Research On 3D Finite Element Simulation And Parameter Optimization For Quenching Processes

As a significant means to change and improve the performance of materials, quenching is used widely in the manufacturing field, such as automobiles, aviation, transportation, and building. But quenching process is a complicated multi-field coupled process, the traditional heat treatment technologies are now not suitable to the request of modernization of mechanical industry, and it is a urgent task to reform the heat treatment technologies using modern computer technologies. At this background, the mathematical model of heat treatment and computer simulation technologies are given much regards, and become the hot topic in the world research and development.Since 70 to 80 times of last century, computer gained widely application in every engineering field as a powerful tool, with the rapid developing of computer technologies, and so is in the field of quenching process. As a kind method of computer aided process design, numerical simulation of quenching process can optimize process design and reduce the rejection rate, thus improve the work efficiency apparently. Computer technologies have made some progress recently, and displayed great superiority. But computer technologies still are in the developing stage, many problems, such as the measurement and calculation of heat transfer coefficient and model building coupling with temperature, phase transformation and stress, need further research and the latent capacities are not exploited really.According to the above problems, a model is built coupling with temperature, phase transformation and stress, a method is brought forward based finite element method and the optimization method to calculate the heat transfer coefficient, and process parameters improving gas quenching qualities are acquired by introducing genetic algorithm into the gas quenching process optimization. Which are as follows:A three-dimensional non-linear finite element program is developed in Visual C++ program language to simulate the temperature field in the quenching process. Based on an example with an analytical result, the simulation of the program develop in this paper is compared with the analytical result and the ANSYS simulation. The comparison shows the program developed in this paper is right. The influence of the non-linear factors, such as surface heat transfer coefficient, phase transformation latent heat and thermal-physical parameters, is integrated into the program, and the simulation error is apparently reduced which arose when the non-linear factors is replaced with constant in analysis of quenching process by the commercial finite element soft wares.The calculation and disposal of phase transformation latent heat is discussed. The phase transformation latent heat is treated as the inner heat source in every element, and the iteration method is used to gain the convergence, thus improving the computation precision. Program to simulate phase transformation is written. The methods of local mess refined, dynamic time step and lumped heat capacity matrix were employed to resolve the problem of numerical oscillation happened in course of evaluating the temperature field. It is indicated by the results that the simulation result fits the theoretic result very well when the heat capacity matrix is lumped with appropriate time step; but if the heat capacity matrix was not lumped, the difference is obvious between the theoretic result and the simulation result acquired by the method of refining local mess and decreasing the time step. But with heat capacity matrix lumped, the errors of bound nodes' temperature are enlarged between simulations and theoretic results.An experiment equipment of end-quenching is designed in the paper. Some P20 steel samples are quenched using the new experiment equipment. HRC hardness of several sections in these samples is measured using the sclerometer, and the new phase volumes of several sections in these samples are measured using the volumetric metallurgical microscope. FEM model of P20 steel end-quenching is built according to the end-quenching process, and the end-quenching process of P20 steel is simulated using FEM. The HRC hardness and new phase volumes of several sections in the FEM model are attained by FEM simulation. The results of FEM simulation are compared with the results of experiment to check the FEM program, the comparison shows that the results of FEM simulation are consistent with the results of experiment, and the FEM program developed in the paper is dependable and accurate.According to the characters of quenching process, the inverse heat conduction problems in the quenching process are studied with non-linear finite element method and optimization method. The temperature cooling curves are acquired with referred heat transfer coefficient and the thermal physical parameters. Then the heat transfer coefficient is retrieved by the program with FEM and optimization methods developed in this paper according to above temperature cooling curves. It is shown by the comparison between the reference data and the simulation that the heat transfer coefficient retrieved has a good accuracy to the referred data and the program developed in the paper is right.The computing efficiency of retrieving the heat transfer coefficient in the inverse heat conduction problem in the quenching process is studied with three dimensional FEM. The initial search interval is fixed by the retreat method first, and then the heat transfer coefficient is retrieved by the golden section method, the successive parabolic interpolation method and the hybrid method of the two methods, respectively. A comparison is made between an end-quenching case and a full-quenching case during retrieving the heat transfer coefficient. Due to the heat transfer coefficient of different points of the specimen are retrieved first at the two cases, it is shown that the retrieved heat transfer coefficient in the end-quenching case fits with the reference data better than that in the full-quenching case.The cooling curves of P20 steel in 20℃and 60℃water are acquired using the designed high-speed data acquisition system based on ISA. According to these cooling curves, the heat transfer coefficients of P20 steel in 20℃and 60℃water are evaluated using the program developed in the paper.The stress/strain relations in elastic zone, plastic zone and transition zone are deduced, and the corresponding 3D finite element program is written. According to the characteristic of quenching process, a new method to calculate the elasto-plastic ratio is presented, so and the method to accelerate the calculation of elasto-plastic ratio. A FEM program is developed based on Visual C++ system, and the program can evaluate the temperature fields, the phase transformation and the stress/strain fields of the quenching process. Using the coupled simulation module, the temperature, the phase transformation and stress/strain of quenching process can be evaluated by the coupling method. In order to check the program, two examples that have accurate results or analytical results are selected to compare with FEM simulating results using this program, one example is the problem of 1080 steel quenching in the water, the other example is the problem of thermal stress in one plate. The comparisons between the simulations and experiment results show that, the simulations are consistent with the results of these examples, and this program can gain acceptable stress and strain values.The coupled analysis model is built. An axial hollow cylinder and an eccentric hollow cylindrical tube are chose as the research objects. The according finite element model is build, and the variation of temperature field, phase transformation and stress field during quenching process is studied. The simulation of the final phase distribution is contrasted with the experiment results. The effect of plasticity due to phase transformation and the initial strains due to temperature and phase transformation to the residual stress in the quenching process are studied.Genetic algorithm is introduced into the process parameters optimization of gas quenching, and the corresponding program is wrote. The staggered model of heat transfer coefficient is brought forward, and, according to the model, the gas quenching process of a P20 steel brick is optimized with the optimization objective of distortion. The quenching results before and after the optimization are contrasted with the evaluation functions of hardness and residual stresses. The contrast results show that process parameters improving the quenching quality are attained.