Pearlite Transformation Research Articles

Effects and Mechanisms of Vanadium on Structural Transformation of BVRE Heavy Rail Steel

The different dilatometric curves of continuous cooling transformation with the different cooling rates were determined by means of Gleeble-2000 thermal simulation machine. The CCT curve of BVRE heavy rail steel was obtained by measuring the dilatometric curves and metallographic analysis. And the effects and mechanisms of vanadium on the phase transformation and microstructure of BVRE heavy rail steel were investigated. It is found that, the BVRE heavy rail steel only takes place pearlite and martensite transformation during continuous cooling. The CCT curve of BVRE heavy rail steel is moved to lower right with increasing vanadium content, which indicates that vanadium can obviously improve the stability of super cooled austenite and delay the pearlite transformation. When the content of vanadium is increased from 0.052% to 0.12%, the shortest incubation time of pearlite transformation is increased from 30s to 59s. When the cooling rate ≤ 5 ·s-1, with increasing vanadium content, both starting and finishing temperatures of pearlite transformation are decreased at different extent, meanwhile the pearlite is refined and the pearlite percentage is notably decreased. When the cooling rate is 2 ·s-1, the pearlite percentage is decreased from 65.7% to 35.9% with increasing vanadium content. When the content of vanadium is increased from 0.052% to 0.12%, the critical cooling rate of quenching is decreased from 13 ·s-1 to 7 ·s-1, thus the hardenability of BVRE heavy rail steel is improved.

Effect of strong magnetic field on isothermal transformation of degenerate pearlite in an Fe-C-Mo alloy

The pearlite transformation in a Mo-containing iron alloy was investigated under 12 T magnetic field. The pearlite transformation was accelerated owing to the application of a strong magnetic field. Pearlite was of degenerated morphology without the presence of a strong magnetic field; but the degeneracy of pearlite is reduced when a strong magnetic field was applied, which may be attributed to the effect of strong magnetic field on faster carbon diffusion and less molybdenum segregation caused by a strong magnetic field.

Effect of Compressive Deformation on Degenerate Pearlite Transformation in a Fe-C-Mo Alloy

The isothermal transformation of pearlite in an Fe-0.27%C-3.15%Mo (weight percent) alloy applying large compressive deformation was investigated by means of thermal simulation, optical and scanning electron microscopy. The pearlite transformation was promoted by large compressive deforming on austenite, which is attributed to the effect of large compressive deformation on nucleation of pearlite transformation.

Influence of magnetic field on isothermal pearlite transformation in Fe–C–Ni hypoeutectoid alloy

The influence of an external magnetic field of 8 T on isothermal pearlite transformation was studied in an Fe–0·4 mass%C–2 mass%Ni hypoeutectoid alloy, in which both proeutectoid ferrite and pearlite transformations were accelerated by the field. Whereas the pearlite volume fraction increased with decreasing reaction temperature, the magnetic field increased the proportion of pearlite relative to proeutectoid ferrite only at low undercoolings. The interlamellar spacing of pearlite was reduced 5–15% by the magnetic field, which may have led to a substantial increase in growth rate of pearlite nodules. Whereas the pearlite carbon content fell near the paraequilibrium γ/(γ+cem) phase boundary, other features, e.g. interlamellar spacing, seem to be in accord with the condition of non-partition local equilibrium at the pearlite/austenite boundary as reported in Fe–C base hypoeutectoid alloys.

Hot Deformation and Ductility Analysis of Continuous Cast C40 Steel by Means of Tensile and Compression Tests

The steel production from scrap using continuous cast technology has increased in last decades. Sometimes, steels processed via this route display poor ductility at high temperature. This feature is associated to cooling conditions and chemical composition, which in turns affect the segregation pattern and vary the transformation temperatures and the phase transformation kinetics. The material under study was a C40 steel with a dendrite solidification microstructure coming from an industrial continuous casting plant. The high temperature ductility was evaluated by means of tensile tests up to fracture at strain rate of 0.001 s-1 in a temperature range of 1100 to 710°C. The reduction in area at fracture as a function of temperature graphs show a clear reduction of the steel ductility in the intercritical region, but also after the pearlite transformation. Single deformation compression tests were also carried out on the steel in the austenitic temperature domain, 900 to 1100°C, and at strain rate of 0.001 to 1 s-1. A modification of the Garofalo hyperbolic sine equation has been employed to derive the peak and steady stresses of the flow curve. The work hardening, U, and dynamic recovery, Ω, parameters which describe the flow curve before dynamic recrystallization takes place and the k and t50 parameters, based on the JMAK model, to describe the recrystallization kinetics were also calculated for every test and expressed as a function of the Zener Hollomon parameter, Z.

Experimental Investigation on the Effect of Copper Upon Eutectoid Transformation of As-Cast and Austenitized Spheroidal Graphite Cast Iron

Copper is known as a pearlite promoter in cast iron and has been used as such for a long time, most often together with low amounts of manganese. Literature data, however, has shown that these two elements act differently on the ferritic and pearlitic transformations. In order to provide more insight in the role of this element on the solid-state transformation of spheroidal graphite (SG) cast iron, this paper investigates the effect of adding copper in small step increments, from 0.11 to 1 wt. % Cu, to SG irons containing about 0.15 wt. % Mn.The characteristic temperatures for the stable and metastable eutectoid transformations as recorded during cooling after solidification in standard cups and after austenitizing are presented together with microstructure information. It is found that a copper content of about 0.6 wt. % is the upper limit over which only small amounts of ferrite could be obtained except at very low cooling rates. This is tentatively related to the lowering of the temperature for ferrite and the associated decreased kinetics for austenite decomposition in the stable system.

Influence of Thermomechanical Processing on the Austenite–Pearlite Transformation in High Carbon Vanadium Microalloyed Steels

Multipass torsion tests were carried on with several V-microalloyed high carbon steels, using different deformation sequences in order to modify the austenite state prior to transformation. Both recrystallized and deformed austenite microstructures were studied. After deformation, different cooling rates were applied. The results show that accumulating strain in the austenite before transformation seems to slightly increase the interlamellar spacing for a given cooling rate, this increase being related to the pearlite transformation taking place at higher temperatures because of the increase in the austenite grain boundary area per unit volume (SV). On the other hand, the retained strain significantly contributes to a refinement of the “ferrite units”, this effect being more significant as vanadium and nitrogen contents rise. A relationship between the mean “ferrite unit” size with SV and cooling rate was determined. Similarly, empirical expressions to predict strength as a function of vanadium microalloying addition, SV and cooling rate were derived.

Research on modelling of transformation plasticity via dilatometric tests

The approaches of modelling transformation plasticity (TP) were briefly reviewed. Dilatometric tests were employed to investigate transformation plasticity generated under small external loading during pearlitic, bainitic and martensitic transformation. The influence of specimen diameter and heating ratio on experimental results was investigated, an approach of experimental data processing was proposed to separated deformation caused by TP from that caused by temperature variance and phase transformation. The TP coefficient of SA508-Cl3 steel determined via uniaxial loading tests was compared with the experimental results of biaxial loading tests which were obtained from the published papers. A simplified experimental modelling approach was proposed, and it was applied to predict TP coefficients of several steels. The predicted results of TP during martensitic transformation were validated by experimental data.

Evaluation of the Role of Alloying Elements in Austemperability of Heavy Section Ductile Iron

In this investigation the role of alloying elements on austemperability of heavy section ductile irons was studied. Four different chemical compositions were chosen in a way to specify an optimal chemical composition with suitable austemperability. Austempering was carried out at 315 and 350oC for 1 hour on specimens prepared from cast Y-blocks with 75 mm thickness. Metallographic examinations and hardness tests indicated that thickness of bainitic layer was positively dependant on the amount of the alloying elements which deferred pearlitic transformation in TTT diagram to the longer time. In addition, by increasing the amount of alloying elements, the morphologies of ferrite in bainitic structure changed from featherlike to acicular. According to the results of this study, in order to achieve a full bainitic structure (in the specimens whit 75mm thickness), the optimal amounts of alloying elements were: 0.35 % Mn, 0.22 % Mo, 1.1-1.4% Ni and 0.6% Cu. In addition, regarding to the thick sections of Y-blocks, graphite degeneration defect was also studied, and it was noticed that this defect could be completely eliminated by adding 50ppm of antimony.

Evolution of the defect structure of reverted austenite upon pearlite transformation in a steel 120G4

Evolution of the microstructure of a steel 120G4 after pearlite transformation and subsequent heating to temperatures of 680–1200°C has been investigated by transmission electron microscopy. It has been established that with a rise in the heating temperature the fraction of the carbide phase decreases and the austenite defect structure changes from microtwins and stacking faults to dislocations. Upon heating to T = 700–720°C, in the restored (reverted) austenite, wide twins with a regular crystallographic faceting, whose appearance is seemingly connected with the specific features of the pearlite transformation rather than with relaxation of internal stresses, have been discovered experimentally for the first time.

Effects of Cu addition on the synergistic effects of Ti–B in thermomechanically processed low carbon steels

The present study concerns the influence of Cu addition on the synergistic effects of Ti–B in low carbon thermomechanically processed steels. Effective retardation of the γ→α transformation due to the combined addition of Ti and B is known to enhance the hardenability of a wide range of low carbon microalloyed steel. On the other hand, addition of Cu in low carbon age hardenable steel suppresses the pearlitic transformation resulting into a microstructure constituted by interspersed islands of martensite in ferritic matrix. Therefore, it may be of interest to examine the role of Cu on the synergistic contribution of Ti and B in respect of increasing hardenability of austenite. In the present study, effects due to the addition of Ti, B and Cu in different steels are interpreted in respect of microstructure–property correlation. It is demonstrated that the addition of 1.5wt% Cu perceptibly enhances the synergistic effect as manifested by lowering the transformation temperature of austenite. Addition of 0.8wt% Ni in Cu–Ti–B steel does not alter the transformation temperature of austenite to any appreciable extent. However, Cu–Ni–Ti–B steel has resulted into the enhancement of strength–ductility combination.

Crystal geometric analysis of the orientational relationships between austenite and fine pearlite in steel 120G4

Electron-microscopic examination of the crystallographic relationships between the structural constituents in the high-carbon steel 120G4 has been carried out after a partial isothermal pearlitic transformation. On a large number of experimentally investigated fragments of the microstructure, the parallellism has been established between the close-packed directions and planes of structural constituents of fine pearlite and one of the variants of twinned austenite.

Magnetic field induced pearlite transformation in Fe–0.47C–2.31Si–3.15Mn steel above the eutectoid temperature

It was found that the pearlite transformation (PT) in Fe–0.47C–2.31Si–3.15Mn (wt%) alloys could be induced by magnetic field at temperatures above the eutectoid temperature. The mechanism of magnetic-induced pearlite transformation (MIPT) has been discussed. It was found that the ferromagnetic ferrite phase has lower free energy compared with other phases under the applied magnetic field. Therefore, its formation is responsible for the nucleation and growth of pearlite at temperatures above the eutectoid point. The driving force for the MIPT is the magnetostatic energy difference between the austenite and ferrite due to the different saturation magnetization in the presence of magnetic field. The preceding phase in the formation of pearlite is the ferromagnetic ferrite phase.

Prediction of Bainite Intervened in Ferrite-Pearlite Forging Steel I. Modeling

The successive ferrite (� ) + pearlite (P) transformations from austenite (� ) were modeled to predict the presence of bainite in as-forged medium-carbon manganese steels. The kinetics of diffusional transformations were calculated based on classical nucleation and growth theory coupled with CALPHAD multi-component thermodynamics. The description of the growth rate of proeutectoid-� includes a time dependence due to the carbon enrichment in the remaining � . The � /� interface was assumed to be in negligible-partitioned local equilibrium (NPLE). The

Interlamellar Spacing of Pearlite in a Near-eutectoid Fe–C Alloy Measured by Serial Sectioning

The interlamellar spacings of pearlite formed isothermally at 700 and 680°C in a near-eutectoid Fe–0.82mass%C alloy were measured by serial sectioning coupled with scanning electron and atomic force microscopy. The intersection angle of cememtite lamellae with the metallographic surface was determined from the variation of horizontal displacement of cementite lamellae with removal depth. The true lamellar spacing was then calculated from the apparent lamellar spacing on the surface. When the angle was small, i.e. less than ~20°, it was measured directly by atomic force microscope. The minimum and mean true lamellar spacings are in good agreement with those measured by other methods. The distribution of interlamellar spacings on the specimen surface back calculated from the spectra of true interlamellar spacing assuming the random orientation of cementite lamellae was in fair agreement with the measured one. It is likely that the distribution of true lamellar spacings is primarily due to recalescence and the minimum true spacings rather than the mean value may be relevant to the thermodynamic theory of pearlite transformation.

Exploring the Possibilities of Development of Directly Quenched TRIP-Aided Steel by the Artificial Neural Networks (ANN) Technique

In TRIP-aided steels, generally the composition-process combination is aimed at circumventing pearlitic transformation during cooling of austenite and to retain the desired volume fraction of austenite (∼10 vol%) in the microstructure, which is amenable to stress/strain induced transformation during deformation. The purpose is achieved by individual and interactive contribution of numbers of compositional and process variables. Therefore, it is impractical to predict the best combination of most significant variables by using conventional expertise. In this regard, the artificial neural network (ANN) technique has already been established as a potential tool for composition–process–properties correlation in various materials. In the present study, the ANN technique is utilized to predict the composition–process–properties correlation with an aim to achieve the most attractive strength–ductility combination in TRIP aided steel. In the course of the aforesaid exercise, it is indicated that an attractive strength–ductility combination may be achieved without much requirement of intercritical annealing (ICA) and isothermal holding at bainitic temperature, even at lower level of carbon (say, 0.1 wt%), with judicious alloying by Cu and Ni. The hypothesis is first tested by conducting dilatometric study and microstructural examination of the dilatometric samples and subsequently ascertained by determination of mirostructure and mechanical properties of the as hot roll samples of predicted compositions.

A New Continuous Cooling Transformation Diagram for AISI M4 High-Speed Tool Steel

The increasing evolution of dilatometric techniques now allows for the identification of structural transformations with very low signal. The use of dilatometric techniques coupled with more common techniques, such as metallographic, hardness testing, and x-ray diffraction allows to plot a new CCT diagram for AISI M4 high-speed tool steel. This diagram is useful for a better selection of alternate solutions, hardening, and tempering heat treatments. More accurate determination of the various fields of transformation of austenite during its cooling was made. The precipitation of carbides highlighted at high temperature is at the origin of the martrensitic transformation into two stages (splitting phenomena). For slow cooling rates, it was possible to highlight the ferritic, pearlitic, and bainitic transformation.

Microstructure, Mechanical Properties, and Abrasive Wear Behaviour of Si‐Mn‐Cr‐B Cast Steel as a Function of Carbon Concentration

The microstructures, mechanical properties and abrasive wear behaviour of five kinds of Si‐Mn‐Cr‐B cast steels were studied. The steels investigated contained X wt.% C with X= 0.15, 0.25, 0.35, 0.45, 0.55, 2.5 wt.% Si, 2.5 wt.% Mn, 0.5 wt.% Cr, 0.004 wt.%B . The results showed that the Ac1temperatures increased and Ac3 and Ms temperatures decreased with increasing carbon concentration. From the continuous cooling transformation (CCT) curves, it was discovered that the incubation period of pearlitic transformation was prolonged and the transformation curves of pearlite and bainite were separated significantly with rising carbon concentration. At lower carbon concentration, the normalized structure of Si‐Mn‐Cr‐B cast steel consisted mainly of granular bainite and M‐A islands. The normalized microstructures of the cast steel changed from granular bainite gradually to needle‐like bainite, upper bainite, and lower bainite with rising carbon concentration. The tensile strength and hardness of Si‐Mn‐Cr‐B cast steel increased and impact and fracture toughness decreased with increasing carbon content. The wear testing results showed that the wear resistance of Si‐Mn‐Cr‐B cast steel improved with higher carbon content but was obviously unchanged beyond the carbon concentration of 0.45%. The best balance of properties of Si‐Mn‐Cr‐B cast steel is obtained at the carbon concentration range of 0.35 ‐ 0.45%C.

Modeling of Austenite Decomposition in Low Si-Mn TRIP Steel During Cooling

Transformation behavior in low carbon Si-Mn TRIP steel was investigated by means of microstructural observation and computer modelling. A transformation model in which transformation is controlled by carbon diffusion was suggested, which well described the volume fractional change of ferrite, pearlite, and bainite during continuous cooling. The influence of Si content and austenite grain size was thoroughly investigated. The calculated results indicated that Si retards pearlite transformation, accelerates polygonal ferrite transformation, refines the austenite grain, and increases the ferrite transformation rate.

Effects of deformation and Mo, Nb, V, Ti on continuous cooling transformation in Cu–P–Cr–Ni–Mo weathering steels

The continuous cooling transformation (CCT) diagrams of the weathering steels Cu–P–Cr–Ni–Mo were investigated by means of a combined method of dilatometry and metallography. For five of the steels (B, C, D, E and F), since there is a metastable austenite gap completely open or half-open between the ferrite and the bainite transformation regions, it is possible to obtain dual-phase microstructure directly by hot-rolling and appropriate cooling. Hot deformation increased the transformation start temperature Ar3 significantly, enlarged the polygonal ferrite transformation temperature range, and shortened the gestation period of pearlite and polygonal ferrite transformation. Mo effectively retards the bainite and the pearlite transformations, elevates the pearlite-finish temperature, and induces the metastable austenite gap formed.