Austenite Grain Growth Research Articles

Austenite Grain Structures in Ti- and Nb-Containing High-Strength Low-Alloy Steel During Slab Reheating

Austenite-grain growth was investigated in a couple of microalloyed steels, one containing Ti and the other containing Nb, Ti, and V, using different reheating temperatures between 1273 K and 1523 K (1000 °C and 1250 °C). Nature and distribution of microalloy precipitates were quantitatively analyzed before and after reheating. Interdendritic segregation (or microsegregation) during casting can result in an inhomogeneous distribution of microalloy precipitates in the as-cast slabs, which can create austenite grain size variation (even grain size bimodality) after reheating. Ti addition reduced the grain size variation; however, it could not eliminate the grain size bimodality in Nb-containing steel, due to the differential pinning effect of Nb precipitates. A model was proposed for the prediction of austenite grain size variation in reheated steel by combining different models on microsegregation during solidification, thermodynamic stability, and dissolution of microalloy precipitates and austenite grain growth during reheating.

Enhancing mechanical properties of a low-carbon microalloyed cast steel by controlled heat treatment

In the present work, detailed studies were made on the optimization of microstructure and mechanical properties of a low-carbon microalloyed cast steel through control of heat treatment conditions. Specimens were austenitized at temperatures ranging from 950 to 1200°C for 2h followed by different cooling methods (furnace, air and water). For analyzing the effect of holding time on mechanical properties, some cast specimens were austenitized at 1100°C for different times followed by furnace cooling. After heat treatment, mechanical tests were employed to evaluate the room temperature Charpy impact and tensile properties. The characterization of complex precipitates formed during heat treatment process was investigated by using analytical electron microscopy. The results show that dissolution of vanadium-containing precipitates plays an important role in the abnormal growth of austenite grains at 1150°C. Further growth in austenite grains at 1200°C is caused by the dissolution of Ti-containing particles and the reduction of the amount of precipitates. Correct selection of the austenitizing temperature, holding time and cooling method is very important to improve the mechanical properties of the steel. Heat treatment at 1100°C for 2h followed by furnace cooling leads to the best combination of excellent Charpy impact and tensile properties.

The Solute Drag Effect during Recrystallisation and Grain Growth in Austenite

The relevance of the solute drag phenomenon to the hot rolling of modern steel grades is outlined. An overview of our present knowledge of solute drag in grain growth and recrystallisation in austenite is presented and recommendations for subsequent research are given.

Microstructure and Mechanical Properties of Martensitic Stainless Steel 6Cr15MoVn

Martensitic stainless steel containing Cr of 12% to 18% (mass percent) are common utilized in quenching and tempering processes for knife and cutlery steel. The properties obtained in these materials are significantly influenced by matrix composition after heat treatment, especially as Cr and C content. Comprehensive considered the hardness and corrosion resistance, a new type martensitic stainless steel 6Cr15MoV has been developed. The effect of heat treatment processes on microstructure and mechanical properties of 6Cr15MoV martensitic stainless steel is emphatically researched. Thermo-Calc software has been carried out to thermodynamic calculation; OM, SEM and TEM have been carried out to microstructure observation; hardness and impact toughness test have been carried out to evaluate the mechanical properties. Results show that the equilibrium carbide in 6Cr15MoV steel is M23 C6 carbide, and the M23C6 carbides finely distributed in annealed microstructure. 6Cr15MoV martensitic stainless steel has a wider quenching temperature range, the hardness value of steel 6Cr15MoV can reach to HRC 60.8 to HRC 61. 6 when quenched at 1060 to 1100 °C. Finely distributed carbides will exist in quenched microstructure, and effectively inhabit the growth of austenite grain. With the increasing of quenching temperature, the volume fraction of undis-solved carbides will decrease. The excellent comprehensive mechanical properties can be obtained by quenched at 1060 to 1100 °C with tempered at 100 to 150 °C, and it is mainly due to the high carbon martensite and fine grain size. At these temperature ranges, the hardness will retain about HRC 59. 2 to HRC 61. 6 and the Charpy U-notch impact toughness will retain about 17. 3 to 20 J. A lot of M23C6 carbides precipitated from martensite matrix, at the same time along the boundaries of martensite lathes which leading to the decrease of impact toughness when tempered at 500 to 540 °C. The M3C precipitants also existed in the martensite matrix of test steel after tempered at 500 °C, and the mean size of M3C precipitates is bigger than that of M23C6 precipitates.

Abnormal and post-abnormal austenite grain growth kinetics in Nb–Ti microalloyed steels

The kinetics of abnormal grain growth in austenite was examined in microalloyed steel containing Nb and Ti additions. Abnormal grain growth was triggered by the dissolution of microalloyed carbonitrides during reheating. The progress of abnormal grain growth was determined at 1200, 1300 and 1400°C. Post-abnormal grain growth was defined as the grain growth process which takes place once all of the original fine grains have been consumed by the abnormal grains. The post-abnormal grain growth kinetics appeared to follow a simple parabolic law. The parabolic constant needed to describe post-abnormal grain growth in the present alloy was three times larger than that needed to describe normal grain growth in a similar alloy without Nb and Ti additions. The possible origins of this effect are briefly discussed.On a examiné la cinétique de croissance anormale de grain dans l'austénite d'acier micro-allié contenant des additions de Nb et de Ti. La croissance anormale de grain était déclenchée par la dissolution des carbonitrures micro-alliés lors du réchauffage. On a déterminé le progrès de la croissance anormale de grain à 1200°C, 1300°C et 1400°C. La croissance post-anormale de grain était définie comme le processus de croissance de grain qui a lieu une fois que tous les grains fins originaux ont été consommés par les grains anormaux. La cinétique de la croissance postanormale de grain semble suivre une simple loi parabolique. La constante parabolique nécessaire pour décrire la croissance post-anormale de s grain dans le présent alliage était trois fois plus grande que celle qui est nécessaire pour décrire la croissance normale de grain dans un alliage semblable sans additions de Nb et de Ti. On discute brièvement de l'origine possible de cet effet.

Prediction of grain growth of coarse-grained austenite in Nb–V–Ti microalloyed steel

Grain growth of coarse-grained austenite in Nb–V–Ti microalloyed steel during equalisation was predicted by extending the previous investigation. The prediction worked with initial austenite grain size distribution instead of average grain size. An improved model taking into account the holding time was used in the prediction. The result showed that only part of initial austenite grains grow at each equalisation temperatures, but the grain size of growing grains is expanded to a wider range with increasing equalisation temperature, which indicates that grain size distribution should be considered when grain growth of coarse-grained austenite is evaluated. The predicted austenite grain size distribution is close fit to the measured one and further work is expected to improve the quality of model prediction.

The Kinetics of Austenization Process of T91 Ferritic Heat-Resistant Steel

By fitting the calculated transformed fraction according to developed phase-transformation model to the experimental data obtained by differential dilatometry, the kinetic characteristics of the austenitization process in T91 steels have been investigated. According to the kinetic parameters fitted, we recognize that the nucleation and growth of austenite grain are mainly controlled by the diffusion of carbon in ferritic and austenite respectively. In addition, by increasing the diffusion active energy of carbon in austenite, carbides hinder the motion of interface and thus refine austenite grain.

The Effect of Co and Cu on the Creep Properties of High Chromium (12%Cr) Ferritic Steel

In this paper, three kinds of 12% Cr ferritic steels without Co and Cu, with 3%Co and with 3%Cu are produced. The addition Co and Cu lead to an evident increase in creep-resistant of the ferritic steel, and furthermore also markedly affect the fracture behavior. Microstructures were studied by comparing three kinds of the 12% ferritic steels to better understand different in those creep behaviors and fracture behaviors. It was found that the addition of Co and Cu not only inhibit the formation δ-ferrite but also are benefit for the growth of prior austenite grains. Additionally, the part of the added Cu precipitates in the matrix of the ferritic steel. These changes in microstructure and the solution of Co and Cu strengthen the ferritic steel.

Dispersion of the structure of steels under intense thermal effect. Part 2. A study of the effect of alloying system on the evolution of structure in austenization

Processes that develop in heating of initially hardened steels 12Kh2G2NMFT and 12KhN3A in the range of tempering temperatures, in the intercritical temperature range (ITR), and in the single-phase austenitic region are studied. The rate of relaxation of the hardened state in tempering, the intensity of formation of austenite during continuous heating and isothermal holds in the ITR, and the susceptibility to secondary growth of austenite grains in the single-phase region are analyzed.

Simulation of Austenitization Processes in Fe–C Steels by Coupled Cellular Automaton and Finite Difference Methods

Abstract The aim of the simulation was to calculate the austenitization processes in plain carbon steels with different carbon concentrations. As the first step of simulation, an initial structure was generated where the diameters of pearlite colonies, the thickness and orientation of cementite lamellae, and the thickness of pro-eutectoid ferrite or even secondary cementite were changed. By this method, similar structures to the real ones were obtained. To solve the nucleation problem, a free enthalpy model was used, which makes difference between the places of nuclei. The diffusion equation (Fick II.) in case of the grain growth of austenite was solved by the Finite Differential Method. Nuclei can be formed at the boundary of pearlite colonies, at the boundary of the pro-eutectoid ferrite and cementite, or rarely at the boundary of cementite and ferrite lamellae too. The grain growth model calculates the carbon diffusion through both ferrite and austenite phases. The simulation runs as a coupled Cellular Automaton and Finite Difference Methods.

Effects of Rare Earths on Austenite Grain Growth Behavior in X80 Pipeline Steel

Accorded with the criteria the X80 pipeline steels with RE additions have been prepared with a vacuum induction furnace. The effect of rare earths (RE) on austenite grain growth behavior in pipeline steel has been discussed in this work. After hot rolled, the effect of RE on austenite grain growth behavior have been studied by the thermal simulator, scanning electron microscopy and metallographic microscope. The results indicate that RE additions inhibit the abnormal growth of austenite grain, trace RE could significantly influence the austenite grain growth behavior, and obtain fine grain size in X80 pipeline steel. For instance, the activation energy of grain growth reduced from 475 kJ/mol to 425 kJ/mol with addition of 0.0220 wt.% RE according with no deliberate RE additions, but the grain growth index increased from 3.5 to 4.5.

A study on martensitic phase transformation in 9Cr–1W–0.23V–0.063Ta–0.56Mn–0.09C–0.02N (wt.%) reduced activation steel using differential scanning calorimetry

The salient characteristics of martensite formation kinetics in 9Cr–1W–0.23V–0.063Ta–0.09C–0.02N (wt.%) reduced activation steel have been investigated using differential scanning calorimetry and supplemented by microscopy characterisation. Accurate determination of the martensite start ( M S) and finish ( M f) temperatures as a function of cooling rate (1–99 K min −1) and holding time (1–120 min) in the γ-austenite phase field have been made. The critical cooling rate to form nearly 100% martensite is found to be of the order of 5–6 K min −1. The measured latent heat for the martensite transformation is found to be in the range 62–75 J g −1. In the case of austenitisation at 1323 K, it is found that the grain size of the parent austenite exhibited significant growth and the observed M S temperature too exhibited a systematic increase with austenite grain size. However, for those samples that are solution treated at a slightly lower temperature of 1253 K, the M S is found to be initially insensitive to the increase in holding time for up to about 30 min. This trend is followed later on by the usual increasing character of M S for further rise in the holding periods, up to about 120 min. This unusual initial behaviour is also reflected by the corresponding variation in the hardness values of the quenched in martensite. Moreover, the growth of parent austenite grains has also been restricted during the initial phase of solution annealing at 1253 K. This is attributed to the presence of undissolved carbide particles, which exert a pinning influence on the movement of austenite grain boundaries. The observed athermal martensite formation kinetics is described well by the Koistinen–Marburger relation.

Isothermal Grain Growth of Austenite in Hypoeutectoid and Hypereutectoid Plain Carbon Steels

Grain growth kinetics of austenite in a hypoeutectoid steel (containing carbon of 0.35%) at 920 °C and in a hypereutectoid steel (containing carbon of 1%) at 980 °C for holding time ranging from 1 h to 6 h was investigated. The hypoeutectoid steel exhibited normal grain growth without solute drag hindrance with a time exponent (0.51) close to the theoretical value (0.5). However, the grain growth of austenite in the hypereutectoid steel held up to 3 h was extremely slow, characterizing by a low value of time exponent (0.08). Thereafter, a breakaway occurred and the grain growth in the hypereutectoid steel held from 3 h to 6 h progressed normally with a time exponent (0.52) close to the theoretical value (0.5).

Microstructure and precipitation in as cast low carbon Nb–V–Ti microalloyed medium thin slab

The solidification structure, austenite and precipitates in a quenched compact strip processing (CSP) medium thin slab (170 mm thick) of Nb–V–Ti microalloyed steel have been studied. It was found that secondary dendrite arm spacing and austenite grain size are slightly larger than that of similar steel produced by CSP thin slab. This is partially attributed to the slower cooling rate caused by the increased slab thickness. On the other hand, the formation of carbonitride during solidification reduces the width of secondary dendrite arm spacing, while TiN particles and alloying elements in solution may inhibit the growth of austenite grain during solidification and subsequent cooling. In addition to the semidendritic, larger cubic and fine cubic precipitates, which can be observed in CSP thin slab, dendritic precipitates were also found in CSP medium thin slab.

Austenite Nucleation and Growth Observed on the Level of Individual Grains by Three-Dimensional X-Ray Diffraction Microscopy

Austenite nucleation and growth is studied during continuous heating using three-dimensional X-ray diffraction (3-D XRD) microscopy at the European Synchrotron Radiation Facility (ESRF) (Grenoble, France). Unique in-situ observations of austenite nucleation and growth kinetics were made for two commercial medium-carbon low-alloy steels (0.21 and 0.35 wt pct carbon with an initial microstructure of ferrite and pearlite). The measured austenite volume fraction as a function of temperature shows a two-step behavior for both steel grades: it starts with a rather fast pearlite-to-austenite transformation, which is followed by a more gradual ferrite-to-austenite transformation. The austenite nucleus density exhibits similar behavior, with a sharp increase during the first stage of the transformation and a more gradual increase in the nucleus density in the second stage for the 0.21 wt pct carbon alloy. For the 0.35 wt pct carbon alloy, no new nuclei form during the second stage. Three different types of growth of austenite grains in the ferrite/pearlite matrix were observed. The combination of detailed separate observations of both nucleation and growth provides unique quantitative information on the phase transformation kinetics during heating, i.e., austenite formation from ferrite and pearlite.

Comparison of Grain Growth between Fine-Grained and Coarse-Grained Austenite in a Nb-V-Ti Microalloyed Steel

Comparison of grain growth of fine-grained and coarse-grained austenite in a Nb-V-Ti microalloyed steel during reheating or equalization at the same temperature was investigated using cold-charging and hot-charging specimens respectively in this study. The results show that the different grain growth behavior appears in fine-grained and coarse-grained austenite. The uniform grain growth and lower growth rate at reheating temperature studied was found in fine-grained austenite, while partial grain growth and higher growth rate was present in coarse-grained austenite. During reheating or equalization, the slow growth rate in fine-grained austenite may be contributed to stronger pinning force of fine precipitates while higher grain growth rate in coarse-grained austenite were believed to the result of lager size difference among part of grains. Grain coarsening occurs in fine-grained austenite as result of precipitate unpinning at extending holding time, but coarse-grained austenite remained wide size distribution at the same condition and this should not be considered as grain coarsening. coarse-grained austenite remained wide size distribution at the

Grain growth behavior of coarse-grained austenite in a Nb–V–Ti microalloyed steel

Grain growth of coarse-grained austenite during equalization in a simulated thin slab casting and direct rolling process (TSDR) has been studied on a Nb–V–Ti microalloyed steel. Coarse-grained austenite produced by reheating at 1350 °C for 90 min were reheated to four equalization temperatures (1100, 1150, 1200 and 1250 °C) for 30 min, then the austenite grain sizes and precipitates were examined. The experimental results show that coarse-grained austenite keep its wide range of grain sizes at elevated equalization temperatures. Ti-rich carbonitrides retain its strong pinning effect on grain boundary of coarse-grained austenite at temperatures below 1250 °C. The occurrence of abnormal grain growth in coarse-grained austenite may be difficult when particle coarsening occurs as it is hard for the smaller grains which has the larger driving forces to grow into neighboring larger grain matrix.

Kinetic Analysis of the Austenite Grain Growth in GCr15 Steel

The growth behavior of austenite grains in GCr15 steel was investigated through the isothermal annealing tests of the steel under different heating temperatures and holding times. The tests were performed on a Gleeble-3800 thermo-mechanical simulation machine. Austenitizing temperatures 1223, 1323, 1373, and 1423 K were chosen, and holding time varied from 0 to 480 s. Experimental results suggest that austenite grains grow gradually with the increase of heating temperature, and holding time has an important effect on the growth of austenite grains. The time exponent for the growth is bigger at higher temperature, and the growth rate decreases with increasing time. On the basis of previous models and experimental results, a mathematical model that can describe the growth behavior of austenite grains in the tested steel under different heating temperatures and holding times was obtained using regression analysis. The predicted grain sizes by the model are in good agreement with measured ones.

Micro-structure Refinement in Low Carbon High Manganese Steels through Ti-deoxidation: Austenite Grain Growth and Decomposition

This paper investigates the effect of de-oxidation inclusions on micro-structure evolution in low-carbon steels. Low carbon (0.07 wt%), high Mn (0.9 wt%) steel in a Al2O3 or MgO crucible was deoxidized by adding either aluminum (0.05 wt%) or titanium (0.05, 0.03 or 0.015 wt%) in a 400 g-scale vacuum furnace, and cast in a Cu mold at cooling rates between 2.0–6.0 K/s. These cast samples were re-melted and cooled at various cooling rate, 1 through 100 K/s in the hot-stage of a conforcal laser scanning microscope (CSLM) in order to investigate the effect of cooling rate.Oxide inclusion sizes in all the Ti-killed steels were smaller and inclusion densities higher than those in the Al-killed steel. In Ti-killed steel, inclusion size and densities increased with increasing the oxygen content, inclusion size decreased and their densities increased with increasing the cooling rate.A Confocal Scanning Laser Microscope (CSLM) was used to study the differences in solid state micro-structural evolution between the Ti-killed and theAl-killed samples. The growth of austenite grains were studied under isothermal conditions and it was found that both grain-boundary mobility and final grain size were lower in the Ti-killed sample than for the others. With regards to austenite decomposition, during continuous cooling from a comparable austenite grain structure, the resulting austenite decomposition structure was finer for the Ti-killed sample due to a higher Widmanstatten lath density due to precipitation at. The inclusion size was found to have a significant effect on both austenite grain size and austenite decomposition structure. Different orientations of ferrite precipitates originating at inclusions were observed in the Ti-killed samples. The highest lath concentration was obtained for the sample that had the smallest average inclusion size rather than the sample with highest density of sub micro-meter inclusions.

Optimizing the heat treatment of steel of 20X1MΦA type

Low-alloy Cr‐Mo‐V structural steel is widely used because its mechanical properties may be enhanced by varying the parameters of strengthening heat treatment [1]. Selection of the optimal treatment temperature and time calls for study of austenite-grain growth on heating and the disintegration kinetics of the supercooled austenite. In addition, account must be taken of the influence of the decomposition products of austenite, by various mechanisms, on the steel properties. In the present work, we study the structural and phase transformations in low-alloy Cr‐Mo‐V steels on heating in the austenite region and on continuous cooling at different rates. The table summarizes the chemical composition of the steel. SNOL furnaces are used for heat treatment of steel. A Neophot 21 optical microscope permits metallographic analysis. Durometric data are obtained in accordance with State Standard GOST 2999; a MK30A pendulum-type impact tester is employed for dynamic tests. Fractographic data are obtained by means of a Philips SEM 535 raster electron microscope. The disintegration kinetics of the supercooled austenite in continuous cooling is studied by means of a programmable electronic‐mechanical dilatometer, which permits sample heating at 0.5‐1000 ° C/min and cooling at 0.5‐500 ° C/min.