Numerical Simulation And Experimental Study On The Laser Transformation Hardening Of Ductile Cast Iron QT600-3

In recent years, automobile industry has advanced rapidly. Ductile cast iron has a good combination of mechanical properties and process capability, and used popularly as one key kind of die materials for vehicle bodies. However, the life of the ductile cast iron die is not high enough to satisfy requirement. In order to improve the surface properties of dies, it is necessary to laser harden the die surface. Numerical simulation and experimental study of the Laser transformation hardening of ductile cast iron QT600-3 were carried out in this thesis.A 3-D numerical model of the temperature field for the laser transformation hardening was created based on finite element software ProCAST. The temperature field of the laser transformation hardening of ductile cast iron QT600-3 was calculated. According to the temperature field, the depths and widths of the hardened band were predicted. When the laser power ranged from 800W to 1000W, the scan velocity ranged from 2mm/s to 2.667mm/s, and the diameter of laser beam ranged from 4mm to 5mm, The depths of the hardened distribute between 0.2mm and 0.64mm as well as the widths distribute between 2mm and 3.7mm. The depths and widths of the hardened band rises with increase in laser power or decrease in laser scanning velocity. The simulation results show that the depths of the hardened is fairly high with a fairly large laser beam diameter and consequentially laser power as well as a lower scanning velocity of laser beam.In order to verify the numerical simulation results, A YAG laser unit was used in the study of Laser transformation hardening of pearlite ductile cast iron QT600-3. Most of the hardness of the hardened band surface are above 50HRC, and the Micro hardness of the hardened surface are between 600HV and 920HV. Metallographic change is also researched. The hardened surface band depths as well as hardened surface band widths were obtained. Hardened surface band depths and parameters linear regression was obtained. The experimental and simulation results showed that the simulation value was similar with the experimental results, The numerical simulation results can be regarded as a basis for design of the Laser transformation hardening process.In order to enhance the Laser transformation hardening depths of the hardened band with the use of CO2 laser, laser transformation hardening of pearlite ductile cast iron QT600-3 was investigated experimentally. In the experiment, a CO2 laser unit was used. The laser power ranged from 700W-800W, the scan velocity ranged from 2mm/s-3mm/s, and the diameter of laser beam was 8mm. The hardness of the hardened band surface is between 48HRC and 50HRC, and the Micro hardness of the hardened surface are between 600HV and 913HV. The experimental results show that great amounts of acicular martensites are obtained in the hardened area. This thesis discussed the relation between surface hardness and scan velocity as well as laser power, the relation between hardened surface band and scan velocity as well as laser power, which is a useful revelation for the Laser transformation hardening process designed. The depths of the hardened is between 0.53mm and 0.95mm. Through experimental results, It found that the way to significantly improve the hardened surface band. By comparing the common hardened surface band depths and parameters linear regression, The preferably linear regression was obtained. Moreover, Laser transformation hardening of 45-steel and admixture ductile cast iron was investigated to compare with the microstructure of the pearlite ductile cast iron of the laser transformation hardening. Experiment results has the theoretical significance for the analysis and understanding of laser transformation hardening of the ductile cast iron QT600-3, and can be a strong reference in the optimization for the stamping dies for automobiles of laser transformation hardening process.