The head of nuclear pressure vessel is a key component to guarantee the safety of nuclear power plant, so it is necessary to improve its mechanical properties during manufacturing. In the practical production, due to the huge size of the ingots from which the head is manufactured, coarse grains and voids are common defects existing in the material. Furthermore, cracks may appear in the forming process. It is highly demanded that the forming process must be properly designed with suitable parameters to compact the voids, to refine and homogenize the grains and to avoid cracks. Therefore, the research on the evolution of internal voids, grain size and cracks is very important to determine the forming process of huge components. SA508-3 steel is the material to manufacture the head of pressure vessel in the nuclear island. In the previous studies, we have separately built models to evaluate the evolution of internal voids, grain size and cracks during the hot forming process for SA508-3 steel. This study integrates the models for multi-scale simulation of the forging process of the head of nuclear pressure vessel in order to control the quality of the forgings. Through the software development, the models are integrated with a commercial finite element code DEFORM. Then, the extended forging and final forging processes of the head are investigated, and some appropriate deformation parameters are recommended.

•The metadynamic recrystallization behavior of SA508-Ш steel was investigated.•The kinetic equations and the grain size model for MDRX of SA508-Ш steel were established for the first time.•The strain rate and the temperature sensitivity of MDRX of SA508-Ш steel was studied.The metadynamic recrystallization (MDRX) behavior of SA508-Ш steel was investigated by isothermal double-hit hot compression tests at forming temperatures of 950–1250 °C, strain rates of 0.001–0.1 s−1 and the inter-stage delay time of 1–300 s. Experimental results show that the effects of forming temperature and the strain rate are significant, while the pre-strain and initial grain size are less on metadynamic recrystallization behavior of SA508-Ш steel. Based on the experimental results, the kinetic equations and the grain size model for metadynamic recrystallization of SA508-Ш steel were established. Comparisons between the experimental and predicted results were carried out. The predicted results agree well with the experimental data, which indicates that the proposed kinetic equations can give a reasonable estimation of the softening behavior for this steel. Furthermore, by implementing the established kinetics and microstructural evolution equations of MDRX into commercial FE software, the hot forging process of SA508-Ш steel can be designed and optimized through numerical simulation.

•The shape and orientation evolvements of elliptic-cylindrical voids were studied.•The void evolution results were obtained by using RVE method and FE calculation.•An evolution model considering void deformation and rotation was established.•The model was integrated into FE code to predict void evolution during metal forming.The evolution behavior of the internal voids influences the mechanical property of the materials significantly. Considering the void deformation and rotation, the evolution behavior of the elliptic-cylindrical void in power-law viscous materials was investigated by using the representative volume element (RVE) model. The rigid visco-plastic finite element (FE) method was applied to calculate the velocity field in the RVE under different loading conditions, and the instantaneous changing rate of the void radius and orientation were determined by evaluating the evolving of the void profiles at the instant. The calculated results show the deviatoric stress takes an important role in the void radius evolution, and the shear stress influences the change of the void orientation significantly. Based on the investigation, a void evolution model was established to relate the changing rates of the void radius and orientation to the void aspect ratio and the macroscopic stress/strain conditions. This model was incorporated into the FE code to predict the evolvements of void radius and orientation in each step of the deformation history. The predictions agree well with the results of the numerical simulations containing embedded void geometries in the mesh, which demonstrates that this model is capable to evaluate the void evolution behavior under large deformation. As an application, this model was used to predict the closure behavior of the void defects in the large ingot during the hot forging process.Download high-res image (188KB)Download full-size image

•The temperature and strain rate dependent ductility is attributed to DRX.•DRX affects ductile fracture behavior via influencing void evolution.•The GTN-Thomason model was extended to hot working with the consideration of DRX.•Validation experiments were conducted to corroborate the extended model.Ductile fracture is a key factor to the workability of metallic materials undergoing hot deformation. The ductility of materials at elevated temperature is closely related to dynamic recrystallization (DRX). To systematically investigate the DRX based ductile fracture, hot tensile experiments, microscopic observations and modeling of fracture behavior were conducted for 316LN steel. Based on the experimental results, a monotonic increasing relationship between ductility and the percentage of DRX (Xdrx) was figured out and identified to be attributed to the DRX influenced void evolution. With the softening effect caused by DRX, the local stress concentration, which serves as the driving force of void nucleation, void growth as well as void coalescence of the material, is highly relieved and the behaviors of voids thus change. To describe the DRX based void evolution and predict ductile fracture in hot working process, an extended damage model was established by introducing Xdrx into the void-based GTN-Thomason ductile fracture model, which is termed as the extended GTN-Thomason model in this research. In modeling of the ductile fracture considering DRX, the void nucleation strain, which represents the strain with the highest nucleation rate, and the critical void size ratio, which articulates the onset of void coalescence were figured out to increase with Xdrx. In addition, the strain rate sensitivity and the temperature dependency are involved in representing the kinetics of DRX and the flow stress applied in the model. The developed model was then implemented into finite element (FE) simulation and its related parameters were calibrated via a hybrid experiment and simulation method. Finally, the specific validation experiments were designed and conducted and the predicted fractures agree well with experimental results. This research thus offers an in-depth understanding of the DRX based ductile fracture and further facilitates and supports the design of hot working process by avoiding ductile fracture occurrence.

The internal void defects often exist in the large ingot inevitably due to the non-uniform solidification of the materials. To guarantee the mechanical performance of the products, these voids should be closed and eliminated in the subsequent forging process. The main purpose of this study is to investigate the void closure efficiency in different cogging processes of large ingot by using a 3-D void evolution model. In order to obtain a more reasonable description of the actual engineering condition, the voids were considered as prolate ellipsoidal, and the influences of the instantaneous void shape changing, stress state and deformation history were taken into account for the void evolution. According to the results, alternate compression in different directions makes the changing of the void shape counteracted and decreases the void closure efficiency. The initial void shape impacts the void closure significantly, as the non-spherical void shape causes the anisotropy of the void closing behavior. It can be found that the compression perpendicular to the longer principal axis of the prolate void provides the higher void closure efficiency than the compression aligned with this direction. Therefore, using the extend-forging as the first step in cogging process is more efficient to close the voids, considering the morphology of the real voids in the ingot. Moreover, appropriate processing parameters were determined to enhance the void closure efficiency in extend-forging. Besides, the surface bonding experiments show that the high pressure and temperature, as well as the long holding time, are favorable to eliminate the void defects after the void closure. It implies that the processing sequence, which can make the void closed completely at high pressure and high temperature and keep the sufficient interval time between forming stages, should be emphasized in the process planning.

The static recrystallization behavior of SA508-III steel was investigated by isothermal double-hit hot compression tests at the deformation temperature of 950–1250 °C, the strain rate of 0. 01–1 s−1, and the inter-pass time of 1–300 s. The effects of deformation parameters, including forming temperature, strain rate, degree of deformation (pre-strain) and initial austenite grain size, on the softening kinetics were analyzed. Experimental results show that static recrystallization kinetics is strongly dependent on deformation temperature and degree of deformation, while less affected by the strain rate and initial grain size. The kinetics and microstructural evolution equations of static recrystallization for SA508-III steel were developed to predict the softening behavior and the statically recrystallized grain size, respectively. Based on the comparison between the experimental and predicted results, it is found that the established equations can give a reasonable estimate of the static softening behavior for SA508-III steel.

The main purpose of this work is to develop a pragmatic model to predict austenite grain growth in a nuclear reactor pressure vessel steel. Austenite grain growth kinetics has been investigated under different heating conditions, involving heating temperature, holding time, as well as heating rate. Based on the experimental results, the mathematical model was established by regression analysis. The model predictions present a good agreement with the experimental data. Meanwhile, grain boundary precipitates and pinning effects on grain growth were studied by transmission electron microscopy. It is found that with the increasing of the temperature, the second-phase particles tend to be dissolved and the pinning effects become smaller, which results in a rapid growth of certain large grains with favorable orientation. The results from this study provide the basis for the establishment of large-sized ingot heating specification for SA508-III steel.

Due to its good toughness and high weldability, SA508-III steel has been widely used in the components manufacturing of reactor pressure vessels (RPV) and steam generators (SG). In this study, the hot deformation behaviors of SA508-III steel are investigated by isothermal hot compression tests with forming temperature of (950–1250)°C and strain rate of (0.001–0.1)s−1, and the corresponding flow stress curves are obtained. According to the experimental results, quantitative analysis of work hardening and dynamic softening behaviors is presented. The critical stress and critical strain for initiation of dynamic recrystallization are calculated by setting the second derivative of the third order polynomial. Based on the classical stress–dislocation relation and the kinetics of dynamic recrystallization, a two-stage constitutive model is developed to predict the flow stress of SA508-III steel. Comparisons between the predicted and measured flow stress indicate that the established physically-based constitutive model can accurately characterize the hot deformations for the steel. Furthermore, a successful numerical simulation of the industrial upsetting process is carried out by implementing the developed constitutive model into a commercial software, which evidences that the physically-based constitutive model is practical and promising to promote industrial forging process for nuclear components.

•The CA model coupled with a topology deformation technique was developed.•Two separate coordinate systems were established to describe the effect of compression on the grain shape.•Initial austenite grain size has a pronounced effect on the DRX kinetics.The paper presents a two-dimensional CA approach for quantitative and topographic prediction of the microstructure evolution of 316LN austenitic stainless steel during hot deformation. To describe the effect of deformation on grain topology more accurately, an updated topology deformation technique was used in the built model, in which a cellular coordinate system and a material coordinate system were established separately. The cellular coordinate system remains unchangeable in the whole simulation; the material coordinate system and the corresponding grain boundary shape change with deformation. The grain topography, recrystallization fraction and average grain size were also obtained. The simulated results agree well with the experimental data in terms of average grain size and flow stress, suggesting that the developed CA model is a reliable numerical approach for predicting microstructure evolution during dynamic recrystallization (DRX) for the 316LN steel.

In order to continuously simulate multi-pass plate rolling process, a 3-D elastic hollow-roll model was proposed and an auto mesh-refining module with data passing was developed and integrated with FE software, Marc. The hollow-roll model has equivalent stiffness of bending resistance and deformation to the real solid and much less meshes, so the computational time is greatly reduced. Based on these, the factors influencing plate profile, such as the roll-bending force, initial crown, thermal crown and heat transfer during rolling and inter-pass cooling can be taken into account in the simulation. The auto mesh-refining module with data passing can automatically refine and re-number elements and transfer the nodal and elemental results to the new meshes. Furthermore, the 3-D modeling routine is parametrically developed and can be run independently of Marc pre-processing program. A seven-pass industrial hot rolling process was continuously simulated to validate the accuracy of model. By comparison of the calculated results with the industrial measured data, the rolling force, temperature and plate profile are in good accordance with the measured ones.

A quantitative investigation of the mesomechanism of void closure in large ingots during hot forging is undertaken in the present study. The constitutive relation of the void-free matrix is assumed to obey the Norton power law. A cell model which includes matrix and void is employed and a Ritz procedure is developed to study the volumetric strain-rate of the void. On the basis of a large number of numerical computations, a criterion for void closure in large ingots during hot forging is proposed. In addition, the significant effects of the Norton exponent, the remote stress triaxiality and the remote effective strain on void closure are revealed: (1) the volumetric strain-rate of the void increases monotonically as the stress triaxiality level and the Norton exponent of the material increase, (2) the remote effective strain required for void closure decreases as the stress triaxiality level and the Norton exponent increase and (3) the void becomes unstable and the collapse rate decreases at the final stages of void closure. With the criterion for void closure, process design and optimization in terms of elimination of voids in large ingots will be convenient since the criterion can be easily applied in CAE analysis.

Workability consists of two independent parts: state-of-stress (SOS) workability and intrinsic workability. The traditional processing maps proposed by Prasad on the base of dynamic material model (DMM) demonstrate the intrinsic workability depending on the material properties. The three-dimensional (3-D) processing maps including strain are put forward in this paper, which describe the variations of the efficiency of power dissipation and flow instability domains with strain rate, temperature and strain. The Gleeble-1500 thermo-mechanical tests of magnesium alloy AZ31B were conducted. The hot deformation behaviors in the temperature range of 250–400 °C and strain rate range of 0.001–1 s−1 are studied. According to the 3-D processing maps, the optimum domain of hot deformation is in the temperature range of 250–325 °C and the strain rate range of 0.1–1 s−1. For metals characterized by dynamic recrystallization (DRX) such as magnesium alloys, the 3-D processing maps are able to deal with the sensitivity of the workability to strain at elevated temperature. By integration of the 3-D processing maps and finite element simulation (FEM), the distributions and variations of stress, strain, strain rate, temperature and flow instability domains are obtained under different hot deformation conditions. This method demonstrates both SOS workability and intrinsic workability and can be considered as an effective way to analyze the workability of metal deformation.

•A semi-analytical expression is deduced for void evolution in linear viscous material.•The influences of void shapes and loading conditions are investigated.•A 3-D void evolution model is established for linear and non-linear viscous materials.•This model is also available in multi-stage deformation when the loading has changed.•Results predicted by this model agree with analytical, simulated and experimental results.The paper presents a study on the evolution of dilute ellipsoidal voids in power-law viscous materials under triaxial loading condition. Firstly, referring to the work of Eshelby (1957), a semi-analytical expression is deduced to evaluate the deformation of ellipsoidal void in linear viscous material. Then, for the non-linear viscous materials, the concept of mesoscopic representative volume element (RVE) is applied to study the voids deformation under different stress states, and a rigid visco-plastic finite element (FE) procedure is applied to solve the RVE model. For the condition of stress triaxiality ranging from −1 to +1, it is found that the voids deformation behaves similarly in both linear and non-linear viscous materials. Due to this fact, the framework of the expression of void deformation in linear viscous material is inferred to describe the void evolution in non-linear viscous materials, while the parameters of the expression are re-evaluated for the specific materials. The results show that the void shapes and loading conditions take important roles in the void evolution. Therefore, for an ellipsoidal void, the void radius strain rate is expressed as a function of the void shape index, the macroscopic stress and strain-rate. Meanwhile, the void volume strain rate is obtained as a function of the void radius strain rate. This void evolution model is integrated into FE code and applied to study the void closure problem in the metal forming process. The FE simulation provides the evolution of macroscopic stress, strain and strain-rate, and then the model is used to calculate the changes of void shape and volume in each step of the deformation history. It can be found that the results predicted by this model agree well with the analytical solution, experiment measurements and numerical simulations with embedded void shapes, which demonstrates that this method can be appropriately used to predict the void evolution during the large compressive deformation process.