Pearlite Reaction Research Articles

Accelerating bainite transformation by concurrent pearlite formation in a medium Mn steel: Experiments and modelling

Bainite transformation has yet to be utilized and even thoroughly studied in medium Mn steels. Here, we investigate the isothermal bainite transformation in a 10Mn steel at 450 °C experimentally and theoretically, focusing on the effect of dislocations introduced by warm deformation. We show that the bainite transformation in the studied medium Mn steel exhibits extremely sluggish kinetics (on a time scale of days), concurrent with the pearlite formation. The introduced dislocations can significantly accelerate bainite transformation kinetics while also facilitating the pearlite reaction. This is likely the first report on the simultaneous occurrence of these two solid-state reactions in medium Mn steels. With respect to the roles of dislocations in the acceleration of bainite transformation observed in this work, we propose a new ‘carbon depletion mechanism’, in which dislocations-stimulated pearlite formation makes a twofold contribution: facilitating the formation of bainitic ferrite sub-units to further enhance the autocatalytic effect and preventing the carbon enrichment in the remaining austenite. On this basis, a physical model is developed to quantitatively understand the bainite transformation kinetics considering the effect of concurrent pearlite formation, revealing good agreements between model descriptions and experiment results. Our findings, herein, offer fundamental insights into the bainite transformation in medium Mn steels and uncover a previously unidentified role played by introduced dislocations in influencing the kinetics of bainite formation, which may guide its future application in manipulating microstructure for the development of advanced high-strength steels.

<i>δ</i>–pearlite Reaction by Carburization in Fe–Cr Binary Alloy

<i>δ</i>–pearlite Reaction by Carburization in Fe–Cr Binary Alloy

Influences of Alloying Elements on Continuous Cooling Phase Transformation and Microstructures of Extremely Fine Pearlite

In the present research, the cooling phase transformation characteristics and microstructures of a Co- and Al-containing high-carbon steel were investigated under slow cooling rates (0.02–2.0 °C/s). In addition, by means of a Thermecmastor thermal simulator, the phase transformation temperatures were measured, and the corresponding continuous cooling transformation (CCT) curves were plotted. It was found that an extremely fine, fully pearlitic structure with an interlamellar spacing of 45 nm, and a hardness of 425 HV1 (Vickers hardness under 1 kg load) were obtained at a cooling rate of 0.1 °C/s. The addition of Co and Al accelerated the pearlitic reaction and reduced the interlamellar spacing by increasing the free energy of phase transformation, thus a fully pearlitic structure was obtained at slow cooling rates.

Pearlite Development in Commercial Hadfield Steel by Means of Isothermal Reactions

The Fe–12Mn–1C Hadfield steel is an abrasion-resistant alloy of high technological relevance for mining and heavy machinery. This composition is susceptible to pearlite formation which is detrimental for the material’s ductility. Although its spread use, the study of pearlite formation has been preserved to laboratory conditions which cannot be transferred to industrial practices. This manuscript provides updated information about this phenomenon by constructing the time–temperature-transformation diagram of the alloy between 400 and 600 °C. The pearlitic reaction occurs above 450 °C and begins on the grain boundaries. Only 7 min is needed for the transformation to start at 550 °C, and a maximum pearlite fraction of 35% is reached after 150 min at this temperature. Results are compared with the Fe–12Mn–0.8C composition mostly found in literature. The discussion comprises the effects of carbon and manganese content on the pearlitic reaction with the support of thermodynamics calculations.

Isothermal Pearlite Formation Kinetics in High-Chromium Cast Irons without Additional Alloying

A model of isothermal pearlite reaction kinetics in high-carbon ternary Fe–Cr–C alloys (cast irons) containing (Cr, Fe)7C3 carbide phase is presented. The model is based on traditional Avrami approach completed with a simple type temperature dependence of K coefficient. The model parameters for individual alloys were determined from F. Maratray and R. Usseglio-Nanot’s experimental data covering composition range from 2.1 to 4.3 %C and from 12 to 26 %Cr. The dependence of parameters (Avrami exponent n, activation energy U and time-scale constant C) on gamma phase composition by the end of austenitization (calculated with account for kinetics of carbide dissolution) is determined. The model thus permits to calculate the isothermal pearlite Ccurve (TTT diagram) for an alloy of arbitrary composition after given austenitization regime. The comparison of calculation results to experimental data shows their sufficient correlation.

Modelling of recalescence effect on austenite decomposition

ABSTRACTA model for recalescence has been established by integrating a model for the decomposition of austenite and one dealing with heat transfer with latent heat release taken into account. The effects of recalescence on each individual austenite transformation product have been studied. It was found that Widmanstätten ferrite and pearlite reactions are most affected. The calculated cooling curve, as affected by the recalescence, has also been verified with a commercial steel subjected to two different environment conditions.

Spheroidisation of hypereutectoid state of nanostructured bainitic steel

Nanostructured bainite is extremely resistant to severe tempering making it difficult to reduce its hardness sufficiently to allow certain processing before heat treatment into its final state. An attempt is therefore made to introduce the divorced pearlite reaction that relies on the presence of pro-eutectoid cementite in the austenite in order to produce a microstructure containing spheroidal cementite particles simply by continuous cooling from the two phase austenite and cementite field. It is demonstrated that dramatically reduced hardness values can be achieved using this method.

Influence of Co and Al on pearlitic transformation in superbainitic steels

The effect of cobalt and aluminium on the austenite to pearlite transformation in superbainitic steel was investigated. Experimental work showed that the transformation rate of pearlitic and the volume fraction of pearlite both were increased by adding ∼4 wt-%Co and 1·5 wt-%Al to the parent superbainitic steel. The addition of cobalt and aluminium to superbainitic steels desirably accelerated the growth rate of pearlitic microstructure and undesirably reduced the interlamellar spacing of pearlite, resulting in hardness higher than that of the parent steel in the pearlitic condition. Nevertheless, the accelerated pearlitic reaction, as a softening process of superbainitic steels before machining/forming, may make the commercial use of these steels more viable.

Pearlitic reaction in Cu-10wt%Al alloy with Ag additions

The pearlitic reaction in Cu-10wt%Al alloy with additions of 4, 6, 8, and 10wt%Ag was studied using scanning electron microscopy, energy dispersive X-ray microanalysis, in situ X-ray diffractometry, and microhardness measurements. The results indicated that the presence of Ag changes the pearlitic phase microstructure and its mechanical properties, because of the influence of Ag in the pearlitic phase growth mechanism.

The Cementite Spheroidization Process in High-Carbon Steels with Different Chromium Contents

The cementite spheroidization process is investigated in hypereutectoid steels with different chromium (Cr) contents. A spheroidized structure in high-carbon steel is usually obtained by a divorced eutectoid transformation (DET) reaction, which occurs during slow cooling of austenite with fine cementite particles. A bimodal distribution of spheroidized cementite particles is experimentally observed. It is shown that Cr addition allows one to obtain the spheroidized structure after austenitization at a higher temperature and a longer annealing time as compared with low-Cr steel. It is found that the DET reaction takes place at low undercoolings compared with the pearlitic reaction. The occurrence of the DET reaction also depends strongly on the spacing between cementite particles during the austenitization process, i.e., on austenitization temperature and time.

Identification of Transformation Plasticity Coefficient for Some Steels during Pearlite Transformation and the Dependence on Chemical Composition

A newly developed testing system available to control temperature and the rate as well as applied stress or strain with high quality of extensometer is used to measure the transformation plastic strain occurring in the course of diffusion type transformation (including pearlite reaction). Specimens of six kinds of steels widely used for quenching operation are heated to austenite temperature of 900°C and kept at the steady temperature followed by cooling to lower temperature below transformation finish point subjected to several applied loads. Since the difference in the strain between zero and nonzero stress corresponds to the transformation plastic strain, the value of transformation plasticity coefficient is identified by the ratio of the strain to applied stress. Dependence of the coefficient on the chemical composition is discussed by defining an equivalent carbon content and compared with the data for plastic hardening coefficient.

Control of Microstructure during the Eutectoid Transformation in the As‐cast Spheroidal Graphite Cast Iron with »in‐situ« Dilatation Analysis and Quenching Experiments

Transformation of austenite can proceed into ferrite and graphite (γFe→αFe+ G) and/or in pearlite (γFe→αFe+Fe3C). Investigation of the eutectoid transformation has been undertaken by evaluation of the “in‐situ” dilatation curves in connection with metallographic examinations, DTA and dilatation analyses of the solid state. On a basis of numerous quantitative relations, such as the relation between ferrite and pearlite fractions in as‐cast spheroidal graphite cast iron (SGI), the ratio between ferrite and pearlite in the microstructure is determined in a very short time. Thus the melt composition was corrected using the “in situ” dilatation analyses by addition of Cu and/or Mn or Si respectively. From the kinetics of austenite transformation and temperature dependence of the ferrite or pearlite growth, the following characteristic temperatures of the eutectoid transformation have been established: the ferrite nucleation T0α, the beginning of the ferrite growth Tα, and pearlite growth Tp, respectively. Both temperatures T0α and Tp are determined from the differential inflection point of the dilatometric curves. For mainly ferrite and pearlite SGI, the kinetic curves, which show the fraction of the single microstructure constituents in the microstructure in dependence of the transformation time, are well represented by the physical sigmoidal Boltzmann model. In case of the kinetics temperature dependence, however, pearlite growth does not follow the sigmoidal transformation course, which results from the recalescence of the pearlite reaction. The time of transformation is a more suitable criterion for the description of the transformation.

A new empirical formula for the bainite upper temperature limit of steel

The definition of the practical upper temperature limit of the bainite reaction in steels is discussed. Because the theoretical upper temperature limit of bainite reaction, B 0, can neither be obtained directly from experimental measurements, nor from calculations, then, different models related to the practical upper temperature limit of bainite reaction, B S, are reviewed and analyzed first in order to define the B 0 temperature. A new physical significance of the B S and B 0 temperatures in steels is proposed and analyzed. It is found that the B 0 temperature of the bainite reaction in steels can be defined by the point of intersection between the thermodynamic equilibrium curve for the austenite→ferrite transformation by coherent growth (curve Z $$\gamma \to \overrightarrow \alpha $$ ) and the extrapolated thermodynamic equilibrium curve for the austenite→cementite transformation (curve ES in the Fe-C phase diagram). The B S temperature for the bainite reaction is about 50–55 °C lower than the B 0 temperature in steels. Using this method, the B 0 and B S temperatures for plain carbon steels were found to be 680 °C and 630 °C, respectively. The bainite reaction can only be observed below 500 °C because it is obscured by the pearlite reaction which occurs prior to the bainite reaction in plain carbon steels. A new formula, B S(°C) =, 630-45Mn-40V-35Si-30Cr-25Mo-20Ni-15W, is proposed to predict the B S temperature of steel. The effect of steel composition on the B S temperature is discussed. It is shown that B S is mainly affected by alloying elements other than carbon, which had been found in previous investigations. The new formula gives a better agreement with experimental results than for 3 other empirical formulae when data from 82 low alloy steels from were examined. For more than 70% of these low alloy steels, the B S temperatures can be predicted by this new formula within ±25°C. It is believed that the new equation will have more extensive applicability than existing equations since it is based on data for a wide range of steel compositions (7 alloying elements).

The role of the divorced eutectoid transformation in the spheroidization of 52100 steel

Austenite containing fine particles of carbides may undergo the eutectoid transformation by either the pearlite reaction or the divorced eutectoid transformation (DET) reaction. The latter reaction automatically produces a spheroidized structure, while the pearlite reaction produces a structure that resists spheroidization. This study has evaluated the heat treatment conditions that promote the DET in 52100 steel. The results show that the DET is dominant over the pearlite reaction for austenitization at temperatures of 830 °C and below and for cooling rates of 500 °C/h and below. This information should be useful in the design of more energy efficient heat treatment processes for spheroidization annealing than the very long time processes currently recommended in the literature.

Modelling of austenite decomposition of hot-rolled plain carbon steels under complex cooling conditions

The purpose of the present work is to develop a mathematical model allowing the simultaneous prediction of both transformation product portions and mean ferrite grain size from the same common principles as a result of austenite decomposition during continuous cooling of plain carbon steels. The transformation products considered specifically are polygonal ferrite and pearlite. The model is based on the classical equations of nucleation-growth theory and also contains some empirical parameters. The chemical driving forces for nucleation and composition of elements at the phase interfaces are derived from thermodynamic analysis. Three modes of ferrite nucleation are taken into account that correspond to the nucleation on the austenite grain corners, edges and faces. The model considers the reduction of the nucleation sites due to the occupation of austenite grain boundary surface by ferrite grains. Pearlite transformation starts at the γ/α interface and suppresses further ferrite grain growth. The parameters related to ferrite reaction were determined on the basis of a series of austenite transformation kinetic curves and grain size measurements for a steel with the composition 0.084%C-0.58%Mn-0.02%Si obtained by dilatometric technique for cooling rates from 0.032 to 2.5 K/s. The parameters related to pearlite reaction were determined on the basis of the data for a steel with 0.66%C. After determination of the model parameters the model was applied to complex cooling conditions of the run-out table of the hot strip mill at Voest-Alpine Stahl Linz GmbH. Predicted ferrite grain size appeared to be 1.2 - 1.3 times smaller than the observed one. With regard to experimental data on grain growth in iron, it was suggested that the underestimation of grain size is due to additional ferrite grain growth occurring after the coiling of the steel sheet. Taking that into account provided satisfactory agreement with observed values.

Abnormal ferrite in hyper-eutectoid steels

The microstructural characteristics of ultra-high carbon hyper-eutectoid Fe–C and Fe–C–Cu experimental steels have been examined after isothermal transformation in a range just beneath the eutectoid temperature. Particular attention was paid to the formation of so-called “abnormal ferrite”, which refers to coarse ferrite grains which can form, in hyper-eutectoid compositions, on the pro-eutectoid cementite before the pearlite reaction occurs. Thus it is confirmed that the abnormal ferrite is not a result of pearlite coarsening, but of austenite decomposition before the conditions for coupled growth of pearlite are established. The abnormal ferrite formed on both allotriomorphic and Widmanstätten forms of pro-eutectoid cementite, and, significantly, it was observed that the pro-eutectoid cementite continued to grow, despite being enclosed by the abnormal ferrite. Under certain conditions this could lead to the eventual formation of substantially reduced amounts of pearlite. Thus, a model for carbon redistribution that allows the pro-eutectoid cementite to thicken concurrently with the abnormal ferrite is presented. The orientation relationships between the abnormal ferrite and pro-eutectoid cementite were also determined and found to be close to those which have been reported between pearlitic ferrite and pearlitic cementite.

A micro-model for eutectoid phase transformations in as-cast ductile iron

As ductile iron is cooled below the eutectoid temperature range, there are two branches available for decomposition of the eutectoid austenite: the stable eutectoid reaction (ferrite reaction) in which ferrite and graphite are the product phases and the metastable eutectoid reaction (pearlite reaction) leading to pearlite formation. The following sections describe mathematical formulation and calibration of a semi-empirical micro-model to simulate solid-state transformations in as-cast ductile iron. A comprehensive set of data and experimental results obtained from laboratory experiments in conjunction with an optimization methodology has been used to calibrate the micro-model. The experimental results consisted of cooling curve and microstructure characterization data obtained from a series of heats conducted as a part of industry sponsored research aimed at developing thermal analysis based methodologies for process control in foundries. Four different types of thermal analysis cups were poured. As the principal objective in this work is to develop a model to predict microstructure evolution, heat transfer in the thermal analysis cups is simulated using a simple lumped system volume element model in which the temperature is a function of time only.

Effects of Carbon Concentration and Cooling Rate on Continuous Cooling Transformations Predicted by Artificial Neural Network.

Employing 151 continuous cooling transformation (CCT) diagrams, an artificial neural network (ANN) has been modeled and trained. The CCT diagrams of a class of Fe-xC-0.4Si-0.8Mn-1.0Cr-0.003P-0.002S (x within 0.1 through 0.6) steels are predicted by the model developed. It indicates that an increase in carbon concentration (C%) gives rise to a decrease in ferrite start (Fs), bainite start (BS), and martensite start (MS) temperatures, but the carbon concentration has weak effect on the pearlite end (Pe) temperature. The rate of decrease, ∂Fs/∂C, further depends on the carbon concentration. The carbon dependence predicted by the ANN is consistent with what is predicted by thermodynamic models. The Fs temperature is also affected by the cooling rate (υ), especially for high carbon steels and υ>0.1°C/s. C prolongs the incubation period of ferrite formation, but accelerates the overall growth kinetics of the pearlite reaction. The Fs and Pe temperatures at low cooling rates predicted by the ANN model are in agreement with those predicted by thermodynamic models. The deviations of Pe and Fs from their thermodynamic equilibrium counterparts are nearly independent of the carbon concentration. The minimum undercooling for both ferrite and pearlite reactions is around 50°C. It increases up to 100°C at higher cooling rates. Pre-bainite decomposition of austenite retards bainite formation. Employing the Ms temperature, the critical driving force for heterogeneous athermal nucleation is also estimated and related to the Ms temperature. Ms temperatures predicted by this model prove to be consistent with those predicted by several empirical linear models. It can be concluded that the current ANN model is reliable and effective.

Effect of Nitrogen on Formation of Martensite-Austenite Constituent in Low Carbon Steels.

The effect of dissolved nitrogen on phase transformations and microstructural evolution in low-carbon high manganese silicon steel wire-rod was examined for varying nitrogen levels of 30, 70, and 100 ppm, while keeping the base composition constant. It was found that nitrogen suppresses the pearlite reaction start temperature by up to 30°C. A strong correlation was discovered between the nitrogen level and the formation of martensite-austenite microconstituent (MA phase). The possible mechanisms by which nitrogen causes the formation of MA phase are discussed. The main conclusion drawn from this study is that levels of nitrogen between 70 and 110 ppm can have a significant effect on microstructure, such that up to approximately 10vol% of MA phase is formed in these steels. Microstructural changes in titanium-microalloyed steels were also investigated - particularly changes in the development of the MA and pearlite phases. The pearlite content was up to double that found in a Ti-free alloy. Microalloying with titanium was found to be effective in completely eliminating MA phase.

Effect of Vanadium Alloying on the Precipitation Reactions during Pearlite Formation in Medium and High Carbon Steels

Vanadium microalloyed medium carbon steels are currently used in the automotive industry due to the improved tensile and impact properties that are achieved with moderate additions of vanadium. So far, from results published in the literature, the improvement in mechanical properties is attributed to the precipitation of VC within the proeutectoid ferrite that constitutes the major microstructural phase of these steels, while the pearlitic component has received relatively less attention. The purpose of the present study is to elaborate on the precipitation of vanadium carbide occurring during the isothermal pearlitic reaction in vanadium alloyed medium and high carbon steels. Since during this isothermal transformation the formation of proeutectoid ferrite precedes that of pearlite, the precipitation of VC within the proeutectoid ferrite was also studied, in order to verify existing literature results and to establish a basis of comparison with the VC precipitation occurring within the pearlitic ferrite during the subsequent pearlite formation. Three vanadium alloyed steels were used during the present research. These included two experimentally produced alloys, and a commercially available steel. The present study confirmed that, vanadium carbide interphase precipitates always exist within the pearlitic ferrite component of the lamellar and within non-lamellar pearlite. In the case of the lamellar pearlite, aligned arrays of interphase VC precipitates are present within the pearlitic ferrite, whereas in the non-lamellar pearlitic ferrite random distributions are most frequently observed. No partitioning of vanadium was observed into the adjacent pearlitic cementite regions even in cases of prolonged ageing at the high temperatures of the isothermal transformation. Morever, the experimental results show clearly that interphase VC precipitation exists not only within pearlitic ferrite but also within the proeutectoid ferrite,that forms in low carbon steels, in a form of well defined linear or curved arrays of regular shaped and uniformly sized particles.