Strength Steel Research Articles

STRUCTURAL CHANGES DURING THERMAL STRENGTHENING OF THE RAILWAY WHEEL

Purpose. Justification of mechanism of the structure transformations in the carbon steel of the railway wheel during disk thermal strengthening. Research methods. The material for the study was carbon steel of a railway wheel with a content of 0.57 % C, 0.65 % Si, 0.45 % Mn, 0.0029 % S, 0.014 % P and 0.11 % Cr. The railway wheel was heated to temperatures higher than Ac3, kept at this temperature to complete austenite homogenization process, and disk was rapidly cooled to the specified temperature. Degree defectiveness structure of the metal after accelerated cooling was assessed using technique of X-ray structural analysis. Strength stress, yield stress and relative elongation of the carbon steel were determined at stretching at rate of deformation 10-3 s-1. Results. At accelerated cooling of the carbon steel, the sources of strengthening are the processes of blocking mobile dislocations due to the condensation of carbon atoms on them and dispersion strengthening from the formed particles of the carbide phase. At temperatures of termination of forced cooling of carbon steel above 300…350 °C, the reduction rate of strength properties is determined by the excess of total effect of softening from decomposition of the solid solution, acceleration of spheroidization and coalescence of cementite particles over the blocking of dislocations by carbon atoms and dispersion hardening. Scientific novelty. The level of strength and plasticity characteristics of carbon steel of the railway wheel disc, depending on the temperature end forced cooling, is determined by the ratio of the influence of degree super saturation of the solid solution and the dispersion strengthening by carbide phase. Practical value. For temperatures termination of accelerated cooling of 200…300 °C, degree of super saturation of the solid solution is the main factor that determines the level of strength and plasticity characteristics. When manufacturing an all-rolled railway wheel, the strength limit of the disc metal can be increased by accelerated cooling to the middle range of temperature.

Effect of yttria content on microstructural evolution, mechanical properties and temperature dependent strengthening mechanisms in 9Cr-oxide dispersion strengthened (ODS) steel developed by hot powder forging

9Cr-oxide dispersion strengthened (ODS) ferritic/martensitic steel with varying yttria (Y2O3) contents (0, 0.35, 0.5, and 1 wt%) were prepared by mechanical alloying followed by hot powder forging. The powder forged samples were subsequently normalized and tempered by a standard heat treatment. The microstructure consisted of a dual phase structure of tempered martensite and residual ferrite, along with fine distribution of carbides (MC/M23C6). The grain size was strongly influenced by the yttria contents. Tensile tests were conducted at room temperature, 300°C, 500°C, and 700°C for all the compositions. A trend of increasing tensile strength and decreasing total elongation was observed with increase in yttria contents at all the temperatures. Contrary to the results in literature, superior ductility and a comparable tensile strength of ODS steels with the higher yttria contents (≥0.5 wt%) at room as well as high temperatures indicate the benefits of the powder forging process. As per the quantitative estimation of yield strength at room as well as elevated temperature via strengthening mechanisms, root mean square method (σrms) is proven more reliable than linear superposition method (σls). Additionally, strength from grain boundaries, dislocation density, and Orowan was found major at room temperature while dispersoids seemed to be prevailed at high temperature.

Uniaxial tensile behaviors and Hall-Petch relationship of polycrystalline 316LN stainless steel via molecular dynamics simulation

A molecular dynamics (MD) simulation method was used to investigate the effects of element position, tensile strain rate (ε̇) and average grain diameter (d) on the uniaxial tensile properties of 316LN stainless steel at room temperature. It was found that the mechanical properties (elastic modulus, yield strength and tensile strength) of 316LN stainless steel were independent of the random position of the elements used in the model. Since the time of atoms relaxing to the equilibrium position was determined by the tensile strain rate, the mechanical properties deeply depended on the tensile strain rate. The fluctuations of simulated mechanical properties decreased with the increase number of randomly oriented grains in the molecular dynamics model, and the minimum number of grains should not be less than 500. Intragranular dislocation movement and grain boundary sliding were the main factors leading to plastic deformation, and their competition led to the normal to inverse Hall-Petch (HP) transformation of yield strength at the critical average grain diameter of 17 nm. When d was bigger than 17 nm, the relationship between yield strength and d was a normal HP relationship, σy(MPa) = 162 + 465.5d −0.5; while d was smaller than 17 nm, the forgoing relationship transformed into an inverse HP relationship, σy(MPa) = 3930-25d−0.5.

Axial compressive behavior of reinforced hollow circular high strength concrete filled steel tubular short columns

This study aims to investigate the mechanical behavior of reinforced hollow circular high-strength concrete-filled steel tubular (RHCFST) short columns under axial compression. The axial compression test of 6 RHCFST short columns was carried out, and the bearing capacity, ductility and failure modes of the columns were analyzed. The results revealed that the failure modes of RHCFST short columns primarily included mixed failure, shear failure, and crushing failure. Comparatively, the failure modes of specimens with ordinary reinforcement exhibited a shift towards mixed failure, significant improvement in ductility, and a decrease in the increase trend of ultimate bearing capacity with the growth of steel tube thickness. Based on the test results, finite element models were developed using ABAQUS software for parameter analysis. The results demonstrated a positive correlation between the short column’s bearing capacity and parameters such as steel strength, sandwich concrete strength, steel tube thickness, and ordinary reinforcement diameter. Additionally, the initial stiffness of the short column increased with the increase of steel tube thickness. Considering the mechanical properties and engineering applicability of the member, the recommended design of steel tube yield strength, sandwich concrete strength, steel tube thickness and ordinary reinforcement diameter were given. Furthermore, the axial bearing capacity calculation formulae for RHCFST short columns were proposed. The calculated values were found to closely match both experimental and simulated values, with errors falling within 10 %. Consequently, the formulae can offer a reliable prediction of the member’s axial bearing capacity.

Ti-Mo microalloyed medium Mn steels: Precipitation and strengthening mechanism

Medium-Mn steels possess an exceptional strength-ductility balance but normally relatively low yield strength, which hinders their practical applications especially in ultra-high strength structural components. In this study, the simultaneous addition of Ti and Mo was proposed to improve the yield strength of medium-Mn steels, together with the systematic experimental and theoretical investigations on the precipitation behavior of (Ti, Mo)C carbides and its effects on the microstructure-properties relationship. The addition of Ti and Mo increases yield strength by 150∼350 MPa and ultimate tensile strength by 55∼320 MPa without a significant loss of ductility. Based on the intercritically annealed microstructure close to equilibrium as well as the nucleation and growth kinetic theory, the Ti–Mo addition can reduce the fraction of austenite, but increase the mechanical stability of austenite by refining the mean grain size of austenite. Mo was found to increase the precipitate density of (TixMo1-x)C by lowering the interfacial energy between the nano-precipitates and the matrix, and reduce the mean size of precipitates. A maximum increment in yield strength (∼350 MPa) was achieved at 610 °C, which was mainly attributed to the combined effects of grain refinement and precipitation hardening due to the addition of Ti and Mo.

Data enhanced lightweight network-based prediction of cold-rolled steel mechanical properties

The production of cold-rolled carbon steel is a complex nonlinear process, with chemical composition and process parameters being the key factors influencing its mechanical properties. Establishing the mapping relationship between key parameters and mechanical properties is the key to predicting the mechanical performance of rolled steel. Existing research has overlooked that chemical composition and process parameters are two different scales of data, lacking the exploration of potential features between these scales. This paper proposed a data enhancement-based expand-channel depth-separable convolutional neural network (MBE-EDSC) to predict the mechanical properties of cold-rolled steel. Multi-branch autoencoder was proposed to enhance the original data, which could be mined for potential features of the data. Meanwhile, the self-adaptive convolution channel attention mechanism was proposed to improve the sensitivity of the model to the features, and the channel extension layer to compensate for the problem of accuracy loss caused by deep convolution. Compared with the performance of the existing networks, the proposed network achieved 97.3 %, 98.5 %, and 99.2 % accuracy in predicting the yield strength, tensile strength, and elongation of cold-rolled carbon steel, respectively, and the R2 was improved by 4.9 %, 4.3 %, and 5.9 %, respectively.

Design of Quenching and Tempering Process and Elucidation of the Relationship between Microstructure and Properties of 20Mn2SiNiMo Bainitic Wheel Steel

The rapid development of train transportation toward higher speed and heavier load requires superior wheel materials with high strength and toughness. Herein, a 20Mn2SiNiMo bainitic wheel steel is developed and bainitic wheels are produced. The quenching and tempering process of bainitic wheels is studied considering the bainitic ferrite content and stability of retained austenite (RA). The quenching process includes water spraying and air cooling. Proeutectoid ferrite can be suppressed during the water‐spraying process. During air cooling, the bainitic ferrite transformation is promoted and the stability of RA is improved with the extension of cooling time. Corresponding microstructure of as‐quenched steel at wheel rim is bainite with different morphologies, martensite, and RA. The evolution of RA at lower and higher tempering temperatures is dominated by carbon enrichment and carbon consumption, respectively. Yield strength of bainitic wheel steel mainly comes from body‐centered cubic phase (containing martensite and bainitic ferrite). The increase of dislocation density and the decrease of effective grain size are beneficial for yield strength improvement. Bainitic packets with various direction, deformation‐induced martensite transformation of RA, and plastic deformation all play an important role in enhancing the toughness of bainitic wheel steel.

Interfacial Heterogeneous Microstructure and Subsequent Machinability of 316L/CuSn10 Bimetallic Structure Manufactured by Laser Powder Bed Fusion

Additively manufactured bimetallic structures combine the advantages of dissimilar materials and can achieve localized properties through a customized composition distribution. However, additively manufactured parts may still lack the dimensional accuracy and surface integrity essential for precision mechanical assemblies that the post-machining process can address. Therefore, this study aims to systematically investigate the microstructure and machinability of 316L/CuSn10 bimetallic structures fabricated using laser powder bed fusion. The results show that the fusion zone of the bimetallic structure had refined grains of microscale size owing to the mixture of the primary elements of the bimetals, which resulted in the highest microhardness of 3.4 GPa. The difference in microstructure and microhardness between the single-material and fusion zones also causes significant differences in the cutting response during the ultraprecision process. The 316L stainless steel side exhibited the highest cutting force and more severe material accumulation in the chips. The cutting force drops when cutting through the fusion zone, with an observable fracture in the chips and separation of dissimilar materials on the machined grooves, indicating that the heterogeneous properties of additively manufactured 316L/CuSn10 bimetallic structures pose challenges to the improvement of surface quality. The simulation results also showed that stress accumulation occurred in the tool path through the fusion zone owing to the higher yield strength and hardness of stainless steel, indicating that lower cutting speeds and depths of cut are favorable for reducing cutting force and improving surface quality. This study provides deep insight into the microstructure evolution mechanism and a theoretical basis for improving the surface quality of additively manufactured bimetallic structures using an ultraprecision machining process.

Enhanced strength and plasticity of 316L austenitic stainless steel through combined ECAP and annealing treatment

The mechanical properties of 316L austenitic stainless steel (ASS) were enhanced by equal channel angular pressing (ECAP) combined with annealing treatment. High-density dislocations, twins, and slip bands are generated during 4 passes ECAP. An appropriate annealing treatment is beneficial for overcoming ductility loss and maintaining strength of ECAP samples. In this study, we found that annealing at 700 °C for 1 h and 600 °C for 12 h can achieve this goal, mainly due to the recovery of stacked dislocations at boundaries and the retention of nanoscale twins and slip bands.

Numerical parametric study on ultimate load of precast concrete piers with demountable connections

To investigate the flexural capacity of precast piers with demountable steel-concrete composite connection, based on the model test, the refined finite element model was developed. The flexural capacities were compared between cast-in-place (CIP) piers and precast piers with different parameters including the thickness of base steel plate and notched perfobond connector, material strength, open-hole size of base steel plate, height of jacket, and axial compression ratio. The results show that the flexural capacity of concrete precast piers with rationally designed connections should not be lower than that of CIP piers. The cracking load and flexural moment-axial compression ratio curve can be evaluated as CIP piers. The ultimate flexural capacity is positively correlated with the base steel plate thickness and jacket height. The notched perfobond connector and the stiffeners set on the base steel plate can effectively improve the flexural capacity and optimize the shape of yielding line of base steel plate. The steel strength should be greater than 300 MPa in order to ensure that the steel connection yields later than the longitudinal reinforcement. Finally, the simplified model of base steel plate and the influence mechanism of the steel plate are discussed. This paper provides reference for the design of the proposed piers and other similar connections.

Proposition for the effective moment of inertia to determine the deflection in a composite slab

The objective of this work is to propose a mathematical expression for calculating the effective moment of inertia of a composite slab that utilizes steel formwork incorporated into the concrete, and consequently, to verify the deflection. The analysis of the behavior and strength of composite steel and concrete slabs covers several parameters, such as load-deflection curves, load-end slip curves, and load-steel strain curves. Among these parameters, the load versus deflection curve at the mid-span perform a crucial role in the evaluation and verification of deflection. Regarding the calculation of deflection, generally, the technical standards recommend that the moment of inertia of the composite section be given by the simple average of the moments of inertia of the uncracked and cracked sections. However, experimental investigations have shown that this procedure inadequately characterizes composite slab behavior, resulting in underestimated effective moment of inertia and reduced deflection, especially when subjected to higher loads. Using the results of experimental tests on a composite slab built with a specific steel sheeting carried out at the Structures Laboratory of the Federal University of Minas Gerais, this work evaluated several expressions suggested by technical standards and authors of scientific articles aiming to validate a proposal for determining the moment of effective inertia in composite slabs that adequately represents the behavior load versus deflection during the entire loading phase until collapse. From this evaluation it is possible to recommend the proposed equation for design verification in the case of the service limit state for excessive deformations.

Static Loading Simulation on Temporary Platform and Ladder Absorbent Chamber Design

This study utilizes SolidWorks software to thoroughly examine the structural layout and static load simulation of a temporary platform and ladder in an adsorbent chamber. SolidWorks, a computer-aided design (CAD) and computer-aided engineering (CAE) software, offers robust tools for 3D modeling, simulation, and analysis, making it ideal for this type of structural assessment. Finite element analysis (FEA), a simulation model within SolidWorks, is employed to determine stress distribution and safety factors. The results demonstrate the platform's maximum stress at 198.65 MPa, below the yield strength of ASTM A36 Steel, leading to a safety factor of 1.3. These findings validate the design's safety and reliability for industrial use.

Analysis and Real-Time Monitoring of the Influence of Wind Load on a High-Altitude Steel Connecting Bridge with Small Spacing

With the development of steel-structure construction technology in high-rise buildings, the design and construction of high-rise steel structures tend to be complicated. Based on the Zhuhai Tiejian Square project, the fluid-structure coupling calculation analysis and on-site monitoring are carried out in view of the wind load of the high-altitude steel-structure connecting bridge in the hoisting stage of the Zhuhai Tiejian Square project. The main structure of the project is four towers and five high-altitude, small-spacing steel structures connecting the four towers. The lifting process of the third zone is taken as the analysis object, and the hoisting idea of “low-altitude hoisting, overall lifting” is adopted. Because the span of the high-altitude steel-structure connecting bridge is small and the installation height is high, the influence of wind load on the hoisting process cannot be ignored. Therefore, the unidirectional fluid-structure coupling model of the high-altitude small-space steel-structure connecting bridge in the third zone was established by using CFD calculation software ANSYS Fluent 2022 R1, and the corresponding flow field and solid calculation results were obtained and analyzed. The analysis results show that the lifting structure still produces a certain deformation when the wind speed is 5 m/s or 10 m/s, and the calculation results show that the stress calculation results are still within the safe range of steel strength for the sensitivity of the lifting structure under wind load. With the increase of wind speed, the local maximum stress of the structure increases greatly, but the overall deformation remains stable, which indicates that the greatest challenge of hoisting steel structures under wind load may be the stability direction rather than the strength, so it is necessary to strictly monitor the displacement deformation of the structure during construction. Then, through the monitoring of the overall lifting process of the high-altitude, large-span, steel-connected structure of Zhuhai Tiejian Square from pre-lifting to formal lifting, real-time monitoring and data analysis show that the lifting process of the high-altitude steel bridge of Zhuhai Tiejian Square is safe and reliable, the force transformation of each component is reasonable, the lifting process is relatively stable, the external environment has little impact, and the expected monitoring effect has been achieved. The calculation simulation and on-site monitoring in this paper can provide theoretical and practical guidance for the construction safety under the influence of wind load in the construction process of high-altitude steel-structure hoisting, and provide an important reference value for similar projects.

Injection Micropile Bar Fatigue Resistance at Loads Lower and Greater than the Yield Strength of Steel

One of the techniques commonly applied today for deep foundation construction is based on self-drilling injection micropiles. Micropiles are structural elements intended primarily for reinforcing foundations and buildings, particularly under difficult terrain conditions. The goal of the tests presented herein is to inspect the fatigue resistance, strength and ductility of injection micropiles formed from 28Mn6 steel at loads significantly exceeding the values defined for the fatigue test in the requirements of the relevant European Assessment Document (EAD). The test results and the micropile bar strain model εM presented in this paper are primarily of interest to designers for the purposes of determining the fatigue resistance of steel micropiles, which find particular application in land degraded by mining activity that is characterised by frequent terrain vibration and mining-induced tremors. None of the R25N injection micropile bars failed during the fatigue resistance testing at 2·106 cycles at a load Fu = 0.7·FRe0.2 (under the yield strength of the 28Mn6 steel) as well as at Fu = 1.0·FRe0.2 and Fu = 1.2·FRe0.2, where the bars operated at the limit of and significantly above the load FRe0.2 which results in stress at the yield point of the 28Mn6 steel. Furthermore, the bar tests conducted at static and cyclic loading demonstrated the high strength and good ductility of the 28Mn6 steel.

Heterostructure mediated high strength and large ductility in novel medium-Mn steels with low Mn content

Medium-Mn steels (MMnS) exhibit superior strength and ductility via the transformation-induced plasticity (TRIP) of retained austenite. However, to realize the partitioning of Mn atoms into retained austenite for thermal stability optimization, long-term intercritical annealing is required. In addition, the high Mn content of MMnS also causes other issues, e.g., high cost, welding, etc. In this work, instead of utilizing the TRIP effect of retained austenite, heterostructure consisting of alternatively distributed lamellar martensite zones and lamellar ultrafine dual-phase (martensite + ferrite) zones was engineered in a MMnS with low Mn content by a conventional rolling and short-term intercritical annealing process. The formation of the heterostructure was attributed to the heterogeneous distribution of Mn, the formation of highly dispersed martensite-austenite (MA) islands in Mn-poor zones and the low diffusion rate of Mn atoms at the intercritical annealing temperature. Tensile test results show that the steel exhibits superb synergy of a high ultimate tensile strength of 1452 MPa, a large ductility of 17 % and a high work hardening capability. The high work hardening capability and large ductility were mainly attributed to the apparent microhardness difference between the lamellar martensite zones and the ultrafine dual-phase zones that caused a strong hetero-deformation induced (HDI) strengthening effect. This study proposes a novel approach to developing high-performance steels, which provides an effective paradigm for addressing the long-established conflicts between mechanical performance, high Mn content and long intercritical annealing period of MMnS.

Simultaneous improvement of strength and uniform elongation of ferrite-bainite steel with heterogeneous networked microstructure

A novel heterogeneous networked dual-phase structure, composed of bainite grains surrounding the ferrite grains and forming a networked structure, was successfully introduced into the low-carbon ferrite-bainite steel through a two-step heat treatment process. The ferrite-bainite steel with the networked structure exhibited a high ultimate tensile strength of 872 ± 39 MPa, a good uniform elongation of 15.59 %, and a high total elongation of 22.79 %. Meanwhile, it possessed excellent work hardening behaviors compared to the ferrite-bainite steel with an island feature. The improvement of the comprehensive mechanical properties was attributed to the influence of hetero deformation induced (HDI) hardening and the synergistic deformation of the dual-phase structure.

Seismic Performance of Steel-Reinforced High Strength Concrete Joints Considering Bond Slip Effect

This study presents a solution for push-out failure for the staged bond-slip constitutive relationship between the structural steel and high strength concrete, taking into account the concrete strength grade, anchoring length, and stirrup ratio. The critical point coordinates for different stages are determined by the tests of 14 steel-reinforced high strength concrete (SRHSC) specimens. It is observed that with the increase in the concrete strength grade and anchorage length, the ultimate load of the specimens increased significantly, but the influence on the residual bond strength was not significant. The effect of the stirring ratio was mainly manifested in a slight increase in the initial bond strength. The formula for calculating SRHSC characteristic bond strength and characteristic slip value is established, and the bond-slip constitutive relation of SRHSC is proposed based on the tests. The material constitutive model considering the effect of bond-slip is implanted into the software in the case of the ABAQUS finite element platform. The material is applied to the numerical simulation analysis of the SRHSC exterior joints. The rationality and accuracy of the new material are verified by comparing the simulation results with the test results.

A transfer learning strategy for tensile strength prediction in austenitic stainless steel across temperatures

Austenitic stainless steel plays a pivotal role in the nuclear industry with exceptional mechanical properties and corrosion resistance, wherein high-temperature tensile strength (∼290 °C) is a critical factor in ensuring its performance and reliability. This study focuses on the predictive modeling of tensile strength in cast austenitic stainless steel at room and elevated temperatures by developing a transfer learning strategy. The application of a gradient-boosting decision tree algorithm, enhanced by key features derived from room-temperature analyses and augmented with partial high-temperature data, results in significantly increased accuracy. The effective selection of features underscores the stability of critical variables across different temperatures, affirming the efficacy of the proposed transfer learning strategy. The work sheds light on material selection and design for high-temperature applications, and lays the groundwork for future exploration into predictive modeling of material properties in extreme conditions.

CORRELATION BETWEEN COMPRESSIVE AND TENSILE SPLITTING STRENGTH OF GEOPOLYMER FIBER CONCRETE

Geopolymer concrete mixture contain fine and coarse aggregate, fly ash and activator, composed from sodium silicate and sodium hydroxide. Like normal concrete, geopolymer concrete is weak in terms of tensile strength, so fiber needs to be added to improve the mechanical properties of geopolymer concrete. In this research nylon and steel fiber are added to the geopolymer concrete mixture, with percentage from 0.5%, 0.75%, 1%, 1.5% and 2% of volume. The correlation between compressive and tensile splitting strength of geopolymer steel and nylon fiber concrete were evaluated in this research program. The relationship between tensile splitting strength and compressive strength of geopolymer concrete will be built in a mathematical model using multivariable non-linear regression from SPSS software. The specimens are cylindrical concrete, which have diameter of 100 mm and height of 200 mm. The specimen undergo a curing process in an oven at 60oC for duration of 24 hour. Geopolymer concrete mixture with a fiber percentage of 0.75% have a good workability, since they still meet the slump value requirements (10 + 2 cm). The compressive strength reaches the highest value at 0.75%, for both steel and nylon fibers. The addition of up to 2% steel fiber produces a splitting tensile strength 12.06% higher than geopolymer concrete with nylon fiber. The equation model proposed for geopolymer fiber concrete is fspt = 0.049f'c.exp(0.7p), with the R-square values from 0.722 - 0.715.

Response Modification Factor of High-Strength Steel Frames with D-Eccentric Brace Using the IDA Method

The design innovation of high-strength steel frames paired with D-eccentric bracing exhibits remarkable resistance to plastic deformation during seismic events. This method strategically combines regular steel connections (with yield strengths below 345 MPa) and high-strength steel beams and columns (such as Q460 or Q690, with yield strengths over 460 MPa), effectively reducing cross-sectional sizes while preserving the elasticity of non-energy-dissipating members. This configuration results in substantial ductility and superior energy dissipation capabilities. The response modification factor (R) is vital for achieving both effective and economical seismic resilience, particularly in the development of efficient and cost-effective seismic designs. However, the 2016 edition of the Code for Seismic Design of Buildings (GB50011-2010) fails to incorporate the concept of R, opting instead to apply a uniform value to all structural systems. This oversight is fundamentally flawed, necessitating a comprehensive investigation into the R value specifically for the high-strength steel frame with a D-eccentric brace. This research primarily aims to improve structural performance design, provide guidance for future projects, and encourage the adoption of this advanced seismic performance structure in earthquake-prone areas. To achieve these objectives, a performance-based seismic design approach is employed. This method involves designing structures with varying numbers of stories (4, 8, and 12), different link lengths (900, 1000, and 1100 mm), and various steel strengths (Q460 and Q690). This study uses the Incremental Dynamic Analysis (IDA) method to determine the R values for each prototype. The derived performance coefficients act as crucial references for the development of future innovative structural designs. This research greatly enhances seismic design practices and facilitates the wider adoption of high-strength steel frames with D-eccentric braces due to their outstanding seismic performance.