Decomposition Of Austenite Research Articles

Optimization of the CCT Curves for Steels Containing Al, Cu and B

New continuous cooling transformation (CCT) equations have been optimized to calculate the start temperatures and critical cooling rates of phase formations during austenite decomposition in low-alloyed steels. Experimental CCT data from the literature were used for applying the recently developed method of calculating the grain boundary soluble compositions of the steels for optimization. These compositions, which are influenced by solute microsegregation and precipitation depending on the heating/cooling/holding process, are expected to control the start of the austenite decomposition, if initiated at the grain boundaries. The current optimization was carried out rigorously for an extended set of steels than used previously, besides including three new solute elements, Al, Cu and B, in the CCT-equations. The validity of the equations was, therefore, boosted not only due to the inclusion of new elements, but also due to the addition of more low-alloyed steels in the optimization. The final optimization was made with a mini-tab tool, which discarded statistically insignificant parameters from the equations and made them prudently safer to use. Using a thermodynamic-kinetic software, IDS, the new equations were further validated using new experimental CCT data measured in this study. The agreement is good both for the phase transformation start temperatures as well as the final phase fractions. In addition, IDS simulations were carried out to construct the CCT diagrams and the final phase fraction diagrams for 17 steels and two cast irons, in order to outline the influence of solute elements on the calculations and their relationship with literature recommendations.

Kinetics of Austenite Phase Transformations in Newly-Developed 0.17C-2Mn-1Si-0.2Mo Forging Steel with Ti and V Microadditions.

This work presents the results of phase transformation kinetics during continuous cooling in newly developed high strength low-alloy steel (HSLA). Initial theoretical calculations for the determination of heat treatment parameters were conducted. To determine the structural constituents formed due to the austenite decomposition the dilatometry approach was used. The material was cooled down from the austenitization temperature of 1000 °C with cooling rates between 0.1 °C/s to 60 °C/s. Then, light and scanning electron microscopy investigations were carried out. The microstructure after cooling at rates between 0.1 °C/s up to 1 °C/s is mainly ferritic with some fraction of granular bainite. Increasing the cooling rate led to formation of a higher fraction of bainitic ferrite. At 60 °C/s the microstructure was mainly bainite with some fraction of ferrite. To determine the presence of retained austenite, color etching using Klemm solution was used. The results show that the increase of cooling rate decreases the amount of retained austenite in the microstructure of the steel. Hardness measurements were made to determine the changes in the mechanical properties as a function of the cooling rate.

A Comparison Study of the Austenite Grain Growth and Its Transformation Behavior during Uniform Continuous Cooling of a Wrought and Selective Laser Melting 4340 Steels

Abstract This study compares austenite grain growth and its continuous cooling transformation (CCT) behavior between selective laser melting (SLM) 4340 and conventional wrought 4340 steels. Standard dilatometry tests were used to determine the austenite decomposition behavior at different cooling rates. The analysis of the data from the controlled cooling paths was used to generate CCT diagrams for the two steels investigated in this study. Advanced microstructural characterization techniques, Scanning Electron Microscopy and Orientation Imaging Microscopy, were employed to support the CCT diagrams. Electron Backscatter Diffraction (EBSD) and a special EBSD-Image Quality characterization technique were used to assess the percentage of the microstructural components observed. Three important observations were made: (1) the hardenability of the wrought 4340 steel was higher than the SLM 4340 steel; (2) the presence of granular bainite was observed after slow cooling conditions in both steels; (3) the SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied. MTEX, a MatLab program, was used to reconstruct the prior austenite grains. The SLM steel exhibited finer austenite grain size distribution than the wrought steel in the temperature range studied.

In-Situ High-Energy X-ray Diffraction Study of Austenite Decomposition During Rapid Cooling and Isothermal Holding in Two HSLA Steels

In-situ high-energy X-ray diffraction experiments with high temporal resolution during rapid cooling (280 °C s−1) and isothermal heat treatments (at 450 °C, 500 °C, and 550 °C for 30 minutes) were performed to study austenite decomposition in two commercial high-strength low-alloy steels. The rapid phase transformations occurring in these types of steels are investigated for the first time in-situ, aiding a detailed analysis of the austenite decomposition kinetics. For the low hardenability steel with main composition Fe-0.08C-1.7Mn-0.403Si-0.303Cr in weight percent, austenite decomposition to polygonal ferrite and bainite occurs already during the initial cooling. However, for the high hardenability steel with main composition Fe-0.08C-1.79Mn-0.182Si-0.757Cr-0.094Mo in weight percent, the austenite decomposition kinetics is retarded, chiefly by the Mo addition, and therefore mainly bainitic transformation occurs during isothermal holding; the bainitic transformation rate at the isothermal holding is clearly enhanced by lowered temperature from 550 °C to 500 °C and 450 °C. During prolonged isothermal holding, carbide formation leads to decreased austenite carbon content and promotes continued bainitic ferrite formation. Moreover, at prolonged isothermal holding at higher temperatures some degenerate pearlite form.

The effect of quench cracks and retained austenite on the hydrogen trapping capacity of high carbon martensitic steels

Hypereutectoid martensitic steels possess excellent hardness levels which make them attractive materials for specific industrial applications. However, they can contain cracks and/or retained austenite after quenching which show a particular interaction with hydrogen (H). Hence, this work evaluates the interaction between H and a martensitic Fe-1.1C alloy by combining a wide variety of (H) characterization techniques with a systematic approach for specially designed H charged and heat treated samples. A detailed analysis of the microstructure for every condition serves as the basis for the interpretation. The results show that the presence of H leads to additional cracking and branching or growth of pre-existing quench cracks. Moreover, it is shown that when the temperature exceeds the retained austenite decomposition temperature while the austenitic grains contain H, additional cracking occurs which increases the amount of reversible H trapping sites thus raising the HE susceptibility.

Mathematical Simulation of Microstructural Phase Transformations in the Course of Welding Heating for the Case of Cladding of Protective Layer in the WWER-1000 Reactor Vessel

By using the developed finite-element models, we perform the numerical investigation of nonstationary temperature fields and the kinetics of microstructural phase transformations in the base material of the WWER -1000 reactor vessel (15Kh2NMFА steel) under the conditions of submerged-arc cladding of protective anticorrosion layers of austenitic materials. We perform the comparative analysis of the results of simulation of microstructural phase transformations obtained by using two methods of prediction of the decomposition of austenite in the process of cooling. The first method is based on the application of regression equations and the characteristic cooling time Δ t8/5. The second method is based on the diagrams of isothermal decomposition of austenite, the Avrami equation, and the Koistinen–Marburger equation. As a result of simulation of microstructural transformations under cladding, we obtain the bainite-martensite composition in the heat-affected zone of the base material of reactor vessel, which is confirmed by results of the dilatometric analysis and metallography.

The Properties of Nonvacuum Electron Beam Melted Composite Coating after Thermal Treatment

The paper studies the influence of thermal treatment of the structure and properties of the composite coating consisting of chromium and titanium carbides and obtained by nonvacuum electron beam melting. It is shown that the coating tempering leads to a decrease in the microhardness and wear resistance. This is because the austenitic structure decomposition with the formation of the soft ferrite-carbide structure. The formation of very hard martensite strengthens the tempered coating and improves its microhardness, which is comparable with that of the initial coating after the electron beam melting. The wear resistance of the hardened composite coating is lower than that of the electron beam melted coating because of brittle martensite, which cannot retain hard carbide particles. The composite coating produced under the optimum treatment conditions possesses higher properties and does not require an additional thermal treatment, that can modify the structure and reduce the mechanical properties of the coating.

Competitive mechanisms occurring during quenching and partitioning of three silicon variants of 0.4 wt.% carbon steels

Quenching and partitioning (Q&P) treated steels are traditionally alloyed with silicon (Si), but its precise role on microstructural mechanisms occurring during partitioning is not thoroughly understood. In this study, dilatometric analysis has been combined with detailed microstructural characterization to unravel the competing mechanisms occurring during partitioning either in parallel or in succession. Three 0.4 wt.% carbon steels with varying Si contents were quenched to 150 °C for ~20% untransformed austenite, and partitioned for 10–1000 s in the temperature range 200–300 °C. The steel with low Si content (0.25 wt.%) exhibited substantial bainitic transformation during partitioning at 300 °C and only 4% retained austenite (RA) at room temperature (RT) even after 1000 s hold. In contrast, a high Si fraction (1.5 wt.%) enabled retention of ~18% austenite under similar conditions. While η-carbides precipitated within the martensite laths in the high-Si steel, cementite precipitated in the low-Si variant. Furthermore, carbide precipitation and growth were strongly suppressed by high Si content. Secondary martensite formation occurred from carbon-enriched austenite during final cooling, irrespective of Si-content. Results illustrate that Si retards austenite decomposition at higher partitioning temperatures but does not improve carbon partitioning at lower temperatures.

Prediction of Microstructure Constituents’ Hardness after the Isothermal Decomposition of Austenite

An increase in technical requirements related to the prediction of mechanical properties of steel engineering components requires a deep understanding of relations which exist between microstructure, chemical composition and mechanical properties. This paper is dedicated to the research of the relation between steel hardness with the microstructure, chemical composition and temperature of isothermal decomposition of austenite. When setting the equations for predicting the hardness of microstructure constituents, it was assumed that: (1) The pearlite hardness depends on the carbon content in a steel and on the undercooling below the critical temperature, (2) the martensite hardness depends primarily on its carbon content, (3) the hardness of bainite can be between that of untempered martensite and pearlite in the same steel. The equations for estimation of microstructure constituents’ hardness after the isothermal decomposition of austenite have been proposed. By the comparison of predicted hardness using a mathematical model with experimental results, it can be concluded that hardness of considered low-alloy steels could be successfully predicted by the proposed model.

Effect of Silicon Content on the Decomposition of Austenite in 0.4C Steel during Quenching and Partitioning Treatment

Although quenched and partitioned (Q&P) steels are traditionally alloyed with Si, its precise role on microstructural mechanisms occurring during the partitioning process is not thoroughly investigated. In this study, a systematic investigation has been carried out to reveal the influence of Si on austenite decomposition, phase transformation and carbide precipitation during Q&P treatment. Using a Gleeble thermomechanical simulator, three medium carbon steels with varying Si contents (0.25, 0.70 and 1.5 wt.%) were hot-rolled, reaustenitized, quenched into the Ms -Mf range, retaining about 20% austenite at the quench-stop temperature (TQ), and held for 1000 seconds above TQ in the temperature range of 200-300°C in order to better understand the mechanisms operating during partitioning. Dilatometric measurements combined with microstructural characterization using SEM-EBSD, TEM, and XRD clearly revealed the occurrence of various mechanisms. The effect of partitioning temperature/time on the hardness of the Q&P samples was correlated with the microstructural features. Steel containing low Si content (0.25%) was incapable of promoting carbon enrichment of austenite during partitioning, leading to its continuous decomposition into isothermal martensite and/or bainite without any detectable austenite retained even holding at 300°C. In comparison, 1.5% Si content promoted retention of about 19% austenite under similar Q&P conditions. Small fractions of bainite and high-carbon martensite formed during final cooling in both steels after partitioning at 200°C. Moreover, carbide precipitation was strongly retarded by high Si content.

Thermal Stability of Retained Austenite in Advanced TRIP Steel with Bainitic Ferrite Matrix for Automotive Industries

Multiphase steels consisting of retained austenite and martensite/bainite microstructures such as TRIP, low-temperature-bainite, and Q&P steels are attractive candidates for the new-generation of AHSS. These steels exhibit a remarkable combination of strength and toughness which is essential to meet the objective of weight reduction of engineering-components, while maintaining the compromise of tough-safety requirements. Such good mechanical properties are due to the enhanced work hardening rate caused by austenite-to-martensite transformation during deformation and the strengthening contribution of martensite/bainite. The retained austenite can thermally decompose into more thermodynamically stable phases as a consequence of temperature changes, which is referred to as the thermal stability of retained austenite. TRIP-aided steel is an effective candidate for automotive parts because of safety and weight reduction requirements. The strength–ductility balance of high strength steel sheets can be remarkably improved by using transformation induced plasticity behavior of retained austenite. In manufacturing hot rolled TRIP-aided sheet steels, austenite transforms into bainite during the coiling process. Because black hot coils cool slowly after the coiling process, they are exposed at about 350–450°C for a few hours or days. Therefore, the metastable residual austenite can be decomposed into other phases. This decomposition of residual austenite can produce serious deteriorate of mechanical properties in hot rolled TRIP-aided sheet steels. The present work identified the decomposition behavior and study the thermal stability of retained austenite in the TRIP-aided steel with bainitic/ferrite matrix depending on coiling temperatures and holding times by means of DSC and XRD analysis.

Effects of Al, Si and N Content on the Formation of M-A Constituent and HAZ Toughness of Ti-Containing Low-Carbon Steel

The present work investigated the effects of Al, Si, and N content on the impact toughness of the coarse-grained heat-affected zone (CGHAZ) of Ti-containing low-carbon steel. Simulated CGHAZ of differing Al, Si, and N contents were prepared, and Charpy impact toughness was determined. The results were interpreted in terms of microstructure, especially martensite-austenite (M-A) constituent. All elements accelerated ferrite transformation in CGHAZ but at the same time increased the amount of M-A constituent, thereby deteriorating CGHAZ toughness. It is believed that Al, Si, and free N that is uncombined with Ti retard the decomposition of austenite into pearlite and increase the carbon content in the last transforming austenite, thus increasing the amount of M-A constituent. Regardless of the amount of ferrite in CGHAZ, its toughness decreased linearly with an increase of M-A constituent in this experiment, indicating that HAZ toughness is predominantly affected by the presence of M-A constituent. When a comparison of the effectiveness is made between Al and Si, it showed that a decrease in Si content is more effective in reducing M-A constituents.

Formation of Nodular Bainite in a Fe-9.10ni-0.06c (Wt. %) Alloy: A New Microstructure for Cryogenic Steels

Quenched and tempered Fe-9Ni (wt. %) steels have been widely used in cryogenic applications. For the first time, the formation of nodular bainite in an Fe-9Ni-0.06C (wt. %) steel during isothermal transformation at 550 and 500 °C for 24 h is reported. Electron microscopy revealed that proeutectoid ferrite forms predominantly by a massive transformation at prior austenite grain boundaries or at intragranular inclusions as the first stage of austenite decomposition. Then, nodular bainite composed of a ferritic matrix and rows of fine cementite precipitates forms by interphase boundary precipitation during decomposition of adjacent C-enriched austenite. Both the Bagarayatskii and Isaichev orientation relationships between ferrite and cementite were observed. Also, the microhardness of nodular bainite in the sample transformed at 500 °C (214 ± 4 VHN) was higher than that in the sample transformed at 550 °C (195 ± 5 VHN), due to smaller sizes of cementite precipitates in the former.

Phase transformations occurring during the formation of a welded joint from rail steel

The results of dilatometry, metallography and hardness testing of the decomposition process of supercooled austenite of R350LHT steel are presented. During continuous cooling and in isothermal conditions, continuous cooling transformation diagrams of supercooled austenite decomposition of steel R350LHT are constructed.

Proposition of an Empirical Functional Equation to Predict the Kinetics of Austenite to Ferrite Transformation in a Continuous Cooled IF-Ti-Stabilized Steel

The kinetics of phase transformations in isothermal processes is well described by the classic JMAK model. However, it is known that most industrial facilities employ continuous cooling processes and, for this condition, the JMAK model is adapted but, sometimes, without success. Considering the importance to predict critical temperatures, the present work proposes an alternative empirical functional equation to describe the kinetics of steel phase transformation under continuous cooling, being useful when the JMAK use is not successful. In this study, the continuous cooling austenite to ferrite transformation was experimentally characterized by dilatometry for an IF-Ti-stabilized steel. The proposed equation was compared with the classic adapted JMAK model regarding to evaluate its efficacy to fit the dilatometric experimental data. Using the constants obtained by the both model fittings as input parameters, a computational simulation was performed to determine the CCT diagram of the IF-Ti steel. The proposed functional equation was efficient to predict the critical temperatures, the kinetics of austenite decomposition and the CCT diagram of the studied steel. The results predicted by the proposed model greatly met the experimental data measured by dilatometry.

ПРО МОЖЛИВІСТЬ ПРОГНОЗУВАННЯ МЕХАНІЧНИХ ВЛАСТИВОСТЕЙ ХОЛОДНОДЕФОРМОВАНИХ МЕТАЛОВИРОБІВ ЗІ СТАЛЕЙ ПЕРЛІТНОГО КЛАСУ

In accordance with the latest global trends and modern needs of hardware factories, the demand for wire rod made of pearlitic steels has significantly increased, which can undergo cold plastic deformation with large degrees of reduction and is intended for the manufacture of various commercial products (cold-worked reinforcement, reinforcing ropes, spring wire, steel cord, wire for high pressure hoses, construction fiber, etc.). The most promising direction for increasing the strength class of hot-rolled steels is strain hardening during cold plastic deformation. The structure of cold-worked steels has a more uniform distribution over the cross-section, in contrast to thermally hardened ones, which in the latter case are characterized by annular structural zones formed due to different mechanisms of austenite decomposition. In a number of cases, structural heterogeneity causes instability of the mechanical properties of rolled steel, therefore, there is currently no alternative way to strain hardening. The manufacture of high-strength cold-worked metal products is a complex process and depends on the quality of the wire rod. If the cold-worked hardware does not meet the requirements of the normative documentation for the strength class, then it is no longer possible to ensure the correction of this defect using heat treatment. In this regard, scientific and practical interest has arisen to determine the possibility of creating a method for predictive determination of the mechanical properties of cold-deformed metal products made of pearlitic steels. The features of the influence of cold plastic deformation by on the formation of the strength class of pearlite grade steels with a carbon content of 0.7…0.9 % are considered It is established that the temporary resistance to fracture, at the known parameters of structure and carbon content, lends itself well to mathematical calculations and allows to create predictive models. According to the results of the research, a computer program was created that allows to automatically calculate the energy parameters of drawing and determine the tensile strength of the wire after depending on the total relative compression, parameters of the structure of wire rod and the carbon content in the steel.

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].

Characteristics of bainitic transformation and its effects on the mechanical properties in quenching and partitioning steels

Abstract Steel treated by quenching and partitioning exhibits an excellent combination of strength and ductility by taking advantage of the transformation-induced plasticity effect of retained austenite. Therefore, the precise tailoring of retained austenite is crucial for quenching and partitioning steels. However, austenite decomposition including bainitic transformation during partitioning is usually neglected, and consequently, its effects on the mechanical properties are not clear. Here, we elucidate the significant role of bainitic transformation during the partitioning process in the microstructure and mechanical properties of 0.2C–2.82Mn-1.58Si steel subjected to one-step quenching and partitioning process and two-step quenching and partitioning process. The results show that the occurrence of bainitic transformation during the QP as a result, the amount of retained austenite is increased, and more blocky austenite is retained due to the optimization of carbon redistribution during bainitic transformation. Moreover, an increase in the bainite volume fraction and the various morphologies of retained austenite lead to excellent work hardening performance, thereby significantly improving the elongation.

Limiting Retained Austenite Decomposition in Quenched and Tempered Steels: Influences of Rapid Tempering and Silicon

Limiting Retained Austenite Decomposition in Quenched and Tempered Steels: Influences of Rapid Tempering and Silicon

Effect of deformation and grain size on austenite decomposition during quenching and partitioning of (high) silicon‑aluminum steels

The effect of austenite deformation on carbon partitioning and transformation to athermal and isothermal martensite, and bainite during quenching and partitioning (QP) is described for three steel compositions: Fe-0.3C-0.6Si-1.1Al, Fe-0.3C-1.0Si and Fe-0.3C-0.5Si-0.5Al. Microstructures were characterized using SEM-EBSD, TEM and XRD. Austenite decomposition kinetics was investigated using dilatometry. Deformation close to the no-recrystallization temperature refines the grain size, lowers the martensite start temperature and affects the stability of austenite during QP processing. The results in respect of dilatation measurements and retained austenite contents at room temperature corroborate the QP microstructures and their hardness and can help in designing optimal QP treatments for these novel steels.