Analysis of the Three Techniques of Surface Strengthening Treatment for Die Wholesale

At present, the service life of moulds in China is not high, and the wholesale consumption of moulds is large. Therefore, it is a very urgent task to improve the service life of moulds in China. The heat treatment of die manufacturers has a great impact on the service life. The mold damage we often come into contact with is mostly caused by improper heat treatment. According to statistics, the failure of dies due to improper heat treatment accounts for more than 50% of the total failure efficiency. Therefore, more and more vacuum furnaces, semi vacuum furnaces and non oxidation atmosphere furnaces are used for heat treatment of dies abroad.

The heat treatment process of die includes matrix strengthening and toughening and surface strengthening. The strengthening and toughening of the matrix is to improve the strength and toughness of the matrix and reduce fracture and deformation, so its conventional heat treatment needs to be carried out in strict accordance with the process. The main purpose of surface strengthening is to improve the wear resistance, corrosion resistance and lubrication performance of the die surface. There are many methods for surface strengthening treatment, mainly including carburizing, nitriding, sulfurizing, boronizing, nitrocarburizing, metallizing, etc. By using different surface strengthening treatment processes, the service life of the die can be increased several times or even dozens of times. In recent years, some surface strengthening processes have appeared. This paper focuses on three of them.

1. Ion nitriding can be used to improve the corrosion resistance, wear resistance, thermal fatigue resistance and adhesion resistance of the die. The outstanding advantages of ion nitriding are that the nitriding time is significantly shortened, the nitriding layer structure can be controlled by adjusting different gas components, the surface brittleness of the nitriding layer is reduced, the deformation is small, the hardness distribution curve of the nitriding layer is relatively stable, and it is not easy to produce spalling and thermal fatigue. The permeable base material is wider than that of gas nitriding, non-toxic, non explosive, and safe in production. However, for molds with complex shapes, it is difficult to obtain uniform heating and uniform carburizing layer, and the carburizing layer is shallow and the transition layer is steep. Temperature measurement and temperature uniformity still need to be solved.

The appropriate temperature for ion nitriding is 450 ～ 520 ℃. After 6 ～ 9h of treatment, the depth of nitriding layer is about 0.2 ～ 0.3mm. Too low temperature and too thin infiltration layer; If the temperature is too high, the surface layer is easy to appear loose layer, reducing the anti sticking ability. The thickness of ion nitriding layer should be 0.2~0.3mm. The worn ion nitriding die can be put into use after repair and re ion nitriding, thus greatly increasing the total service life of the die.

2. Nitrocarburizing. Nitrocarburizing process has low temperature (560~570 ℃) and small deformation. The surface hardness of the treated die steel is as high as 900~1000HV, with good wear resistance, strong corrosion resistance and high high temperature hardness. It can be used for die casting die, cold heading die, cold extrusion die, hot extrusion die, high speed forging die and plastic die, which can increase the service life by 1~9 times. However, deformation often occurs after gas nitrocarburizing, and the expansion amount accounts for about 25% of the compound thickness, so it is not suitable for precision die. Annealing and residual removal are required before treatment.

3. Carbon, nitrogen and boron three-way penetration can be carried out in the nitriding furnace. The penetration agent is boron containing organic penetration agent and ammonia, the ratio is 1:7, the penetration temperature is 600 ℃, the penetration time is 4h, and the thickness of the compound layer of the penetration layer is 3-4 μ m. The depth of diffusion layer is 0.23mm, and the surface hardness is HV011050. The service life of the die is significantly improved after the penetration treatment.