U71Mn Rail Research Articles

Relationship Between the Microstructural Evolution and Wear Behavior of U71Mn Rail Steel

During train operation in a wheel-rail system, the service lives of the wheel and rail materials are heavily influenced by microstructural evolution. This study investigated the relationship between microstructural evolution and the wear properties of U71Mn rail steel under conditions of dry sliding wear using an MRH-5A wear tester. The results demonstrated that tribological white etching layers (T-WELs), which comprise nanocrystalline α-Fe and fine carbide, are formed after a certain number of cycles. The T-WEL thickness increased with the number of cycles; however, more cycles caused T-WEL flaking, which resulted in a reduced T-WEL length. The high microhardness of T-WELs (approximately 950 HV) is mainly attributed to ferrite grain refinement and cementite dissolution. During initial wear, the existence of grinding marks on the surfaces of the samples indicated an obvious increase in the wear loss of the steel. The high microhardness of the T-WELs can enhance wear resistance; thus, as the number of cycles was increased, the wear loss of the steel was reduced. At this point, an adhesive wear mechanism acted on the rail steel. The formation of fatigue cracks at the interface between the T-WEL and the plastic deformation layer induces T-WEL flaking, which increases wear loss. Consequently, after 6000 cycles, wear loss increased as the wear mechanism changed to fatigue wear. It is confirmed that T-WEL flaking is the primary cause of wear failure in U7Mn rail steel.

Effect of Alloying Composition on Microstructure and Mechanical Properties of Ultranarrow Gap Welded Joints of U71Mn Rail Steel

A new welding method, ultranarrow gap welding with constrained arc by flux band, is proposed to compensate for the low quality of rail thermite welded joints. This article presents the results of research on the microstructure and mechanical properties of ultranarrow gap welded joints of U71Mn rail steel made using three types of alloying composition content flux bands. Results indicated that the base metal metallographic microstructure consisted mainly of pearlitic, the HAZ was mainly composed of fine pearlite, and the microstructure of the welded bead was composed of acicular ferrite, while the weld grain size decreased as the alloy composition increased. The average hardness noticeably changed in weld metal as the alloy composition increased, and when the alloy composition reached 19%, the hardness was equivalent to the base material. The average hardness value of the HAZ (35.8 HRC) was higher than that of the base metal (24.8 HRC). The tensile strength increased, and the percentage elongation after fracture decreased with increasing alloying composition from 9% to 19%. The impact absorbing energies were decreased as the alloying composition increased. Consequently, all the mechanical properties of rail ultranarrow gap welding were higher than those of the standard requirements of the rail flash welding. And the optimal alloying composition of flux band was 19%.

Numerical investigation on the rolling contact wear and fatigue of laser dispersed quenched U71Mn rail

The influences of laser dispersed quenching (LDQ) technology on the rolling contact wear and fatigue of U71Mn rail were investigated numerically. Firstly, based on the improved Abdel Karim-Ohno’s material model, a three-dimensional elasto-plastic model with of LDQ rail was established, and the moving load method was used to apply the equivalent contact load. Subsequently, combined with the ductility exhaustion theory and Smith-Watson-Topper fatigue assessment method, the rolling contact wear and fatigue performances of U71Mn rail were investigated, respectively. The results show that the LDQ treatment has little effect on fatigue crack initiation life, but can significantly improve wear resistance of rails.

Comparison of wear and rolling contact fatigue behaviours of bainitic and pearlitic rails under various rolling-sliding conditions

Rolling-sliding wear experiments were performed to investigate the wear and rolling contact fatigue (RCF) behaviours of a premium pearlitic rail (PH), a carbon-free bainitic rail (BH) and two standard pearlitic rails (U71Mn and U75V). The wear regime and RCF damage evolution of the PH and BH materials in terms of Tγ/A (i.e. creepages and contact pressures) were compared and analyzed. The high-hardness BH steel presented a wear resistance similar to U71Mn and U75V rail materials, whereas lower wear rate was shown by the PH steel. Concerning the RCF performance, the damage of BH steel (comparable to U75V) was more severe than that of PH steel (comparable to U71Mn). Finally, with the increase in creepages and pressures, the wear and RCF damage of both PH and BH steels increased. These information could provide a guide in choosing rail materials and in development of bainitic rails.

Time-dependent ratcheting assessment of steel samples undergoing multi-step loading cycles through use of hardening rules

The present study evaluates ratcheting of U71Mn rail and 316 L stainless steel samples subjected to various multi-step loading cycles by means of kinematic hardening rules of Ohno-Wang (O-W), Abdel Karim-Ohno (AK-O) and Ahmadzdeh-Varvani (A-V). Chaboche’s isotropic hardening and visco-plastic description was introduced into the kinematic hardening models to further address yield surface expansion/contraction and time-dependency of steel samples. Over Low-High-High loading sequence, ratcheting was increased continuously, while as loading steps changed to subsequent steps of High-Low-Low, ratcheting slightly dropped at the transition of steps. Ratcheting over High-Low steps was preserved since ratcheting over the prior step was largely promoted. Predicted ratcheting strains and hysteresis loops were compared with those of measured values under multi-step loading conditions. Both the O-W and AK-O models under-predicted ratcheting strains over loading cycles particularly over the last loading steps. The A-V model predicted ratcheting over Low-High and High-Low sequences and closely agreed with experimental data. Evolution in size and width of loops as stress levels altered with loading steps was evident from predicted hysteresis loops. While the O-W and AK-O models were yet to generate loops along axial direction comparable with measured data, the A-V model predicted hysteresis loops in close agreement with those of experimental values.

Dynamic Mechanical Characteristics and Constitutive Modeling of Rail Steel over a Wide Range of Temperatures and Strain Rates

Rail steel plays an indispensable role in the safety and stability of the railway system. Therefore, a suitable constitutive model is quite significant to understand the mechanical behavior of this material. Here, the compressive mechanical behavior of heat-treated U71Mn rail steel over a wide range of strain rates (0.001 s−1–10000 s−1) and temperatures (20°C–800°C) was systematically investigated via uniaxial quasistatic and dynamic tests. The split Hopkinson pressure bar (SHPB) apparatus was utilized to perform dynamic mechanical tests. The effects of temperature, strain, and strain rate on the dynamic compressive characteristics of U71Mn were discussed, respectively. The results indicate that the flow response of U71Mn is both temperature-sensitive and strain rate-sensitive. However, the influence of temperature on the flow response is more remarkable than that of strain rate. On the basis of the experimental data, the original and modified Johnson-Cook (JC) models of the studied material were established, respectively. Using correlation coefficient and average absolute relative error parameters, it is revealed that better agreement between the experimental and predicted stress is reached by the modified JC model, which demonstrates that the modified one can characterize the mechanical behavior of the studied material preferably.

Study on the Performance of High‐Speed Rail Damage of Four Different Materials

For four types of railway rails, corresponding rolling tests have been executed by using the JD‐1 wheel/rail simulation device based on the Hertz simulation method, which can ensure the reciprocal contact condition between simulation rail and wheel under experimental conditions approximate to practical contact conditions. The results indicate that, for the rail material which has a higher hardness, the wear volume was less and the plastic deformation layer was thinner after the rolling test, but the crack propagation was more significant and the fatigue damage was severer, showing that its wear resistance was better. The analysis shows that the fatigue resistance of U71Mn rail was better, and considering fatigue is the main failure type for high‐speed rail, so it is more suited to the high‐speed railway.

On the wear and damage characteristics of rail material under low temperature environment condition

This study presents the wear and damage behaviours of U71Mn rail material with a friction pair of ER8 wheel material under different temperature conditions, which were explored using a rolling-sliding wear testing device. It turns out that, compared to those at 20°C room temperature, the adhesion coefficient, wear rate and hardness all increase at low temperatures. Meanwhile, three types of damage mechanisms are proposed according to wear and rolling contact fatigue (RCF) behaviours under different temperature conditions: (a) room temperature (RT) damage mechanism, (b) ductile-brittle transition temperature (DBTT) damage mechanism, (c) below ductile-brittle transition temperature (BDBTT) damage mechanism. With the temperature decreasing from room to low temperature, the wear mechanism transforms from abrasive wear to adhesive wear and surface fatigue. Meanwhile, the rail material gradually becomes brittle and the subsurface damage at low temperature is more serious than that at room temperature. Furthermore, the subsurface damage at −15°C is the worst. At low temperature, there is a wider size range of wear debris than that at room temperature.

The microstructure analysis of white etching layer on treads of rails

Abstract Through the utilization of the scanning electron microscope (SEM) and transmission electron microscope (TEM), the microstructure of Martensite White Etching Layers (WELs) on the surface of treads of U71Mn rail(used for a long time and taken from curves of normal railroad tracks) was analyzed and also its formation mechanism was discussed. As the result showed, the WELs observed from the cross-section was crescent-shaped and discontinuously distributed, with the thickness between 30 and 100 μm. Within the microstructure of WELs, the grain size of prior austenite at high temperature was ultra-fine, with the maximum diameter of approximate 1 μm at the outermost layer. As depths increased in the WELs, the grain size of prior austenite inside the WELs gradually became smaller; when the temperature on rail surface reached the temperature of austenitic transformation, austenite grains were less than 200 nm. Under the condition of room temperature, the microstructure inside the WELs was mainly composed of martensite. However, nanocrystalline α-phase, the structure of which was body-centered tetragonal, existed within 300 nm in the outermost layer of the WELs, and its misorientation generally featured random distribution. The organizations in the subsurface of the WELs were plate martensite, and undissolved and ultra-fine cementite, the amount of which rose as depths increase. The formation of the above-mentioned WELs was caused by rapid temperature rises and the subsequent cooling down of rail surface when a train suddenly braked or crept on a track.

Wear and damage transitions of wheel and rail materials under various contact conditions

This study discusses a Tγ/A method of plotting wear data from a twin-disc machine for identifying the wear and damage transitions of wheel and rail materials. As found in previous work, three wear regimes (mild wear, severe wear and catastrophic wear) of U71Mn rail material were identified in dry rolling–sliding contact tests. It was determined that the damage mechanism transforms in the different wear regimes. Here earlier studies were extended to establish wear behavior for the presence of a number of third body materials (oil, water, friction enhancers) and a rail cladding process designed to make wheels and rails more durable. This has provided much needed data for Multi-Body Dynamics (MBD) simulations, and will allow better predictions of profile evolution of wheel and rail over a wider range of conditions.

Investigation of welding residual stress in flash-butt joint of U71Mn rail steel by numerical simulation and experiment

Abstract U71Mn rail steel has been widely used in high speed railway construction, and it is commonly jointed by flash welding process. It is inevitable to produce very high residual stress in the welded joint. In this study, residual stresses in a rail steel flash-butt joint were measured by hole-drilling method. Meanwhile, an in-house finite element programme JWRIAN was developed to simulate welding residual stresses in the flash-butt joint. To effectively and accurately predict welding residual stress, a fast computing algorithm named iterative substructure method and an advanced material model with consideration of the influence of solid-state phase transformation were included in the computational approach. By comparing the calculated results with the experimental data, the validity of the developed computational approach was verified. Both experimental and numerical results suggest that the solid-state phase transformation plays a very important role in the course of formation of welding residual stress. In addition, simulation results indicate that the welding residual stress distribution in the flash-butt joint is very complex, and the peak values of tensile residual stress are relatively high. The residual stress distributions of the rail steel joint obtained by numerical simulation will be helpful in assessing the structural integrity.

Fracture analysis of U71Mn rail flash-butt welding joint

This paper mainly investigates the fracture problem of U71Mn rail flash-butt welding joint. Fracture surface morphology, microstructure and micro hardness are analyzed by using the scanning electron microscopy (SEM/EDS), the optical microscope (OM) and the micro Vickers hardness tester (Vickers-tester). The analysis results show that the welding joint is fatigue fracture, and the fracture surface morphology is the cleavage fracture characteristics. The metallographic morphology, inclusions and micro-hardness near the fracture surface are all in the normal levels. On the other side, the free solidification microstructure which extended from the outside to inside in the joint of the left side of the rail web and the rail head is the crack source of the rail welding joint fatigue fracture. Under the action of bending stress, the crack firstly generates in this area, and gradually extended to the rail web, to final fracture.

Decarburization Depth Control Technology Research and Application for U71Mn Rail

U71Mn is the main steel grade for high-speed railway rails. In the production of U71Mn rail, its decarburization depth is the deepest, and the qualified percentage is lower than 60% for the requirement of less than 0.3mm of decarburization depth. On the basis of current heating furnace, the influence of different heating processes on the decarburization depth of U71Mn rail was studied, and the result shows that just optimization of heat process can not decrease greatly the decarburization depth. Therefore the high temperature anti-oxidation coating protection technology was carried out in the process of heating of U71Mn rail. Under the condition of protection heating and optimization of heating process, the decarburization depth of U71Mn rail decreases significantly, and the qualified rate reaches to more than 95% for the depth of less than 0.30mm, and 100% for the depth of less than 0.50mm.

Material and Structure Design for Welding Joints in Railways Basing on Mechanical-Property Tests

Industrial design is important for industry, especially in the circumstance concerning the human security, of which the material selection and design basing on serious experiments, especially mechanical-tests have drawn more and more attentions of many artists and scientists. In this study, by tension, impact, hardness and scanning electron microscopy (SEM), the mechanical property of the flash butt welding joint of U71Mn rail steel to bainitic alloy steel used for railway frog is studied. The results indicate the distribution of hardness in the alloy steel side is uniform, and the hardness at the HAZ (hot affected zone) of the rail steel is higher than that of the base material. The tension property is the same as the U71Mn steel, and both the tension and impact property meet the corresponding requirements of Railways Ministry, which indicates the feasible of the bainitic alloy steel to the rail steel joint by flash butt welding.

Mechanical properties and fracture toughness of rail steels and thermite welds at low temperature

Brittle fracture occurs frequently in rails and thermite welded joints, which intimidates the security and reliability of railway service. Railways in cold regions, such as Qinghai-Tibet Railway, make the problem of brittle fracture in rails even worse. A series of tests such as uniaxial tensile tests, Charpy impact tests, and three-point bending tests were carried out at low temperature to investigate the mechanical properties and fracture toughness of U71Mn and U75V rail steels and their thermite welds. Fracture micromechanisms were analyzed by scanning electron microscopy (SEM) on the fracture surfaces of the tested specimens. The ductility indices (percentage elongation after fracture and percentage reduction of area) and the toughness indices (Charpy impact energy Ak and plane-strain fracture toughness KIC) of the two kinds of rail steels and the corresponding thermite welds all decrease as the temperature decreases. The thermite welds are more critical to fracture than the rail steel base metals, as indicated by a higher yield-to-ultimate ratio and a much lower Charpy impact energy. U71Mn rail steel is relatively higher in toughness than U75V, as demonstrated by larger Ak and KIC values. Therefore, U71Mn rail steel and the corresponding thermite weld are recommended in railway construction and maintenance in cold regions.

Analysis of Crack on Rail Web after Flash Welding

A failure U71Mn rail steel was investigated by stereomicroscopic, scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and laser scanning confocal microscope (LSCM). The results show that, the crack is initiated from heat affected zone (HAZ) of the rail web. The inclusions (Al2O3, SiO2) and the holes in the rail web are the main reasons for the fracture. The deepest of the hole is 64.1μm.

Failure Analysis of U71Mn Rail Welded Joints

The fracture of U71Mn rail welded butt joints on flash welding was analyzed by using visual observation, metallographic microscope, SEM and EDS from macro to micro.The results show that, the transverse fracture is rail oval flaw and the fracture mechanism of the rail is fatigue-crack propagation. The fatigue cracking is caused by silicate-non-metallic inclusions which are residues in the welded joints after flash welding.

Analysis of the Generation Cause of Scale Shelling Defects on Running Surface of 60kg/m U71Mn Rail

There were different degree scale shelling defects on running surface of 60kg/m U71Mn rail after used on the curve for a period of time, the characteristics and the generating reasons of the defects were analyzed, and the improvement measures were presented. There test results indicated that the scale shelling defects found on rail running surface were a sort of typical rolling contact fatigue damage, which caused mainly by the excessive contact stress, as a result of the wheel long-term contact with rail on the gauge corner of the rail on curve. It is effective to prevent and reduce rolling contact fatigue damage by following measures, such as improving the wheel/rail shape matching, and guaranteeing the wheel/rail interface locating on the rail tread center position, and strengthening the railway maintenance, and reasonable preventive grinding and corrective grinding for rails, and strict executing the system that rail grading use, the heat-treated rails should be used on small curve radius and heavy-load railway.

A study of rolling contact fatigue crack growth in U75V and U71Mn rails

Rolling contact fatigue (RCF) failure is a permanent and quite severe problem and hence of major concern to the modern railway industry. The selection of rail material should be based on a well-founded understanding of RCF. An analysis of the chemical composition, mechanical performance and microstructure was performed on U75V and U71Mn rail samples with oblique cracks taken from Guangzhou–Shenzhen railway. The results indicate that the strength character of U75V rail is superior to U71Mn rail. The crack propagation angle to the surface of U71Mn rail is smaller than that of U75V rail. A mixture of intergranular and transgranular fracture was found dominant in the process of fatigue crack propagation of U71Mn rail; in comparison, the dominant fracture mode of U75V rail is transgranular and the crack is easier to propagate at a relatively higher rate. The analysis shows that U71Mn rail is more suitable for the high-speed railway.

An Investigation to the Behavior of Fatigue Crack and Rail Selection for High-Speed

The rolling tests of railway rail were performed using a JD-1 wheel/rail simulation facility without any lubricant. The failure behaviour of rail rollers with different materials, was investigated in detail by examining wear volume and wear scar using optical microscopy (OM) and scanning electronic microscopy (SEM). The results indicate that rail material with a high hardness appears less plastic flow after rolling test. When the plastic flow is small, the wear resistance of material appears better. However, the crack propagation is more significant and fatigue damage is more severe. There is a mutual competitive relationship between wear and surface fatigue crack. A high wear rate can reduce rolling contact fatigue damage by removing constantly surface cracks. The analysis shows that U71Mn rail is more suitable for the high-speed railway.