Pearlite Transformation Research Articles

Mean field model of phase transformations in steels during cooling, which predicts evolution of carbon concentration in the austenite

The objectives of the paper were twofold. The first was exploring possibility of fast and reliable modelling of phase transformations during cooling of steels, accounting for the evolution of the carbon concentration in the austenite. Existing discrete models require long computing times and their application to optimization of industrial processes is limited. Therefore, a model based on the modified JMAK equation was proposed. Control of the carbon concentration in the austenite during ferritic and bainitic transformations allowed to predict incomplete austenite transformation and occurrence of the retained austenite. Moreover, prediction of the onset of pearlitic transformation after the bainitic was possible. The model was validated by comparison the predictions with the results of physical simulations. Numerical simulations for various industrial processes were performed. Problem of the difference in the incubation time between isothermal and constant cooling rate tests was raised.

Simulation of phase transformations in high carbon pearlite steel at various cooling rates

The paper reports on the experimental research into the austenite decomposition in high carbon steel at a constant temperature and rates of cooling. The study was carried out by the methods of dilatometry, micro-durometry, optical and scanning microscopy. From the dilatometric curves it is apparent they are sufficiently presented by the Kolmogorov-Avrami equation in a temperature range from 700 to 550 °С, and by the logistic function for temperatures ranging 500 to 250 °С. The data of the dilatometric analysis were used to draw isothermal and continuous cooling transformation diagrams of phase conversions. It has been pointed out an isothermal diagram to comprise four C-curves of pearlite and bainite transformations, which are approximated by second- and third-degree polynomials. The critical temperatures of austenite, bainite, and martensite transformations have been determined, being 711, 550 and 196 °С, respectively. Considering the data obtained experimentally, a mathematical model of austenite decomposition has been developed for a constant cooling rate, and volume fractions of structure-phase components estimated theoretically, demonstrating the congruence with experimental results. The suggested mathematical model can be adopted to calculate a structure-phase composition in industrial differential heat treatment of rails. The authors are grateful to Vladimir Sarychev, Egor Polevoy and Victor Gromov who took an active part in the preparation of the manuscript. This work is supported by the Russian Foundation for Basic Research (RFBR) [project number 19-32-60001] and President grant for State Support to young researches [grant number МK-118.2019.2].

Ultrafine Pearlitic Transformation Behavior of 2000 MPa Ultra‐High‐Strength Steel Wire Under Nb Microalloying

The influence of Nb in the transformation behavior of ultra‐high‐strength pearlitic steels is studied. The phase transition temperature is measured by thermal simulation device, and the continuous cooling transformation curve is established. Results show that with addition of Nb, the equilibrium transformation temperature of the test steel increases, and both size of the original austenite, pearlite colonies and pearlite lamellar spacing are refined. Pearlitic transformation mechanism has changed by the addition of Nb through the interaction of solid solution and precipitation. Nb microalloy delays the beginning and ending point of pearlite transformation when the cooling rate is greater than 1.00 and 0.10 °C s−1, respectively. In contrast, the transformation point is advanced when the cooling rate is less than 0.50 and 0.05 °C s−1. This variation is mainly due to the morphology and distribution of Nb‐containing precipitates at different cooling rates.

Direct Observation of Austenite and Pearlite Formation in Thermally Simulated Coarse Grain Heat-Affected Zone of Pearlite Railway Steel

In the present study, coarse grain heat affected zone (CGHAZ) of the pearlite railway steel was thermally simulated at different cooling rates ranging from 1 to 4 °C/s to in–situ observe the austenite and pearlite formation by high temperature laser scanning confocal microscopy (HT–LSCM). During heating, austenite nucleation was started at 811 °C at fast heating rate of 9.1 °C/s. Pearlite was completely decomposed into austenite in 21 s at 1005 °C. At peak temperature of 1300 °C, austenite was composed of two microstructural compositions, i.e., high-carbon austenite and low-carbon austenite. Homogenization of austenite and formation of grain boundaries were continued even during cooling when the temperature was above the pearlite transformation. During cooling, Low-carbon austenite has increased the pearlite transformation by providing additional nucleation sites. By increasing the cooling rate from 1 to 4 °C/s, the pearlite transformation temperature was reducing, pearlite growth rate was increasing, and pearlite interlamellar spacing was becoming narrow, respectively. Mathematical interpretation of pearlite growth rate was developed that gives true changing behavior of pearlite growth rate with the varying cooling rate. Present study provides direct observation and unique quantitative information of austenite and pearlite formation in CGHAZ in pearlite railway steel.

Effects of Austenitization Temperature and Pre-Deformation on CCT Diagrams of 23MnNiCrMo5-3 Steel.

The combined effect of deformation temperature and strain value on the continuous cooling transformation (CCT) diagram of low-alloy steel with 0.23% C, 1.17% Mn, 0.79% Ni, 0.44% Cr, and 0.22% Mo was studied. The deformation temperature (identical to the austenitization temperature) was in the range suitable for the wire rolling mill. The applied compressive deformation corresponded to the true strain values in an unusually wide range. Based on the dilatometric tests and metallographic analyses, a total of five different CCT diagrams were constructed. Pre-deformation corresponding to the true strain of 0.35 or even 1.0 had no clear effect on the austenite decomposition kinetics at the austenitization temperature of 880 °C. During the long-lasting cooling, recrystallization and probably coarsening of the new austenitic grains occurred, which almost eliminated the influence of pre-deformation on the temperatures of the diffusion-controlled phase transformations. Decreasing the deformation temperature to 830 °C led to the significant acceleration of the austenite → ferrite and austenite → pearlite transformations due to the applied strain of 1.0 only in the region of the cooling rate between 3 and 35 °C·s−1. The kinetics of the bainitic or martensitic transformation remained practically unaffected by the pre-deformation. The acceleration of the diffusion-controlled phase transformations resulted from the formation of an austenitic microstructure with a mean grain size of about 4 µm. As the analysis of the stress–strain curves showed, the grain refinement was carried out by dynamic and metadynamic recrystallization. At low cooling rates, the effect of plastic deformation on the kinetics of phase transformations was indistinct.

Computational thermodynamics/kinetics aided design for the microstructure of pearlitic steels during cooling process

Abstract The effect of cooling rate on the microstructure of SGLX82A steel was systematically investigated by coupling of experimental characterizations and thermodynamic/ kinetic calculations. The as-received casting ingots were heated and hot-deformed using a Gleeble simulator before being subjected to continuous cooling with rates of 5, 10, 15 and 20 K s-1. The microstructure of the SGLX82A steel was analyzed by using laser scanning confocal microscopy and scanning electron microscopy. The homogeneous microstructure with different proeutectoid ferrite fraction, pearlite colony size as well as pearlite lamellar spacing was examined at varied cooling rate. The calculation of the para-equilibrium phase diagram was performed by using Thermo-Calc software, while the DICTRA module was applied to calculate the diffusion-controlled ferrite formation and pearlite growth. Based on the calculations, the deviation between equilibrium and realistic eutectoid point was well explained, and the detected small amount of proeutectoid ferrite was attributable to the short duration before pearlite transformation. For the calculated pearlite lamellar spacings, good agreement was obtained compared with the experimental determinations. In addition, the present thermodynamic/kinetic method was also valid for a Crmodified steel SWRS87B, indicating the composition, processing parameters and microstructure of pearlitic steels can be reasonably designed with the aid of the present computational techniques.

Computational thermodynamics/kinetics aided design for the microstructure of pearlitic steels during cooling process

Abstract The effect of cooling rate on the microstructure of SGLX82A steel was systematically investigated by coupling of experimental characterizations and thermodynamic/ kinetic calculations. The as-received casting ingots were heated and hot-deformed using a Gleeble simulator before being subjected to continuous cooling with rates of 5, 10, 15 and 20 K s-1. The microstructure of the SGLX82A steel was analyzed by using laser scanning confocal microscopy and scanning electron microscopy. The homogeneous microstructure with different proeutectoid ferrite fraction, pearlite colony size as well as pearlite lamellar spacing was examined at varied cooling rate. The calculation of the para-equilibrium phase diagram was performed by using Thermo-Calc software, while the DICTRA module was applied to calculate the diffusion-controlled ferrite formation and pearlite growth. Based on the calculations, the deviation between equilibrium and realistic eutectoid point was well explained, and the detected small amount of proeutectoid ferrite was attributable to the short duration before pearlite transformation. For the calculated pearlite lamellar spacings, good agreement was obtained compared with the experimental determinations. In addition, the present thermodynamic/kinetic method was also valid for a Crmodified steel SWRS87B, indicating the composition, processing parameters and microstructure of pearlitic steels can be reasonably designed with the aid of the present computational techniques.

Austenite Transformation Behavior and Mechanical Properties of Constructional V, Nb-Alloyed TRIP-Assisted Steel

The present article is aimed at studying the austenite transformation kinetics and tensile properties of constructional 0.2 wt%C-Si2Mn2CrMoVNb TRIP-assisted steel subjected to isothermal holding in the subcritical temperature range (350-650 °C with the step of 50 °C) after intercritical annealing at 770 °C. The study was fulfilled using optical microscopy (OLYMPUS GX-71), electron scanning microscopy (JEOL JSM-), dilatometric analysis, tensile testing, Vickers hardness measurements. The critical temperatures of the steel were found to be Ac1=750-760 °C and Ac3=930 °C. The results showed that austenite demonstrated increased stability to pearlite and bainite transformations with an incubation period of decades of seconds at any of the mentioned temperatures. The bainitizing treatment at 400 °C with holding of 300-600 s resulted in ferrite/bainite/retained austenite structure with precipitates of nanosized carbide (V,Nb)C providing an improved combination of mechanical properties as compared to direct quenching (YS=548-555 MPa, UTS=908-1000 MPa, total elongation=16-18 %, PSE index=14.6-18.0 GPa%, YS/UTS ratio=0.55-0.60). The contributions of different strengthening components were estimated in order to reveal the benefits of a multi-phase microstructure for constructional applications.

Enhancement of pearlite transformation by Warm Rolling in 1.0C-1.5Cr Steel

Isothermal and non-isothermal pearlite transformations under 840°C warm-rolled and 950°C hot-rolled were researched. It was found that the pearlite nuclei at grain boundary and defects were increased by warm deformation. The results showed that the incubation time was shortened in the isothermal pearlite transformation and the volume fraction of pearlite was increased in continuous cooling condition by warm deformation. The pearlite transformation behaviour under continuous cooling condition was predicted by the formula of Umemoto. The critical cooling rates of the fully pearlite structures formed under warm deformation condition and hot deformation condition were calculated.

Low-Temperature Carbide Transformation in Pearlite of Medium-Carbon Steels

Metallographic studies of annealed medium-carbon steels (45, 40Kh, and 35KhGSA) showed that there is low-temperature peritectoid carbide transformation of pearlite of these steels, in which solid solutions of ferrite and cementite form solid solution with a broad homogeneity range on the basis of e-carbide Fe2C. It was suggested to plot the horizontal line of peritectoid transformation at the temperature of 382°C and the horizontal line of peritectoid transformation of cementite at 1650°C on the Fe–C diagram of state.

Effects of austenite deformation on continuous cooling transformation of the pearlite heat-resistant steel

ABSTRACT Continuous cooling transformation (CCT) diagrams of undeformed and 40% compressively deformed pearlite heat-resistant steels at 950°C were obtained by a combined method of dilatometry and metallography. The microstructure and hardness of the steel under these two experimental conditions were compared and analysed. The effects of austenite deformation on the CCT of pearlite heat-resistant steel are discussed. The CCT diagrams show that the austenite deformation extends the ferrite and pearlite transformation fields and increases the critical cooling rates to achieve the ferrite and pearlite microstructure, which causes the CCT curve to shift to the left. However, austenite deformation suppresses the transformation of bainite and martensite. Moreover, it reduces the hardness of the tested steel at the same cooling rate and grain size of the ferrite at low cooling rates.

Formation of a Gradient-Layered Structure during Thermal Deformation Treatment of Reinforced Steel

In this study, we consider the mechanism and kinetics of structure formation in the hardened zone during thermal deformation treatment of reinforced steel. Depending on the cooling rate and temperature conditions of austenite decomposition, pearlite and martensitic transformations are shown to occur with the formation of a gradient-layered structure, leading to structure modification of the surface layer of steel at a constant chemical composition, structure, and properties of the central layers of the workpiece. A high cooling rate due to a large temperature gradient near the surface is the reason for the formation of a finely dispersed layered structure. A diffusion-free martensitic transformation develops in the surface zone, leading to the formation of acicular martensite. In the underlying layers, the decomposition of austenite proceeds by diffusion and is accompanied by the formation of a lamellar ferrite–carbide mixture of varying dispersion degrees. An increase in the cooling rate leads to a strong refinement of the structure (grain point according to GOST 5639–82 is 11 in the surface layer and 8 in the core) characterized by an increase in the dispersion degree of the ferrite–carbide mixture, which causes an increase in strength and reduction of the plastic characteristics of steel. It is noted that the formation of a gradient-layered structure in the surface layer of strain-hardened reinforced steel allows excluding a sharp transition from the martensite structure to troosto-martensitic and mixed pearlite structures. This increases the contact-fatigue strength of reinforced steel and its crack resistance.

Determination of Internal Stresses During Pearlite Transformation of 0.8c–1.5Mn Steel Through <i>In-Situ</i> Neutron Diffraction

In this study, in-situ neutron diffraction during pearlite transformation and isothermal holding at 873 K after cooling of a 0.8C–1.5Mn steel specimen from 1323 K were conducted to measure the transformation stresses related to a eutectoid transformation. The stress relaxation behavior in cementite was also monitored to elucidate the relaxation of transformation elastic strains during isothermal annealing after pearlite transformation. By referring to the stress-free lattice parameter of an extracted electrolytic single-phase cementite sample after the long-time isothermal holding at 873 K, the transformation stresses were successfully evaluated. As one of the main findings, the elastic volume strain in cementite was determined to be approximately − 0.362%.

<i>In situ</i> Neutron Diffraction on Ferrite and Pearlite Transformations for a 1.5Mn-1.5Si-0.2C Steel

The phase transformation behavior from austenite upon cooling in a 1.5Mn-1.5Si-0.2C steel was in situ monitored using dilatometry, X-ray and neutron diffractions. The starting temperature of ferrite transformation was in good agreement between dilatometry and neutron diffraction, whereas much higher in X-ray diffraction. Such a discrepancy in transformation temperature is attributed to the change in chemical composition near the surface of a specimen heated to elevated temperatures in a helium gas atmosphere for X-ray diffraction. In situ neutron diffraction enables us to investigate the changes in lattice constants of ferrite and austenite, which are affected by not only thermal contraction but also transformation strains, thermal misfit strains and carbon enrichment in austenite. Pearlite transformation started after carbon enrichment in austenite reached approximately 0.7 mass% and contributed to diffraction line broadening.

Determination of Internal Stresses During Pearlite Transformation of 0.8c–1.5mn Steel Through In-Situ Neutron Diffraction

Determination of Internal Stresses During Pearlite Transformation of 0.8c–1.5mn Steel Through In-Situ Neutron Diffraction

Study on Deformation-Induced Pearlite Transformation in Vanadium Microalloyed Eutectoid Steel

In this paper, the hot compression tests were performed to study on deformation-induced pearlite transformation in vanadium microalloyed eutectoid steel. The results showed that volume fraction of deformation -induced pearlite were higher and the pearlite were spheroidized better under lower strain rate and higher strain in vanadium microalloyed steel. Ferrite grains and granular cementites were further refined through vanadium microalloying combined with deformation-induced pearlite transformation .Vanadium dissolved in γmatrix could retard deformation-induced pearlite transformation under low strain, vanadium carbides precipitated due to strain-induced precipitation eliminate the retardation when the strain was increased to a certain extent. Under heavy deformation, ferrite grains and granular cementites in vanadium microalloyed steel were finer compared with vanadium free steel.

Acceleration of pearlite transformation in a high-carbon steel by uniaxial compressive stress confirmed by volume measurements

Abstract Effect of external stress on pearlite transformation in a high-carbon steel was examined in terms of the measurement of volume change during isothermal holding. Two dimensional measurement of the steel holding at various temperatures under constant stress condition clarified anisotropic transformation plasticity, which means that the volume measurements are necessary to examine the transformation behaviors quantitatively. According to the measurement of volume change, it was found that the larger loading brings the shorter incubation time when compared at the same transformation temperature. These changes can be represented by the Johnson-Mehl-Avrami-Kolmogorov type equation with four constants when the transformation temperature is adequately deviated from the bainite start temperature.

Study of phase transformation behavior of SWRH82B high-carbon steel and its abnormal core structure

To study the influence of hot deformation parameters on the phase transformation and microstructure evolution of SWRH82B steel, a thermal dilatometer is used to measure the thermal expansion curves of undeformed and deformed austenite, and a scanning electron microscope is utilized to analyze the influence of the process parameters on microstructure transformation. In addition, an electronic probe is used to analyze the cause of the abnormal microstructure that appeared in the core of the SWRH82B wire rod. Thermo-Calc software is used to establish the diffusion model of Cr and Mn elements and study the effects of heating temperature and holding time on the elimination of the abnormal structure. The results show that pearlite, bainite, and martensite transformation occurred in the experimental steel; thermal deformation promoted the transformation of pearlite and martensite, and the CCT diagram moved to the upper left. The larger the deformation was, the greater the movement of the CCT diagram. When the total deformation was 50%, the CCT diagram subjected representing two thermal deformations moved more to the left and upward than the one representing a single thermal deformation. The minimum cooling rate for martensite transformation increased from 5 °C s−1 (undeformed) to 10 °C s−1 (one thermal deformation) and 12 °C s−1 (two thermal deformations). Thermal deformation can significantly refine the pearlite lamellar spacing of experimental steel and improve its hardness and strength. The abnormal structure of the core of the SWRH82B wire rod was island-shaped martensite, and the martensite was produced by the segregation of the Cr and Mn elements. The abnormal structure was eliminated when the wire rod sample was heated to 1200 °C and maintained at that temperature for more than 1 h.

Effect of Cooling Rate on Microstructure and Mechanical Properties in the CGHAZ of Electroslag Welded Pearlitic Rail Steel

The effect of cooling rate, ranging from 6 to 1 °C/s, on microstructure and mechanical properties in the coarse-grained heat affected zone (CGHAZ) of electroslag welded pearlitic rail steel has been investigated by using confocal scanning laser microcopy (CSLM) and Gleeble 3500 thermo-mechanical simulator. During heating, the formed austenite was inhomogeneous with fractions of untransformed ferrite, which has influenced the pearlite transformation during cooling by providing additional nucleation sites to pearlite. During cooling, at 6 °C/s, the microstructure was composed of martensite and bainite with little pearlite. From 4 to 1 °C/s, microstructures were completely pearlite. Lowering the cooling rate of the CGHAZ from 4 to 1 °C/s increased the pearlite start temperature and reduced the pearlite growth rate. Meanwhile, this increase in pearlite start temperature enlarged the pearlite interlamellar spacing. Alternatively, increasing pearlite interlamellar spacing in the CGHAZ by lowering the cooling rate from 6 to 1 °C/s reduced the hardness and tensile strength, whereas toughness was found unaffected by the pearlite interlamellar spacing. It has been found that a cooling rate of 4 °C/s leads to the formation of pearlite with fine interlamellar spacing of 117 nm in the CGHAZ of electroslag welded pearlitic rail steel where hardness is 425 HV, tensile strength is 1077 MPa, and toughness is 9.1 J.

Effects of cooling rate and strain rate on phase transformation, microstructure and mechanical behaviour of thermomechanically processed pearlitic steel

In the present investigation, thermomechanical controlled processing of a high carbon steel and a Nb microalloyed high carbon steel have been conducted in a Gleeble 3800 simulator. Different microscopic techniques have been utilised for the characterisation of the microstructure and hardness data has been used for the evaluation of mechanical properties. In order to suppress the transformation enthalpy, experiments are performed under varying cooling rate and strain rate. The effect of niobium microalloying leads to the lowering of recalescence and suppresses austenite to pearlite transformation start and finish temperatures at every cooling rate which leads to the refinement of interlamellar spacing and thereby improve hardness and predicted yield strength values. It is evident that a higher strain rate accelerates the kinetics of pearlite transformation and elevates the pearlite start temperature. The increase of strain rate in the range of 1s–1 to 100s–1 followed by a constant cooling rate (free cooling) leads to the refinement of interlamellar spacing as well as improves mechanical properties. The true stress-true strain diagram at a lower strain rate indicates higher strain hardening with sharp yield point, whereas the same at a higher strain rate indicates the sudden occurrence of strain softening. The variation in recalescence due to the alternation of the cooling rate and strain rate has been correlated with the final microstructure and mechanical properties.