Deformation behavior and microstructure evolution in multistage hot working of TA15 titanium alloy: on the role of recrystallization

Interrupted compression tests of TA15 titanium alloy with initially equiaxed microstructure were carried out at deformation temperatures between 1173 to 1273 K and strain rates between 0.001 to 0.1 s⁻¹ to investigate the deformation behavior and microstructure evolution under multistage deformation. The TA15 alloy exhibits significant flow softening in both β and (α + β) working. It is found that the flow softening relates to dynamic recrystallization of β phases under current experimental conditions. In multistage β working, metadynamic recrystallization is the main softening mechanism during inter-pass holding. The grain refinement by metadynamic recrystallization leads to the decrease in peak stress upon reloading. In multistage (α + β) working, static recrystallization is the main softening mechanism during inter-pass holding. The static recrystallization kinetics increases with temperature and strain rate. The inter-pass holding has little influence on the morphology of the primary α phases. The β grain size is determined by spacing of primary α phases, which is more affected by working temperature but less dependent on strain rate and inter-pass holding time.     

Prediction of metadynamic softening in a multi-pass hot deformed low alloy steel using artificial neural network

The metadynamic softening behaviors in 42CrMo steel were investigated by isothermal interrupted hot compression tests. Based on the experimental results, an efficient artificial neural network (ANN) model was developed to predict the flow stress and metadynamic softening fractions. The effects of deformation parameters on metadynamic softening behaviors in the hot deformed 42CrMo steel have been investigated by the experimental and predicted results from the developed ANN model. Results show that the effects of deformation parameters, such as strain rate and deformation temperature, on the softening fractions of metadynamic recrystallization are significant. However, the strain (beyond the peak strain) has little influence. A very good correlation between experimental and predicted results indicates that the excellent capability of the developed ANN model to predict the flow stress level and metadynamic softening, the metadynamic recrystallization behaviors were well evidenced.     

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.     

300M钢的热变形行为及热锻成形工艺研究现状

300M钢凭其优异的综合力学性能而被广泛应用于飞机起落架大型构件的生产。在大型构件的热锻成形过程中,材料的流动行为及组织演变受到众多因素影响,变形机制复杂。主要从300M钢的热变形本构模型、微观组织演变以及锻造工艺三方面对现有研究进行综述。在热变形本构模型方面,综述了300M钢在单道次及多道次热变形下的本构模型的研究现状。在微观演化方面,综述了300M钢热变形各个阶段所对应的组织演化机制,包括晶粒长大、动态再结晶、静态再结晶、亚动态再结晶以及相变过程。此外,从数值分析角度综述了热锻成形工艺的研究现状。最后,针对现有研究提出了后续值得继续深入研究的方向。     

Dynamic and metadynamic recrystallization of Hastelloy X superalloy

Hot compression tests in the temperature range of 900–1150 °C and strain rates varying between 0.001 and 0.5 s⁻¹ were performed on Hastelloy X superalloy in order to investigate the kinetics of hot deformation. An Arrhenius-type equation was used to characterize the dependence of the flow stress on deformation temperature and strain rate. The results showed that dynamic recrystallization (DRX) as well as metadynamic recrystallization (MDRX) occurred during hot working. A novel technique has been developed for calculating the DRX kinetics parameters on the basis of the Johnson-Mehl-Avrami-Kolmogorov (JMAK) and isothermal transformation rate equations. The variation of grain size in the DRX and MDRX regimes correlated with the standard Zener–Hollomon parameter.     

奥氏体不锈钢热轧加工性能的数学模型研究

钢的热加工性能是钢的热轧工艺设计的基础.奥氏体钢在热加工中涉及到众多的物理现象,如动态回复、动态再结晶、静态回复、亚动态再结晶、静态再结晶和晶粒长大.一个优秀的描述钢的热加工性能的数学模型可以优化热轧工艺,提高生产效率,改善产品质量.综述了奥氏体不锈钢在热加工中发生的各类物理现象及其相对应的数学模型,讨论了变形温度、变形参数与流变应力、再结晶以及再结晶晶粒度之间复杂的关系,并分析了在工业多道次轧制工艺中,如何应用这些数学模型模拟和预测轧钢过程中残余应变和其内部组织的演变过程.     

Study of microstructural evolution during static recrystallization in a low alloy steel

Hot compression tests of 42CrMo steel were carried out on Gleeble-1500 thermo-mechanical simulator. The effects of forming temperature, strain rate, deformation degree, and initial austenite grain size on the microstructural evolution during static recrystallization in hot deformed 42CrMo steel were discussed. Based on the experimental results, the grain size model for static recrystallization was established. It is found that the effects of the processing parameters on the microstructural evolution during static recrystallization are significant, while those of the initial austenitic grain size are not obvious. Additionally, a good agreement between the experimental and predicted grain sizes was also obtained.     

低碳钢奥氏体再结晶模型的建立

为了描述低碳钢变形过程的组织演化,建立了一套完整的奥氏体动态再结晶、静态再结晶、亚动态再结晶模型.本文利用Gleeble试验机研究不同初始晶粒度、变形温度、应变和应变速率对奥氏体再结晶量和晶粒尺寸变化的影响.流变应力模型考虑了变形条件对模型系数的影响.利用测得的应力-应变曲线及晶粒度由多元非线性回归得出了奥氏体再结晶模型系数,并且由模型计算的峰值应变、稳定应变、硬化区流变应力、再结晶体积分数、晶粒尺寸和实际接近.     

A new method to predict the metadynamic recrystallization behavior in 2124 aluminum alloy

The metadynamic recrystallization behaviors in deformed 2124 aluminum alloy were investigated by isothermal interrupted hot compressive tests, which were carried out at the deformation temperatures of (653–743) K, strain rates of (0.01–10) s^?1 and inter-stage delay time of (30–180) s. A new approach, “peak stress method”, is proposed to calculate the softening fractions induced by the rapid metadynamic recrystallization. The kinetic equations were developed to predict the metadynamic recrystallization behaviors in hot compressed 2124 aluminum alloy. Both the experimental and predicted results show that the effects of deformation parameters, including strain rate, deformation degree and temperature, on the softening behaviors in the two-pass hot compressed 2124 aluminum alloy are significant. A good consistency between the experimental and predicted results indicates that the proposed kinetic equations can precisely estimate the softening behaviors and metadynamic recrystallization kinetics of the hot compressed 2124 aluminum 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.     

Zr对Ti微合金化低碳钢形变奥氏体再结晶和析出相的影响

通过多道次模拟压缩实验，研究不同Zr和Ti含量的三种Ti微合金化低碳钢在950℃~1050℃形变奥氏体再结晶和析出相的变化和最佳变形温度。结果表明，Ti含量的提高和Zr的加入使Ti微合金钢形变奥氏体的再结晶和晶粒长大延迟。Zr的加入还能增加Ti微合金钢中析出相的数量、改善析出相尺寸分布的均匀性进而得到相对均匀的奥氏体组织。变形温度为1000℃时的Ti-Zr微合金钢奥氏体组织最细小均匀。     

Flow Behavior and Evolution of Microstructure during Hot Deformation for a High Mo Stainless Steel

1. IntroductionHigh Mo austenitic stainless steels are widelyapplied to oceanology, petroleum chemical industryef..[1'2], because of their good resistance to local corrosion and uniform corrosion. However, they arehard to process and normally have worse plasticity.Therefore, that are the main weaknesses of this kindof material. It is necessary to improve the processing properties during hot working and control themicrostructure. Some researches have been carriedout on austenitic stainless ste…     

SA508Gr.4N钢热变形过程微观组织演变及流变应力模型

利用Gleeble-1500D热模拟试验机研究Ni-Cr-Mo系低合金SA508Gr.4N钢在变形温度为850~1200℃,应变速率为0.001~1 s^-1,真应变为0.9条件下的等温热变形行为,建立包含动态回复和动态再结晶的基于物象的流变应力模型与动态再结晶晶粒尺寸模型,并提出避免粗大晶粒组织遗传性的适宜锻造工艺。结果表明:随着变形温度的升高,应变速率的降低,动态再结晶体积分数和晶粒尺寸逐渐增加;SA508Gr.4N钢的真应力-真应变曲线具有明显的不连续动态再结晶现象;通过实验值和模型预测值对比可得流变应力模型的相关系数(R)及平均相对误差(MRE)分别为0.998和4.76%,动态再结晶晶粒尺寸模型的相关系数(R)及平均相对误差(MRE)分别为0.991和8.69%,两个模型均具有较高的准确性。     

Effects of holding time and Zener-Hollomon parameters on deformation behavior of cast Mg-8Gd-3Y alloy during double-pass hot compression

Double-pass hot compression tests were carried out over a wide range of holding time (0–180?s) and Zener-Hollomon parameter (1.6E15–1.3E20) to study the deformation behavior of cast Mg-8Gd-3Y alloy. The flow curves show obvious work hardening and strain softening stages, leading to the peak stress of double-pass hot compression. Holding time and Zener-Hollomon parameter can significantly affect the second pass peak stress. It is found that increasing the holding time can cause a higher peak stress in the second pass deformation. The second pass stress reaches the peak stress of 71?MPa at Zener-Hollomom parameter of 1.6E15. When the parameter rises to 1.3E20, the second pass peak goes up to 237?MPa. In addition, the second pass peak stress is significantly higher than the unloading stress, which is opposite to the flow behavior of aluminum alloys. Residual stored deformation energy caused by the first pass deformation could be consumed by metadynamic recrystallization. Therefore, more strain energy is required for subsequent dynamic recrystallization, resulting in hardening behavior. A hardening fraction is defined to describe the deformation behavior quantitatively, which shows a positive correlation with the metadynamic recrystallization fraction. The metadynamic recrystallization leads to grain growth at the inter pass holding stage, diminishing dynamic recrystallization nucleation positions in the second pass deformation.     

Metadynamic recrystallization in C steels

Metadynamic recrystallization has been investigated in three plain carbon steels (ENIA, EN2 and EN24) through the use of hot interrupted compression tests on a wedge plastometer. Holding time was 0.5 s between passes. Strain rates of 0.05 and 0.12/s and small strain increments of 3, 5 and 7% were employed. Test temperatures were varied between 800 and 1100°C. Various incremental and continuous stress strain curves were highlighted at different temperatures and strain rates for 3 steels, ENIA, EN2 and EN24, resulting in varying flow stresses and strains. Highest peak stress was 180 MPa for EN24 at peak strain of 0.25 and 900°C, with a strain rate 0.12/s. Peak strain values for all steels at 1100°C was 0.133 at a strain rate of 0.05/s and 0.15 at a strain rate of 0.12/s. Strain accumulation resulted in dynamic and metadynamic recrystallization with refinement to about 15 μm for dynamic and 22 μm for metadynamic recrystallization. Fractional softening,X, decreased from 0.27 to 0.12 as recrystallization times in metadynamic recrystallization increased from 0.9 s to 1.5 s at 1100°C. Time for 50% metadynamic recrystallization was also reduced as temperature increased. For ENIA, a drop from 10000 s to 20 s, as temperature increased from 800 to 1100°C was observed. For EN24 and EN2 steels, a drop from 4000 s to 6 s for similar temperature rise was observed. Metadynamic recrystallization (at strains higher than critical strain) is observed to be a strong function of strain rate and a very weak function of temperature and strain. It significantly refined the austenite grain size prior to transformation.     

3003铝合金动态再结晶实验研究

采用Gleeble-1500热模拟试验机对3003铝合金进行变形温度为300～500℃、应变速率为0.01～10.0s-1的高温等温压缩实验,由真应力-真应变曲线计算应变硬化速率,并采用截线法测量热压缩后平均晶粒尺寸,结果表明:3003铝合金动态再结晶临界应变εc随着Z参数的增大而提高,合金发生动态再结晶的临界条件为:...     

Cr8钢的动态再结晶行为及组织转变

利用Gleeble-3500热/力模拟试验机对Cr8支承辊用钢在应变速率0.01～1s-1、变形温度950～1 200℃条件下进行了热压缩变形试验,研究了其热变形力学行为和再结晶规律,并对该钢热变形后的显微组织及物相变化进行了分析。结果表明:在应变速率较低为0.01s-1,当变形温度低于1 050℃时,Cr8钢热变形后的组织主要为动态回复型,当变形温度高于1 100℃时,热变形后的组织为动态再结晶型,且随着变形温度的升高,动态再结晶晶粒逐渐长大;当应变速率增加到0.1s-1时,热变形后的组织在温度低于1 050℃时为动态回复型,在温度高于1 100℃时为动态再结晶型;当应变速率增加到1s-1时,变形温度高于1 050℃时,热变形后的组织即发生了明显的再结晶,奥氏体晶粒大部分已长成为等轴的再结晶晶粒;Cr8钢热变形后的物相主要为α-Fe和γ-Fe,显微组织主要为马氏体和残余奥氏体。     

Microstructural modeling and simulation for GCr15 steel during elevated temperature deformation

The microstructural evolution of GCr15 steel, one of the most commonly used bearing steels, was investigated and simulated by physical experiments and finite element method (FEM). Physical experiments were conducted on the Gleeble-3500 thermo-simulation system. Effects of initial grain size and plastic strain on the microstructural of the materials were investigated by setting different heating temperature, holding time and deformation degree, respectively. Based on the results of stress–strain curves and metallographic analysis, the constitutive equations for flow stress, austenite grain growth and dynamic recrystallization of GCr15 steel were formulated by linear regression method and genetic algorithm. In addition, the coupled thermo-mechanical finite element method integrated with the developed constitutive models was used to simulate the microstructural evolution of GCr15 steel during hot compression. Good agreement between the calculated and experimental results was obtained, which confirmed that the developed constitutive models can be successfully used to predict microstructural evolution during hot deformation process for GCr15 steel.     

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℃ and strain rates 0.01-1s-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.     

Mn18Cr18N护环钢静态再结晶组织及模型

在Gleeble-1500D热模拟机上,采用双道次热压缩试验研究Mn18Cr18N护环钢高温变形后不同停留时间内的静态软化行为,分析热变形温度、应变速率、变形程度以及初始奥氏体晶粒对静态再结晶行为的影响.通过应力补偿法计算静态再结晶软化率,并结合金相组织作了修正.建立其静态再结晶动力学模型,获得静态再结晶激活能249.3 k J/mol.研究表明:Mn18Cr18N钢静态再结晶软化曲线呈"S"形,符合Avrami方程.静态再结晶体积分数随着停留时间延长而增加,热变形温度越高,静态再结晶分数越大,而在较低温度和较小变形程度时,孕育时间较长,主要发生静态回复,将静态再结晶动力学模型的预测结果与实测值进行比较,二者吻合较好,为护环钢后续热镦粗工艺模拟提供更为详尽的模型.