Conditions Of Dynamic Recrystallization Research Articles

Investigation on hot deformation behavior and microstructure evolution of superaustenitic stainless steel S31254 during elevated temperature compression testing

The study thoroughly investigated the hot deformation behavior and microstructure evolution of superaustenitic stainless steel S31254. Hot compression tests were conducted at temperatures ranging from 950 °C to 1200 °C and strain rates of 0.01–10 s−1. The flow behavior of S31254 is influenced by adiabatic heating, particularly at high strain rates. The critical conditions of dynamic recrystallization for various deformation conditions were calculated based on the “double differential method.” An rise in deformation temperature or a reduction in strain rate reduces critical stress. Using the Arrhenius equation and Zener-Hollomon parameter to represent the deformation parameters, a linear connection between peak stress and dynamic recrystallization critical conditions was determined. Electron backscattering diffraction and transmission electron microscopy were used to characterize the microstructure under various deformation conditions. The dynamic softening of superaustenitic stainless steel is mainly achieved through dynamic recrystallization. The recrystallized grains are driven by distorted energy to nucleate preferentially at high-angle grain boundaries and grow towards high misorientations. Discontinuous dynamic recrystallization characterized by grain boundary bowing out is the main recrystallization mechanism in S31254. However, at high temperatures and low strain rates (1200 °C, 0.01 s−1), the dynamic recrystallization process shifts from discontinuous to continuous, with subgrain coalescence serving as the nucleating mode.

Hot deformation behaviors and dynamic recrystallization mechanism of Ti-35421 alloy in β single field

Hot deformation behaviors and microstructure evolution of Ti−3Al−5Mo−4Cr−2Zr−1Fe (Ti-35421) alloy in the β single field are investigated by isothermal compression tests on a Gleeble−3500 simulator at temperatures of 820−900 °C and strain rates of 0.001−1 s−1. The research results show that discontinuous yield phenomenon and rheological softening are affected by the strain rates and deformation temperatures. The critical conditions for dynamic recrystallization and kinetic model of Ti-35421 alloy are determined, and the Arrhenius constitutive model is constructed. The rheological behaviors of Ti-35421 alloys above β phase transformation temperature are predicted by the constitutive model accurately. The EBSD analysis proves that the deformation softening is controlled by dynamic recovery and dynamic recrystallization. In addition, continuous dynamic recrystallization is determined during hot deformation, and the calculation model for recrystallization grain sizes is established. Good linear dependency between the experimental and simulated values of recrystallized grain sizes indicates that the present model can be used for the prediction of recrystallized grain size with high accuracy.

Critical Conditions for Dynamic Recrystallization of S280 Ultra-High-Strength Stainless Steel Based on Work Hardening Rate

Isothermal and constant-strain-rate compression experiments for S280 ultra-high-strength stainless steel were carried out under deformation temperatures of 1000–1150 °C and strain rates of 0.001–10 s−1 with a Thermecmaster-Z thermal simulator. The flow–stress behavior of the alloy was studied and the hot deformation activation energy was calculated. A critical strain model of the dynamic recrystallization (DRX) of the alloy was established using the work hardening rate for the first time. The results show that S280 ultra-high-strength stainless steel was positively sensitive to the strain rate and negatively sensitive to temperature, and its flow–stress curve showed characteristics of flow softening. The hot deformation activation energy corresponding to the peak strain was 519.064 kJ/mol. The DRX critical strain of the steel was determined from the minimum value of the −∂(lnθ)/∂ε − ε curve. The relationship between the DRX critical strain and peak strain could be characterized as εc=0.599εp and the relationship between the DRX critical stress and peak stress could be characterized as σc= 0.959σp The critical strain model of DRX could be expressed as εc=0.010Z0.062. The research results can provide theoretical support for avoiding the generation of actual thermal processing microstructure defects such as coarse grains and for obtaining products with excellent microstructure and properties.

Experimental study on dynamic recrystallization of titanium alloy Ti6Al4V at different strain rates

The dynamic recrystallization condition and law of Ti6Al4V alloy at all strain rates are investigated through thermal processing simulation test, split Hopkinson compression bar test and milling test. The theoretical calculation of the experimental results and the microstructure observation showed that the dynamic recrystallization become possible under the conditions of high strain rate and low strain rate. The model uses two expressions to describe the material constitutive characteristics of different critical strain value intervals; the subprogram written in Fortran language is imported into AdvantEdge FEM software, and the finite element analysis of John-Cook constitutive and improved constitutive is carried out respectively. The comparative study of simulation proves that the improved constitutive equation is closer to the high temperature and high impact environment of high-speed cutting of titanium alloy, which has certain guiding significance for high-speed cutting of titanium alloy.

Prediction of the critical conditions for dynamic recrystallization in metals

Abstract Engineering approaches to model dynamic recrystallization (DRX) explicitly assume the existence of a critical strain for DRX. Since the strain is not a state variable of crystal plasticity, however, there is no critical strain, rather there are critical conditions for microstructural instability. It was proposed recently that these critical conditions can be expressed in terms of a point of inflection on the θ(σ) curve, where σ is the flow stress and θ is the hardening rate. A theoretical framework is presented to predict these critical conditions on the basis of a dislocation work hardening model. Once tuned to a specific hardening curve the model allows to predict the onset of DRX for different temperatures and strain rates, and, in principle, for changing materials chemistry.

Kinetic model for describing continuous and discontinuous dynamic recrystallization behaviors of 2195 aluminum alloy during hot deformation

2195 aluminum alloy is widely used to manufacture the external tanks of space shuttles and aircraft skins, and its mechanical properties are substantially affected by dynamic recrystallization occurring in its microstructure. In this study, hot compression experiments were performed on 2195 aluminum alloy to investigate its dynamic recrystallization (DRX) behavior. The results indicated that discontinuous (DDRX) and continuous dynamic recrystallization (CDRX) both occurred in the alloy from 300 °C to 520 °C at strain rates of 0.01–10 s −1 . The microstructure analysis revealed that DDRX grains formed at grain boundaries, while CDRX grains appeared in the triangular grain boundary region with transition grain boundaries or within grains. The dominant DRX mechanism changed from DDRX to CDRX when the strain was greater than 1.2. Moreover, at higher temperatures or lower strain rates, the number of CDRX grains increased, while the number of DDRX grains decreased. Finally, to quantitatively describe the DDRX and CDRX kinetics of 2195 aluminum alloy, the critical conditions were used to establish a composite kinetics model that was consistent with the experimental results, and which suitably predicted the volume fraction of DRX grains formed via the two different DRX mechanisms. • A strain of 1.2 was the transition point between the two recrystallization mechanisms for 2195 aluminum alloy under thermal compression. • Upon increasing the temperature or decreasing the strain rate, CDRX grains increased, while DDRX grains decreased. • Critical conditions of dynamic recrystallization were found and the critical stress, critical strain and other stress of model were established. • A composite kinetics model was proposed by considering the characteristics of DDRX and CDRX.

Achieving single-pass high-reduction rolling and enhanced mechanical properties of AZ91 alloy by RD-ECAP pre-processing

It is widely recognized that rolling Mg alloys is difficult due to their poor ductility. Conventional rolling of Mg alloys often needs to be conducted at high temperatures with multi-pass small thickness reduction, and the strong basal texture often leads to poor ductility and plate cracking. Herein, we conducted a single-pass high-reduction rolling (SPHRR) on AZ91 alloy after pre-homogenization (PH) and RD-ECAP pre-processing (PH + ECAP). Results shows that the AZ91 rolled plate processed by SPHRR (PH + ECAP + R) exhibits a good combination of high tensile strength (ultimate strength of ~420 MPa, yield strength of ~335 MPa) and high ductility (elongation-to-failure of ~19%), together with significantly reduced edge cracks. Microstructure examinations revealed that the improved mechanical property is mainly attributed to a typical bimodal grain structure featured with relatively high Schmidt factors of basal slips (SF basal ). The fine-grain structure produced by the high-pass RD-ECAP pre-processing contributes to the improvement of formability, provides favourable conditions for dynamic recrystallization (DRX) in further rolling and also promotes the formation of the bimodal structure. Overall, the high-pass RD-ECAP is applicable to improve plastic formability of hard-to-deform materials like Mg alloys, and thus has a promising prospect for industrial applications.

Dynamic Recrystallization Critical Conditions and a Physically-Based Constitutive Model of Al-4.8Mg Alloy Under Hot Working Conditions.

The microstructure evolution and the mechanical behavior of Al–4.8Mg alloy were investigated by means of isothermal compression tests at various temperatures (280–520 °C) and strain rates (0.01–10 s−1). The results shown that there are three main mechanisms of dynamic softening of samples within the range of selected process parameters: dynamic recovery, dynamic recovery + dynamic recrystallization, and dynamic recrystallization, and the equiaxed dynamic recrystallization grain tends to be formed under higher temperature and higher strain rate. In order to accurately describe the dynamic recrystallization condition of Al-4.8Mg alloy under a wide range of hot deformation parameters, an improved dynamic recrystallization critical conditions model is proposed based on deformation activation energy and work-hardening rate. Additionally, a two–stage physically constitutive model considering the influence of work hardening–dynamic recovery and dynamic recrystallization is established. Comparisons between the predicted and experimental data indicate that the proposed model can adequately predict the flow stress in the range of selected process parameters with the average absolute relative error of 4.02%.

Comparative Assessment on the Hot Deformation Behaviour of 9Cr–1Mo Steel with 1Cr–1Mo Steel

Hot forged 1Cr–1Mo and 9Cr–1Mo steels are being progressively used in the making of the turbine rotors of steam power plants. Forging of these steels requires precise control of processing parameters (i.e. strain, strain rate and temperature) for a defect free product. In line with the above mentioned applications, a comparative study to analyze the hot deformation behavior of both the steels was carried out in the temperature domain of 850–1050 °C with varying strain rate (0.001–0.1 s−1) by compression test data obtained from Gleeble 3800® thermomechanical simulator up to true strain of 0.69. The flow stress behavior of hot compressed specimens was analyzed by using material constants, activation energy and formulated Zener–Hollomon parameter. Critical conditions for dynamic recrystallization (DRX) of both the steels were calculated with the help of a mathematical model developed by Poliak and Jonas. Further, the variations in critical conditions were correlated with the Zener–Hollomon parameter. For both the steels, the current study also focused on the comparison of dynamically recrystallized austenite ( $$X_{DRX}$$ ) volume fraction with respect to true strain by applying the concept of Avrami equation. Finally, to validate the kinetics of DRX and softening mechanism, the microstructural evaluation and characterization have been carried out using optical and electron microscopy.

Determination of Critical Conditions for Dynamic Recrystallization of Commercially Pure Titanium

Abstract Microstructural evolution and mechanical properties are controlled by dynamic recrystallization (DRX) during industrial processing. To identify the critical condition for initiation of DRX, hot isothermal compression tests of commercially pure (CP)-70 titanium were conducted in a temperature range of 750°C–1,100°C and strain rate range of 0.001–10 s−1 to a true strain of 0.69. The critical condition for initiation of DRX can be identified from the inflection point on the strain-hardening rate (θ=∂σ/∂ε) versus flow stress (σ) curve. A third-order polynomial equation was fitted to the experimental θ−σ data to find out the inflection point. The results show that the critical stress (σc) for the initiation of DRX can be expressed as σc=−B/3A, where A and B are the coefficients of the third-order polynomial equation. It was observed that critical stress value depends on the deformation condition. The stress-strain curve was then normalized with respect to the peak stress (U=σ/σp vs.W=ε/εp), leading to a normalized value of critical stress, (Uc)=σc/σp=−B′/3A′, where A′ and B′ are the coefficients of the normalized third-order equation. The value of Uc(≈0.95) is a constant and independent of deformation conditions. These parameters were then used to calculate the DRX fraction, and the relationship between DRX fraction and processing variables was established.

Critical conditions and grain growth model for dynamic recrystallization of Cu-Ni-Si alloy

Hot compression tests of Cu-Ni-Si alloy with high Ni and Si contents were performed at the deformation temperatures of 750–950 °C, and the strain rates of 0.01–10 s−1 on a Gleeble-3800 thermo-mechanical simulator. The critical conditions for dynamic recrystallization (DRX) of Cu-Ni-Si alloy were obtained by processing the collected data and combining with the inflection point of θ-σ curve with the minimum value criterion of −∂θ/∂σ-σ curve. There was a certain correlation between the critical strain (εc) and the peak strain (εp), that was, εc/εp = 0.723, and the functional relationship between εc and Z parameter was εc = 1.05 × 10−3Z0.059. The average recrystallized grain size under various deformation conditions was measured, and the DRX grain growth model of Cu-Ni-Si alloy was established as dDRX = 5650.4ε̇−0.12891exp(−73142/RT).

Critical Conditions for Dynamic Recrystallization of Hot Deformed 1 wt% Chromium - 1 wt% Molybdenum Rotor Steel

A medium carbon 1 wt% Chromium and 1 wt% Molybdenum low-alloy steel is an important material for making of rotors of turbines. Its hot forging requires precise control of temperature, strain, and strain rate for defect-free forging. In a view of this, this steel was subjected to a hot compression test in a thermomechanical simulator, Gleeble 3800®. Hot compression was carried out at temperatures of 900°C, 1,000°C, and 1,100°C. At each temperature four strain rates, i.e., 0.001 s−1, 0.01 s−1, 0.1 s−1 and 1 s−1, were used. Constitutive equations were used to analyze hot flow behavior, material constants, and activation energy. A mathematical model developed by Poliak and Jonas has been used to determine critical condition parameters like critical stress (σc), critical strain (εc), peak stress (σp), and peak strain (εp). For dynamic recrystallization (DRX), the variation of strain hardening rate up to steady state stress (σss), steady state strain (εss), and volume fraction of DRX (XDRX), with the help of Avrami-type models, have been discussed. Zener Hollomon parameters (Z) explained in terms of strain rate and steady state that flow stress (σss) has been expressed. Microstructures at different hot deformation conditions have been reported and explained.

Thermal Deformation Behavior and Critical Conditions of Dynamic Recrystallization of QCr0.8 Alloy

In this paper, the flow stresses, the constitutive equation, processing map and the critical conditions of dynamic recrystallization (DRX) of the hot forged QCr0.8 alloy are studied by hot compressive test in the 750-900°C temperature and 0.01-10s-1 strain rate ranges using Gleeble-1500D thermo-mechanical simulator. The compression reduction of thermal compression deformation is 50%. The results show that the thermal deformation temperatures and strain rates have a significant effect on the high temperature deformation behavior of the alloy. The higher the temperature, the smaller the strain rate and the easier the DRX of the alloy is found.The peak stresses of the alloy decreases with the increase of temperature and increases with the increase of the strain rates.The flow stresses during hot deformation can be described by a hyperbolic sine function. The activation energy Q of the thermal compression deformation is determined to be 370.8KJ/mol. The constitutive equation and processing map of the alloy are established. Critical strains of DRX εc are studied by the inflection point characteristic of the lnθ-ε curve of the alloy and the corresponding minimum value of the ∂θ (∂θ)/∂ε-ε curve.

STRUCTURE FORMATION OF HIGH STRENGTH NITROGEN-BEARING STEEL UNDER HOT DEFORMATION

Deformation resistance characteristics of nitrogen-bearing steel of Cr-Ni-Mn alloying composition have been defined with the help of universal research complex Gleeble 3800 at various temperatures between 1200 and 800 : C depending on the strain rate. Temperature and deformation conditions of dynamic recrystallization depending on the strain rate have been determined by analyzing the deformation diagrams, namely that of the deformation limit beginning the process. Optimum temperature regimes for industrial hot stamping, forging and rolling have been recommended. It has been found that at a degree of true strain e = 0.9 the dynamic recrystallization of steel occurs in the range of 10~ 2 + 2 s _1 of strain rate at temperatures not lower than 900°C. The experimental data have been confirmed by metallographic research. Studies have shown that the processes of structure formation during isothermal deformation of steel at different speeds have some differences at temperatures above 900°C. The higher the temperature and the lesser the strain rate, the greater is the development of relaxation. It was found that at a strain rate of 0.01 s _1 corresponding to hot stamping, dynamic recrystallization in the temperature range of 1200 - 1100°C deformation starts at values equal to e = 0.1 (about 10 % of relative reduction). When the temperature lowers to 1000 and 900°C it is necessary to accumulate deformation to 20 and 30 %, respectively. Increasing the strain rate to 0.1 s _1 (when forging) leads to the fact that at temperatures above 1100° : C dynamic recrystallization begins at a degree of deformation about 20 %; at temperatures of 1000 and 900°C about 28 and 35 %, respectively. When the strain rate is 1-2 s _1 (when rolling) dynamic recrystallization starts when the temperature range is 1100 -1000°C and degree of deformation equals to 30 %. Lowering the temperature to900°C, as well as increasing to1200°C, increases the limit degree to 36 %.

Hot plastic deformation behavior and its effect on microstructure of low carbon multi-microalloyed steel

Abstract In this work, we studied the hot plastic deformation behavior of low carbon multi-microalloyed steel by the single-pass compression test and the effect of deformation on following microstructure after constant cooling rate. The results showed that this experimental steel has high dynamic recrystallization conditions, such as the deformation temperature higher than 1000 °C even at the low strain rate of 0.1 s-1. The lower deformation temperature is in favor of enhancing the formation of acicular ferrite. It is surprising to find that a part of recrystallized grains are formed at the deformation temperature of 1000 °C with 0.5 and 5 s-1 stress rate, even though the true stress-strain curves show the dynamic recovery type.

A novel unified dislocation density-based model for hot deformation behavior of a nickel-based superalloy under dynamic recrystallization conditions

In this study, a novel unified dislocation density-based model is presented for characterizing hot deformation behaviors in a nickel-based superalloy under dynamic recrystallization (DRX) conditions. In the Kocks–Mecking model, a new softening item is proposed to represent the impacts of DRX behavior on dislocation density evolution. The grain size evolution and DRX kinetics are incorporated into the developed model. Material parameters of the developed model are calibrated by a derivative-free method of MATLAB software. Comparisons between experimental and predicted results confirm that the developed unified dislocation density-based model can nicely reproduce hot deformation behavior, DRX kinetics, and grain size evolution in wide scope of initial grain size, strain rate, and deformation temperature. Moreover, the developed unified dislocation density-based model is well employed to analyze the time-variant forming processes of the studied superalloy.

Prediction of Critical Conditions for Dynamic Recrystallization in 316LN Austenitic Steel

AbstractHot compression experiments conducted on a Gleeble-3500 thermo-mechanical simulator and metallographic observation tests were employed to study the critical conditions of dynamic recrystallization (DRX) of 316LN austenitic stainless steel. The true stress-true strain curves of 316LN were obtained at deformation temperatures ranging from 900 °C to 1200 °C and strain rates ranging from 0. 001 s−1 to 10 s−1 . Based on the above tests, the critical conditions of DRX were determined and compared with those obtained from work-hardening theory and the Cingara-McQueen flow stress model. Furthermore, the microstructure was observed to validate the calculated results. The ratio of critical strain to peak strain (ɛc/ɛp) for 316LN was determined, and the quantitative relationship between the critical strain and the deformation parameters of 316LN was elucidated. The results demonstrated that the onset of DRX corresponds to the constant normalized strain hardening rate (Γ), namely, the critical strain hardening rate Γc for 316LN is equal to 0. 65.

Effect of temperature–strain-rate conditions of deformation on structure formation in commercially pure copper deformed in Bridgman anvils

Commercially pure copper (99.9 wt % Cu) subjected to high-pressure torsion at room temperature is investigated. A relation between the copper structure and temperature–strain-rate conditions has been established. It has been shown that impurity dragging prevents grain growth during postdynamic recrystallization. This has allowed us to determine the conditions under which either the hardening of the deformed material, which is accompanied by a continuous increase in the hardness and structural refinement, or the dynamic recrystallization resulting in the stabilization of the hardness and the average grain size predominantly occurs in commercial-purity copper compared to high-purity copper (99.99 wt %). It has been shown that, under the hardening conditions, the structure of the investigated copper is determined by the true strain, whereas under dynamic recrystallization conditions, by temperature–compensated strain rate.

Modeling of Flow Stress and Grain Size Based on Z Parameter and Material Constants of Al-5.2Mg-0.6Mn Alloy during Hot Deformation

The flow stress curves of Al-5.2Mg-0.6Mn alloy were tested by isothermal compression method with Gleeble-1500 thermal simulator at the temperatures of 300, 350, 400, 450 and 500°C with strain rate of 0.001, 0.01, 0.1 and 1s-1, respectively. The morphologies of grains deformed were analyzed by TEM. Four material constants including structural factor (A), stress exponential (n), stress multiplier (α) and average activation energy (Q) of the alloy were calculated by linear regression processing. The models of flow stress (σ) and grain size (d) based on Zener-Hollomon parameter (Z) and the constants were established. The results show that the values of A, n, α, Q of the alloy were equal to 3.058×109s-1, 3.314, 0.0184 mm2N-1, 160.94 kJmol-1 respectively, and the models of flow stress and average sub-grains size can be described as σ=54.31ln {(0.327×10-9Z)0.302 +[(0.327×10-9Z)0.604+1]0.5} and d=(0.045lnZ-0.675)-1, respectively. The flow stress appreciably reduced but average grain size increased with decreasing Z value. The conditions of dynamic recrystallization occurring for the alloy were the temperature above or equal to 350°C and lnZ below or equal to 24.47.

Constitutive modeling and prediction of hot deformation flow stress under dynamic recrystallization conditions

Simple modeling approaches based on the Hollomon equation, the Johnson–Cook equation, and the Arrhenius constitutive equation with strain-dependent material’s constants were used for modeling and prediction of flow stress for the single-peak dynamic recrystallization (DRX) flow curves of a stainless steel alloy. It was shown that the representation of a master normalized stress–normalized strain flow curve by simple constitutive analysis is successful in modeling of high temperature flow curves, in which the coupled effect of temperature and strain rate in the form of the Zener–Hollomon parameter is considered through incorporation of the peak stress and the peak strain into the formula. Moreover, the Johnson–Cook equation failed to appropriately predict the hot flow stress, which was ascribed to its inability in representation of both strain hardening and work softening stages and also to its completely uncoupled nature, i.e. dealing separately with the strain, strain rate, and temperature effects. It was also shown that the change in the microstructure of the material at a given strain for different deformation conditions during high-temperature deformation is responsible for the failure of the conventional strain compensation approach that is based on the Arrhenius equation. Subsequently, a simplified approach was proposed, in which by correct implementation of the hyperbolic sine law, significantly better consistency with the experiments were obtained. Moreover, good prediction abilities were achieved by implementation of a proposed physically-based approach for strain compensation, which accounts for the dependence of Young’s modulus and the self-diffusion coefficient on temperature and sets the theoretical values in Garofalo’s type constitutive equation based on the operating deformation mechanism. It was concluded that for flow stress modeling by the strain compensation techniques, the deformation activation energy should not be considered as a function of strain.