Numerical Simulation Of Heat Treatment Process In18CrNiMo7-6Gear And Determination Of Heat Transfer Coefficient

The numerical simulation of the quenching process can describe the transient temperature field, structure field, stress and strain field of the workpiece and the interactions between them in the quenching process. It also can simulate and predict the distribution of structure, hardness, stress and distortion after quenching. During the computer simulation process, especially for temperature field, structure field and stress and strain field, heat transfer coefficient is an extremely important boundary condition. It directly affects the accuracy of the computer simulation. Therefore, accurate heat transfer coefficient is a very important prerequisite to ensure the accuracy of computer numerical simulation.In this thesis, the cooling curve of18CrNiMo7-6specimen in quenching process are measured to calculate the heat transfer coefficient between specimen and KR128oil with inverse heat conduct method. The finite difference equations and nonlinear estimation method are applied and thermal properties and mechanical properties of the material are linearly regressed. The effect of latent heat released in the process of phase transformation on the temperature field is considered in the calculation.In order to conduct the numerical simulation on the process of end quenching, heat treatment finite element simulation software named COSMAP is used as a platform at the time calculated heat transfer coefficient as the boundary condition and then the calculated results are verified. In addition, numerical simulation quenching and carburizing process of a planetary spur gear is finished to study the variation of each field of the gear in the quenching and carburizing process, the distribution of the residual stress field is analyzed. The results are: in the low temperature area, surface heat transfer coefficient increases as temperature increases. Rapid changes of heat transfer coefficient are in the range of127℃to477℃. It reached its peak at the480℃, and then decreases. After727℃, the heat transfer coefficient decreases slowly. As for the stress field simulated, the maximum tensile stress is525Mpa, located below the sample boss. The maximum compressive stress is400MPa, located a distance of about14mm from the upper end. Compressive stress is mainly distributed across the end quenching surface. In the distribution of the amount of deformation, the maximum is0.116mm, which occurs at the spray end. As the distance from the spray end increases, the martensite gets smaller and the amount of deformation of the specimen decreased. The calculated temperature curve and the measured temperature curve changes fast in the high temperature area, slowly after200seconds. At300seconds, calculated temperature and the measured temperature are all about450℃, close to the Ms point. Calculated cooling curve and the measured cooling curve in the center of the specimens are in good agreement. At the site of1.5mm from the quenched end, the structure is mainly composed of martensite. Structure in contour display, martensite content is about90%. The microstructure and structure field agrees well. In the gear simulation results, the deformation from the tooth crown to the tooth root volume is expansion, while the maximum amount of expansion is in the pitch circle. The smallest amount of expansion is in the crown and tooth root. The maximum tensile stress of the gear is584Mpa, located at the lower end of the web core part; the maximum compressive stress is1531Mpa located in the part of the tooth surface and restraint surface of web.