High-strength Rail Steel Research Articles

Use of a modified critical fracture strain model for fracture toughness estimation of high strength rail steels

This paper explores the feasibility of estimating fracture toughness (KIC,pred) for nine high-strength rail steels using data from smooth tensile and instrumented ball indentation test methods via a modified critical fracture strain model. The modification focuses on determining the equivalent plastic fracture strain and equivalent plastic strain from tensile and indentation tests, respectively, as well as the characteristic distance needed for estimation of fracture initiation. It shows that stress triaxiality plays a vital role by decreasing the ductility required to initiate fracture, based on the kinetic damage evolution law for a multiaxial stress state. A void growth index model was established from average grain sizes, and then used as the characteristic distance for fracture initiation. At the end, the KIC,pred defined based on equivalent plastic strain from tensile tests showed a strong correlation with KIC determined from ASTM standard method. KIC,pred from indentation also offers the opportunity for ascertaining the possibility of estimating the fracture toughness of high strength rail steels, non-destructively by focusing on the equivalent plastic strain and pressure at the tip of the indenter.

Indentation Testing Method for Determining Mechanical Properties and Tensile Flow Curve of High-Strength Rail Steels

Indentation Testing Method for Determining Mechanical Properties and Tensile Flow Curve of High-Strength Rail Steels

A coupled stress-triaxiality-dependent damage viscoplasticity model on crack initiation and propagation in high-strength rail steel

In this paper, a continuum damage enhanced formulation of effective (undamaged) material properties is proposed to predict fracture behaviour of various notched tensile specimens and a compact tension (CT) specimen for a high-strength rail steel that is commonly known to be insensitive to change in hydrostatic stress. The presented constitutive model considers a thermodynamically nonlinear, isotropic continuum damage, and stress-triaxiality-dependent effective stress using a viscoplastic regularization method to reduce strain and damage localization at the sharp notch tip. Conventional loading-unloading tensile test for damage measurement and monotonic tensile test on a notched specimen are used to calibrate the constitutive equation which consists of a set of eight model parameters. Further validation is performed on the geometry transferability of the model parameters to three different notched tensile specimens in the high stress triaxiality regime. The calibrated constitutive model is then applied to simulate crack initiation and propagation in the mode I plane-strain condition, where the computed result of load versus crack mouth opening displacement shows good agreement with those determined from CT specimens. The study concludes that the critical damage parameters for fracture are relatively constant among different notched specimens of high-strength rail steel, and thus as a proper fracture criterion for crack initiation at and crack propagation from the sharp notch tip. The study also found that the complementing mechanisms for the stress-triaxiality-dependent damage evolution and strain-hardening-induced enhancement of effective matrix material properties, respectively, result in the overall hydrostatic-stress insensitivity, and thus is often regarded as a material that belongs to the classical J2 plasticity.

Comparative evaluation of austenite grain in high-strength rail steel during welding, thermal processing and plasma surface hardening

In the process of plasma surface hardening, a surface layer that consists of zones of different sizes with different phase and structural composition is formed. The minimum grain size for plasma surface heating is determined by the initial austenitic grain. Its size depends on the dispersion of the initial structure. The rate of plasma heating at 200-1000 °C/s affects the size of the initial grain. The further growth of austenite crystallites with an increase of temperature essentially depends on the heating rate: small rates and high temperatures of plasma surface heating can lead to a significant enlargement of the grain. Increase of the heating temperature in the zone of thermal influence during welding from 900 to 1040 °C of rail steel 76F leads to the growth of the austenitic grain from No. 9 to No. 7 and individual grains – from No. 8 to No. 3. The largest austenite grains while heated at 1000 and 1040 °C form separate zones where conglomerates of large grains predominate. Stiffness of 76F steel, heat-strengthened on the troosto-sorbitol structure, deteriorates substantially when the temperature of rapid heating increases from 900 to 1040 °C. This also increases the instability of stiffness.

Fracture toughness estimation for high-strength rail steels using indentation test

In this paper, tensile properties and mode I critical stress intensity factor (KIc) are measured on the railhead of six types of high-strength rail steels using standard testing methods. An instrumented ball indentation testing method is then developed to estimate fracture toughness for the six rail steels based on continuum damage mechanics. Critical damage parameter, determined by combining repeated loading-unloading tensile tests and a ductile damage model, is used to characterize a critical contact depth under indentation that is used to calculate the specific indentation energy, and thus the fracture toughness for indentation (KInd). Comparison between KInd and KIc for the six high-strength rail steels suggests that the current indentation testing method can successfully rank the six high-strength rail steels for their fracture toughness.

Constitutive analysis of pressure-insensitive metals under axisymmetric tensile loading: A stress triaxiality-dependent plasticity damage model

In this paper, by separately characterizing the influence of stress triaxiality (ratio of hydrostatic stress to von Mises stress) on the stress response to plastic deformation and to damage evolution, the constitutive equation of the pressure-insensitive metal under axisymmetric tensile loading is defined as a new stress triaxiality-dependent plasticity damage model. For this purpose, experimental testing and finite element modelling are performed based on smooth and notched round bars, machined from a high-strength rail steel. The test results show that mechanical behaviour of both types of tensile specimens can be closely simulated based on the same constitutive equation calibrated following the classical J2 flow theory; even though stress triaxiality and damage evolution generated in these two types of specimens are very different. Such an issue is investigated by implementing the new constitutive model to finite element modelling of smooth and notched specimens using the ABAQUS user subroutine VUMAT. It is found that the rates of both damage-free stress response to deformation and damage evolution increase with increasing stress triaxiality. Also, the increase of damage-free stress response to deformation, caused by the increase of stress triaxiality, is offset by the decrease of stress due to the increase of damage evolution. This results in the constitutive equation insensitive to the change of stress triaxiality.

Indentation for fracture toughness estimation of high-strength rail steels based on a stress triaxiality-dependent ductile damage model

Fracture toughness can be estimated using indentation test (KInd) based on continuum damage mechanics, usually with the assumption that the critical damage parameter (Dcr) of a standard tensile (smooth) specimen is equivalent to that at the crack tip. In view of the effect of stress triaxiality on damage development, KInd is determined in this work based on the Dcr at the crack tip. In addition, an adjusting parameter κ is introduced to reduce the Dcr value to reflect the potential difference in damage development between indentation compression and tensile fracture. The above approach is applied to three types of high-strength rail steels to determine their KInd values. Two types of notch-free (smooth and short-gauge) specimens are used to calibrate a ductile damage model for rail steels, and two additional types of round-notched specimens are used to establish the locus of plastic fracture strain versus stress triaxiality. The damage evolution and Dcr at the crack tip are extrapolated for the three rail steels. The so-determined Dcr is then applied to the indentation test, with κ as the adjusting parameter, to determine the critical contact depth for calculating KInd. Results show that Dcr is indeed dependent on stress triaxiality, increasing with stress triaxiality under tensile loading. Also, it is found that the change in KInd based on Dcr either at the crack tip or from the smooth specimen is generally consistent with the difference in the measured fracture toughness (KIc) among the three rail steels, thus, justifying the validity of using Dcr from the smooth specimen to estimate KIc of ductile materials by indentation test.

Hydrogen embrittlement behavior of high strength rail steels: A comparison between pearlitic and bainitic microstructures

The present study was attempted to evaluate the hydrogen embrittlement (HE) behavior of two high strength rail steels with pearlitic and bainitic microstructures by using slow strain rate test (SSRT) with notched round bar specimens. The results show that the bainitic rail steel with a mixed microstructure of bainite, tempered martensite and M/A constituents exhibits superior mechanical properties in comparison with the pearlitic rail steel. The results of SSRT revealed that the bainitic rail steel is more susceptible to HE than the pearlitic rail steel, which is ascribed mainly to the higher strength level and the microstructural characteristics of the bainitic rail steel. The attempt of re-tempering treatment of the as-received bainitic rail steel exhibits no notable improvement in the resistance to HE at no expense of strength. It is thus suggested that further efforts concerning obtaining low susceptibility to HE besides obtaining excellent combination of strength and toughness should be conducted to guarantee the safety service of the bainitic steel rails.

Fracture behaviour at the sharp notch tip of high strength rail steels – Influence of stress triaxiality

This work explores the possibility of using mechanical properties of two types of notch-free specimens to evaluate crack growth resistance of a sharp notch tip. Equivalent plastic fracture strain (εeqf,ηav) of pre-cracked single-edged-notched bending (SENB) specimen is estimated using a model that correlates εeqf,ηav for notch-free specimens with their average stress triaxiality. Finite element modelling is used to establish constitutive equations and variations of stress triaxiality in the two types of notch-free specimens and the pre-cracked SENB specimen. All of the information is then used to predict the critical strain energy density factor (Sc) at a characteristic distance ahead of the sharp notch tip and the corresponding stress intensity factor (KSc) for three high strength rail steels. The predicted KSc values are compared with the experimentally measured mode I critical stress intensity factor (KIc) at temperatures 23, −10, and −40°C. The results show good agreement between KSc and KIc in their variation with temperature, possibly because Sc takes into account both distortional and dilatational strain energy density of the sharp notch tip under small-scale yielding. Results from the study also suggest that a correlation should exist among mechanical properties for notch-free and notched specimens, though their apparent trend of change with temperature may not bear any similarity.

Characterisation of head-hardened rail steel in terms of cyclic plasticity response and microstructure for improved material modelling

Stress- and strain-controlled tests of heat treated high-strength rail steel (Australian Standard AS1085.1) have been performed in order to improve the characterisation of the said material׳s ratcheting and fatigue wear behaviour. The hardness of the rail head material has also been studied and it has been found that hardness reduces considerably below four-millimetres from the rail top surface. Historically, researchers have used test coupons with circular cross-sections to conduct cyclic load tests. Such test coupons, typically five-millimetres in gauge diameter and ten‐millimetres in grip diameter, are usually taken from the rail head sample. When there is considerable variation of material properties over the cross-section it becomes likely that localised properties of the rail material will be missed. In another case from the literature, disks 47mm in diameter for a twin-disk rolling contact test machine were obtained directly from the rail sample and used to validate ratcheting and rolling contact fatigue wear models. The question arises: How accurate are such tests, especially when large material property gradients exist? In this research paper, the effects of rail sampling location on the ratcheting behaviour of AS1085.1 rail steel were investigated using rectangular-shaped specimens obtained at four different depths to observe their respective cyclic plasticity behaviour. The microstructural features of the test coupons were also analysed, especially the pearlite inter-lamellar spacing which showed strong correlation with both hardness and cyclic plasticity behaviour of the material. This work ultimately provides new data and testing methodology to aid the selection of valid parameters for material constitutive models to better understand rail surface ratcheting and wear.

Effect of temperature on deformation and fracture behaviour of high strength rail steel

Fracture toughness (KIc), Vickers hardness, and constitutive equation under tensile loading for high strength rail steel were determined at −40, −10 and 23°C. KIc and hardness were directly determined from experimental testing, while the constitutive equation also relying on finite element (FE) modelling because of the non-uniform deformation during the necking process. The constitutive equation was verified by enabling the FE model of tensile specimen to regenerate the experimentally measured load–elongation curve and cross-section reduction, and applied to the FE model of Vickers indentation to mimic the load–depth relationship. This paper also discusses relevance of the results with those reported in literature.

Ratcheting behaviour of high strength rail steels under bi-axial compression–torsion loadings: Experiment and simulation

Experimental studies were carried out to investigate the ratcheting behaviour of three high strength rail steels of similar nominal hardness but with different chemical compositions subjected to uniaxial and non-proportionally bi-axial compression–torsion cyclic loading conditions. Different axial stress and equivalent shear stress amplitudes and different non-proportional loading paths were considered. Experimental results show that an obvious cyclic softening (i.e., the stress amplitude decreases with the increase of cyclic number) occurs in all three steels under uniaxial strain cycling. The ratcheting strain and ratcheting strain rate increase with the axial stress and the equivalent shear stress amplitudes under bi-axial compression–torsion stress cycling. Moreover, both ratcheting strain and ratcheting strain rate are strongly influenced by the non-proportional loading path. Among the three rail steels, it is found that the low alloy heat-treated rail steel grade has a better resistance to ratcheting than the two hypereutectoid rail steel grades. The hypereutectoid rail steel grade with a higher carbon content gives a lower ratcheting strain and a lower ratcheting strain rate than the hypereutectoid rail steel grade with a lower carbon content under higher loading amplitude. To simulate the ratcheting behaviour of the high strength rail steels, an existing cyclic plasticity model was modified by coupling a non-proportionally multi-axial parameter into isotropic softening and kinematic hardening rules. The method to calibrate the material parameters for the plasticity model and the simulated results validated with experimental data for the three studied rail steels are presented in the paper. This modified plasticity model with the calibrated material data from the experimental study can be applied to investigate the ratcheting behaviour of the three rail steels under wheel–rail cyclic rolling contact in practice.

On Constitutive Models for Ratcheting of a High Strength Rail Steel

The ratcheting behaviour of a hypereutectoid high strength rail steel with carbon content of 0.85% was experimentally studied under both uniaxial and bi-axial cyclic loadings recently by the authors. To numerically simulate the multiaxial ratcheting behaviour of the rail steel, the Abaqus built-in Lemaitre-Chaboche model was applied first in current study. Following Abaqus documentation, the material data for the Lemaitre-Chaboche model were calibrated from the uniaxial loading test results. Comparing with experimental data, the Lemaitre-Chaboche model with the calibrated data provides overpredictions for the ratcheting responses of the rail steel under both uniaxial and bi-axial loadings. After that, a modified cyclic plasticity model with a coupling multiaxial parameter in the isotropic and kinematic hardening rules was applied for the material. The material data for this modified model were calibrated from both uniaxial and bi-axial loading tests. Comparison between the simulated results and the experimental data show that this modified cyclic plasticity model has the capacity to simulate both uniaxial and multiaxial ratcheting behaviour of the hypereutectoid rail steel with an acceptable accuracy.

High strength rail steels—The importance of material properties in contact mechanics problems

The importance of material properties for solving rolling contact fatigue (RCF)—contact mechanics problems is shown. In addition to parameters like loading, geometry and friction, the contribution of the material to the system is very important. The determination and modelling of these material properties under the characteristic load conditions of a wheel–rail contact are unsolved problems in the scientific field of contact mechanics. A closer look at these material properties could contribute to a global solution.

Effect of initial-carbide morphology on the structure and properties of high-strength rail steels after quenching from induction heating

1. High strength in combination with sufficient plasticity and ductility are achieved in high-carbon rail steel by the production of a fine-grain highly disperse structure and substructure as a result of rational alloying with Cr, Si, and Mn, and complex modification with V, Ti, and Ce together with combined heat treatment, where spheroidizing annealing to disperse granular pearlite is used as preliminary treatment. 2. Globular carbides, which ensure the slowing of austenite-grain growth and size reduction in the structure, are retained to a certain degree in the γ-solid solution when lowalloy rail Cr−Si−Mn steel, which has been pretreated to disperse granular pearlite, is subjected to final heat treatment using induction heating. In this case, the structure of the martensite is formed with plates having acicular and packet morphology and with an insignificant number of twins.

Effect of chemical composition on the hardenability of high-strength rail steel

1. The hardenability nomograms developed for high-strength rail steels make it possible to select the composition of steel with a given hardenability, the minimal permissible value of which depends on the operating conditions. 2. An even structure of martensite in the head of the rail to a depth of 12–15 mm from the rolling surface (under modern operating conditions) is achieved by complex alloying of high-carbon steel with chromium, silicon, and manganese at a total concentration of at least 2.8 at.%. 3. On the basis of the results we recommend the following chemical composition for high-strength rails with differentiated strength through the section: 0.75–0.85% C, 1.0–1.2% Mn, 0.3–0.5% Si, 1.2–1.5% Cr, and 0.005% Ti or 0.03–0.08% V.

Effect of chemical composition on properties of high-strength rail steels with thermomechanical damage

1. Alloying of rail steel containing 0.75% C with manganese or chromium has a positive effect on its susceptibility to thermomechanical damage at low temperatures. Microadditions of titanium have no effect on susceptibility to thermomechanical damage. 2. Raising the carbon content of high-strength rail steel reduces the work of fracture of samples with thermomechanical damage in impact bending tests, although this is partially due to a change in the overall toughness of the steel. It was found that raising the carbon content from 0.65–0.70 to 0.75–0.85% which is necessary to increase the contact-fatigue strength of the steel, does not substantially lower the resistance to thermomechanical damage.