Effect of deformation parameters on the austenite dynamic recrystallization behavior of a eutectoid pearlite rail steel

Understandings of the effect of hot deformation parameters close to the practical production line on grain refinement are crucial for enhancing both the strength and toughness of future rail steels.In this work,the austenite dynamic recrystallization(DRX)behaviors of a eutectoid pearlite rail steel were studied using a thermo-mechanical simulator with hot deformation parameters frequently employed in rail production lines.The single-pass hot deformation results reveal that the prior austenite grain sizes(PAGSs)for samples with different deformation reductions decrease initially with an increase in deformation temperature.However,once the deformation temperature is beyond a certain threshold,the PAGSs start to increase.It can be attributed to the rise in DRX volume fraction and the increase of DRX grain with deformation temperature,respectively.Three-pass hot deformation results show that the accumulated strain generated in the first and second deformation passes can increase the extent of DRX.In the case of complete DRX,PAGS is predominantly determined by the deformation temperature of the final pass.It suggests a strategic approach during industrial production where part of the deformation reduction in low temperature range can be shifted to the medium temperature range to release rolling mill loads.

Microstructural Evolution of Pearlite in Eutectoid Fe-C Alloys during Severe Cold Rolling

The microstructural evolution of pearlite during severe cold rolling in Fe-0.8C binary alloy and Fe-1Mn-0.8C ternary alloys was investigated by using SEM, TEM and XRD etc. The results show that the deformed pearlite consists of irregularly bent lamella, coarse lamella with shear-band and fine lamella. As the rolling reduction increases, the proportion of fine lamella increases. The strong plastic deformation, amorphization and dissolution of cementite take place during the severe cold rolling. The maximum carbon content in ferrite reaches 0.15 mass% after 90% cold rolling.

INFLUENCING FACTORS FOR OBTAINING ULTRAFINE AUSTENITE GRAINS WITH INITIAL MICROSTRUCTURE OF WARM-ROLLED FERRITE/PEARLITE

Three warm-rolled ferrite/pearlite microstructures were prepared by rolling at 500℃, and the austenitizing characteristics were discussed in conjunction with deformation during the heating stage. The results indicated that the final austenite grain size was sensitive to the deformation direction of the initial warm-rolled microstructure. The transient microstructure at a given temperature was the most important influencing factor on the austenitizing characteristic combined with deformation. Moreover, the hot-rolled mierostructure also had to be prepared in an optimal state because of its direct effect on the warm-rolled microstructure.

Microstructure Evolution and Its Effects on the Mechanical Behavior of Cold Drawn Pearlite Steel Wires for Bridge Cables

The microstructure evolution and its effects on the mechanical performance of 2000 MPa bridge cable steel wires were investigated by transmission electron microscope(TEM),electron backscatter diffraction(EBSD),X-ray diffractometer(XRD)and mechanical tests.Experimental results reveal that,with the increasing strain from 0 to 1.42,a fiber structure and a<110>fiber texture aligned with the wire axis are gradually developed accompanied by cementite decomposition and the formation of sub-grains;the tensile strength increases linearly from 1510 to 2025 MPa,and the reduction of the area is stable with a slight decline from 44%to 36%.After annealing at 450℃for different times,pronounced changes in the microstructure occur.Cementite lamella fragment into coarser globules corresponding to a remarkable spheroidization process,while ferrite domains recover and recrystallize,and this process is associated to modifications in the mechanical properties.Furthermore,based on the observations on dislocation lines crossing through cementite lamellae,a possible mechanism of cementite decomposition is discussed.

The Influence of Abnormal Segregation Band on Mechanical Properties of Hot Rolled Ferrite/Pearlite Steel Plate

In order to further reveal the influence of abnormal segregation band on mechanical properties of hot rolled ferrite/pearlite steel plate, especially on laminated tensile fracture, the experimental method of delamination tension was adopted. In this paper, the thin tensile samples with 3 mm thickness from the surface, 1/4 positions and center along the thickness orientation of test plate were measured, also the relationship between microstructure and mechanical properties was probed. The results show that the center region of hot rolled ferrite/pearlite steel plate exists granular bainite and ferrite mixed grains, which leads to lower plasticity and toughness of this region. During the tensile process, microcracks are generated and extended at the center of steel plate due to the inconsistency of deformation and fracture on the adjacent structures, finally leading to laminated fracture of steel plate.

Effect of Hot Deformation on Pearlite Transformation of 86CrMoV7 Steel

The continuous cooling transformation (CCT) diagrams of 86CrMoV7 steel samples including hot deformed and not hot deformed were constructed by dilatometry, metallography and transmission electron microscopy (TEM). The results showed that hot deformation accelerated pearlite transformation and fine pearlite microstructure. Moreover, the undissolved carbides became the nucleating sites of pearlite, accelerated pearlite formation and fine pearlite if the steel had been deformed at high temperature. In contrast, undissolved carbides did not make any influence on pearlite transformation if the steel had not been deformed at high temperature.

THERMODYNAMIC EXPLANATION OF LEADING PHASE IN PEARLITE TRANSFORMATION

In this paper a concept of 'leading probability' is presented. The difference in the leading probability between ferrite and cementite depends mainly upon the difference between their driving forces at the beginning of precipitation. The results of theromdynamic calculations showed that the leading probability of cementite increased with the increase of carbon concentration of austenite, and the decrease in transformation temperature was favourable to cementite's being the leading nucleus during pearlite transformation.

Effect of Pearlite Interlamellar Spacing on Predominant Abrasive Wear Mechanism of Fully Pearlitic Steel

The aim of this investigation was the determination of the predominant wear mechanism on three-body abrasion of fully pearlitic low alloy steel. Furthermore, the effect of pearlite interlamellar spacing on wear behavior was investigated. For this purpose, the samples were subjected to the different heat treating to attaining different interlamellar spacing. Mechanical properties such as hardness, yield strength, tensile strength, elongation, and impact toughness were evaluated. Three body abrasion tests were conducted under ASTM standard condition using a rubber wheel abrasion test apparatus. Abraded surface and wear debris were investigated by light optical microscopy and scanning electron microscopy. The results showed that wear resistance of fully pearlitic steel depended to pearlite interlamellar spacing the and lower spacing has the greater wear resistance, so it may be due to subsurface work hardening and interlamellar spacing and cementite in fine and/or coarse pearlite, that influence on surface destruction during wear. Although during wear process the several mechanisms play roles, but study of surface and debris shows that with decreasing interlamellar spacing, the predominant mechanism wear changed from ploughing to cutting mode.

Effect of Metallic, Nonmetallic, Water Cooled and Cryogenic Chills on Pearlite Content (PC), Eutectic Cell Count (ECC) and Grain Size (GS) of Hypo Eutectic Nickel Alloyed Cast Iron

This paper presents the results obtained, deductions made from solidification behaviour and a series of micro structural studies such as pearlite content, eu-tectic cell count and grain size of hypoeutectic gray cast iron which was sand cast (CO2 moulding) using metallic, nonmetallic, water cooled and subzero (cryogenic) end chills. Hypo-eutectic cast irons containing C 3.42, Si 2.4 and Ni 1.5 with impurity contents (S, P, Mn etc.) were solidified unidirectionally in an American Foundrymen Society (AFS) standard mould, the end of which was provided with different end chills to study the effect of chilling during solidifi-cation. The melts were inoculated with 0.3% Fe-Si to promote graphitization. It was observed that the transition from one structure to another is more gradual than normally obtained in the structure of cast irons solidified mul-ti-directionally in a sand mould at room temperature. Austenite dendrite interactions were shown to be a major factor in determining the microstructure, in which the higher dendrite reaction leads to changes in DAS, ECC and GS. It is observed that, the number of eutectic cells is an index of graphite nucleation and the effect of these on structure, since the eutectic cells are developed on the graphite nuclei during solidification.

Predication of Plastic Flow Characteristics in Ferrite/Pearlite Steel Using a Fern Unit Cell Method

The flow stress of ferrite/pearlite steel under uni-axial tension was simulated with finite element method (FEM) by applying commercial software MARC/MENTAT. Flow stress curves of ferrite/pearlite steels were calculated based on unit cell model. The effects of volume fraction, distribution and the aspect ratio of pearlite on tensile properties have been investigated.

THEORETICAL PREDICTION OF THE KINETICS CURVES OFPEARLITIC TRANSFORMATION IN HYPO-PROEUTECTOID STRUCTURAL STEELS

Supposing carbon contents of ferrite phases in pearlite precipitating from austenite in multicomponent steel at temperature T and in Fe-C ystem at T' are the same the pearlite formation temperature diference, can be calculated from the FeX phase diagrams and the equilibrium temperature Al. Using Tp and Fe-C binary thermodynamic model, the driving forces for phase transformation from austenite to pearlite in multicomponent steels have been successfully calculated. Through the combination of simplified Zener and Hillert's model for pearlite growth with Johnson-Mehl equation, using data from known TTT diagrams, the interfacial energy parameter and activation energy for pearlite formation can be determined and expressed as functions of chemical composition in steels by regression analysis. The calculated starting curves of pearlitic transformation in some commercial steels agree well with the experimental data.

VARIATION OF SUBSTRUCTURES OF PEARLITIC HEAT RESISTANT STEEL AFTER HIGH TEMPERATURE AGING

The observations of dislocations, substructures and other microstructural details were conducted mainly by means of transmission electron microscope (TEM) and scanning electron microscope (SEM) for 12CrlMoV pearlitic heat-resistant steel. It is shown that during the high temperature long-term aging, the disordered and jumbled phase-transformed dislocations caused by normalized cooling are recovered and rearranged into cell substructures, and then the dislocation density is reduced gradually. Finally a low density linear dislocation configuration and a stabler dislocation network are formed and ferritic grains grow considerably.

MICROSTRUCTURE EVOLUTION AND MECHANICAL HARDENING OF HYPEREUTECTOID PEARLITIC STEEL DURING COLD ROLLING

The microstructure evolution of different cold rolling reductions （from 0 to 81.6%） was studied by SEM （scanning electron microscopy） and TEM （transmission electron microscope）. The study showed that the orientation multiplicity of pearlitic lamellas resulted in inhomogeneous deformation of different pearlitic lamellas, and with the increase of reduction, the microstructure underwent a course of ＂homogeneity → inhomogeneity → homogeneity＂. The result of XRD （X-ray diffraction） analysis indicated that cementite did not decompose and dissolve into ferrite; the results of the mechanical property test suggested that the relationship between Rp0.2 （yield strength） and ε （true strain） was in good agreement with Hollomon relationship. With the equation Rp0.2 = 1465ε^0.18, the yield strength of the steel in different reductions could be well predicted.

Formation kinetics of austenite in pearlitic ductile iron

This work evaluated the isothermal transformation of austenite in unalloyed pearlitic ductile iron and drew the isothermal phase diagram of austenitization in the ductile iron, Austenite forms at grain boundaries and then grows up to graphite regions during austenitiza- tion. The formation kinetics of austenite complies with the Avrami equation, in which the parameter （n） ranges from 4.71 to 4.99. The start time and finish time of transformation can be calculated at each temperature using the Avrami equation.

New pearlitic steels for 21st century rail applications

To improve competitiveness,the nation's railroads have increased the axle loads and speed of the trains.This has led to a rapid decrease in the life expectancy of premium rails through accelerated wear,rolling contact fatigue and fracture.To counter this effect,the railroads need rails that exhibit better performance in these areas.A research program has been initiated to study the microstructural aspects of near-eutectoid steels that would improve these properties.The first phase of the work was to carefully characterize the existing commercial rail steels in terms of pearlite interlamellar spacing,steel cleanliness and the presence of pro-eutectoid cementite on prior-austenite boundaries.These characterizations were then correlated with both mechanical properties and overall rail performance.The second phase of the program was to develop a better microstructure through control of composition,thermomechanical processing and cooling path.This was achieved through the use of laboratory-melted heats of experimental near-eutectoid compositions and a computer controlled MTS compression machine modified for axisymmetric compression testing and subsequent controlled cooling.The optimum processing route for these new steels has been determined,and pilot-scale heats have been melted,hot rolled and cooled using the information gained from the MTS investigations.The mechanical properties of these new steels have been determined and the rail performance tests are being conducted using laboratory-scale evaluation.Ultimately,these new rail steels will be tested under commercial conditions on the TTCI test track in Pueblo,Colorado.This paper will report on the alloy and processing design and resulting properties of the steels developed in this research program.Guidelines for future rail compositions and processing to obtain improved properties and performance will be presented.

A microstructure-based analysis of cyclic plasticity of pearlitic steels with Hill's self-consistent scheme incorporating general anisotropic Eshelby tensor

A pearlitic steel is composed of numerous pearlitic colonies with random orientations, and each colony consists of many parallel lamellas of ferrite and cementite. The constitutive behavior of this kind of materials may involve both inherent anisotropy and plastic deformation induced anisotropy. A description of the cyclic plasticity for this kind of dual-phase materials is proposed by use of a microstructure-based constitutive model for a pearlitic colony, and the Hill＇s self-consistent scheme incorporating anisotropic Eshelby tensor for ellipsoidal inclusions. The corresponding numerical algorithm is developed. The responses of pearlitic steel BS 11 and single-phase hard-drawn copper subjected to asymmetrically cyclic loading are analyzed. The analytical results agree very well with experimental ones. Compared with the results using isotropic Eshelby tensor, it is shown that the isotropic approximation can provide acceptable overall responses in a much simpler way.

A MICROSTRUCTURE-BASED ANALYSIS FOR CYCLIC PLASTICITY OF DUAL-PHASE PEARLITIC RAIL STEELS

Based on the assumption that a representative element of apearlitic steel is an aggregate of numerous spherical pearliticcolonies with randomly distributed orientations, and that each colonyis com- posed of many parallel fine lamellas of ferrite andcementite, a description for the dual-phase pearlitic steel isobtained by making use of a microstructure-based constitutiveequation for a single dual-phase pearlitic colony and the Hill'sself-consistent scheme. The elastoplastic response of dual-phasepearlitic steel BS11 subjected to asymmetrically cyclic loading isanalyzed, and a comparison with the experimental results showssatisfacto- ry agreement. The non-proportional cyclic plasticity ofBS11 is also analyzed, in which stress develops along a semi-circlein a biaxial tension/compression and shear stress plane, as istypically experienced by the sur- face elements in rolling andsliding contact.

Characteristic of Near-surface Microstructure and Its Effects on the Torsion Performance of Cold Drawn Pearlitic Steel Wires for Bridge Cables

The characteristic of near-surface microstructure and its effects on the torsion performance of cold-drawn pearlitic steel wires for bridge cables were investigated by focused ion beam-scanning electron microscope,transmission electron microscopy and differential scanning calorimetry.The samples with similar tensile strength before and after hot-dip galvanizing process are,respectively,characterized as delaminated and non-delaminated in torsion test which indicates that the tensile strength is independent of the toughness value(i e,reduction area and torsion ability).It is interesting to find that there exists submicron granular ferrite on near-surface of the wires,which can be attributed to dislocation rearrangement and sub-grains rotation during cold drawing and hot-dip galvanizing process.And their distribution can suggest homogeneousness of deformation degree to a certain extent:the closer to the surface of their distribution,the more homogeneous deformation of the wires.There is a close relationship between the thermal stability of the cementite layer and distribution of granular ferrite:differential scanning calorimetry(DSC)analysis shows that the sample is accompanied by submicron granular ferrite which is located closer to the surface has higher thermal stability under galvanizing temperature(450°C).A new mechanism of the torsion delamination of pearlitic steel wires is discussed in terms of the thermal stability of the cementite layer and distribution of granular ferrite.

Continuous Wave Diode Laser Surface Texturing of Austenitic and Pearlitic Steels

Microstructuring of steel resulting in directional solidification and texturing, previously observed in various metallic materials during pulsed laser processing, melt-spinning, high-gradient liquid metal melting, zone melting etc., is reported for the first time in continuous wave diode laser processing of steels. Influence of laser interaction time on surface morphology/topology of austenitic manganese and pearlitic steels is investigated utilizing a wide rectangular multi-mode diode laser beam. X-ray diffraction analysis of the laser treated austenitic steel surface showed strong texturing influence, with preferred crystallographic orientation of γ-Fe crystals in the (200) plane, which increased with interaction time. In case of pearlitic steel, no such texturing influence could be observed. The free surface topologies were also observed to be different in each case, with well-aligned domes of γ-Fe observed in laser treated austenitic steel as compared to randomly oriented fine domes of metal oxides in pearlitic one. In situ surface temperature measurement during laser irradiation indicated higher temperature on pearlitic steel than in austenitic manganese steel owing to its lower effective thermal conductivity associated with higher oxide film formation.

Modelling of temperature and strain rate dependent behaviour of pearlitic steel in block braked railway wheels

Block braked railway wheels are subjected to thermal and rolling contact loading.The thermal loading results in high temperatures and thermal stresses which cause slow time dependent processes such as creep,relaxation and static recovery of the wheel material.At the same time,the rolling contact loading implies a very fast mechanical load application.This paper is focused on material modeling of pearlitic steel for a wide range of loading rates at elevated temperatures.The starting point is a viscoplasticity model including nonlinear isotropic and kinematic hardening.The Delobelle overstress function is employed to capture strain rate dependent response of the material.The model also includes static recovery of the hardening to capture slower viscous(diffusion dominated)behaviour of the material.Experiments for the pearlitic wheel steel ER7 in terms of cyclic strain-controlled uniaxial tests with hold-time,uniaxial ratchetting tests including rapid cycles and biaxial cyclic tests with tension/compression and torsion are used to calibrate the material model.These experiments were performed under isothermal conditions at different temperatures.In the ratchetting tests,higher loading rates are obtained and these have been used to calibrate the high strain rate response of the viscoplasticity model.The paper is concluded with a numerical example of a block braked wheel where the importance of accounting for the viscoplasticity in modelling is highlighted.