Research On The Stress Field During Atomizing Gas Quenching Considering Multiple Factors

Quenching is one of the most common heat treatment methods used to improve microstructures and mechanical properties of metallic materials. Due to the cooling performance of quenching medium is not ideal and not easy to control, the traditional quenching methods normally lead to various defects and workpiece scrapped after quenching process. Using an atomized mixture of gas and liquid as quenching medium, atomizing gas quenching method can adjust its composition, velocity of flow, fluid pressure and other parameters to achieve the desired quenching cooling rates. At the same time, it is a widely applied, big potential and promising research direction, which can save production cost directly and reduce defectiveness. Atomizing gas is also a green quenchant which is environment friendly and highly efficiency.In actual manufacture, the quenching distortion and cracking are considered to be a key issue and a difficult problem, and it is especially prominent during the quenching process of large forgings, alloy steels and precision parts. Quenching stress is the basic cause of the part deformation and cracking, forecasting and controlling quenching stress have been the issue that relevant scientific and engineering staff extremely focus on. At the present time, both domestic and overseas research on the stress field of atomization gas quenching process are very limited. More in-depth study need to be conducted for the establishment of its model, the selection of numerical methods, the testing of residual stress, and so on.Aiming at these problems above, this dissertation analysis particularly on the stress field of atomizing gas quenching, considering multiple influence factors, using theoretical analysis, computer simulation and experiment examination, applies multidisciplinary crossover study focusing on the mathematical-physical model, boundary condition, numerical simulations and experimental testing. Main work and results of this dissertation go as follows:(1) Study on establishment of the mathematical-physical model of stress field in the process of atomizing gas quenching considering multiple factors. This dissertation fully considers particularity of atomizing gas quenching and analyzes causes for quenching stress and mutual coupling relationship among multi-physical fields in the process of atomizing gas quenching. On the basis, by applying multidisciplinary knowledge, this dissertation deduces the quenching heat conduction equation, phase change conditions and materials performance parameter calculating formula, and introduces phase change factors into thermal elastoplastic stress-strain relationship. the constitutive equation containing influence of phase change for stress field in the process of atomizing gas quenching has been proposed, which truly reflects the actual situation of sample stress field in the atomizing gas quenching process.(2) Study on atomizing gas flow field in the atomizing gas quenching process and determining fluid pressure boundary conditions of stress field. In this dissertation, this paper adopts a hybrid model and applies the fluid dynamic simulation software FLUENT based on the finite volume method to conduct coupled numerical simulation of atomizing gas flow field during the quenching process at different medium velocities of flow. Then, distribution and law of velocity field and stress field of quenching medium fluid are obtained and gas pressure boundary conditions of the quenching stress field is determined. The simulation results of flow field indicates that along with the increase of inlet flow velocity of atomizing gas, flow velocity and pressure of quenching medium around the sample are also increased; In the case of same inlet velocity, flow field velocity and pressure value in the tank become larger and the medium is distributed evenly and flows stably when the distance between the wall of inner cylinder of quenching tank and the deflector ring is smaller and the deflector ring tilts 60 degrees.(3) Optimizing structure of experimental device and experimentally study on atomizing gas quenching. According to the flow field simulation results of the sample placed at different places and quenching tanks with different structures, this paper obtained optimized parameters of quenching tank structure and improved the quenching equipment, then a large number of experiments and studies related to atomizing gas quenching have been carried out with this equipment. By designing experimental schemes for atomizing gas quenching, continuously cooling curve collecting, residual stress test and quenching medium flow rate test, and conducting relevant experiments, the basis data for relevant simulation and calculation have been obtained. Meanwhile, this dissertation also compares the simulation results with the calculation results, and modifies the model and theory.(4) Study on heat transfer boundary conditions of atomizing gas quenching. As heat transfer process of atomizing gas quenching is a summarization of convection, radiation, boiling heat transfer, and so on. And it's often accompanied by the release of latent heat during material phase transformation process, which is often impossible to test the heat transfer coefficient on a real-time online basis. Based on theories and methods related to inversion problem and the measured data of internal temperature field of the sample, this dissertation solves the inverse problem of heat conduction and acquires the stress field heat transfer boundary condition-synthetical surface heat transfer coefficient which is influenced by multiple factors by adopting Nonlinear inverse estimation method and finite-difference method and making use of Matlab software programming. The results prove the way above is an effective research idea and method, which determines boundary condition with an internal known quantity and then studies on multi-parameter coupling effect.(5) Study on numerical simulation of stress field in the process of atomizing gas quenching and measuring on residual stresses of samples after the heat treatment. In this dissertation, by taking synthetical surface heat transfer coefficient and the airflow pressure on the sample surface as the heat transfer and pressure boundary conditions of stress field and making use of finite element analysis software ANSYS, the geometric model and finite element model of the quenching sample are established. After imposing corresponding displacement boundary conditions and inporting material performance parameter influenced by phase change, numeral simulation and analysis are carried out against the quenching stress field which considers multiple factors. Then, the hole-drilling method is utilized to measure residual stress of the cooled samples after atomizing gas quenching and collect the experimental data. Upon comparison between numeral simulation calculation results and experimental data, the results show that finite element numerical simulation results of residual stress are consistent with experimental results, which proves the numerical simulation of stress field during atomizing gas quenching in this dissertation is effective.(6) Carrying out simulation study on transient stress field of web-type gear in the process of atomizing gas quenching. The finite element solving method of stress field which is proven as effective and correct during the heat treatment is utilized to simulate the transient stress field of web-type gear with complex structure in the atomizing gas quenching process. On the basis of it, the calculation results curves and 3D surface and cross sectional contours of stress field are acquired to intuitively display the stress distribution and change inside the gear. The simulation results provide important basis for formulation of parts process scheme and selection of process parameters in the atomizing gas quenching process.