U71Mn Rail Research Articles

Localized corrosion induced by MnS inclusions in U71Mn rail steels

To understand the corrosion mechanisms of rail steels, immersion tests were conducted on U71Mn rail steels produced in 1994 and 2021. It was observed that, at the onset of corrosion, only matrix near clustered MnS inclusions dissolved. Further characterization revealed that this was due to the combined effects of lower Volta potential, higher lattice distortion of the surrounding matrix, and strip-shaped carbides in the surrounding matrix induced by the clustered distribution of MnS. Consequently, the U71Mn2021 rail steel, with fewer MnS clusters formed, exhibited better corrosion resistance, as evidenced with electrochemical measurements. These findings suggest that refined processing techniques to control inclusion formation could be effective methods for developing U71Mn rail steels with greater corrosion resistance.

Prediction of basic turnout rail straightening with a neural network.

The main factors affecting the rebound of a switch rail in the straightening process are analysed and used as the input data of a neural network. An experimental group is designed, finite element simulation is carried out, and the data results are used to train the designed neural network to obtain a model with high accuracy. Taking the U71Mn rail as an example, the parameters are selected according to a certain interval, the test dataset is obtained via finite element simulation, and the selected parameters are predicted by the neural network. The error between the detection prediction data and the simulation data is small, and the prediction results are divided to obtain the parameter range that meets the requirements. This method provides a reference for the automatic straightening of a basic turnout rail.

The correlation between material deformed microstructure and rolling contact fatigue crack propagation of U71Mn rail when matching with CL60 wheel

Deformed microstructure and rolling contact fatigue (RCF) cracks are the two predominant damage characteristics of the field wheel and rail materials from the cross-sectional view. The proportion of two damage characteristics varied under different operating conditions. Thus, this study aims to investigate the correlation between material deformed microstructure and RCF crack propagation of U71Mn rail when matching with CL60 wheel using a twin-disc testing machine. An initial deformed microstructure layer on U71Mn rail material was first obtained by testing under dry condition, and then the RCF crack propagation tests were conducted under water condition. Results indicated that when the crack depth was smaller than the initial deformed microstructure layer depth, the deformed orientation dominated the crack propagation direction; when the depth of crack propagation towards the matrix exceeded the depth of newly formed deformed microstructure region, crack propagation direction dominated the microstructure deformed orientation. The increase in the initial deformed microstructure layer depth would intensify both the crack length and depth, and also would result in thicker newly formed deformed microstructure region. The Pearson correlation coefficient between initial deformed microstructure layer depth and crack depth was 0.998. The influence of initial deformed microstructure layer depth on wear rate was slightly greater than crack depth.

Multi-Objective Optimization of Process Parameters in Laser DED Ni-Based Powder on Steel Rail Using Response Surface Design

With the rise of global industrialization, the requirements for the operating speed and carrying capacity of high-speed trains are increasingly higher. Because the wear and tear of rails gradually increases during the running of high-speed trains, strengthening or repairing rail surfaces is of paramount significance. Laser-directed energy deposition (DED) exhibits significant advantages in improving surface hardness, corrosion resistance, and abrasion resistance. Because of the multiple interacting optimization objectives, the development of a multi-objective optimization method for process parameters is significant for improving DED deposition quality. Response surface design employs multivariate quadratic regression equations to fit the functional relationship between the factors and the responses, which can be employed to find the optimal process parameters and solve multivariate problems. This study develops a multi-objective optimization model with response surface design and 2D process mappings to visually analyze the effects of scanning speed, laser power, and powder feed rate on aspect ratio, dilution rate, and microhardness. The optimal combination of process parameters for Ni-based alloys on U71Mn rail is a laser power of 431 W, a scanning speed of 5.34 mm/s, and a powder feed rate of 1.03 r/min. In addition, a multi-physics field finite element model is developed to analyze the evolution mechanism of the microstructure from the bottom to the top of the single track. This study can provide theoretical and technical support for the surface strengthening or repair of U71Mn rail.

Improving wear and corrosion resistance of U71Mn rail steel subjected to laser in-situ fabrication of Fe based Cr–V coatings

With purpose of improving wear and corrosion resistance of U71Mn railroad switch, Fe based Cr–V coating was designed and in-situ fabricated on the surface of U71Mn rail steel using laser alloying technology. The phase constituents, effect of V content on the grain size, the microstructure, wear and corrosion resistance were investigated. Grain size refinement effect was observed due to more content of V content in the raw powder. The microhardness and wear resistance of the obtained Fe based Cr–V coatings presented a positive correlation with the content of V addition. The coating with more content of V addition presented a nobler Ecorr and a lower passivation current density icorr, indicating a more excellent corrosion resistance performance.

Study on fatigue damage tolerance of rail steel materials using peridynamics

With railways transitioning into the maintenance era, this study proposes a method to ensure structural safety and control maintenance costs. We develop a Peridynamics (PD) fatigue crack growth prediction model to determine the safe crack threshold for rail steel material under cyclic loading, known as the fatigue damage tolerance size. By conducting fatigue tests, we obtain the PD parameters for the rail steel and rigorously validate the model's reliability. Using both the compact tensile (CT) specimen model and a transient wheel-rail contact model, we analyze factors influencing crack propagation and fatigue damage tolerance. The results reveal that higher cyclic loads and smaller stress ratios result in shorter fatigue life and smaller damage tolerance sizes. Additionally, larger initial crack angles, faster train speeds, and heavier axle loads reduce fatigue crack propagation life and damage tolerance sizes. Ultimately, based on damage tolerance analysis, it is recommended to limit crack propagation in U71Mn rail steel within a 3.8-millimeter safety range for reliable operation.

Improving the microstructures and mechanical properties of U71Mn rail steel liner friction welded joint by normalizing treatment

A large amount of martensitic and elemental segregation exert negative effects on the mechanical properties of U71Mn steel LFW joints. In this study, the normalizing treatment was applied to promote martensite phase transformation and eliminate the element segregation area, thereby improving the mechanical properties of the joints. Specifically, after normalizing treatment, both the tensile strength and impact toughness of the joints were improved. The highest tensile strength could reach 900.1 MPa, about 89% of the BM. The impact energy could be increased from 2.1 J•mm2 to 12.1 J•mm2, indicating a significant enhancement. Compared with LFW joints, the recrystallized grain proportions in the WCZ and TMAZ of the N-LFW joint showed a smaller difference, and the microhardness values of the N-LFW joint were almost identical. This illustrates that normalizing treatment was beneficial to improve microstructural uniformity (about 315Hv). Meanwhile, the strain energy stored in the grains was released, which improved the failure resistance of the joints. This study provided an efficient and stable method for improving the microstructures and mechanical properties of U71Mn LFW joints.

Study on the microstructure and properties of rail cladding layer after laser quenching

The U71Mn rail surface processed by laser cladding suffers from large rough area and grain size at the top of cladding layer as well as residual tensile stress in the cladding layer. This paper proposes to remanufacture rail surface by combining laser cladding with laser quenching. The experiment results show that the quenched cladding layer was transformed from dendritic to equiaxed crystals at a specific quenching depth, with refine grains. Laser quenching results in dense martensite structure on the surface of the cladding layer. The hardness of the cladding layer after quenching is about 895 HV0.2, 18 % higher than that before quenching. The wear rate after quenching is 0.713 μm3/(N∙μm), with a decline of 22.3 % compared with that before quenching 0.918 μm3/(N∙μm), indicating that wear resistance is improved after laser quenching. In addition, the cladding layer after quenching exhibits stronger corrosion resistance than that before quenching, and internally it is transformed from tensile stress to compressive stress, with a maximum residual compressive stress of about −486 MPa, which endows the rail with stronger fatigue performance and longer service life.

Modeling and simulation of the negative angle cutting with a combined constitutive model for U71Mn rail steel

Due to the increasing train axle load, running speed and traffic volume, rails are prone to severe rolling contact fatigue (RCF) defects. Repairing the rail profile by milling is the latest technology to solve the above hidden problems, but there are few related studies in recent years. In this paper, the milling maintenance process of rails is studied by using finite element method(FEM). This study examines the fracture behavior of U71Mn steel rail under various stress triaxiality conditions, temperatures, and strain rates. An electronic universal testing machine is utilized to conduct the experiments, and a Johnson-Cook (J-C) failure model for U71Mn steel rail is developed. A 3D cutting simulation model was developed using the Johnson-Cook (J-C) combined constitutive model. The accuracy and reliability of the numerical simulation model were confirmed by comparing the cutting force and chip morphology obtained from both simulation and experimental results. The cutting simulation model had an error rate of within 10 % when predicting the cutting force. Following that, the fracture zone and chip portion of the tensile specimen were metallographically analyzed. The applicability of the ductile failure criterion in describing the observed failure in the U71Mn rail has been proven.

Effect of intrinsic thermal cycles on the microstructure and mechanical properties of heat affect zones in laser cladding coatings on full-scale rails

Laser cladding (LC) wear-resistant alloy on the surface of pearlite rails has been proven to be able to improve their wear resistance significantly. However, the rapid cooling rate in LC will result in the formation of martensite structures with high hardness but low toughness in heat affected zone (HAZ), making the rails' service safety endangered. In this paper, a novel method was proposed to eliminate the un-tempered martensite in U71Mn rail based on the in-situ gradient tempering (IGT) effect induced by the intrinsic thermal cycles in multilayer LC. The influence mechanisms of interlayer and inter-track thermal cycles on the evolutions of microstructure and mechanical properties of HAZs were studied systematically. The results illustrate that the martensite in HAZs can be gradually tempered within the optimal IGT temperature range, and the tempering processing mechanisms are as follows: firstly, the ε-carbides precipitate from HAZs and then dissolve to form granular cementites. Then, the cementites form colonies of lathlike particles with similar orientations. Finally, a composite structure consisting of fine-tempered sorbite and granular cementites forms, in which the lamellar spacing of sorbite is <1/5 of that of the original pearlite, and the proportion of the high angle grain boundaries has increased by 2.2 times. During the tempering process, the microhardness of HAZs gradually decreases, accompanied by the increase of the bending properties therein. The ultimate bending strength and fracture strain of the specimens with fully tempered HAZs are enhanced to be about 1.6 and 5.5 times as much as those of the specimens with fully quenched HAZs, even much better than the substrate.

An investigation on the formation and hardening mechanisms of white etching layer in hypoeutectoid rail surface

The stress distribution in a hypoeutectoid U71Mn rail surface due to the wheel-rail contact was simulated via finite element, and the microstructure and hardening of the after-service rail surface were comparatively studied with heat treated (holding at 850°C for 30min and cooling at 15°C/s) samples under atmospheric pressure (AP-850) and under hydrostatic pressure of 1GPa (HP-850), respectively. The rail surface experiences 3-dimensional stress-state of 1GPa and large shear stress. A white etching layer (WEL) was formed, showing martensite-type microstructure with spherical cementite particles and a hardness of 1101HV. The phase composition and hardening show great resemblance to those of the HP-850 sample, but differ significantly from those of AP-850 sample. High pressure martensitic transformation is proposed to interpret the formation of WEL and the hardening is analyzed according to the structural cause.

Optimization and wear behaviors of 316L stainless steel laser cladding on rail material

Laser cladding is a potential technology for repairing the rail with side wear. The laser processing parameters should be optimized and the wear behaviors of clads should be analyzed before its application. Therefore, single-pass and multi-pass laser cladding experiments of 316L stainless steel powder with different laser powers, scan speeds, scan modes (directions), and numbers of layers are conducted on the U71Mn rail to optimize the processing parameters. After that, sliding wear tests are conducted on different depths in clad to explore the wear behaviors. The results show that with the increase in laser power, the clad width is increased, the height is increased first and then decreased. With the increase in scan speed, both the clad height and width are decreased. Scan mode 1# produces the thinnest clads, the mode 3 produces the thickest ones. The microstructure of clad is composed of dendritic grains. With the increase in depth in clad, the grain size decreases and the hardness increases. The grain size is increased with laser power and decreased with scan speed, and the hardness is increased with the power and speed. Smallest grains and highest hardness are produced using mode 1#, and largest grains and lowest hardness using mode 3#. The Cr and Ni elements from clad permeate into the heat affected zone of U71Mn rail material. Wear depth of clad has a positive relationship with the grain size and a negative relationship with the post hardness. A wear resistance coefficient, Xphv2.55/Xgs1.56,where Xphv refers to the post hardness and Xgs refers to the grain size, is proposed to characterize the inherent anti-wear property of 316L stainless steel clad on U71Mn rail.

Experimental study of the effects of temperature and humidity on the wear and damage behavior of U71Mn rail steel

Wheel-rail interfaces operating in open systems are constantly affected by environmental factors, such as water, relative humidity, temperature, leaves, and sand. In this study, a twin-disc rolling-sliding friction and wear test device with a controlled environmental chamber was used to conduct experiments to study the influence of high temperature and humidity on the wear and damage behavior of U71Mn rail steel. The results showed that surface oxidation of rails increased with increasing temperature and humidity. The surface oxides and very humid air provided lubrication that could reduce wear. With increasing temperature and humidity, the trend of the friction coefficient of the rail samples was basically consistent with that of the plastic deformation layer. The wear of rail samples first decreased and then increased with increasing humidity. However, with increasing temperature, the wear of the rail samples first increased and then decreased. With increasing temperature and humidity, the crack length on the rail surface gradually increased and extended to the center of the material, which made the competition between wear and rolling contact fatigue more intense. When the wear was less than rolling fatigue, rail surfaces underwent severe cracking and peeling, which resulted in the loss of a large amount of surface material.

Investigation on fatigue crack propagation behaviour of U71Mn and U75V rails using peridynamics

Fatigue crack propagation in rails is an important factor that increases the cost of railway maintenance and threatens operating safety. Research into fatigue crack propagation rules for different rail materials helps to provide scientific guidance for rail selection and maintenance plans. Based on the results of fatigue crack propagation tests on U71Mn and U75V rails, the bond fatigue fracture criterion was established for the two materials, and their accuracy was verified. Using ordinary state-based peridynamic theory, a model was established to analyse the process of crack propagation in the two types of rails. The behaviour of U71Mn and U75V rails was then simulated with different wheel loads and wheel-rail friction coefficients, and the characteristics and rules for fatigue crack propagation in the two types of rails were analysed in a comparative manner. The results showed that the material type exerted little impact on the crack propagation path within the rails and that the U71Mn rail, which showed a lower crack propagation rate than the U75V rail, is more suitable for high-speed railways. The rate of crack propagation increased with both wheel load and wheel-rail friction coefficient. Certain measures, such as track regularity improvement, could be employed to reduce dynamic wheel-rail impact, while others, such as lubrication, could be adopted to decrease wheel-rail friction coefficients to prolong rail service life.

Comparative Studies on Shear Failure Behaviors of U71Mn Rail Steel at High Strain Rates Using Hat-Shaped Specimens

Abstract The shear failure behaviors of U71Mn rail steel are investigated by quasi-static and dynamic compression tests utilizing two different hat-shaped specimens: S1 which combines shear and compressive stress states and S2 which combines shear and tensile stress states. A split Hopkinson pressure bar is used to acquire shear stress–strain curves at various initial temperatures and shear strain rates, and it is found that a lower shear strain rate is observed in hat-shaped specimen S1 than that in hat-shaped specimen S2 under the same impact pressure. Scanning electron microscopy is employed for observing the microstructures of specimens. The results indicate that the hat-shaped specimen S1 is difficult to form voids and dimples. Moreover, as far as the hat-shaped specimen S2 is concerned, the number of voids reduces with the rising shear strain rate, and no voids appear on the fracture surface at the shear strain rate of 36,000 s−1. Furthermore, the creation of voids is aided by a rise in initial temperature. The factors affecting the formation of adiabatic shear bands are explored based on the numerical simulation, which suggests that the magnitude of the temperature gradient plays a crucial role in the generation of adiabatic shear bands.

Mechanistic Study of Rail Gouging during Hypersonic Rocket Sled Tests

Gouging—an obstacle to the development of hypersonic rocket sled test techniques—was mechanistically investigated through experimental and theoretical analyses. Typical gouges were analyzed using macroscopic and microscopic experiments to investigate the evolution of gouging. Quasistatic compression and Hopkinson bar experiments were performed to systematically study the thermoviscoplastic properties of U71Mn rail steel under wide ranges of the strain rate and temperature. The critical condition for gouging was derived based on a thermoviscoplastic constitutive model supplemented by a three-variable criterion for an adiabatic shear instability. The results showed the following. (1) Adiabatic shear bands (ASBs) form when stress reduction under the combined action of frictional heating and high-speed deformation exceeds the strain-hardening effect of the rail material. (2) The nonuniform deformation of the edges of ASBs leads to the generation of cracks that split the rail surface. As the ASBs expand, the cracks grow and coalesce, eventually causing the material to peel off, forming gouges. (3) The relationships among the temperature, strain rate, and strain at the onset of gouging can be determined based on the critical condition for the formation of ASBs in rail steel.

Investigation into the microstructure evolution and damage on rail at curved tracks

As an important running part, rails serve as an important link to guarantee the safe and stable operation of railway system. The revelation of microstructural evolution and damage on the surface layer of rails after service will provide theoretical bases for analyzing rail failure mechanisms and improving rails' service life. This paper selected U71Mn rail at curved tracks as the research object, analyzed the microstructures and properties of the rail in different regions based on characters of forces and damage during its service period, revealed the microstructural evolution laws on the surface layer of the rail after service, focused on the characterization of the microstructures inside the transition zone between white etching layers (WELs) and the matrix, classified the types of WELs based on the formation mechanisms of WELs, and investigated the features of fatigue damage on the rail surface through analyzing surface damage.

Influence of initial crack on fatigue crack propagation with mixed mode in U71Mn rail subsurface

A three-dimensional finite element model of wheel-rail that contains an initial crack in rail subsurface was established, the influences of initial crack angle, depth and size on mixed fatigue crack growth behaviour under actual load were evaluated. The results showed that the initial crack propagated with II + III mixed mode, and the slip propagation of mode II was dominant. Different initial crack angle, depth and size lead to variable proportions of mode II and mode III because of the load mixity at the crack tip, which further affects the equivalent stress intensity factor (SIF) range ΔKeq and residual fatigue lifetime. 45° was the most dangerous angle of initial crack, which leads to the highest ΔKeq and the fastest crack growth initial crack depth increased, the ΔKeq and crack growth rate decreased sharply. With the increase of initial crack size, the ΔKeq increased and crack propagation was accelerated.

A strain rate-dependent cohesive zone model for shear failure of hat-shaped specimens under impact

The shear behaviors of U71Mn rail steel are investigated during different impact pressures using hat-shaped specimens, where the dynamic shear stress-strain responses of the specimens can be obtained by utilizing a split Hopkinson pressure bar apparatus. The maximum shear tractions are determined through the shear stress-strain curves of the experiments, and cohesive energies are obtained by trial and error method, i.e., the strain waves of the simulations are compared with those of the experiments. It is found that the maximum shear tractions and shear cohesive energies of U71Mn steel are sensitive to strain rates. Further, a strain rate-dependent cohesive zone model is developed to describe the dynamic fracture behaviors. Finally, the crack velocities are acquired under different impact pressures, and the dynamic stress intensity factors and dynamic energy release rates vary with the crack velocity.

Effect of laser claddings of Fe-based alloy powder with different concentrations of WS2 on the mechanical and tribological properties of railway wheel

Laser cladding could be applied on railway wheels to improve their service lives. Fe-based alloy powder is widely used as the laser cladding material due to its good self-fluxing ability and low price. However, the friction coefficients and wear rates of Fe-based alloy laser claddings are high. WS2 is used as the additive for common laser cladding treatment, while it has not been used for claddings on railway wheels. Understanding the influence of WS2 concentration on the mechanical and tribological properties of Fe-based alloy laser claddings on railway wheels has significance for extending the application of laser treatment on railway transit. Therefore, laser claddings of Fe-based alloy powders with different contents of WS2 powder (0, 2%, 4%, 6%, 8%) were produced on the railway CL60 wheel material. Rolling-sliding tests were conducted on a twin-disc tribometer using the uncladded and cladded wheel samples against an uncladded U71Mn rail sample. The results indicated that the microstructure of laser claddings was composed of dendritic and eutectic phases. With the increase in the WS2 content from 0 to 6%, the size of dendritic phases was decreased from around 20 μm to less than 10 μm. With 8% WS2, the size of dendritic phases was increased to around 10–20 μm. Laser claddings of Fe-based alloy powders with WS2 included (Fe, Ni) and (Fe, Si) solid solutions, hard carbide (Cr7C3), Ni–Cr–Fe, and CrS phases. The hardness was increased to 730–820 HV0.5 and residual stresses were changed to be compressive state. Rolling contact fatigue (RCF) cracks preferentially initiated and developed along the boundary between dendritic and eutectic phases. The smallest wear rate and shortest RCF cracks occurred on the cladding from powders with 6% WS2. Thus, the optimum content of WS2 in the Fe-based alloy powder was 6% for laser cladding on the railway wheel material.