| Large aspect ratio parts such as transmission shaft and screw usually need to be quenched and tempered in order to obtain excellent comprehensive mechanical properties.However,the uneven temperature field of large aspect ratio parts in the quenching process often results in non-uniform microstructure or excessive deformation,finally resulting in mechanical properties or dimensional accuracy can not meet the technical requirements.In recent years,Vacuum High-pressure Gas Quenching(VHGQ)technology is considered to be able to solve this problem,because compared with traditional oil-cooled quenching and salt bath quenching,it is easier to control the temperature field uniformity of the parts.Therefore,the equipment manufacturer has designed and manufactured the first VHGQ furnace with static alternating flow pattern in China in order to meet the heat treatment requirement for Large aspectratio parts.It is necessary to carry out research study for such parts in VHGQ furnace urgently.In this paper,the basic heat transfer model of fluid sweeping isothermal plate is used to validate the applicability of the physical model in the CFD software.On this basis,CFD software is used to establish a 1:5 scale model and a 1:1 scale model of single workpiece,a 1:1 model of multi-workpiece for coupled thermo-hydro-mechanical numerical simulation.Based on the numerical simulation,the effects of quenching pressure,gas flow rate,gas type,gas inflow temperature,alternating blowing time interval of static alternating flow pattern and position of workpiece on temperature field uniformity are studied.And taking the screw as an example,the uniformity of the temperature field and the deformation of the workpiece with 10 second alternating blowing time interval are tested and verified.The paper laid a foundation for large aspect ratio parts in VHGQ.The following are conclusions through this study:1)Using 1:5 scale model to simulate and analyze the effects of quenching pressure(3,6,9,12bar),gas flow rate(5,10,20,30m/s),gas species(H2,He,N2,Ar),inflow temperature(320,400,500K)on the uniformity of workpiece temperature field.The results show that the greater the quenching pressure,the faster the flow rate,the stronger the heat transfer capacity and the inflow temperature are,the faster the cooling speed is and the more uneven the temperature field is.The large axial temperature difference of Large aspectratio parts is the main reason for the uneven temperature field of the whole workpiece.2)Using a 1:1 single workpiece model to simulate one-way flow pattern and static alternating flow pattern(alternating blowing time interval 10 s,20s,30s).The results show that the static alternating flow pattern of up-down alternating blowing is beneficial to improve the axial temperature uniformity of large aspect ratio parts.And the shorter the alternating blowing time interval,the more uniform the temperature field of the parts.For this example,the surface temperature difference of workpiece with 10 seconds alternating blowing time interval is acceptable in engineering.3)Using a 1:1 multi-workpiece model to simulate one-way flow pattern.The results show that when multiple workpieces are loaded in the furnace at the same time,the difference of position will lead to the difference of flow field,which will affect the temperature distribution of the workpiece.Therefore,the workpiece located at the edge of the furnace is more likely to form non-uniform axial and radial temperature field.4)Using LZGQ80 VHGQ furnace and Cr12Mo1V1 screw with large aspect ratio to carry out the test.The experimental results show that when the cooling rate is constant,the static alternating flow pattern can improve the axial temperature uniformity of large aspect ratio parts to a certain extent.The overall temperature difference of the workpiece is also affected by the cooling rate and the structure of the workpiece.The workpiece can obtain good mechanical properties and dimensional accuracy by adjusting the quenching pressure to change the cooling rate. |
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