High Temperature Deformation Behavior of 4340 Steel: Activation Energy Calculation and Modeling of Flow Response

The 4340 steel is extensively utilized in several industries including automotive and aerospace for manufacturing a large number of structural components. Due to the importance of thermo-mechanical processing in the production of steels, the dynamic recrystallization (DRX) characteristics of 4340 steel were investigated. Namely, hot compression tests on 4340 steel have been performed in a temperature range of 900–1200 °C and a strain rate range of 0. 01–1 s^?1 and the strain of up to 0. 9. The resulting flow stress curves show the occurrence of dynamic recrystallization. The flow stress values decrease with the increase of deformation temperature and the decrease of strain rate. The microstructure of 4340 steel after deformation has been studied and it is suggested that the evolution of DRX grain structures can be accompanied by considerable migration of grain boundaries. The constitutive equations were developed to model the hot deformation behavior. Finally based on the classical stress-dislocation relations and the kinematics of the dynamic recrystallization; the flow stress constitutive equations for the dynamic recovery period and dynamic recrystallization period were derived for 4340 steel, respectively. The validity of the model was demonstrated by demonstrating the experimental data with the numerical results with reasonable agreement.     

Hot Deformation Behavior and Microstructure Evolution of 2Cr12Ni4Mo3VNbN Steel Used for Last-Stage Large Blades of Steam Turbine

The hot deformation behavior and microstructure evolution of a recently developed 2Cr12Ni4Mo3VNbN martensitic stainless steel are examined through hot compression tests conducted within the temperature range of 900–1200 °C and the strain rate range of 0.01–10 s⁻¹. The constitutive equation and processing maps corresponding to hot deformation are established. The activation energy for hot deformation of 2Cr12Ni4Mo3VNbN steel is determined to be ≈457491.77 J mol⁻¹. Simultaneously, the microstructure evolution during hot deformation is studied. Based on the processing maps and microstructure evolution analysis, it is concluded that the optimal windows for hot processing are within the temperature range of 1106–1150 °C and the strain rate range of 0.01–2.7 s⁻¹, as well as at 1200 °C within the strain rate range of 1–2.7 s⁻¹, exhibiting a power dissipation efficiency of 0.32. As the temperature increases and the strain rate decreases, the degree of dynamic recrystallization escalates.     

Hot Deformation and Dynamic Recrystallization Behaviour of Medium Carbon Steel in Austenite Region

The hot deformation behaviour of a 0.47%C (JIS‐S45C) steel in the stable austenite region was systematically investigated under various deformation conditions to collect fundamental data on its high‐temperature deformation and microstructure evolution. The medium carbon steel showed dynamic recrystallization in a wide range of temperatures (850°C～1150°C) and strain rates (10^‐3 s^‐1～100 s^‐1) in the stable austenite region. The dynamically recrystallized grain size was monotonically decreasing with increasing steady state stress. The minimum grain size obtained through dynamic recrystallization was 8.3 μm when the S45C specimen was deformed at 850°C and 1 s^‐1. The stress‐strain relationships were formularized based on a phenomenological model. The stress‐strain curves estimated by the obtained equation were in good agreement with the experimental results.     

Dynamic Recrystallization Behavior and Microstructure Evolution of Bridge Weathering Steel in Austenite Region

The austenite dynamic recrystallization (DRX) behavior and microstructure evolution of a bridge weathering steel was systematically investigated at a deformation temperature range of 800–1100°C and strain rate of 0.1–10 s^?1 by using hot compression test and optical microscopy. The stress exponent and hot deformation energy were obtained by regression method to determine thermal deformation constitutive equation. The curve of stress versus strain is used, combined with high order polynomial fitting, to accurately determine the critical value of DRX. The relationships between critical strain, critical stress, and Z parameter of the bridge weathering steel were obtained by regression method. Moreover, the influence factors of DRX kinetics of the bridge weathering steel were studied in the light of the experimental results. It is shown that the strain rate has a more significant effect on the rate of DRX than that of the deformation temperature, and there is almost 0.85 orders of magnitude increment in the rate of DRX as the strain rate increases an order of magnitude. The dynamically recrystallized grain size can be decreased with decreasing the deformation temperature and increasing the strain rate during the austenite deformation.     

Hot Deformation Behaviors of Fe–30Mn–3Si–3Al TWIP Steel during Compression at Elevated Temperature and Strain Rate

The hot deformation behavior of twinning‐induced plasticity (TWIP) steel was investigated at 973–1373 K and strain rates of 0.01–20 s^?1 by hot‐compression experiments performed on a Gleeble‐3800 thermo‐simulation test system. Microstructural evolution during recrystallization in the hot deformed TWIP steels was investigated by metallurgical analysis. The hot‐flow behavior can be represented by a Zener–Hollomon parameter in the hyperbolic‐sine equation. The hot‐deformation activation energy is 436.813 kJ mol^?1. Deformation bands are initially generated in the deformed austenitic grains during the dynamic recrystallization (DRX) of TWIP steel. With increasing temperature, the recrystallized grains emerge at the boundary junctions after the disappearance of the deformation bands. Subsequently, they gradually spread along the austenitic boundaries and exhibit a necklace shape. The dynamic recrystallized grains continuously grow until they finally reach equilibrium. The DRX mechanism of TWIP steel is a boundary bulge mechanism. The optimum hot‐working technology parameters (especially for rolling) for the TWIP steel is the deformation temperature range of 1223–1323 K, and strain rate range of 1–10 s^?1.     

Dynamic Recrystallization of Ferrite with Particle-Stimulated Nucleation in a Low-Carbon Steel

Microstructure evolution of a low-carbon steel with the initial microstructure of ferrite matrix plus cementite particles during hot compression deformation was investigated at the strain rates of 0.001 s^?1, 0.01 s^?1, and 1 s^?1 at 973 K (700 °C) by means of field-emission scanning electron microscope, electron backscattered diffraction, and transmission electron microscopy. The results indicated that dynamic recrystallization (DRX) of ferrite took place at all of three strain rates, which can be classified as discontinuous DRX at 0.001 s^?1, 0.01 s^?1, and as continuous DRX at 1 s^?1. The formation of the nuclei of DRX of ferrite was mainly ascribed to the occurrence of particle-stimulated nucleation (PSN), accompanied with the lattice rotation and the formation of new high-angle boundaries. The occurrence of PSN was dependent on the development of a subgrain in the regions with high density of dislocations around cementite particles, without the need for the formation of the deformation zone.     

Dynamic recrystallization of ferrite in a low-carbon steel

Plane strain compression tests were performed on a low-carbon steel from 550 °C to 700 °C (ferritephase range) at strain rates of 10 to 5 × 10⁻⁴ s⁻¹, and the deformation microstructure evolution was investigated by means of scanning electron microscopy, transmission electron microscopy (TEM), and electron backscattered diffraction (EBSD). The results indicate that under the present deformation conditions, dynamic recrystallization of ferrite can occur in the low-carbon steel and lead to grain refinement. With increasing Zener-Hollomon parameter Z, the mechanism of this process changes from discontinuous dynamic recrystallization to continuous dynamic recrystallization; the turning point is approximately at Z=1 × 10¹⁶ s⁻¹. The increase of parameter Z leads to the decrease of recrystallized grain size of ferrite under steady state of deformation, and can lead to the formation of ultrafine microstructures with average grain size of about 2 μm.     

Effect of Strain Rate on Hot Ductility Behavior of a High Nitrogen Cr-Mn Austenitic Steel

18Mn18Cr0.6N steel specimens were tensile tested between 1173 K and 1473 K (900 °C and 1200 °C) at 9 strain rates ranging from 0.001 to 10 s^?1. The tensile strained microstructures were analyzed through electron backscatter diffraction analysis. The strain rate was found to affect hot ductility by influencing the strain distribution, the extent of dynamic recrystallization and the resulting grain size, and dynamic recovery. The crack nucleation sites were primarily located at grain boundaries and were not influenced by the strain rate. At 1473 K (1200 °C), a higher strain rate was beneficial for grain refinement and preventing hot cracking; however, dynamic recovery appreciably occurred at 0.001 s^?1 and induced transgranular crack propagation. At 1373 K (1100 °C), a high extent of dynamic recrystallization and fine new grains at medium strain rates led to good hot ductility. The strain gradient from the interior of the grain to the grain boundary increased with decreasing strain rate at 1173 K and 1273 K (900 °C and 1000 °C), which promoted hot cracking. Grain boundary sliding accompanied grain rotation and did not contribute to hot cracking.     

Effect of V on Hot Deformation Characteristics of TWIP Steels

Twinning induced plasticity (TWIP) steels, which rely on high Mn contents to promote twinning as the deformation mechanism, exhibit high ultimate strengths together with outstanding combinations of ultimate strength and ductility. In terms of mechanical properties, one of the most important microstructural features is grain size. The knowledge of the kinetics of recrystallization mechanisms, i.e., dynamic recrystallization (DRX) and static recrystallization (SRX), can be used in order to control the grain size of the final product by a proper rolling schedule design. The focus of this work is the characterization of the DRX kinetics of two TWIP steels. The basic composition of the steels is Fe–21Mn–0.4C–1.5Al–1.5Si, and one of them is further alloyed with 0.12% V. With this objective, compression tests were carried out at 900, 1000, and 1100°C and strain rates ranging from 1 × 10^?1 s^?1 to 1 × 10^?4 s^?1. Furthermore, metallographic observation by optical microscopy (OM) was done to assess the evolution of grain size for the different deformation conditions. According to the results, the existence of V in the composition does not affect the hot flow behavior of the steel, although recrystallization fraction and recrystallized grain size decrease for the V‐containing steel.     

Hot Deformation Behavior of Beta Titanium Ti-13V-11Cr-3Al Alloy

Hot compression tests were conducted on Ti-13V-11Cr-3Al beta-Ti alloy in the temperature range of 1203 K to 1353 K (930 °C to 1080 °C) and at strain rates between 0.001 and 1 s^?1 The stress–strain curves showed pronounced yield point phenomena at high strain rates and low temperatures. The yield point elongation and flow stresses at the upper and lower yield points were related to the Zener–Hollomon parameter. It was found that dynamic recovery at low strain rates and dynamic recrystallization at high strain rates were the controlling mechanisms of microstructural evolution. The results also showed that strain rate had a stronger influence on the hot deformation behavior than temperature. The microstructural observations and constitutive analysis of flow stress data supported the change in the hot deformation behavior of the studied alloy varies with strain rate. For various applied strain rates, the activation energy for hot deformation was calculated in range of 199.5 to 361.7 kJ/mol. At low strain rates (0.001 and 0.01 s^?1), the value of activation energy was very close to the activation energy for the diffusion of V, Cr, and Al in beta titanium. The higher value of activation energy for deformation at high strain rates (0.1 and 1 s^?1) was attributed to the accumulation of dislocations and the tendency to initiate dynamic recrystallization.     

Microstructural Modeling and Simulation of Al-2.8Cu-1.4Li Alloy during Elevated Temperature Deformation

The objective of the present work was to investigate the dynamic recrystallization phenomenon of a new Al-2.8Cu-1.4Li alloy. Isothermal compression experiments were carried out at a temperature of 643 K to 723 K (370 °C to 450 °C), strain rate of 0.001 to 1 s⁻¹, and deformation degree of 20 to 50 pct to determine material parameters for empirical models. Different holding times from 10 to 30 minutes were set to obtain the effect of initial grain size on microstructural evolution. Based on the results of stress-strain curves and metallographic analysis, the constitutive model and dynamic recrystallization mathematical model of Al-2.8Cu-1.4Li alloy were derived. The coupled thermomechanical finite element method integrated with the dynamic recrystallization model was used to simulate the change of microstructure during hot upsetting. Good agreement between the predicted results and experimental results was obtained, which demonstrated that the dynamic recrystallization model can be successfully used to predict microstructural evolution during hot working for Al-2.8Cu-1.4Li alloy.     

Structure formation in high-strength nitrogen-bearing steel on hot deformation

The resistance to deformation of nitrogen-bearing Cr–Ni–Mn steel at 800–1200°C is investigated by means of the Gleeble 3800 system. By analysis of the deformation diagrams—in particular, determination of the threshold strain for dynamic recrystallization—the temperature and strain corresponding to the onset of dynamic recrystallization are established as a function of the strain rate, and optimal temperatures for hot stamping, forging, and rolling are recommended for industrial conditions. With true strain e = 0.9, the dynamic recrystallization in the steel at strain rates of 10^–2–2 s^–1 occurs at temperatures no lower than 900°C. Metallographic data confirm the experimental results and show that the structure formation in the steel on isothermal deformation at different rates is different above 900°C. With increase in temperature and decrease in strain rate, relaxation processes are more developed. At a strain rate of 0.01 s^–1 (stamping on a press), dynamic recrystallization begins at around e = 0.1 (relative reduction around 10%) in the range 1100–1200°C. Strain of around 20 and 30%, respectively, is required with decrease in temperature to 1000 and 900°C. With increase in strain rate to 0.1 s^–1 (forging), dynamic recrystallization begins with around 20% strain above 1100°C, 28% at 1000, and 35% at 900°C. At a strain rate of 1–2 s^–1 (rolling), dynamic recrystallization begins at around 30% strain in the range 1000–1100°C. In that case, the threshold strain is 36% at both 900 and 1200°C.     

Mechanical Behavior and Microstructural Change of a High Nitrogen CrMn Austenitic Stainless Steel during Hot Deformation

The hot deformation behavior of a high nitrogen CrMn austenitic stainless steel in the temperature range 1173 to 1473 K (900 to 1200 °C) and strain rate range 0.01 to 10 s⁻¹ was investigated using optical microscopy, stress-strain curve analysis, processing maps, etc. The results showed that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate in 18Mn18Cr0.5N steel. The dynamic recrystallization (DRX) grain size decreased with increasing Z value; however, deformation heating has an effect on the DRX grain size under high strain rate conditions. In the processing maps, flow instability was observed at higher strain rate regions (1 to 10 s⁻¹) and manifested as flow localization near the grain boundary. Early in the deformation, the flow instability region was at higher temperatures, and then the extent of this unstable region decreased with increasing strain and was restricted to lower temperatures. The hot deformation equation as well as the quantitative dependence of the critical stress for DRX and DRX grain size on Z value was obtained.     

Deformation Behaviour and Microstructural Evolution During Hot Compression of AISI 904L

The hot deformation behaviour and microstructural evolution of AISI 904L super‐austenitic steel has been investigated by means of hot compression tests. The tests were carried out on a Gleeble 1500D thermo‐mechanical simulator in the temperature range from 850 °C to 1150 °C and at strain rates range from 0.001 s^?1 to 5 s^?1. The microstructure evolution was examined by means of light optical microscopy (LOM). The results show that after an initial deformation hardening, softening mechanisms occur. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be represented by a Zener–Hollomon parameter in the hyperbolic‐sine equation with the activation energy for deformation of 463 kJ/mol. The steady state was achieved at maximum strain of 0.9 only at the lower strain rates (under 1 s^?1) and the higher temperatures (above 1100 °C). Microstructural analyses showed a gradual increase in the dynamically recrystallized area with an increase of the temperature and a decrease of the strain rate. The grain size did change, as expected, correlating to the deformation conditions.     

Numerical Simulation of Austenite Recrystallization in CSP Hot Rolled C-Mn Steel Strip

An integrated mathematical model is developed to predict the microstructure evolution of C-Mn steel during multipass hot rolling on the CSP production line,and the thermal evolution,the temperature distribution,the deformation,and the austenite recrystallization are simulated.The characteristics of austenite recrystallization of hot rolled C-Mn steel in the CSP process are also discussed.The simulation of the microstructure evolution of C-Mn steel ZJ510L during CSP multipass hot rolling indicates that dynamic recrystallization and metadynamic recrystallization may easily occur in the first few passes,where nonuniform recrystallization and inhomogeneous grain size microstructure may readily occur;during the last few passes,static recrystallization may occur dominantly,and the microstructure will become more homogeneous and partial recrystallization may occur at relatively low temperature.     

Numerical Simulation of Microstructure Evolution for SA508-3 Steel during Inhomogeneous Hot Deformation Process

Based on hot compression tests by a Gleeble-1500D thermo-mechanical simulator, the flow stress model and microstructure evolution model for SA508-3 steel were established through the classical theories on work hardening and softening. The developed models were integrated into 3D thermal-mechanical coupled rigid plastic finite element software DEFORM3D. The inhomogeneous hot deformation （IHD） experiments of SA508 3 steel were designed and carried out. Meanwhile, numerical simulation was implemented to investigate the effect of temperature, strain and strain rate on microstructure during IHD process through measuring grain sizes at given positions. The simulated grain sizes were basically in agreement with the experimental ones. The results of experiment and simulation demonstrated that temperature is the main factor for the initiation of dynamic recrystallization （DRX）, and higher temperature means lower critical strain so that DRX can be facilitated to obtain uniform fine microstructure.     

Dynamic Recrystallization Behavior of GCr15SiMn Bearing Steel during Hot Deformation

The hot deformation behavior of GCr15SiMn steel was studied through high temperature compression tests on the Gleeble-1500 thermal-mechanical simulator. The initiation and evolution of dynamic recrystallization （DRX） were investigated with microstructural analysis and then the process variables were derived from flow curves. In the present deformation conditions, the curves of strain hardening exponent （n） and the true strain （e） at the deformation temperature of 1423 K and strain rates of 0.1, 1 and 10 s^-1 exhibit single peak and single valley. According to Zener-Hollomon and Ludwik equation, the experimental data have been regressed by using linear method. An expression of Z parameter and hot deformation equation of the tested steel were established. Moreover, the Q values of GCrlSSiMn and GCr15 steels were compared. In order to determine the recrystallization fraction under different con ditions, the volume fraction of DRX as a function of process variables, such as strain rate （ε）, temperature （T）, and strain （ε）, was established. Itwas found that the calculated results agreed with the mierostructure of the steel at any deformation conditions.     

Hot Deformation Characteristics and Dynamic Recrystallization Mechanisms of a Newly Developed Austenitic Heat-Resistant Alloy

The hot deformation characteristics, microstructure evolution, and dynamic recrystallization (DRX) mechanism of the newly developed austenitic heat-resistant steel Fe–18Cr–10Ni–0.3Nb–2.5Cu were systematically investigated by thermal compression tests combined with microstructure characterizations. The activation energy (Q) map, Zener–Hollomon parameter (Z) map, and processing map were plotted according to the stress–strain curves to reveal the inherent connection between the three maps and the hot deformation characteristics of this alloy. The high η region in the processing map does not precisely correspond to the region where DRX developed. Nevertheless, the flow instability map accurately predicts the microstructure. The variation pattern of Z corresponded more closely to the hot deformation microstructure evolution than did the variation pattern of Q. The degree of DRX increases with decreasing Z. The optimal process parameters are 1000 °C/0.01 s⁻¹/0.8 and 1100 °C/10 s⁻¹/0.8 (temperature/strain rate/strain), and they result in complete DRX and a narrow range of Z values. The DRX mechanism at high strain rate is characterized by the combined enhancement of discontinuous DRX (DDRX), continuous DRX (CDRX), and twin-DRX (TDRX). The dominance of the particle-stimulated nucleation (PSN) mechanism at intermediate strain rate results in the formation of incompletely recrystallized microstructures with approximate orientation. Sufficient time at low strain rate promotes the development of DDRX and CDRX.

     

904L不锈钢动态再结晶行为

采用单道次热压缩试验,研究了904L钢在不同变形温度、不同应变速率下的真应力-应变曲线以及组织形貌,阐明了热加工过程中热变形参数对其在变形过程中发生的动态再结晶行为及微观组织演变规律的影响,揭示了其相应的软化机制。结果表明:变形温度越高,流变应力越小,动态再结晶体积分数越高,晶粒尺寸越大;同温度下,变形速率越小,应力峰值越小,晶粒尺寸越大且晶界越平直化;904L钢的动态再结晶行为随着变形温度的升高,应变速率的减小,应变量的增大而进行得越充分且较高的变形温度有利于动态再结晶的进行。     

Static Recrystallization Behavior of 316LN Austenitic Stainless Steel

The static recrystallization of 316LN austenitic stainless steel was studied by double-pass hot compression tests on a Gleeble-3500 thermomechanical simulator. The specimens were compressed at the deformation temperatures of 950, 1050, 1150 °C, strain rates of 0.01, 0.1, 1s^?1, strains of 0.1, 0.15, 0.2, and intervals of 1 — 100 s. The results show that the volume fraction of static recrystallization of 316LN increases with the increase of deformation temperature, strain rate, strain and interval, which indicates that static recrystallization occurs easily under the conditions of higher deformation temperature, higher strain rate and larger strain. Deformation temperature has significant influence on static recrystallization of 316LN. The volume fraction of static recrystallization could easily reach 100% at higher deformation temperatures. By microstructure analysis, it can be concluded that the larger the volume fraction of static recrystallization, the more obvious the grain refinement. The static recrystallization activation energy of 317 882 J/mol and the exponent n of 0.46 were obtained. The static recrystallization kinetics was established. The predicted volume fraction of static recrystallization is in good agreement with the experimental results.