Research On Key Technologies Of Finite Element Simulation And Parameter Optimization For Quenching Processes

According to the characters of quenching process, a new method of evaluating phase-transform latent heat is presented in the paper. The methods of lump parameters, fine mesh and adaptive time-step are used and a finite-element simulating program for evaluating the temperature and phase-transformation is developed in this paper. A new method of self-control and adaptive time-step is presented according to maximum and minimum difference values of temperature field between the previous simulating step and the current simulating step. Some examples that have accurate results or analytical results are selected to compare with FEM results of simulating using the developed software. The comparison result shows that the method presented in this paper can effectively avoid the numerical oscillation, and the evaluating precision is satisfied.A new experiment equipment of end-quenching and a high-speed data acquisition system based on ISA 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.A new method to calculate the temperature-dependent surface heat transfer coefficient during quenching process is presented, which applies finite element method, advance-retreat method and golden section method to the inverse heat conduction problem. During the calculation process, the phase-transform volume and phase-transform latent heat of every element in every time interval can be calculated easily by FEM, the temperature and phase-transform volume of every element are calculated with the coupling calculation of phase-transform latent heat. In order to apply the advance-retreat method to inverse heatconduction problem during quenching process, the arithmetic is improved, so that the searching interval of optimization can be gained by the improved advance-retreat method. The optimum values of surface heat transfer coefficient can be easily obtained in the searching interval by golden section method. The accurate FEM model is built, and FEM program of computing surface heat transfer coefficient is developed in the paper. In order to verify the software, the temperature curves of several positions in the quenching part are calculated using the FEM software package - MARC and user subroutine programmed to evaluate the phase-transformation. During the process of evaluating, the surface heat transfer coefficient is gained from reference. According the temperature curves gained from MARC software package, the temperature-dependent surface heat transfer coefficient is calculated using the software that is programmed in the paper. The calculation result is compared with the result of reference, the comparison shows that the heat transfer coefficients gained using FEM and optimization method is correspond well with the results of reference. The cooling curves of P20 steel in 20 °C and 60 °C water are acquired using high-speed data acquisition system designed in the paper. According to these cooling curves, the surface heat transfer coefficients of P20 steel in 20°C and 60 °C water are evaluated using this software developed in the paper.A new method to compute the elastic-plastic ratio is presented according to the character of quenching stress-strain relationship, and a method to quicken the convergence speed of elastic-plastic ratio is also presented. In the process of computing the elastic-plastic ratio, there are two instances that will affect the convergence speed of iteration. The two instances maybe result in too much iteration numbers or non-convergence. The correspond methods are presented according to the two instances, and these methods are applied to the FEM software developed in the paper, the FEM software can simulate the residual stress and strain during the quenching process. In order to check the software, two examples that have accurate results or analytical results are selected to compare with FEM results of simulating using this software, 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 simulation results and experiment results show that, the evaluating results are consistent with the results of these examples, this program can gain the rather accurate stress and strain values. The convergence speed of elastic-plastic ratio is quickened by the method presented in the paper, the non-convergence can be avoided, and the computation precision can be ensured.In order to research the effects of technical parameters during the gas quenching, the evaluation functions are built on the average surface hardness, standard deviation of surface hardness, average equivalent residual stress of surface, standard deviation of surface equivalent residual stress and distortion of quenching part. The effects of technical parameters to quenching result are researched, and the heat transfer coefficient, preheat temperature and quenchant temperature are regarded as the object variable in the research. The effects of heat transfer coefficient, preheat temperature and quenchant temperature to the quenching results are evaluated using the FEM software developed in the paper. The research results show that, the heat transfer coefficient is the most important factor of affect the quenching results, preheat temperature and quenchant temperature have a little effect on the quenching results. The heat transfer coefficient is a perfect object variable to optimize.A step function model with time is presented in the paper, and an axisymmetric component is regarded as the study objective in this model. The heat transfer coefficient during the gas quenching process is described as a function of time in this model, and five design variables are selected to do the design of Box-Behnken experiment with five factors and three levels. The levels of design variables that attain from the result of Box-Behnken experiment design are regard as the technical parameters of gas quenching to simulate the gas quenching process using the FEM software developed in the paper. The response results are gained by the FEM simulation results and the evaluation functions of quenching quality. Some mathematical models of response surface are gained by the mixed regression method and response surface method. These mathematical models show the dependencies of distortion, surface average equivalent residual stress, standard deviation of equivalent residual stress, average surface hardness and with standard deviation of surface hardness respect to the design variables. The optimization model is presented with the distortion as the optimization objective, and the model is optimized with an upper limit, a lower limit and the constraint function by the non-linear method and the Lagrange multiplier method. The optimization data are regard as the technical parameters of gas quenching to simulate the gas quenching process using the FEM software. The quenching results of after optimization are compared with those of before optimization, the comparison shows that the quenching quality using technical parameters of after optimization is much better than the quenching quality using technical parameters of before optimization. After optimization, four items of evaluation objectives areimproved, only the standard deviation of surface hardness is reduced little.A new heat transfer coefficient model with part region is presented in the paper, and a plain strain component is regarded as the study objective in this model. The heat transfer coefficient during the gas quenching process is described as a function of part region in this model, and five design variables are selected to do the design of central composite experiment with five factors and five levels. The levels of design variables that attain from the result of central composite experiment design are regard as the technical parameters of gas quenching to simulate the gas quenching process using the FEM software developed in the paper. The response results are gained by the FEM simulation results and the evaluation functions of quenching quality. Some mathematical models of response surface are gained by the mixed regression method and response surface method. These mathematical models show the dependencies of distortion, surface average equivalent residual stress, standard deviation of equivalent residual stress, average surface hardness and with standard deviation of surface hardness respect to the design variables. The optimization model of multi objectives is presented with the distortion, surface hardness and surface equivalent residual stress as the optimization objectives, and the standard deviation of surface hardness is the main optimization objective in the model. The model is optimized with an upper limit and a lower limit but without the constraint function by the non-linear method and the modified objective approximation method. The optimization data are regard as the technical parameters of gas quenching to simulate the gas quenching process using the FEM software. The quenching results of after optimization are compared with those of before optimization, the comparison shows that the quenching quality using technical parameters of after optimization is much better than the quenching quality using technical parameters of before optimization. After optimization, four items of evaluation objectives are improved, only the standard deviation of equivalent residual stress is increased little.A FEM software is developed based on Visual Fortran system, the software can evaluate the temperature, the phase transformation and the stress/strain fields of quenching process. There are three function modules in the software, such as pre-processing module, post-processing module and coupling simulation module. Using the coupling simulation module, the temperature, phase transformation and stress/strain of quenching process can be evaluated by the coupling method. Using the post-processing module, the temperature, phase transformation and stress/strain fields of discretional times in the quenchingprocess can be outputted in multi ways, and the mechanical performance of part after quenching can be also outputted in multi ways. Some FEM simulation models are built in the paper, the varieties of temperature field and phase transformation field in the quenching part are studied by this FEM software. The effects 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. The process and reason of elastic-plastic region changing trend in the quenching part are analyzed, and the distortion of quenching part is studied also.