Research On Void Evolution In Large Ingot During Hot Forging

Large forgings, the key parts for heavy machine and equipment, are widely used in power generation, metallurgy, shipbuilding, petrochemical engineering, nuclear power, aerospace and defense industry. The quality requirements of large forgings are extremely strict because large loads, poor working conditions, high safety and reliability requirements are its fundamental characteristics. However, internal defects such as shrinkage cavities and porosity inevitably occur in large ingots during steel casting. The continuity of the metal is destroyed by these internal voids and stress concentration and crack damage are easy to form, resulting in loss of life and discarding. Therefore, understanding of void evolution, exploration of the condition for void closure, and design of forging process to improve the forging quality, are of great theoretical significance and practical value. In this paper, on the basis of theoretical modeling, numerical simulation and experimental study, void evolution in large ingot during hot forging is investigated systemically, which yields the following main achievements:Based on mesomechanic plastic theory and the characteristics of large forgings, the representative volume element (RVE) model for void evolution in large ingot during hot forging is put forward, and the relationships between void volume and shape evolution and macroscopic field variables are established. Based on the principle of virtual work, the variational functional for the solution of the local velocity field is derived, and a general method for investigating void evolution is established. The RVE model is a successful solution for studying void evolution in large ingot with a large number of small void and respective deformations during hot forging.Through the calculation for void evolution, the relative void volume (the ratio of current void volume to initial void volume) is suggested as the indicator for void evolution in large ingot during hot forging, and effects of the material properties, stress and strain state on void evolution are revealed. The results show that, the volumetric strain-rate of the void increases monotonically with increasing stress triaxiality level and Norton exponent of the material; relative void volume decreases as the stress triaxiality level and effective strain increase; the remote effective strain required for void closure decreases as the stress triaxiality level and Norton exponent increase.Based on the numerical results and the interpolation model, a criterion for void closure, which takes the material property, stress and strain state, and temperature into account, is proposed. The effect of temperature is considered in the Norton exponent and the stress-strain relationship. The criterion has a clear physical meaning and a general applicability for large ingot during hot forging.By developing the key simulation program for void evolution, the simulation platform for void evolution is established based on the commercial software DEFORM, and for the first time the relative void volume is displayed clearly and synchronously in the simulation of the hot forging process for large ingot. It provides a new approach for selection and evaluation of the traditional forging processes, and a convenient and powerful tool for optimization of forging processes and innovation of forging methods. Based on the established simulation platform for the void evolution, numerical simulations for upsetting processes with different dies and for stretching processes with multi-stroke and multi-pass forging are performed, the effects of different processes on void closure are analyzed and compared, and the cymbal-shaped die upsetting process is proposed.A new experimental method is presented for quantitative investigation of the void evolution. Effects of different forging processes on void closure are investigated. The sophisticated optical projector is adopted in the measurement of the size and volume of the void to ensure the accuracy of the test results. Good agreements of the predicted results with the measured ones confirm the validity of the void evolution model, the void closure criterion, and the simulation platform for void evolution in large ingot during hot forging.Using the established simulation platform for void evolution, forging processes of 1000MW low-pressure rotor is analyzed and evaluated, and new forging processes are recommended by applying the cymbal-shaped die in the upsetting processes. It shows that, based on the simulation platform for void evolution, the optimum forging method can be designed, the most economical forging process can be arranged and the best forging quality can be achieved through the CAE analysis and optimization.