Numerical simulation for stress/strain distribution and microstructural evolution in 42CrMo steel during hot upsetting process

Based on experimental results, the dynamic recrystallization mathematical models of 42CrMo steel were derived. The effects of strain rates on the strain/stress distribution and microstructural evolution in 42CrMo steel during hot upsetting process were simulated by integrating the thermo-mechanical coupled finite element model. The results show that the deformation of the specimen is inhomogeneous, and the degree of the deformation inhomogeneity decreases with the increase of strain rates. The distribution of the effective stress in the specimen is also inhomogeneous, and the locus of the maximum effective stress changes with the variations of strain rates. The dynamic recrystallization volume fraction decreases with the increase of strain rates. The distribution of the dynamic recrystallization grain is inhomogeneous in the deformed specimen, and the average dynamic recrystallization grain size decreases as the strain rate is increased. A good agreement between the predicted and experimental results confirmed that the derived dynamic recrystallization mathematical models can be successfully incorporated into the finite element model to predict the microstructural evolution in the hot upsetting process for 42CrMo steel.     

Finite element analysis and experimental study of microstructure evolution of nitrogen alloyed ultralow carbon stainless steel during hot deformation

Abstract

Hot compression experiments of a nitrogen alloyed ultralow carbon stainless steel were performed in the temperature range of 1223–1423 K, at strain rates of 0.001–1 s^?1, and with deformation amounts of 30–70% on a Gleeble-3500 thermal-simulator. Based on the results from thermo-physical simulation experiments and metallographic analyses, a physically-based constitutive model and a dynamic recrystallisation (DRX) model of the studied steel were derived, and the developed models were further embedded into a finite element method (FEM) software. The microstructure evolution of the studied steel under various hot deformation conditions was simulated by FEM, and the effects of deformation amount, strain rate and temperature on the microstructure evolution were clarified. The results obtained from the finite element analysis were verified by the experiments. The finding confirms that the thermal-mechanical FEM coupled with the developed constitutive model and DRX model can be used to accurately predict the microstructure evolution of the studied steel during hot deformation.     

Constitutive modeling for flow behavior of GCr15 steel under hot compression experiments

The thermal compressive deformation behavior of GCr15 (AISI-52100), one of the most commonly used bearing steels, was studied on the Gleeble-3500 thermo-simulation system at temperature range of 950–1150 °C and strain rate range of 0.1–10 s⁻¹. According to the experimental results, the stress level decreases with increasing deformation temperature and decreasing strain rate. The peak stresses on the true stress–strain curves suggest that the dynamic softening of GCr15 steel occurs during hot compression tests. To formulate the thermoplastic constitutive equation of GCr15 steel, Arrhenius equation and the Zener–Hollomon parameter in an exponent-type equation were utilized in this paper. In addition, a modified Zener–Hollomon parameter considering the compensation of strain rate during hot compression was employed to improve the prediction accuracy of the developed constitutive equation. Analysis results indicate that the flow stress values predicted by the proposed constitutive model agree well with the experimental values, which confirms the accuracy and reliability of the developed constitutive equation of GCr15 steel.     

Effect of Strain Rate on the Ferrite Grain Refinement in a Low Carbon Nb-Ti Microalloyed Steel during Low Temperature Deformation

1. IntroductionThe thermomechanical controlled processing(TMCP) of microalloyed steels has been employed fosome times in the production of plates and sheet material to optimize mechanical properties. The centrafeature of thermomechanically processed steel is the ul-trafine grain size in the final product. Therefore, theferrite grain refinement of structural steels has attractedconsiderable interest from engineering scientists due toits unique role of increasing both strength and toughnessDem…     

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.     

Implementation of a constitutive model in finite element method for intense deformation

Since the constitutive information is one of the most important aspects of material deformation analysis, here a new constitutive model is proposed that can investigate the behavior of material during intense deformation better than existent models. The model that is completely based on physical mechanisms can predict all stages of flow stress evolution and also can elucidate the effects of strain and strain rate on flow stress evolution of material during intense plastic deformation. Here as an application, implementation of the constitutive model in finite element method (FEM) is used to compare two methods of sever plastic deformation (SPD) processes of copper sheet; repetitive corrugation and straightening (RCS) and constrained groove pressing (CGP). The modeling results are in good agreement with the experimental data and show that the hardness uniformity and its magnitude for RCSed sheet are higher than that for CGPed sheet. However, the prominence of these processes in strain uniformity depends on pass number.     

Refining constitutive relation by integration of finite element simulations and Gleeble experiments

Thermo-mechanical coupled finite element calculations were carried out to simulate the Gleeble compression of the samples of a titanium alloy (Ti60), and the results are analyzed and compared with the actual compression tests conducted on a Gleeble 3800 thermo-mechanical simulator. The changes in temperature, stress and strain distribution in the samples and the source of error on the constitutive relations from Gleeble hot compression test were analyzed in detail. Both simulations and experiments showed that the temperature distribution in the specimen is not uniform during hot compression, resulting in significant deformation inhomogeneity and non-ignorable error in the flow stress strain relation, invalidating the uniform strain assumption commonly assumed when extracting the constitutive relation from Gleeble tests. Based on the finite element simulations with iterative corrections, we propose a scheme to refine the constitutive relations from Gleeble tests.     

Finite element modelling on microstructure evolution during multi-pass hot compression for AZ31 alloys using incremental method

Based on the principle of piecewise linearization, the incremental forms of microstructure evolution models were integrated into the thermo-mechanical coupled finite element(FE) model to simulate nonlinear microstructure evolution during multi-pass hot deformation. This is an unsteady-state deformation where dynamic recrystallization(DRX), meta-dynamic recrystallization(MDRX), static recrystallization(SRX) and grain growth(GG) take place during hot deformation or deformation interval. The distributions of deformation and microstructure for cylindrical AZ31 sample during single-pass and double-pass hot compressions were quantitatively calculated and compared with the metallographic observation. It is shown that both the deformation and microstructure are non-uniformly distributed due to the presence of friction between the die and the flat end of sample. The average grain size and its standard deviation under the double-pass hot compression are slightly smaller than those under single-pass compression.The simulated average grain sizes agree well with the experiments, which validates that the developed FE model on the basis of incremental forms of microstructure evolution models is reasonable.     

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。     

φ70～80轴承钢棒材轧制过程的孔型设计及三维模拟

开发了采用300mm方坯生产φ70～80mm规格GCr15轴承钢棒材的孔型系统.利用有限元软件MSC.Marc,建立了该生产过程的三维有限元模型.借助MSC.Marc软件的二次开发功能,将GCr15钢的微观组织演变模型与轧制过程的热力耦合有限元模型相结合,预测了该生产过程中的轧制力、轧件变形情况及轧件内部温度、应变、应...     

Effect of integrated extrusion–equal channel angular pressing temperature on microstructural characteristics of low carbon steel

Abstract

In the present research, a combined forward extrusion–equal channel angular pressing was developed and executed for the deformation of a plain carbon steel. In this method, two different deformation steps, including forward extrusion and equal channel angular pressing, take place successively in a single die. The deformation process was performed at different deformation start temperatures (800, 930 and 1100°C). Three-dimensional finite element simulation was used to predict the strain and temperature variations within the samples during deformation. With microstructural observations and the results of finite element simulation, the main grain refinement mechanisms were studied at different deformation temperatures. The results show that the forward extrusion–equal channel angular pressing is effective in refining the ferrite grains from an initial size of 32 μm to a final size of ～0·9 μm. The main mechanisms of grain refinement were considered to be strain assisted transformation, dynamic strain induced transformation and continuous dynamic recrystallisation, depending on the deformation temperature.     

A study on the numerical simulation of thermal stress during the solidification of shaped castings

In order to study the development of thermal stress and to predict the hot tearing and residual stress of shaped casting, two models were used to carry out the stress analysis of the two stages of solidification. The rheological model [H]–[H|N]–[N|S] was used for the quasi-solid zone while the thermo-elasto-plastic model was used for the period after solidification. Coupling the thermal analysis based on the finite different method with the stress analysis based on the finite element method, a FDM/FEM integrated system of thermal stresses analysis during the solidification process was developed. After experimental verification, the system was put into practical application. The analysis results during the quasi-solid zone show that the visco-plastic strain is an important factor for the occurrence of hot tearing. The hot tearing of a case steel casting and the residual stresses and deformation of a hydro-turbine blade steel casting were analyzed and predicted using the system. The simulation and the practical results were basically in agreement.     

A study on the numerical simulation of thermal stress during the solidification of shaped castings

In order to study the development of thermal stress and to predict the hot tearing and residual stress of shaped casting, two models were used to carry out the stress analysis of the two stages of solidification. The rheological model [H]–[H|N]–[N|S] was used for the quasi-solid zone while the thermo-elasto-plastic model was used for the period after solidification. Coupling the thermal analysis based on the finite different method with the stress analysis based on the finite element method, a FDM/FEM integrated system of thermal stresses analysis during the solidification process was developed. After experimental verification, the system was put into practical application. The analysis results during the quasi-solid zone show that the visco-plastic strain is an important factor for the occurrence of hot tearing. The hot tearing of a case steel casting and the residual stresses and deformation of a hydro-turbine blade steel casting were analyzed and predicted using the system. The simulation and the practical results were basically in agreement.     

GCr15SiMo钢的动态及高温力学行为EI北大核心CSCD

以GCr15SiMo钢为对象,研究热处理工艺对其微观组织的影响规律,并利用霍普金森杆和GNT100-2型高温拉伸试验机,分析不同组织结构GCr15SiMo钢的动态及高温力学行为。结果表明:淬火温度由800℃升高至920℃,GCr15SiMo钢中M_(3)C型碳化物颗粒的质量分数由2.319%减少至0%;动态压缩过程中,GCr15SiMo钢的失效应变均随应变速率的增加而增大,在真应变分别为0.2和0.8时,随着淬火温度的升高,GCr15SiMo钢流变应力分别下降13.45%,21.44%,27.49%和31.79%,流变应力迅速下降主要与组织结构和动态压缩变形时的绝热剪切机制有关;在高应变速率条件下,GCr15SiMo钢的宏观变形由镦粗转变为沿45°方向的剪切破坏,绝热剪切机制是导致变形行为变化的主要原因之一,且组织结构是影响材料绝热剪切敏感性的关键因素之一;GCr15SiMo钢动态压缩变形过程中形变升温在117~333℃之间,M_(3)C碳化物颗粒回溶是其高温性能呈现抗拉强度增加、屈服强度降低的关键因素之一;淬火温度为920℃时,GCr15SiMo钢的组织为均匀一致的孪晶马氏体,孪晶马氏体中的亚晶界可有效阻碍位错运动,在拉伸应力作用下表现出明显的应变硬化现象,应力-应变曲线较淬火温度800℃时呈现更显著的上升趋势。     

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.     

金属热成形过程的综合数值模拟

金属在热成形过程中的微观组织演变是影响产品力学性能的关键因素,该演变过程取决于温度、应变和应变速率。本文基于有限变形理论和微观组织演变的数学模型,建立了能够模拟变形过程、温度变化过程和微观组织演变过程的有限元法,研制了通用的三维有限元计算软件,并在H型钢三维热轧模拟方面进行了深入开发,给出了原材料为C-Mn钢的H型钢热轧过程综合模拟结果。综合对比了8组不同工艺下的热轧实验结果和计算机模拟结果,二者均吻合良好,表明本文方法能够较好地预报金属热成形过程。     

Study on hot deformation behaviour of 316LN austenitic stainless steel based on hot processing map

Abstract

316LN is a type of austenitic stainless steel whose grain refinement only depends on hot deformation. The true stress–strain curves of 316LN were obtained by means of hot compression experiments conducted at a temperature range of 900–1200°C and at a strain rate range of 0·001–10 s^?1. The influence of deformation parameters on the microstructure of 316LN was analysed. Both the constitutive equation for 316LN and the model of grain size after dynamic recrystallisation were established, and the effect of different deformation conditions on the microstructure was analysed. The results show that the suitable working region is the one with a relatively higher deformation temperature and a lower strain rate, in which the dynamic recrystallisation is finely conducted. Moreover, the working region that should be avoided during hot deformation was indicated.     

新型奥氏体耐热钢CHDG-A的动态再结晶行为及其动力学模型

进行新型奥氏体耐热钢(CHDG-A)的热压缩实验,研究了在900~1100℃、应变速率为0.01-10 s^-1条件下这种钢的热变形特征。结果表明:随着变形温度的提高或应变速率的降低这种钢的流变应力显著降低。基于Arrhenius模型构建了这种材料的本构方程,得到CHDG-A热变形激活能Q为515.618 kJ/mol。微观组织分析结果表明,动态再结晶(DRX)是该材料在实验热变形条件下最主要的软化方式,DRX形核主要通过晶界弓出,变形温度的升高和应变速率降低均有利于再结晶形核。基于真应力-应变曲线求得动态再结晶用Z参数表示的峰值和临界值(σ_p、ε_p、σ_c和ε_c),并确定了ε_c/ε_p,σ_c/σ_p的比值分别为0.52和0.98。同时,还基于Avrami方程建立了CHDG-A的DRX动力学模型。     

MICROSTRUCTURAL MODELING AND NUMERICAL ANALYSIS FOR THE SUPERPLASTIC FORMING PROCESS

Superplastic forming is a manufacturing process during which a sheet is blow formed into a die to produce lightweight and strong components. In this paper, the microstructural mechanism of grain growth during superplastic deformation is studied. A new model, which considers grain growth, is proposed and applied to conventional superplastic materials. The relationships among the strain, strain rate, test temperature, initial grain size, and grain growth in superplastic materials are discussed. According to the proposed model, theoretical predictions for superplastic forming processes are presented, and comparison with experimental data is given. The new constitutive equation of superplasticity is introduced into a finite element method program to study superplastic blow forming. The effects of the geometric shape parameters of the die on the superplastic blow forming process are investigated, and the inhomogeneity in the thickness distribution of the specimen is analyzed.     

Directly spheroidizing during hot deformation in GCr15 steels

The spheroidizing heat treatment is normally required prior to the cold forming in GCr15 steel in order to improve its machinability. In the conventional spheroidizing process, very long annealing time, generally more than 10 h, is needed to assure proper spheroidizing. It results in low productivity, high cost, and especially high energy consumption. Therefore, the possibility of directly spheroidizing during hot deformation in GCr15 steel is preliminarily explored. The effect of hot deformation parameters on the final microstructure and hardness is investigated systematically in order to develop a directly spheroidizing technology. Experimental results illustrate that low deformation temperature and slow cooling rate is the favorite in directly softening and/or spheroidizing during hot deformation, which allows the properties of asrolled GCr15 to be applicable for post-machining without requirement of prior annealing.