The rate of dynamic recrystallization in 17-4 PH stainless steel

The hot working behavior of 17-4 PH stainless steel (AISI 630) was studied by hot compression test at temperatures of 950–1150 °C with strain rates of 0.001–10 s⁻¹. The progress of dynamic recrystallization (DRX) was modeled by the Johnson–Mehl–Avrami–Kolmogorov (JMAK) kinetics equation. The flow softening was directly related to the DRX volume fraction and the DRX time was determined by strain rate. For quantification of recrystallization rate, the reciprocal of the time corresponding to the DRX fraction of 0.5% or 50% was used. Analysis of the sigmoid-shaped recrystallization curves revealed that the rate of DRX increases with increasing deformation temperature and strain rate. The Zener-Hollomon parameter (Z) was found to be inappropriate for analysis of DRX kinetics. Therefore, the dynamic recrystallization rate parameter (DRXRP) was introduced for this purpose. The DRXRP may be determined readily from the Avrami analysis and can precisely predict the rate of DRX at hot working conditions.     

Deformation behavior of an Al–Cu–Mg–Mn–Zr alloy during hot compression

Deformation behavior of an Al–Cu–Mg–Mn–Zr alloy during hot compression was characterized in present work by high-temperature testing and transmission electron microscope (TEM) studies. The true stress–true strain curves exhibited a peak stress at a critical stain. The peak stress decreased with increasing deformation temperature and decreasing strain rate, which can be described by Zener–Hollomon (Z) parameter in hyperbolic sine function with the deformation activation energy 277.8 kJ/mol. The processing map revealed the existence of an optimum hot-working regime between 390 and 420 °C, under strain rates ranging from 0.1 to 1 s⁻¹. The main softening mechanism of the alloy was dynamic recovery at high lnZ value; continuous dynamic recrystallization (DRX) occurred as deformed at low lnZ value. The dynamic precipitation of Al₃Zr and Al₂₀Cu₂Mn₃ dispersoids during hot deformation restrained DRX and increased the hot deformation activation energy of the alloy.     

Hot rolling simulations of austenitic stainless steel

The dynamic, static and metadynamic recrystallization behavior of austenitic stainless steel during hot rolling was analyzed. In this approach, each of those recrystallization behaviors is described by appropriate kinetics equations. The critical strain for dynamic recrystallization was determined so that a distinction could be made between static and metadynamic recrystallization; then the amounts of strain accumulation compared with the critical strain each pass. The effects of grain size on the fraction recrystallized and of the latter on the flow stress were evaluated for each type recrystallization behavior. In this way, the dependence of the mean flow stress (MFS) on temperature could be analyzed in terms of the extent and nature of the prior or concurrent recrystallization mechanisms. Finally, an example is given of an industrial process in which DRX/MDRX can play an important role. More grain refinement can be achieved by increasing the strain rate, decreasing the interruption time and lowering the temperature of deformation.     

Hot deformation characteristics and strain-dependent constitutive analysis of Inconel 600 superalloy

The hot deformation characteristics and constitutive analysis of Inconel (IN) 600 superalloy were investigated at elevated temperatures. Hot compressive tests were carried out in the temperature and strain rate ranging from 900 to 1150 °C and 1 × 10⁻³–10 s⁻¹, respectively. The flow behavior analyses and microstructural observations indicate that the softening mechanisms were related to dynamic recrystallization (DRX) and grain growth. DRX played a dominant role in the microstructural evolution at low temperatures (or high strain rates). DRX was the dominant softening effect at low strains on testing at high temperatures with low strain rates, whereas growth of the dynamically recrystallized grains was responsible for softening at high strains. The flow stress of IN 600 was fitted well by the constitutive equation of the hyperbolic sine function under the deformation conditions performed in this study. A constitutive equation as a function of strain was established through a simple extension of the hyperbolic sine constitutive relation.     

Characterization of dynamic recrystallisation in as-homogenized Mg–Zn–Y–Zr alloy using processing map

The hot working behavior of a as-homogenized Mg–Zn–Y–Zr alloy has been investigated in the temperature range 200–400°C and strain rate range 0.0015–7.5 s⁻¹ using processing map. The power dissipation map reveals that a domain of dynamic recrystallisation (DRX) in the temperature range 300–400°C and strain rate range 0.0015–0.15 s⁻¹, with its peak efficiency of 38% at 350°C and 0.0015 s⁻¹, which are the optimum hot working parameters. The apparent activation energy in the hot deformation process is 148 ± 3 KJ/mol that is larger than that of ZK60 alloy because of the obstruction of Y atoms for diffusion. DRX model indicates that DRX of Mg–Zn–Y–Zr alloy is controlled by the rate of nucleation, which is lower one order of magnitude than growth. And the rate of nucleation depends on the process of mechanical recovery by cross-slip of screw dislocations.     

Metadynamic recrystallization behavior of AZ61 magnesium alloy

Metadynamic recrystallization (MDRX) behavior of AZ61 magnesium alloy and its effects on flow behavior and microstructure evolution have been investigated in this study. Towards this end, a set of double-hit hot compression tests was conducted under strain rate of 0.1 s⁻¹ at 400 °C. To differentiate the static and metadynamic recrystallization dominant strain regions, the first stage of deformation was carried out up to the different pre-strains with a constant inter-pass annealing time of 200 s. The results indicated that the MDRX is predominant recrystallization mechanism where the pre-strains are higher than 0.35. Furthermore, to investigate the influence of MDRX on subsequent flow behavior and the related microstructure, an elaborated inter-pass annealing treatment was executed employing a range of inter-pass annealing time (2–500 s). The results show that the progress of MDRX leads to an increase in the flow stress as well as the rate of work hardening encountered in the subsequent deformation. Additionally, the microstructural examinations confirm that the observed hardening phenomenon is a consequence of grain growth evolved from MDRX and its direct effect on the onset of dynamic recrystallization at the second stage of deformation.     

Recrystallization of 30Cr2Ni4MoV ultra-super-critical rotor steel during hot deformation. Part I: Dynamic recrystallization

This paper presents an investigation that characterizes the evolution of the dynamically recrystallized structure of 30Cr2Ni4MoV ultra-super-critical rotor steel during hot deformation, as a starting point for studies of the static recrystallization (SRX) and the metadynamic recrystallization (MDRX) behaviors, by hot compression tests which are performed at the temperatures from 1243 K to 1543 K and strain rates from 0.001 s⁻¹ to 0.1 s⁻¹ on Gleeble-3500 thermo-mechanical simulator, and the corresponding flow curves are obtained. A third-order polynomial is then fitted to the work hardening region of each curve. The critical stress for initiation of dynamic recrystallization (DRX) can be calculated by setting the second derivative of the third order polynomial. By regression analysis, the activation energy in whole range of deformation temperature is determined to be Q = 368.45 kJ/mol. The complete DRX grain size (D_drx) of the test steel is a function of Zener-Hollomon parameter (Z) and is independent of the true strain. The relationship of D_drx and Z is found to be described in a form of power law function with an exponent of −0.24.     

GH4169合金变应变速率热变形过程动态再结晶动力学行为及建模

目的 研究变应变速率高温变形过程中GH4169合金动态再结晶行为和晶粒组织的演变机理.方法 在不同变应变速率工况下对GH4169合金进行高温压缩实验,分析了变形参数对动态再结晶行为与晶粒组织的影响规律,建立变应变速率工艺下GH4169动态再结晶动力学和晶粒尺寸预测模型.结果 随着第一阶段真应变的增加或第一阶段/第二阶段...     

Microstructural evolution of a Ti-4.5Al-3Mo-1V alloy during hot working

The study investigated the variation of microstructure of a Ti-4.5AL-3Mo-1V alloy during hot compressing in α + β phase field. By scanning electron microscopy (SEM) and transmission electron microscopy (TEM), we systematically examined the influences of hot working parameters on the microstructural features including of the morphology of α grain, volume fraction of α phase and dynamic recrystallization (DRX). The experimental evidence showed that both α and β phases underwent DRX under the experimental conditions. Moreover, the recrystallization degree of α phase was more sufficient than that of β phase. The DRX was accelerated with increase of deformation temperature and decrease of strain rate. It was also noted that a certain degree of the (α → β) phase transformation occurred concurrently with the mechanical deformation. In accordance with the microstructural evolution, flow curves of the alloy were characterized by a linear increase until regular oscillations at high strain rates or single peak at low strain rates and softening was followed.     

Effect of initial microstructures on hot forging of Ca-containing cast Mg alloys

Two cast noncombustible Mg–9Al–1Zn–1Ca alloys (composition in mass%) with coarse and fine initial microstructures were hot forged by compression at temperatures of 523–603 K and a true strain rate of 1–10⁻² s⁻¹. The compressive stress–strain curves for the two alloys were similar and typical of metals undergoing dynamic recrystallization (DRX). The alloy with the coarse initial microstructure suffered from edge crack formation during hot forging, while the alloy with the fine initial microstructure exhibited smooth peripheral surfaces after hot forging at temperatures of 573 K and above. The reduction of grain size by DRX was similar in the two hot-forged alloys, but the recrystallized volume fraction was lower in the alloy with the coarse initial microstructure. Insoluble second phases (seemingly Al₂Ca) provide additional DRX sites, and thus it is expected that the finer initial cast microstructure will improve the microstructure in the resulting hot-forged Mg parts.     

Kinetics,mechanism and modelling of microstructural evolution during thermomechanical processing of a 15Cr–15Ni–2.2Mo–Ti modified austenitic stainless steel

The paper discusses the kinetics, mechanism and modelling of the microstructural evolution of a 15Cr–15Ni–2.2Mo–0.3Ti modified austenitic stainless steel (alloy D9) during dynamic recrystallization (DRX). The experimental methodology included different hot working operations employing industrial equipment such as forge hammer, hydraulic press and rolling carried out in the temperature range 1,173–1,473 K to various strain levels. The kinetics of DRX has been investigated employing modified Johnson–Mehl–Avrami–Kolmogorov (JMAK) model. It has been found that the value of Avrami exponent varies in a close range of 1.17–1.34 which implies that D9 exhibits growth controlled DRX. Optical metallography has revealed that nucleation of DRX grains occurred along the prior grain boundaries by bulging mechanism. Microstructural characterization has shown that a significant correlation between microstructural features and processing parameters exists. However, this interrelation is ambiguous and fuzzy in nature. Therefore an artificial neural network model has been developed to predict the microstructural features, namely fraction of DRX and grain size, at different processing conditions. A good correlation between experimental findings and predicted results has been obtained. An instantaneous microstructure, therefore, can be designed in order to optimize the process parameters based on microstructural evolution.     

Effect of δ-ferrite co-existence on hot deformation and recrystallization of austenite

This work evaluates the effect of co-existence of a large volume fraction of δ-ferrite on the hot deformation and dynamic recrystallization (DRX) of austenite using comparative hot torsion tests on AISI 304 austenitic and 2205 duplex stainless steels. The comparison was performed under similar deformation conditions (i.e. temperature and strain rate) and also under similar Zener-Hollomon, Z, values. The torsion data were combined with electron backscatter diffraction (EBSD) analysis to study the microstructure development. The results imply a considerable difference between DRX mechanisms, austenite grain sizes and also DRX kinetics of two steels. Whereas austenitic stainless steel shows the start of DRX at very low strains and then development of that microstructure based on the necklace structure, the DRX phenomena in the austenite phase of duplex structure does not proceed to a very high fraction. Also, the DRX kinetics in the austenitic steel are much higher than the austenite phase of the duplex steel. The results suggest that at a similar deformation condition the DRX grain size of austenitic steel is almost three times larger than the DRX grains of austenite phase in duplex steel. Similarly, the ratio of DRX grain size in the austenitic to the duplex structure at the same Z values is about 1.5.     

Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity steel

Hot deformation behavior of an austenitic Fe–20Mn–3Si–3Al transformation induced plasticity (TRIP) steel was investigated by hot compression tests on Gleeble 3500D thermo-mechanical simulator in the temperature ranges of 900–1100 °C and the strain rate ranges of 0.01–10 s⁻¹. The results show that the flow stress is sensitively dependent on deformation temperature and strain rate, and the flow stress increases with strain rate and decreases with deformation temperature. The peak stress during hot deformation can be predicted by the Zener–Hollomon (Z) parameter in the hyperbolic sine equation with the hot deformation activation energy Q of 387.84 kJ/mol. The dynamic recrystallization (DRX) is the most important softening mechanism for the experimental steel during hot compression. Furthermore, DRX procedure is strongly affected by Z parameter, and decreasing of Z value lead to more adequate proceeding of DRX.     

Dynamic recrystallization behavior of GH4169G alloy during hot compressive deformation

The microstructure evolutions and nucleation mechanisms of GH4169 G alloy were studied by optical microscope, electron backscatter diffraction (EBSD) and transmission electron microscope (TEM). The hot compression tests were performed different imposed reductions in the range of true strain from 0.12 to 1.2 at the temperatures of 930 ℃-1050 ℃ with strain rates of 0.01 s⁻¹-1 s⁻¹. It is found that cumulative and local misorientation increase firstly and then decrease when the strain is increased due to the progress of dynamic recrystallization (DRX). The low angle boundaries (LAGBs) rapidly develop to high angle boundaries (HAGBs) at relatively high deformation temperature or the low strain rate. There are three DRX mechanisms observed for GH4169 G alloy during hot deformation. Discontinuous dynamic recrystallization (DDRX) as the dominant mechanism for GH4169 G alloy is characterized by typical necklace structures and bulged-original boundaries. Besides, different deformation bands with dislocation cells formed in deformed matrix at low temperature and large strain, which indicates that continuous dynamic recrystallization (CDRX) contributed to the DRX process. The twin boundaries lost their coherent characteristics and provide sites for nucleation, which also accelerates the nucleation of DRX.     

Hot deformation behavior of an 8% Cr cold roller steel

An 8% Cr cold roller steel was compressed in the temperature range 900–1200 °C and strain rate range 0.01–10 s⁻¹. The mechanical behavior has been characterized using stress–strain curve analysis, kinetic analysis, processing maps, etc. Metallographic investigation was performed to evaluate the microstructure evolution and the mechanism of flow instability. It was found that the work hardening rate and flow stress decreased with increasing deformation temperature and decreasing strain rate in 8% Cr steel; the efficiency of power dissipation decreased with increasing Z value; flow instability was observed at higher Z-value conditions and manifested as flow localization near the grain boundary. The hot deformation equation and the dependences of critical stress for dynamic recrystallization and dynamic recrystallization grain size on Z value were obtained. The suggested processing window is in the temperature range 1050–1200 °C and strain rate range 0.1–1 s⁻¹ in the hot processing of 8% Cr steel.     

TB6钛合金β区变形的动态再结晶动力学

使用圆柱形TB6钛合金试样在Thermecmaster-Z型热模拟试验机上进行热模拟压缩实验(变形温度为825~1100℃,应变速率为0.001~1 s^-1)。对采集的流变数据进行加工硬化率处理,确定动态再结晶体积分数,研究了TB6钛合金β区变形的动态再结晶动力学。结果表明,流变应力随着变形温度的降低或应变速率的提高而增大,流变曲线呈现出动态再结晶类型的特征。随着应变速率的降低和变形温度的提高,动态再结晶的体积分数和晶粒尺寸增大。在变形温度高于950℃、应变速率低于0.001 s^-1条件下,动态再结晶的晶粒严重粗化。动态再结晶动力学曲线经历缓慢增加—快速增加—缓慢增加三个阶段,呈现出典型的“S”型特征。确定了动态再结晶的体积分数达到50%时的应变,建立了TB6钛合金的动态再结晶动力学模型。     

Effect of stacking fault energy on the dynamic recrystallization during hot working of FCC metals: A study using processing maps

The influence of stacking fault energy (SFE) on the mechanism of dynamic recrystallization (DRX) during hot deformation of FCC metals is examined in the light of results from the power dissipation maps. The DRX domain for high SFE metals like Al and Ni occurred at homologous temperature below 0·7 and strain rates of 0·001 s⁻¹ while for low SFE metals like Cu and Pb the corresponding values are higher than 0·8 and 100 s⁻¹. The peak efficiencies of power dissipation are 50% and below 40% respectively. A simple model which considers the rate of interface formation (nucleation) involving dislocation generation and simultaneous recovery and the rate of interface migration (growth) occurring with the reduction in interface energy as the driving force, has been proposed to account for the effect of SFE on DRX. The calculations reveal that in high SFE metals, interface migration controls DRX while the interface formation is the controlling factor in low SFE metals. In the latter case, the occurrence of flow softening and oscillations could be accounted for by this model.     

钒氮微合金钢动态再结晶动力学及影响因素

为研究钒氮微合金钢的动态再结晶动力学及影响因素,选取3种对比成分的钒氮微合金钢发生动态再结晶的流变应力曲线,利用硬化速率一应力(θ-σ)曲线获得了饱和流变应力σsat、峰值应力σp、动态再结晶临界应力σc及稳态应力σss的准确值及上述特征应力值与σp的依赖关系,回归得到应变速率敏感的中碳钒氮微合金钢动态再结晶临界应变ε...     

Prediction of hot deformation behaviour of Fe-25Mn-3Si-3Al TWIP steel

Hot deformation behaviour of Fe-25Mn-3Si-3Al twinning-induced plasticity (TWIP) steel was investigated by hot compression testing on Gleeble 3500 thermo-mechanical simulator in the temperature range from 800 to 1100 °C and at strain rate range from 0.01 to 5 s⁻¹, and the microstructural evolution was studied by metallographic observations. The results show that the true stress-true strain curves exhibit a single peak stress at certain strain, after which the flow stresses decrease monotonically until the end of deformation, showing a dynamic flow softening. The peak stress level decreases with increasing deformation temperature and decreasing strain rate, which can be predicted by the Zener-Hollomon (Z) parameter in the hyperbolic sine equation with the hot deformation activation energy Q of 405.95 kJ/mol. The peak and critical strains can also be predicted by Z parameter in power-law equations, and the ratio of critical strain to peak strain is about 0.7. The dynamic recrystallization (DRX) is the most important softening mechanism for the experimental steel during hot compression. Furthermore, DRX procedure is strongly affected by Z parameter, and the decreasing of Z value leads to more extensive DRX.     

A study on hot deformation behavior of Ni-42.5Ti-7.5Cu alloy

To investigate the hot deformation behavior of the Ni-42.5Ti-7.5Cu (wt%) alloy, hot compression tests were carried out at the temperatures from 800 °C to 1000 °C and at the strain rates of 0.001 s⁻¹ to 1 s⁻¹. The results show that the occurrence of dynamic recrystallization (DRX) is the dominate restoration mechanism during the hot deformation of this alloy. There is an increase in peak and steady state stresses with decreasing the deformation temperature and increasing the strain rate. The experimental results were then used to determine the constants of developed constitutive equations. There is a good agreement between the measured and predicted results indicating a high accuracy of developed model. Zener–Hollomon (Z) parameter, calculated based on the developed model, indicates that DRX was postponed when the logarithm of the Zener–Hollomon parameter fell around 33 at strain rate of 0.001 s⁻¹ and temperature of 900 °C. This phenomenon can be regarded as the interactions between solute atoms and mobile dislocations. The established constitutive equations can be used to predict and analyze the hot deformation behavior of Ni-42.5Ti-7.5Cu alloy.