general fatigue, are the same fatigue cracks and fatigue crack expansion of two stages. Prolonged contact fatigue is considered to be the main failure mechanism of the contact surface subjected to cyclic loading.
Contact fatigue damage in the form of pitting stripping (pitting), shallow stripping, and deep stripping are three categories. In the contact surface in the depth of 0.2mm below the needle-like or pox-like pits, known as pitting; depth of 0.2mm ~ 0.4mm peeling for shallow peeling, shallow peeling peeling block bottom roughly parallel to the contact surface. Deep peeling depth and surface reinforcement layer depth are comparable, there is a larger area of the surface layer crushed.
Wheel tread at the same time there are pockmarked peeling, shallow peeling, and deep peeling.Many factors affect the contact fatigue of the wheelset tread, such as the wheel itself, the tread surface hardening, the tread type used by the wheel, the wheel-rail contact surface finish, and vehicle operating conditions. The author believes that, in essence, the decision to fatigue performance or the composition and microstructure of the wheel material itself.
The wheel material itself has many aspects that affect the contact fatigue performance of the wheel, such as the organizational structure of the wheel material, material anisotropy, and inclusions in the material. The complexity of the organizational structure of the material leads to a very complex organizational factor for the effect of contact fatigue, which makes the researchers for the organizational structure of the contact fatigue of the influence of the views are also very different, and there is not a unified understanding of many aspects.
Iron and steel materials have undissolved ferrite, ferrite at room temperature mechanical properties are almost the same as pure iron. Its tensile strength is b for 180 ~ 280Mpa, yield strength 0.2 for 100 ~ 170MPa, and hardness of about 80HBS. it can be seen, that ferrite strength, and hardness are not high. As a weak phase in the organization, ferrite is prone to become a source of fatigue under the action of variable stress and lead to crack initiation, so ferrite has a detrimental effect on the contact fatigue life, and the larger the content of ferrite in the organization, the greater the effect on contact fatigue.
Carbon steel, carbon dissolved in - Fe in the interstitial solid solution known as austenite, with general austenite hardness of 170 ~ 220HBS between. Austenite's mechanical properties and its dissolved carbon and grain size, so its mechanical stability will affect the toughness of the organization, thus affecting the contact fatigue life of the material. During fatigue deformation, strain-induced austenite phase transformation occurs in the residual austenite, which can inhibit fatigue crack generation and extension. 18Cr2Ni4WA steel residual austenite on contact fatigue research shows that the residual austenite stability is moderate when the highest contact fatigue life. Too high residual austenite stability will lead to insufficient strength, and too low residual austenite stability will lead to insufficient toughness. Of course, the stability of residual austenite varies from one grade of material to another.
The amount of dissolved carbon in the carburetor in steel materials is extremely high, with a c of about 6.69%, resulting in high hardness (950 to 1050 HV) but almost zero plasticity and toughness. As the main reinforcing phase in steel materials, carburize in steel and other phases coexist in flaky, spherical, reticulated, and plate, its morphology and distribution of steel properties have a great impact. Such as when there is a reticulate distribution in the material, the toughness of the material is reduced, and the mechanical properties will be significantly worse.
Carburite will decompose under certain conditions, forming graphitic free carbon, and free carbon will be converted into other carbides under certain conditions. The effect of free carbon and carbide on contact fatigue is mainly manifested in its physical parameters (such as modulus of elasticity, expansion coefficient, etc.) is different from the material matrix, which destroys the continuity between the two phases. Fatigue deformation process, carbide can dissolve back, but the large carbide has a dislocation accumulation plugging effect, the tip of the upper bainite carbide is easy to produces stress concentration, conducive to crack sprouting. In addition, the dissolution temperature of the bar carbide is higher than the alloy carburizing body, easy to stay down to become undissolved carbide, which leads to a significant reduction in rolling contact fatigue life.
Austenite eutectic transformation of ferrite and carburite formed by the eutectic body called pearlite. Pearlite properties between ferrite and carburite, toughness is better. Its tensile strength b is 750 ~ 900MPa, hardness is 180 ~ 280HBS, elongation is 20 ~ 25%, impact work AKU is 24 ~ 32J. Mechanical properties between ferrite and carburize, high strength, moderate hardness, plasticity, and toughness are good. According to the related research, the effect of pearlite on the fatigue life of the material does not exist alone but depends on the hardness ratio between pearlite and ferrite. When the hardness ratio between ferrite and pearlite is large, the continuity between the two phases is poor (forming a phase difference), and fatigue cracks are easily formed at the ferrite/pearlite boundary, and preferentially expand along the ferrite/pearlite boundary. In addition, the fatigue performance of hot-rolled steel with coarse reticulated ferrite-pearlite organization is poor.
Non-metallic inclusions in steel have a greater impact on steel properties, including brittle with angular oxides, and silicate inclusions on the contact fatigue life of the most harmful. Because these non-metallic inclusions destroy the continuity of the matrix, the material in the surrounding area of tensile stress and orthogonal shear stress of the weak zone, under the action of heavy load cycling, the contact stress and the material's residual stresses superimposed on each other, so that the elastic energy concentrated in the region of the non-metallic inclusions into the deformation energy to produce cracks, this crack will be extended in the direction of the maximum shear stress and the eventual formation of surface peeling. The crack will expand in the direction of the maximum shear stress and eventually form the surface peeling.
As a raw material for the production of wheelset steel, in its smelting process is inevitable to bring a small number of standing elements (silicon, manganese, sulfur, phosphorus) and some impurities (non-metallic impurities and certain gases, such as nitrogen, hydrogen, oxygen). They have a greater impact on the quality of steel, some are beneficial elements, while others are the opposite. In addition, the chemical heat treatment of steel plays a very important role in strengthening and protecting the surface of the workpiece, such as shot peening, carburizing, nitriding, etc. can effectively improve the hardness of the surface layer of the workpiece, abrasion resistance and fatigue limit, etc., but it is important to emphasize its treatment methods and technical requirements.
The vast majority of scholars in the study of rolling contact fatigue of wheel materials, generally assume that the material is isotropic, but the study shows that, because the wheel on the track is not to do pure rolling, so no matter the direction and position, the railway wheels are anisotropic. The anisotropy of the wheel material has an effect on the orientation and position of the experimental specimen, and therefore on the measurement of the strength and other parameters of the material. The material parameters thus obtained are particularly important when applied to fatigue design.
Contact fatigue damage is one of the most important failure modes of wheel-rail contact surfaces subjected to cyclic loading. Proposing measures to avoid fatigue damage requires a good understanding and knowledge of the failure mechanisms involved. The research on the mechanism of contact fatigue damage has been relatively mature, but the wheels in the actual use of the working condition are very different, it is difficult to use a theory to explain them.
The factors affecting the contact fatigue damage of wheels mainly focus on the material itself and external conditions. As far as the material itself is concerned, strengthen the promotion of vacuum smelting technology in the metallurgical industry, to avoid the production process of raw materials in the wheel infiltration of unfavorable impurities (such as S, P, oxides, nitrides, etc.), but also can be targeted to add some favorable elements (such as Si, Mn, V, etc.), reduce the content of carbonic body in the material, the pearlite - ferrite hardness ratio, etc., to carry out effective control. Under the premise of attention to these factors, the use of wheel surface shot peening, carburising, nitriding; and the use of appropriate hardness and toughness of the material can effectively improve the rolling contact fatigue life of the wheel.
