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.

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.

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.

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.

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.

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.

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.

Effects of Hot Deformation on the Evolution of Microstructure in Pearlitic Steel Wire Rod

The influences of hot deformation parameters on pearlite grain nucleation and growth during austenite-pearlite phase transformation in a steel wire rod have been investigated through quantitative analysis of microstructure parameters such as austenite grain size,ferrite grain size,pearlite colony size,and lamellar spacing.During hot deformation,the austenite grain size decreases due to recrystallization,providing extra nucleation sites for pearlite phase transformation,which decreases the ferrite grain size and pearlite colony size.Moreover,the stored strain energy in undercooled austenite accelerates carbon diffusion during pearlite phase transformation,which facilitates ferrite grain growth and increases pearlite colony size.Consequently,the competing influence of recrystallization and strain energy provides flexibility in adjusting ferrite grain size and colony size by hot deformation.This study highlights the critical role of hot deformation in determining the microstructure of pearlitic steel.

Microstructures in a carburized steel after isothermal pearlitic treatment

The influence of carbon concentration variations on pearlite formation(20 h at 600℃)in a case-carburized steel is investigated.The resultant microstructure shows three distinct regions:carburized case,a transition region,and the original core.The microstructural transition from the case to the core regions is observed to be relatively sharp.The investigated region of the carburized case(0.9 wt.%C)con-tains two types of pearlite:ferrite+cementite and ferrite+M_(23)C_(6),where the pearlitic aggregate with M_(23)C_(6)shows faster formation kinetics.The kinetics of pearlite formation in the transition region(0.3 wt.%C)is very slow and is observed with only M_(23)C_(6)carbide.Only around 40%austenite decomposes into pearlite in the transition region,which,in comparison to the carburized case region of 0.9 wt.%C is a fraction that is lower by a factor of two.Pearlite is absent in the investigated core region(0.16 wt.%C).The microstructure in this region is predominantly martensite and pro-eutectoid ferrite,with a fraction of ferrite well below the equilibrium fraction.Ferrite formation in this region is limited by the redistribution of mainly Ni,Mn,and Cr,and their resulting solute drag effect on the austenite/ferrite interface.A ther-modynamic and kinetic argumentation of these observations is provided with the help of thermodynamic data,precipitation simulations,and a general mixed-mode Gibbs energy balance model.

Structure evolution of the Fe_(3)C/Fe interface mediated by cementite decomposition in cold-deformed pearlitic steel wires

Cold-drawn pearlitic steel wire is irreplaceably used in industry owing to its outstanding mechanical property which is dominated by the cementite/ferrite(Fe_(3)C/Fe) interfaces in the material. However, the fine structures of the Fe3C/Fe interfaces in the deformed wires are less known to date. In this work, transmission electron microscopic investigation was performed on the atomic structures of the interfaces with the Isaichev orientation relationship(OR) in the wires with progressive deformation strains. In addition to the effect of the dislocation/interface interactions, this work revealed that the deformation-induced partial decomposition of cementite plays an important role in the interface reconstruction during deformation. The interfacial carbon vacancies generated by cementite decomposition and particularly, the amorphization of cementite layers in the sample with ε > 1 could effectively annihilated the interfacial dislocations and consequently relaxed the interfacial stress. The correlations between the interface structure changes and the mechanical properties of the wires were discussed.

Fracture Characteristics of Fully Pearlitic Steel Wire in Tension and Torsion

The fracture characteristics of fully pearlitic steel wires with fine and randomly oriented lamellae have been investigated after tension and torsion, respectively. It is found that the predominant fracture mode under small pre- deformation is dimple. The analysis of the colony size and the lamellar structure near the fracture surface indicates that each dimple roots from one colony. A simulation of tensile deformation with several pearlitic colonies based on the real scanning electron microscopy （SEM） observation shows that the plastic deformation concentrates and the stress t^hxialit~＂ is larger ~it the boundaries bf colonies. It demonstrates the microe/＇a^ks initialize at colony boundaries. Thus, the colony size is a significant factor for fracture behaviors under small pre-deformation. On the other hand, the fracture surface is investigated after large pre-deformation via torsion. The results show that fracture characteristics vary with radius from dimples, elongated dimples to the fibrous structure. It indicates that the fracture charac-teristics are dependent on the pre-deformation. The fracture mode under large pre-deformation becomes an anisotropic fibrous structure instead of dimples.

Influence of Lamellar Direction in Pearlitic Steel Wire on Mechanical Properties and Microstructure Evolution

During cold drawing of pearlitic steel wire, the lamellar structure becomes gradually aligned with the draw ing axis, which contributes to the ultra high strength. A direct simulation about the mechanical behaviors and microstructural evolution of pearlitic lamellae was presented. A representative volume element （RVE） containing one pearlitic colony was established based on the real transmission electron microscope （TEM） observation. The deformation of pearlitic colony during tension, shear and wire drawing were successfully simulated. The numerical results show that this metallographic texture leads to a strong anisotropy. The colony has higher yielding stress when the la mellar direction is parallel and perpendicular to the tensile direction. The lamellar evolution is strongly dependent on the initial direction and deformation mode. The formation of typical period shear bands is analyzed. In the wire draw ing, the pearlitic colony at the sub surface experiences a complex strain path： rotation, stretching along the die sur face, and rotation back.

Cementites decomposition of a pearlitic ductile cast iron during graphitization annealing heat treatment

Cementites decomposition of a pearlitic ductile cast iron during graphitization annealing heat treatment was investigated.Fractographies and microstructures of heat treated samples were observed using a scanning electron microscope and mechanical properties were measured by a universal tensile test machine.The results indicated that during isothermal annealing at 750°C,the tensile strength of pearlitic ductile cast iron was increased to a peak value at 0.5h,and decreased gradually thereafter but the elongation was enhanced with the increase of annealing time.Moreover,the diffusion coefficient of carbon atoms could be approximately calculated as 0.56μm2/s that could be regarded as the shortrange diffusion.As the holding time was short（0.5h）,diffusion of carbon atoms was incomplete and mainly occurred around the graphites where the morphology of cementites changed from fragmentized shape to granular shape.In addition,the ductile cast iron with tensile strength of 740MPa and elongation of 7% could be achieved after graphitization annealing heat treatment for 0.5h.Two principal factors should be taken into account.First,the decomposition of a small amount of cementites was beneficial for increasing the ductility up to elongation of 7%.Second,the diffusion of carbon atoms from cementites to graphites could improve the binding force between graphites and matrix,enhancing the tensile strength to 740 MPa.

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.

Understanding microstructure-evolution-dependent fracture behaviors in pearlitic steels

Microstructure evolution or degradation has been well recognized to be closely related to the formation of microcracks in pearlitic rails and wheels.The rolling contact fatigue machine was employed to simulate the rail-wheel contact,and the microstructure evolution and crack formation of pearlitic steels subjected to rolling-sliding contact loading were then experimentally characterized.To further quantitatively predict the fracture behaviors,a phase-field model was herein established to investigate the cyclic loading-driven microstructure evolution and the microstructure-dependent fracture resistance in pearlite.The coupling of microstructure evolution and crack propagation was realized through the introduction of two-set order parameters,i.e.,the crack field and the microstructure field,and the microstructure-dependent fracture toughness.The proposed model can predict the fracture resistance of microstructure at different depths from the contact surface,after different rolling cycles and with different initial pearlitic microstructures,which can shed light on the design of damage-resistant microstructure of pearlitic steels.

Granular Carbides-Assisted Ultrafine-Ferrite Fabrication in the Pearlitic Steel Without Severe Plastic Deformation and Annealing

The evolution of ferrite grain and cementite lamella during cold rolling in a granular carbide–pearlite steel has been investigated.Particular attention has been given to a quantitative characterization of changes in the ferrite grains.Electron backscattered diff raction and transmission electron microscopy observations show that the ultrafi ne ferrite(~388 nm)can be produced through low equivalent strain cold rolling without severe plastic deformation(SPD)and annealing.The average grain size of ferrite depends on the volume fraction,shape and distribution of granular carbides as well as interlamellar spacing of pearlite.A general explanation of granular carbides-assisted grain refi nement is that the embedded carbides between natural barrier will signifi cantly facilitate dislocation nucleation during cold rolling.Dislocation reaction occurs more drastically and quickly near these granular carbides.Such reactions promote the formation of high-angle grain boundaries.The formation of ultrafi ne ferrite grains and subgrains in steel after cold rolling toε=1.4 strain makes the strength and ductility increased simultaneously compared withε=0.6 cold-rolled steel.The results suggest a new material design strategy to obtain ultrafi ne-grained structure via the granular carbides assistance.

Influence of Torsion Deformation on Textures of Cold Drawing Pearlitic Steel Wires

The influence of torsion deformation on textures of cold drawing pearlific steel wires was investigated by twisting the wires to different number of revolutions. Macro-texture （over the entire wire cross section） associated with torsion deformation was investigated by X-ray diffraction, while micro-texture （near the wire surface） was characterized by EBSD. The results show that the （110） macro-texture increases at the beginning of torsion and then decreases with increasing of torsion strain, while the （110） micro-texture decreases linearly with increasing of torsion strain. The relationships between the （110） fiber texture and the microhardness of the wires are also discussed.