U71Mn Steel Research Articles

The effects of in-situ synthesized TiC on the performance improvement of nickel-based composite coatings for rail repair

With the increase in train speed and axle weight, the quality and service life of the rail (U71Mn steel) are more demanding. Aiming at the excellent properties of nickel/nickel-based alloys and ceramic materials, this paper prepares TiC/Ni45 composite coatings for rail repair by in-situ synthesis of ceramic phases using laser cladding technology. The effects of in-situ synthesized TiC on the physical phase, microstructure, microhardness and wear resistance were investigated and discussed in detail. The results show that the reaction of in-situ synthesized TiC can proceed positively, and the microstructure of the cladding layer mainly consists of the segregation of Cr, TiC particles, and substrate phase. With the increase of nickel-coated graphite and titanium powder, the average microhardness value of the cladding layer increased, and the average friction coefficient and wear volume decreased. When the mass fraction of (Ti + C) reaches 15 wt%, the average friction coefficient of the cladding layer reaches the minimum value. When the mass fraction reaches 20 wt%, the average microhardness of the cladding layer reaches 2.65 times that of the substrate, and the wear mechanism is transformed from mainly adhesive wear to abrasive, oxidative, and fatigue wear.

A rapid in-situ hardness detection method for steel rails based on LIBS and machine learning

The railway, as a public transportation system, has played a significant role in global economic development. However, the pursuit of high speed and heavy load in trains has brought significant bending and shear stresses on the rails, making the performance of steel rails a notable focal point. The properties of steel rails are crucial factors determining the operational safety of high-speed trains, while the surface hardness is regarded as a key mechanical characteristic. This study is to develop a new rapid in-situ method to measure the hardness of steel rails, by employing Laser-Induced Breakdown Spectroscopy (LIBS) and specified analysis technology. Three distinct methods, including spectral line intensity ratios, plasma excitation temperature and machine learning, have been compared and analysed. Particularly, a multivariate model is established and optimized using the machine learning methods for U71Mn steel rail hardness analysis. In the machine learning algorithms, variance normalization is utilized, resulted in a significant improvement for the information retention during the data dimensionality reduction process. Subsequently, twelve algorithm combinations were explored, revealing that the Particle Swarm Optimization employed in Support Vector Regression (PSO-SVR) yielded the lowest mean squared error (MSE). Further refinement of the PSO-SVR was achieved through the incorporation of adaptive stochastic weights, resulting in an elevated coefficient of determination (R2) to 0.9876. Finally, the performance of the model on the new five samples was validated with an R2 of 0.9864. Otherwise, its potential applicability may be extended to broader domains, providing robust support for enhancing the precision and reliability of LIBS technology in surface hardness quantitative analysis.

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.

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.

Experimental research on impact adiabatic relationship of U71Mn steel

Little is known about the changes in physical and chemical properties of U71Mn steel during supersonic operation. In this paper, the impact adiabatic relationship of U71Mn steel in the range of 0.5~1km/s impact velocity was studied by flyer impact experiment. The data shows that with the increase of the impact velocity, the Hugoniot pressure of U71Mn steel increases significantly, but the increment of the Hugoniot temperature is relatively small. U71Mn steel does not undergo phase transformation under the impact of 0~23.5GPa pressure range.

Experimental study on Hugoniot parameters of U71Mn steel under high pressure

The Hugoniot parameters of U71Mn steel under high pressure were studied by hypervelocity impact test of U71Mn steel with two-stage light gas gun. The plate impact test of U71Mn steel sample was carried out by means of symmetrical impact. The minimum impact speed was 1.843km/s and the maximum impact speed was 5.018km/s. The Hugoniot relation and Hugoniot parameter C and λ, then the accuracy of Hugoniot parameters of U71Mn steel under high pressure is verified according to Hugoniot equation of state (the relationship curve between impact pressure and density). The research results can provide reference for the application of U71Mn steel in the field of hypervelocity impact.

Prediction of Surface Roughness of U71Mn Steel Milling Based on RBF Neural Network

In order to predict the surface roughness of U71Mn high manganese steel before actual milling operation, an orthogonal experiment was designed. Based on the intelligent algorithm of Radial Basis Functions (RBF) neural network, an accurate prediction model of surface roughness is done with MATLAB. By comparing the predicted data of RBF neural network model with the actual measured data, it is proven that the model is accurate and effective.

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.

Laser shock peening Fe-based coatings for enhancing wear and corrosion resistance

Laser Shock Peening (LSP) was introduced to reduce the residual stress and improve the wear and corrosion resistance of Fe-based 43X coating on worn U71Mn steel by laser cladding technology. The tensile stress decreased with increasing of laser power density and impact times, finally, transferred to compressive stress. The microstructure of the coatings was refined due to LSP treatment. The refined microstructure and existence of retained Austenite phase resulted in significant improvement in microhardness and anti-wear performances. The coatings after LSP process presented better corrosion resistance. However, impact three times with a higher power density has negative effect on improving the corrosion resistance.

Application of Metal Magnetic Memory Testing for Quantitative Evaluation of Stress Concentration in Ferromagnetic Steels

The objective of this research is to develop a quantitative algorithm between the magnetic parameter bm and the stress concentration factor in ferromagnetic steels. Specimens of U71Mn steel were machined into smooth plates with artificial defects (circular holes and rectangular holes) to represent various stress concentration degrees. Variations of the magnetic parameter bm with the stress concentration factor and the defect area were studied, respectively. The magnetic parameter bm of the U71Mn steel specimen was compared with that of the X80 steel specimen. Quantitative correlations of the magnetic parameter bm with the stress concentration factor and the defect area were investigated. The results show that the magnetic parameter bm is exponentially varied to the stress concentration factor and the defect area. The quantitative algorithm between the magnetic parameter bm and the stress concentration factor is influenced by the defect shape and the material type. An equation was proposed to evaluate the defect area based on the magnetic parameter bm.

Investigation on wear modes and mechanisms of abrasive belts in grinding of U71Mn steel

We investigated the wear modes and mechanisms of alumina grits during abrasive belt grinding of U71Mn steel under various normal forces and grinding speeds. Our approach consisted of an experimental procedure that produced sufficient wear followed by a resin-embedding method for observing the cross sections and topographies of grits. Statistics of wear modes and protrusion heights were calculated to visualize and quantify abrasive belt wear. The results demonstrated that different subsurface structures emerged inside the worn grits, and these structures were sensitive to process parameters. Transitions between abrasion and fracture wear modes were frequently observed. The effects of abrasive belt wear and process parameters on wear modes were clarified. The critical protrusion height of the abrasive belts for each wear mode of abrasive grits was experimentally determined.

Numerical Study on Solidification Behavior and Structure of Continuously Cast U71Mn Steel

The solidification behavior and structure of continuous casting high rail U71Mn bloom, under different secondary cooling conditions and superheat, were numerically investigated using the Cellular Automaton-Finite Element (CAFE) model implemented with ProCAST software. Nail shooting and macro etch experiments of the bloom samples under different cooling conditions were conducted to verify the model of macroscopic solidification and structure. The results showed that the simulated results of the solidified shell and solidification structure are basically consistent with experimental results. The secondary cooling condition has little effect on the grain size and distribution of the bloom, while both the bloom surface and corner temperatures are higher and the temperature rise at the beginning of the air-cooling zone is smaller under the super-slow cooling condition. The percentage of center-equiaxed grains decreases from 44.6% to 20.1% and the grain average radius increases from 1.025 to 1.128 mm when the superheat increases from 15 to 40 K, with little change in the grain size occurring between 15 and 20 K. Moreover, for a step increase in the superheat of 5 K, the solidification end is lengthened by about 0.19 m and the surface temperature is enhanced by 3 K. The super-slow secondary cooling condition with the superheat controlled within 20 K is suitable for big-bloom casting.

The Effects of a Submerged Entry Nozzle on Flow and Initial Solidification in a Continuous Casting Bloom Mold with Electromagnetic Stirring

The melt flow, level fluctuation, temperature field, and solidification behavior coupled with electromagnetic stirring (EMS) effects in the continuous casting mold region of U71Mn steel bloom were numerically analyzed by commercial computational fluid dynamics (CFD) software named ANSYS FLUENT. The effects of submerged entry nozzle (SEN) structures and the installation methods for optimized four-port SEN on the flow pattern, level fluctuation, heat transfer and initial solidification behavior in a bloom mold loaded with EMS were investigated. The aim is to propose a better SEN condition for the big bloom casting of high railway steel. The water simulation experiments were conducted to show the flow characteristics under different SEN conditions and verify the numerical model of flow pattern. The experimental and numerical simulation results showed that the optimized four-port SEN with diagonal installation cannot only improve the flow pattern of the molten steel by alleviating the level fluctuation and reducing the impact pressure to the wall. It is also beneficial for temperature variation at both bloom surface and corner, as well as the local solidified shell thinning phenomena due to the elimination of impingement effect.

Modeling Material Removal Rate of Heavy Belt-Grinding in Manufacturing of U71Mn Material

The heavy belt-grinding is a new machining method, which combined the characters of heavy-duty grinding and belt-grinding together, with high efficiency and low cost. In the present paper the removal rate model of heavy belt-grinding in manufacturing of U71Mn steel is established. It is assumed that the distribution of the abrasive particles protrusion height of the abrasive belt surface closes to Gaussian distribution. The model is presented by calculating the removal volumes of all abrasive grains contributing to cutting action based on the probability theory, elastic-plastic mechanics and abrasive cutting theory. It is analysis that the material removal rate depends essentially on the mechanical properties of the workpiece and the belt and the grinding conditions. It is proportional to the average pressure, belt velocity and the indentation depth and is inverse proportion to the grain size.

Characterisation of the fatigue process of U71Mn steel based on non-linear ultrasonic technology

Characterisation of the fatigue process of U71Mn steel based on non-linear ultrasonic technology

Correlation between the Force and Magnetic Field of the U71Mn Notched Sample

In this work experiments were carried out to study the evolution of the piezomagnetic field surrounding steel samples with the tensile stress. The piezomagnetic field was recorded by a highly sensitive fluxgate magnetometer of 428D. The piezomagnetic field (B field) and its gradient of the U71Mn steel are compared with the mechanical response. Results show that the piezomagnetic response seems to be a more sensitive indicator of damage than the mechanical one. Moreover, there exhibits correlations between the piezomagnetic response and mechanical response. This research has demonstrated that the piezomagnetic effect can be utilized as a nondestructive method to monitor damage in the rail.

Oxidation of U71Mn Steel under Pure Oxygen

Isothermal oxidations of U71Mn steels under pure oxygen, between 1027K and 1473K, are experimentally analyzed in this study. The results showed that Oxidation kinetics at different temperature from 1173K to 1473K are similar: mass increase per unit area as function of oxidation time follows parabolic role. Oxidation rate constants of the steel under pure oxygen, from 1173K to 1473K, keep exponential law as function of oxidation time.

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.

Analysis on High Temperature Oxidation of U71Mn Steel under CO<sub>2</sub>

Isothermal oxidation kinetics of U71Mn steels under pure CO2 at 1073--1473K are experimentally studied in this paper. The oxidation kinetic results are similar: mass increase per unit area as function of oxidation time nearly follows linear role for long time, with a more rapid increase at the begin. Oxidation rate constants of the steel under pure CO2, from 1073K to 1473K, keep exponential law as function of oxidation time.

An Investigation of High Temperature Oxidation Dynamics of U71Mn Steel

Oxidation of U71Mn steels under three different reheating curves in air are experimentally analyzed in this study. Total heating time of the heating curves are numerical computed before experiment. The results showed that oxidation began from about 900K, become noticeable and accelerated at near 1100K and became fast from about 1250K. The total mass increase of oxidation under high, typical and low heating rate are 159, 131 and 104 mg/cm2，when the total heating time are 268, 286, and 316 minutes. Keywords: oxidation; steel; high temperature; reheating furnace