Initial Austenite Grain Research Articles

A prediction model for austenite grain size distribution of casting slab in thermal rebound process: Considering α-ferrite precipitation and re-austenization

The precipitation and re-austenitization behavior of α-ferrite during thermal rebound process of casting slab are crucial to the austenite grain size distribution. Accurately obtaining austenite grain size distribution plays an important guiding role for avoiding the formation of slab cracks and effectively improving the quality of steel. In this work, a quantitative model to calculate α-ferrite precipitation at different temperatures has been constructed by employing high-temperature expansion test and peak area method. On this basis, a prediction model for the austenite grain size distribution of casting slab in the thermal rebound process has been theoretically derived with consideration of the α-ferrite precipitation and re-austenitization. Then, the accuracy of the quantitative model for α-ferrite precipitation and the prediction model for austenite grain size distribution have been evaluated by high-temperature quenching experiments. The results show that the maximum error between the calculation values of the prediction model and the experimental values is only 3.67 %, which can reliably predict the austenite grain size distribution of slab under different phase fractions of precipitated α-ferrite and initial austenite grain sizes. When the phase fraction of precipitated α-ferrite reaches about 70 %, the average grain size after re-austenitization comes to nearly 50 % of the initial size, which achieves the best refining effect. The reduction in initial austenite grain size is beneficial to the refinement of austenite grain after thermal rebound. Reasonable control of thermal rebound temperature and multiple cycles of phase transformation are effective means of obtaining austenite microstructure with smaller average grain size and more uniform distribution.

Evaluation of Austenite-Ferrite Phase Transformation in Carbon Steel Using Bayesian Optimized Cellular Automaton Simulation.

Austenite-ferrite phase transformation is a crucial metallurgical tool to tailor the properties of steels required for particular applications. Extensive simulation and modeling studies have been conducted to evaluate the phase transformation behaviors; however, some fundamental physical parameters still need to be optimized for better understanding. In this study, the austenite-ferrite phase transformation was evaluated in carbon steels with three carbon concentrations during isothermal annealing at various temperatures using a developed cellular automaton simulation model combined with Bayesian optimization. The simulation results show that the incubation period for nucleation is an essential factor that needs to be considered during austenite-ferrite phase transformation simulation. The incubation period constant is mainly affected by carbon concentration and the optimized values have been obtained as 10-24, 10-19, and 10-21 corresponding to carbon concentrations of 0.2 wt%, 0.35 wt%, and 0.5 wt%, respectively. The average ferrite grain size after phase transformation completion could decrease with the decreasing initial austenite grain size. Some other parameters were also analyzed in detail. The developed cellular automaton simulation model combined with Bayesian optimization in this study could conduct an in-depth exploration of critical and optimal parameters and provide deeper insights into understanding the fundamental physical characteristics during austenite-ferrite phase transformation.

PECULIARITIES OF REVEALING AND ESTIMATION OF AUSTENITIC GRAIN IN STRUCTURAL ALLOYED STEELS

Purpose. Establishing effective and correct techniques for detecting the grain structure in structural medium-carbon steels taking into account the recommendations of modern standards (ДСТУ ISO, ASTM, ГОСТ) with the aim of reliably determining the austenitic grain size and putting into operation of metal with the well-known and required, according to the normative technical documentation (technical requirements, drawings) quality.
 Research methods. Metallographic analysis of microstructure and evaluation of the austenitic grain size were carried out by comparing the image revealed on polished and etched using various chemical reagents microsections of heat-treatable steel 40KHN2МА-Ш with etalon of scale of standard.
 Results. Studies of structural alloyed steel 40ХН2МА-Ш found that the most effective etchant reagent, which best detects initial austenitic grain boundaries in medium-carbon alloyed steels, is a standard reagent - a saturated aqueous solution of picric acid. As a result of etching in this reagent, a visible image of the grain structure is formed in the form of a clear contrast of light areas of the grain body and dark lines of the grain boundaries, which allows a correct comparison of the structure with the images of the standards on the scales.
 Scientific novelty. On the example of heat- treatable steel 40ХН2МА-Ш, the ways of further development of ideas regarding the choice of an effective chemical reagent are considered for the purpose of detecting and forming a visible grain structure in accordance with the requirements of methodical standards in structural medium-carbon steels, proper and reliable estimation of austenitіс grain size using standard scale.
 Practical value. A correct procedure for detecting the initial austenite grain is proposed, which allows to reliably determine the grain size and quality from this parameter of various structural steels including for the production of responsible parts of aircraft and spacecraft.

Practical approach for determining material parameters when predicting grain size after static recrystallization

The static recrystallization (SRX) behavior of 42CrMo steel was investigated by a non-isothermal hot compression test. The conventional empirical equation for SRX was applied to a finite element (FE) simulator using a nonconventional manner of parameter characterization. Parameter characterization was conducted by applying the FE-coupled optimization method to cylinder compression, which is an axisymmetric upsetting process. An accurate and practical flow stress model was used to precisely determine the critical strain value and define the instant when SRX occurred. The results revealed that the microstructural evolution during SRX was substantially influenced by temperature, strain rate, degree of deformation, and initial austenite grain size. The predicted recrystallized SRX grain sizes exhibited good agreement with the measured values when the model's parameters for the calculation of grain size were obtained using a FE-coupled optimization scheme.

Effect of Initial Austenite Grain Size on Microstructure Development and Mechanical Properties in a Medium-carbon Steel Treated with One-step Quenching and Partitioning

Fe-0.4C-1.2Si-0.8Mn (mass%) alloys austenitized at different temperatures, ranging from 1103 to 1473 K, were subjected to interrupted quenching (IQ) at 473 K and then maintained at that temperature to induce the partitioning of carbon from martensite to austenite (one-step quenching and partitioning (Q&P) process). The initial austenite grain size before the IQ was varied from 20 to 573 μm. As the initial austenite grain size becomes finer, the enrichment of carbon in the untransformed austenite during the partitioning treatment is enhanced, which leads to a greater increase in the volume fraction of retained austenite. The reasons for the increased carbon enrichment were explained by the effective carbon partitioning as well as the promoted bainitic transformation, which were both caused by the increase in the area of the martensite/austenite interface. Tensile tests of the specimens with different initial austenite grain sizes revealed that the mechanical properties of the one-step Q&P specimens improved in both strength and elongation by the refinement of the initial austenite grains.

EFFECT OF TANTALUM ON THE IMPACT RESISTANCE OF 12% CR STEELS SUBJECTED TO THERMOMECHANICAL TREATMENT

The effect of tantalum on the impact resistance of 12% Cr steels with different tantalum content (12CrTaNb and 12CrNb) was investigated in impact tests in the temperature range from -40 to +120°C with determining the brittle-ductile transition temperature as the temperature in the middle between the upper and lower shelf energies. Both 12% Cr steels were thermomechanically treated by alternating 1050°C annealing and forging in addition to standard normalizing treatment with high-temperature tempering. It was found that for Ta-alloyed 12% Cr steel, the entire fracture toughness versus temperature curve is 30-50 J/cm2 higher over the entire temperature range, including the upper and lower shelf energies, and the brittle-ductile transition temperature is 10°С lower. The main structural parameters of Ta-alloyed 12% Cr steel which can have a beneficial effect on toughness are a smaller initial austenite grain size, a larger average size but lower density of M23C6 carbide particles along low-angle martensite lath boundaries, and a larger volume fraction of VX carbonitrides.

Modeling of Austenite Grain Growth Behavior for AISI 302 Stainless Steel

Background: Among the metals used in foundry, we find the austenitic stainless steels, which are used in several fields because of their mechanical properties, which can change during the heat treatments; for that, it is important to understand and control the growth of the austenite grains Objective: Modeling austenite grain growth by considering the effects of heating temperature, holding time, and initial austenite grain size on austenite grain growth. Methods: In this paper, the austenite grain growth process of AISI 302 steel was studied in a temperature range of 900 to 1000 °C and a holding time of up to 360 minutes. Based on the experimental results and a combination of Arrhenius and Sellars type equations, a mathematical model of austenite grain growth was developed Results: From the experimental part, it was found that the increase in heating temperature caused the dissolution of carbides; therefore, the size of austenite grains grew faster, implying a higher growth rate. The prolongation of the holding time also led to the increase in the size of the austenite grains. Conclusion: Based on statistical indicators and a comparison between experimental and predicted results, the ability of the model to predict austenite grain growth was confirmed.

Delayed Fracture of Martensitic Steels during Hydrogen Saturation as a Function of the Initial Austenitic Grain Size

Delayed Fracture of Martensitic Steels during Hydrogen Saturation as a Function of the Initial Austenitic Grain Size

Modeling the microstructure evolution during quenching & partitioning of a conventional CrMo alloy steel

This work presents the simulation approach on the phase transformation and the microstructure evolution of a CrMo steel containing low Si processed with a typical Q&P cycle. The phase transformations at all steps of the Q&P were modelled using kinetics modelling and phase field model in order to predict the austenite grain size, the subsequent fraction and size of microstructural constituents. The initial austenite grain size was predicted considering the solute drag effect due to the presence of Cr obtained from kinetics simulation. The results showed that the austenite grain growth is restricted due to solute drag effect of Cr revealing that the heating stage is important for estimating the austenitic grain size. During quenching, the fraction of initial martensite was predicted. The results were aligned with dilatometry data indicating that the martensite fraction can be predicted via the phase field method. At the partitioning stage, the carbon content in the microstructural constituents determines the fraction of bainitic ferrite and retained austenite at the end of partitioning. In order to predict accurately the carbon content in the microstructure, it is important to simulate the carbon diffusion in the 2nd heating and prior to bainitic transformation. The results showed that the remaining austenite is enriched in carbon. During bainitic transformation, carbon atoms diffuse from bainitic ferrite sheaves towards austenite. By the time C concentration in austenite increases up to para – equilibrium value, austenite is stabilized. Due to fine PAGBs, the enrichment of retained austenite in C can be accelerated; thus smaller partition time can be applied for the stabilization of austenite and to complete the partitioning stage. Phase field results were in agreement with SEM/EBSD results showing that phase transformations can be predicted at all stages of Q&P process.

Deformation behavior during hot torsion of an ultrahigh carbon steel containing 1.3 wt.% C

Abstract The torsion behavior of an ultrahigh carbon steel containing 1.3 wt.% C has been studied at high strain rates (2 – 26 s –1) and high temperatures (900 – 1200 °C). Adiabatic heating strongly affects the shape of the stress– strain curves. Grain size measurements at strains of ε = 2 and ε = 5 reveal that the austenite grain size does not change between these two strains and is finer than the initial austenite grain size. This is an indication that a recrystallization process took place in the interval of strains between ε peak and ε = 2 and that the deformation state associated to the strains of ε = 2 and ε = 5 can be considered as steady state. Stress–strain rate relations have been analysed for ε peak, ε = 2 and ε = 5. It has been found that data obtained at ε peak correlate well with a lattice diffusion-controlled dislocation creep process. On the other hand, analysis of the stress –strain rate relations found at the steady state indicated that the deformation occurs by the contribution of two independent mechanisms: grain boundary sliding controlled by pipe diffusion and slip creep controlled by lattice diffusion.

Анализ влияния предела текучести на коррозионное растрескивание под напряжением мартенситных и ферритных сталей в кислых средах

To evaluate the effect of yield stress on hydrogen embrittlement (HE) of martensitic and ferritic steels, the effect of hydrogen (H) capture by structural inhomogeneities (hydrogen traps) and the effect of plastic deformation and stress on the mechanism of stress corrosion cracking (SCC) are considered. In the presence of hydrogen, the brittle fracture of high-strength martensitic steels consists of flat areas of intergranular fracture at the initial austenitic grain boundaries and quasi-brittle cracks at the boundaries of martensite blocks. In low-strength steels, brittle fracture manifests itself in the form of transgranular fracture of ferrite grains. The decrease in the characteristics of martensitic steels with an increase in the yield strength occurs due to an increase in the hydrogen concentration at the stage of anodic dissolution (AD) due to the growth of the carbide/matrix interface. The reason for the growth hydrogen concentration in ferritic steels is a large mechanical overstress, an increase in the number of active dissolution centers, the formation of an electrochemical pearlite-ferrite pair, and an increase in surface roughness with increasing deformation. It is concluded that the bell-shaped dependences of the critical stress of the transition from AD to SCC and other characteristics of mechanical tests on magnitude are due to different mechanisms of hydrogen accumulation in martensitic and ferritic steels.

Analysis of the effect of yield stress on stress corrosion cracking of martensitic and ferritic steels in acidic environments

To evaluate the effect of yield stress on hydrogen embrittlement (HE) of martensitic and ferritic steels, the effect of hydrogen (H) capture by structural inhomogeneities (hydrogen traps) and the effect of plastic deformation and stress on the mechanism of stress corrosion cracking (SCC) are considered. In the presence of hydrogen, the brittle fracture of high-strength martensitic steels consists of flat areas of intergranular fracture at the initial austenitic grain boundaries and quasi-brittle cracks at the boundaries of martensite blocks. In low-strength steels, brittle fracture manifests itself in the form of transgranular fracture of ferrite grains. The decrease in the characteristics of martensitic steels with an increase in the yield strength occurs due to an increase in the hydrogen concentration at the stage of anodic dissolution (AD) due to the growth of the carbide/matrix interface. The reason for the growth hydrogen concentration in ferritic steels is a large mechanical overstress, an increase in the number of active dissolution centers, the formation of an electrochemical pearlite-ferrite pair, and an increase in surface roughness with increasing deformation. It is concluded that the bell-shaped dependences of the critical stress of the transition from AD to SCC and other characteristics of mechanical tests on magnitude of the yield stress are due to different mechanisms of hydrogen accumulation in martensitic and ferritic steels. Keywords: hydrogen embrittlement, high-angle boundaries, interfaces, traps, hydrogen binding energy, structural inhomogeneities, fracture.

Modeling of the Dynamic Recrystallization of Ti Microalloyed High‐Strength Steel

The dynamic recrystallization (DRX) and austenite microstructure evolution of Ti microalloyed high‐strength steel are investigated and modeled by thermal simulation experiments. The corresponding mathematical models are established based on DRX‐type flow stress curves and the austenite microstructure. A highly accurate austenite grain growth model under different heating conditions is established, which can predict the initial austenite grain sizes before deformation. Considering the effect of initial austenite grain sizes and deformation conditions, the key parameters (peak stress, steady stress, peak strain, and constants C and C1) of the DRX‐type flow stress model are determined, and the predicted DRX‐type flow stresses are in good agreement with the experimental values. The relationships between critical strain and peak strain(εc = 0.77εp), critical stress, and peak stress(σc = 0.82σp) are determined for experimental steel. The relationships between the initial austenite grain size and the key parameters (k, M, a, h) in the DRX kinetic model and the DRX grain size model are determined. The calculated and experimental DRX volume fractions and grain sizes are in good agreement. The results show that these mathematical models can accurately describe the DRX behavior of experimental steel.

In Situ Observations and Microstructure Evolution Behavior of Static Recrystallization of Microalloyed Continuous Casting Slabs in a Solidification End Reduction Process

Solidification end reduction is a novel technique that can effectively refine the microstructure and improve the homogeneity in continuously cast steel. Herein, the static recrystallization (SRX) behaviors and microstructure evolution of microalloyed continuous casting slabs during the heavy reduction (HR) process are simulated and analyzed by means of high‐temperature laser scanning confocal microscopy (HT‐LSCM). According to the statistical results of microstructure, the insulation temperature has a more significant effect on the SRX fraction and recrystallization grain size compared with the strain rate, initial austenite grain size, and prestrain, and a modified SRX kinetic model is proposed. Furthermore, a deforming microstructure is observed at different strains and holding times with electron backscatter diffraction (EBSD), and it is found that the reduction of high‐angle boundaries (HAGBs, θ > 15°) of 25°–35° is related to strain‐induced boundary migration (SIBM). The stored deformation energy of HAGBs is higher than that of very low‐angle boundaries (VLAGBs, θ < 5°) by the detailed misinterpretation profile analysis of the block and lath boundaries. Finally, the combination of in situ observation and EBSD analysis shows that the stored deformation energy provides the driving force for the SRX process, which, in turn, consumes dislocations to stabilize the microstructure.

Effect of deformation parameters on microstructure of martensitic stainless steel during static recrystallization

In this paper, the static recrystallization behavior of X20Cr13 martensitic stainless steel was studied based on the hot compression test. The experimental results show that when the strain of the first pass is less than the critical strain of dynamic recrystallization, the main static softening mechanism is static recrystallization. The effects of prestrain, initial austenite grain size, deformation temperature and strain rate on the microstructure during static recrystallization were analyzed in detail.

Effect of Initial Austenite Grain Size on Microstructure Development and Mechanical Properties in a Medium-carbon Steel Treated with One-step Quenching and Partitioning

Effect of Initial Austenite Grain Size on Microstructure Development and Mechanical Properties in a Medium-carbon Steel Treated with One-step Quenching and Partitioning

Retraction Note To: Artificial neural network to predict the effect of heat treatments on Vickers microhardness of low-carbon Nb microalloyed steels

In the present study, an artificial neural networks-based model (ANNs) was developed to predict the Vickers microhardness of low-carbon Nb microalloyed steels. Fourteen parameters affecting the Vickers microhardness were considered as inputs, including the austenitizing temperature, cooling rate, initial austenite grain size, different chemical compositions and Nb in solution. The network was then trained to predict the Vickers microhardness amounts as outputs. A Multilayer feed-forward back-propagation network was developed and trained using experimental data form literatures. Five low-carbon Nb microalloyed steels and one low-carbon steel without Nb were investigated. The effects of austenitizing temperature (900–1,100°C) and subsequent cooling rate (0.15–227°C/s) and initial austenite grain size (5–130 μm) on the Vickers microhardness of steels were modeled by ANNs as well. The predicted values are in very good agreement with the measured ones, indicating that the developed model is very accurate and has the great ability for predicting the Vickers microhardness.

Modification for prediction model of austenite grain size at surface of microalloyed steel slabs based on in situ observation

The initial solidification process of microalloyed steels was simulated using a confocal scanning laser microscope, and the growth behavior of austenite grain was observed in situ. The method for measuring the initial austenite grain size was studied, and the M 0 * (the parameter to describe the grain boundary migration) values at different cooling rates were then calculated using the initial austenite grain size and the final grain size. Next, a newly modified model for predicting the austenite grain size was established by introducing the relationship between M 0 * and the cooling rate, and the value calculated from the modified model closely corresponds to the measured value, with average relative error being less than 5%. Further, the relationship between Tγ (the starting temperature for austenite grain growth) and equivalent carbon content CP (CP > 0.18%) was obtained by in situ observation, and it was introduced into the modified model, which expanded the application scope of the model. Taking the continuous casting slab produced by a steel plant as the experimental object, the modified austenite grain size prediction model was used to predict the austenite grain size at different depths of oscillation mark on the surface of slab, and the predicted value was in good agreement with the actual measured value.

Особенности замедленного разрушения мартенситных сталей при наводороживании в зависимости от размеров исходных аустенитных зерен

The effect of the size of the initial austenite grains on the local characteristics of delayed fracture of martensitic steel with high and low levels of residual internal microstresses under hydrogenation conditions has been studied. The contribution of hydrogen and residual internal microstresses to the decrease in local strength of the boundaries of the original austenite grains during hydrogenation during the incubation period is divided.

Study of static recrystallization behavior of austenite in a Ti–V microalloyed steel

The static recrystallization (SRX) behavior in Ti–V microalloyed steel was studied through two-pass compression tests using a Gleeble-3500 thermo-mechanical simulator. Under the experimental conditions with the deformation temperatures of 950–1050 °C, strain rates of 0.01–1 s–1, pass strains of 0.1–0.2 and inter-pass time of 1–100 s, respectively. The influence of various processing parameters on the static softening were discussed seriously and the results showed that the softening fractions raise quickly with the deformation temperature, the strain rate, the pass strain and the inter-pass time; Oppositely, it is reduced as the initial austenite grain size raises. In addition, the kinetic equations have been adopted and the required parameters have been determined based on the experiments, the calculated results are consistent with the experimental results, indicating the SRX behavior and microstructural evolutions of Ti–V microalloyed steel could be estimated by the determined kinetics parameters.