Dynamic Recrystallization Of Austenite Research Articles

Effect of Grain Size and Processing Parameters on Critical Strain for Austenite Dynamic Recrystallization

Effect of Grain Size and Processing Parameters on Critical Strain for Austenite Dynamic Recrystallization

Microstructure evolution and parameters optimization of follow-up hammering-assisted hybrid wire arc additive manufacturing

Coarse columnar grains are the typical microstructure of wire arc additive manufacturing, resulting in the poor mechanical properties. A follow-up hammering-assisted (FH) hybrid wire arc additive manufacturing process is proposed to improve the microstructure of deposited weld bead. The effects of different hammering temperatures (higher than Ae3 line) and different deformation amount on the microstructure are systematically studied, and the effect on mechanical properties is investigated correspondingly by fabricating a deposited block. The Electron Back Scatter Diffraction (EBSD), Scanning Electron Microscope (SEM), and Optical Microscopy (OM) are used for characterization. The results show that the FH process can significantly refine the grain size of deposited weld bead, and meanwhile weaken the texture strength. The grain refinement mechanism for hammering higher than the austenite recrystallization temperature is austenite dynamic recrystallization. Instead, for hammering lower than the austenite recrystallization temperature, the more significant grain refinement is attributed to the high-temperature deformation increasing the ferrite phase transformation nucleation site, nucleation rate and the deformation-induced ferrite phase transformation. Hammering temperature 850 °C and hammering deformation 40% are determined to be the optimal process parameters to refine the grains. Using the optimal process parameters, the strength and hardness of the deposited layers are greatly promoted, and the anisotropy is also improved, while the ductility is reduced to some degree, but improved obviously compared to the low-temperature hammering.

Hot-deformation-induced structural and mechanical properties of Ce-modified SAF 2507 super duplex stainless steel

A typical rare-earth element modified Ce-SAF 2507 super duplex stainless steel was isothermally hot-compressed to reach superior mechanic properties over its pristine and peers. Mechanical, macro- and micro-structural evolutions subjected to hot-modification were studied in detail toward an optimal hot working for the Ce-SAF2507. A dynamic softening phenomenon shows that the increase of hot deformation temperature and decrease of the strain rate were dominated by the dynamic recovery of ferrite at a high strain rate and low deformation temperature. The same phenomenon at a low strain rate but high deformation temperature was ruled by the dynamic recrystallization of austenite. These two processes determined significant phase transformations from austenite to ferrite under higher deformation temperature and strain rate. A hot deformation activation energy Q ∼406kJmol−1 was obtained through a unified strain-compensated constitutive equation for Ce-SAF2507. Quantitative grain size refinements further proved the above mechanisms deduced from mechanical and microscopic observations, while structure induced changes of mechanical properties were crosschecked with the microhardness. Insights of microstructure also demonstrated the existences of the Cr2N in both phases, grain boundaries, and α/γ interface. The overall deformation dynamics was explicated based on the structural and quantitative results.

Hot working characteristics of S32760 super duplex stainless steel

Hot deformation behavior of S32760 super austenitic stainless steel was studied in the temperature range of 950 ∼ 1250 °C and strain rate range of 0.1∼10 s-1 employing Gleeble 3800 equipment. The flow stress was modeled using Arrhenius equation and Zener-Hollomon parameter(Z). The microstructures of the specimens under various conditions were investigated, and dynamic recrystallization of austenite, dynamic recovery of ferrite and phase contents varied with temperature and strain rate. The difference of flow stress between experiment and constitutive equation was explained by the phase contents. Finite element analysis was performed to calculate forging load under the same conditions of compression test using modeled flow stress. It was validated that forging load can be predicted precisely with Arrhenius equation.

Microstructures and mechanical properties of friction stir welds on 9% Cr reduced activation ferritic/martensitic steel

In this study, the microstructures and mechanical properties of 9%Cr reduced activation ferritic/martensitic (RAFM) steel friction stir welded joints were investigated. When a W-Re tool is used, the recommended welding parameters are 300rpm rotational speed, 60mm/min welding speed and 10kn axial force. In stir zone (SZ), austenite dynamic recrystallization induced by plastic deformation and the high cooling rates lead to an obvious refinement of prior austenite grains and martensite laths. The microstructure in SZ contains lath martensite with high dislocation density, a lot of nano-sized MX and M3C phase particles, but almost no M23C6 precipitates. In thermal mechanically affect zone (TMAZ) and heat affect zone (HAZ), refinement of prior austenite and martensitic laths and partial dissolution of M23C6 precipitates are obtained at relatively low rotational speed. However, with the increase of heat input, coarsening of martensitic laths, prior austenite grains, and complete dissolution of M23C6 precipitates are achieved. Impact toughness of SZ at −20°C is slightly lower than that of base material (BM), and exhibits a decreasing trend with the increase of rotational speed.

Microstructural Depictions of Austenite Dynamic Recrystallization in a Low-Carbon Steel: A Cellular Automaton Model

A mesoscopic cellular automaton model that takes into account grain deformation during hot deformation has been developed to quantitatively depict the microstructural evolution of the austenite dynamic recrystallization (DRX) in a low-carbon steel. Both the grain deformation and the concept of DRX cycle are introduced, allowing accurate depictions of the grain structures, the overall microstructural properties and the flow stress evolutions that involving in the austenite DRX. The simulation results are compared with the experimental results and the predictions by the macroscopic DRX model and are found to be in good agreement.

Investigating on the Hot Compression Behavior of 2101 Lean Duplex Stainless Steel

The high temperature deformation behavior of 2101 duplex stainless steel has been investigated in the temperature ranges of 900-1150°C and strain rate ranges of 0.001-1s-1. Increasing strain rate from 0.001 to 1s-1 was contributed to austenite dynamic recrystallization (DRX) of specimens with deformation temperature of 900°C, but the DRX effect was weakened due to coarsening of equiaxial grains by increasing deformation temperature to 1050 and 1150°C. The effect of ferrite dynamic recovery (DRV) was enhanced with deformation temperature from 900 to 1050°C, then which was weakened with further increasing deformation temperature to 1150°C. By regression, the deformation equation and activity energy of 457.93 kJ/mol in these deformation conditions was obtained.

Physical Simulation of a Duplex Stainless Steel Friction Stir Welding by the Numerical and Experimental Analysis of Hot Torsion Tests

Physical simulation of friction stir welding (FSW) by means of hot torsion tests was performed on UNS S32205 duplex stainless steel. A thermomechanical simulator Gleeble 3800® with a custom-built liquid nitrogen cooling system was employed to reproduce the thermal cycle measured during FSW and carry out the torsion tests. Microstructures were compared by means of light optical microscopy and electron backscatter diffraction. True strain and strain rate were calculated by numerical simulation of the torsion tests. Thermomechanically affected zone (TMAZ) was reproduced at peak temperature of 1303 K (1030 °C), rotational speeds of 52.4 rad s−1 (500 rpm) and 74.5 rad s−1 (750 rpm), and 0.5 to 0.75 revolutions, which represent strain rate between 10 and 16 s−1 and true strain between 0.5 and 0.8. Strong grain refinement, similar to the one observed in the stir zone (SZ), was attained at peak temperature of 1403 K (1130 °C), rotational speed of 74.5 rad s−1 (750 rpm), and 1.2 revolution, which represent strain rate of 19 s−1 and true strain of 1.3. Continuous dynamic recrystallization in ferrite and dynamic recrystallization in austenite were observed in the TMAZ simulation. At higher temperature, dynamic recovery of austenite was also observed.

Effect of Intercritical Thermomechanical Processing on Austenite Retention and Mechanical Properties in a Multiphase TRIP-Assisted Steel

The effect of dynamic microstructural evolution on austenite retention was investigated in a transformation-induced plasticity-assisted multiphase steel by compressive deformation between 993 K and 1233 K (720 °C and 960 °C) covering the intercritical two-phase region. Based on optical microscopy and electron backscatter diffraction observations, extensive dynamic recovery of ferrite occurred below 1113 K (840 °C), i.e., lower part of two-phase region, due to strain concentration in the ferrite. Deformation-induced ferrite formation occurred at temperatures between 1113 K and 1153 K (840 °C to 880 °C), i.e., upper part of two-phase region, providing up to 27 pct additional fine ferrite grains compared to the undeformed state. Dynamic recrystallization of austenite took place at temperatures above 1173 K (900 °C), above Ac3. The dynamic restoration phenomena were found to have no positive influence on austenite retention; however, shear punch test results indicated that the specimens processed at 1113 K to 1153 K (840 °C to 880 °C) had a very good combination of strength and elongation, which was attributed to the synergic effects of the transformation of retained austenite and the fine ferrite structure generated through deformation-induced ferrite formation. X-ray diffraction analysis and scanning electron microscopy showed that deformation-induced ferrite might have rejected the excess carbon to the boundaries, thereby promoting the austenite formation in these regions. The present findings suggest that austenite can be dynamically stabilized as the result of deformation-induced ferrite formation. The effect is referred to as dynamic transformation-assisted austenite retention.

Hot deformation characterization of duplex low-density steel through 3D processing map development

The high temperature deformation behavior of duplex low-density Fe–18Mn–8Al–0.8C steel was investigated at temperatures in the range of 600–1000°C. The primary constitutive analysis indicated that the Zener–Hollomon parameter, which represents the coupled effects of temperature and strain rate, significantly varies with the amount of deformation. Accordingly, the 3D processing maps were developed considering the effect of strain and were used to determine the safe and unsafe deformation conditions in association with the microstructural evolution. The deformation at efficiency domain I (900–1100°C\\10−2–10−3s−1) was found to be safe at different strains due to the occurrence of dynamic recrystallization in austenite. The safe efficiency domain II (700–900°C\\1–10−1s−1), which appeared at logarithmic strain of 0.4, was characterized by deformation induced ferrite formation. Scanning electron microscopy revealed that the microband formation and crack initiation at ferrite\\austenite interphases were the main causes of deformation instability at 600–800°C\\10−2–10−3s−1. The degree of instability was found to decrease by increasing the strain due to the uniformity of microbanded structure obtained at higher strains. The shear band formation at 900–1100°C\\1–10−1s−1 was verified by electron backscattered diffraction. The local dynamic recrystallization of austenite and the deformation induced ferrite formation were observed within shear-banded regions as the results of flow localization.

Study on the Dynamic Recrystallization of Austenite in the Isothermal Compression of 300M Steel

The 300M steel was isothermally compressed on a Gleeble-3500 simulator at the deformation temperatures ranging from 1123 K to 1473 K, the strain rates ranging from 0.1 s-1 to 25.0 s-1 and a strain of 0.51. The morphology of austenite grains in the isothermally compressed 300M steel was observed using an OLYMPUS PMG3 microscope. The experimental results show that the deformation temperature and strain rate have an interaction effect on austenite grains in the isothermally compressed 300M steel. Dynamic recrystallization occurs more easily at high deformation temperature and low strain rate. Dynamic recrystallization occurs completely and the coarse grains occur at the deformation temperature above 1413 K. The austenite grain size increases as the deformation temperature increases while it decreases as the strain rate increases.

New Type Austenite Dynamic Recrystallization of Microalloyed Forging Steels 38MnVS During Forging Process

Hot compression tests of microalloyed forging steels 38MnVS were carried out on the Gleeble-3800 ther-mo-mechanical simulator at the deformation temperatures from 950 to 1150 °C with the strain rates ranging from 0.1 to 10 s−1. The effects of the deformation temperature and strain rate on the austenite dynamic recrystallization and microstructural changes were researched. The experimental results show that the dynamic recrystallization accelerated with the increase of the deformation temperatures and the decrease of the strain rate. The activation energy of dynamic recrystallization was calculated, which was about 275. 453 kJ/mol. The relation between the dynamic recrystallization and the Z-parameter was investigated, and the state chart of the dynamic recrystallization of the microalloyed forging steel 38MnVS was made according to the experimental data and the deformation parameters.

Reversible Martensitic Transformation Produced by Severe Plastic Deformation of Metastable Austenitic Steel

The peculiarities of martensitic transformations and formation of nanostructured states in metastable austenitic steel (Fe-18Cr-8Ni-Ti) after severe plastic deformation by high pressure torsion are investigated. It is shown that during severe plastic deformation with increased strain rate not only direct (γ→α΄) but also reverse (α΄→γ) martensitic transformations occur, which is revealed by the changes in the volume content of α΄ - martensite during deformation. The fragments thought to be formed by direct and reverse martensitic transformations and those of dynamic recrystallization of austenite are observed.

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

AbstractThe 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.

Austenite dynamic recrystallization of the microalloyed forging steels 38MnVS during forging process

According to the deformation characteristics of the microalloyed forging steels used in automobiles, the austenite dynamic recrystallization of microalloyed forging steels 38MnVS was investigated by thermal simulation test enginery Gleeble-3800 with the stain rate of 0.1-10 s-1 at 950-1150.The effects of the deformation temperature and rate on the austenite dynamic recrystallization were investigated. The austenite dynamic recrystallization's activation energy was calculated. A mathematical expression as variables of the critical strain ɛc, peak strain ɛp, steady strain ɛs and the austenite grain size were obtained, and an equation as a variable of the Zener-Hollomon parameter Z was established. The state chart of the dynamic recrystallization of the microalloyed forging steel 38MnVS was made according to the experimental data and the deformation parameters, and the product quality is improved by optimizing the forging processing.

Mechanical and microstructural analysis of 2205 duplex stainless steel under hot working condition

Hot deformation characteristics of 2205 duplex stainless steel were analyzed by performing hot compression tests at a temperature range of 950–1200 °C and a strain rate of 0.001–1 s−1. Flow stress was modeled by the constitutive equation of hyperbolic sine function. The constants of n, A, α, and the apparent activation energy were determined at different strains. They were then fitted by polynomial equations. Using the hyperbolic sine function and the relations derived between constants and strain flow curves were successfully modeled. Microstructural evolutions were characterized using optical microscopy and electron back scattered diffraction techniques. The results showed that dynamic recovery in ferrite is accelerated at higher temperatures followed by transformation to continuous dynamic recrystallization. Dynamic recrystallization in austenite was postponed by the accommodation of strain in ferrite and very few internal boundaries in austenite. At high strain rates, dynamic recovery in ferrite and dynamic recrystallization in austenite are very slow. Consequently, the total recrystallized fraction decreases. At low temperatures this situation may cause flow instabilities. At low strain rates, softening processes dominate in austenite and ferrite whereas at intermediate strain rates, the formation of substructures is observed in both phases.

Characterization of Austenite Dynamic Recrystallization under Different Z Parameters in a Microalloyed Steel

A low carbon Nb-Ti microalloyed steel was subjected to hot torsion testing over the temperature range 850–1100°C and strain rates 0.01–1 s −1 to study the influence of deformation conditions on the dynamic recrystallization characteristics of austenite. The results show that dynamic recrystallization occurs more easily with the decrease of strain rate and the increase of deformation temperature. The complete dynamically recrystallized grain size as a function of Zener-Hollomon parameter was established. It was found that dynamically recrystallized grain sizes decrease with increasing strain rate and decreasing deformation temperature. The effect of microalloying elements on peak strain was investigated and the solute drag corrected peak strain was determined. Also, the dynamic recrystallization map of austenite was obtained by using recrystallization critical parameters.

Hot deformation behavior of austenite in HSLA-100 microalloyed steel

Dynamic recrystallization of austenite in the Cu-bearing HSLA-100 steel was investigated by hot compression testing at a temperature range of 850–1150 °C and a strain rate of 0.001–1 s −1. The obtained flow curves at temperatures higher than 950 °C were typical of DRX while at lower temperatures the flow curves were associated with work hardening without any indication of DRX. At high temperatures, flow stress exhibited a linear relation with temperature while at temperatures below 950 °C the behavior changed to non-linear. Hence, the temperature of 950 °C was introduced as the T nr of the alloy. All the flow curves showed a yield point elongation like phenomenon which was attributed to the interaction of solute atoms, notably carbon, and moving dislocations. The maximum elongation associated with the yield point phenomenon was observed at about 950 °C. Since the maximum yield point elongation was observed about the calculated T nr, it was concluded that carbon atoms were responsible for it. It was also concluded that the temperature at which the yield point elongation reaches the maximum value increases as strain rate rises. The stress and strain of the characteristic points of DRX flow curves were successfully correlated to the Zener–Hollomon parameter, Z, by power-law equations. The constitutive exponential equation was found more precise than the hyperbolic sine equation for modeling the dependence of flow stress on Z. The apparent activation energy for DRX was determined as 377 kJ mol −1. The kinetics of DRX was modeled by an Avrami-type equation and the Avrami's exponent was determined around 1.1.

Hot deformation characteristics of 2205 duplex stainless steel based on the behavior of constituent phases

High temperature behavior of 2205 duplex stainless steel was studied by considering behavior of each constituent phase. The specimens were subjected to hot compression tests at temperatures of 800–1100 °C and strain rates ranging from 0.001 to 1 s −1 at intervals of an order of magnitude. The flow stress analysis showed that hot working empirical constants are different at low and high temperatures. The strain rate sensitivity m was determined and found to change from 0.12 to 0.21 for a temperature rise from 800 °C to 1100 °C. The apparent activation energy Q was calculated as 554 and 310 kJ/mol for low and high temperature, respectively. The validity of constitutive equation of hyperbolic sine function was studied and stress exponent, n, was assessed to be 4.2. Assuming the hyperbolic sine function for determination of strain rate and application of the rule of mixture, the interaction coefficients of δ-ferrite, P, and austenite, R, were estimated at different hot working regimes. It was found that the interaction coefficients are functions of Zener–Hollomon parameter Z and obey the formulas P = 1.4 Z −0.08 and R = 0.76 Z 0.005. Therefore, it was concluded that at low Z values δ-ferrite almost accommodates strain and dynamic recovery is the prominent restoration process which may even inhibit dynamic recrystallization in austenite. Otherwise, at high Z, austenite controls the deformation mechanism of material and dynamic recrystallization leads in finer microstructure.

Application of mathematical model for microstructure and mechanical property of hot rolled wire rods

A set of integrated mathematical models for predicting microstructures evolution during hot rolling and controlled cooling of high carbon wire rods has been developed through laboratory research work and industrial validation experiments. It consists of many sub-models such as critical strain, static and dynamic recrystallization of austenite, fraction of transformed austenite, interlamellar spacing of pearlite, microstructure–property relations. Based on the models, a simulating software is programmed which can run on PC computers. The physical metallurgy process during hot rolling and controlled cooling including the temperature distribution, the evolution of austenite grains, fraction of transformation austenite, final microstructure and mechanical properties were predicted. The predicted results are agree well with measured of the industrial tests.