Surface treatment technology in die casting die
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Surface treatment technology in die casting die
Die casting die is a large class of die. With the rapid development of China's automobile and motorcycle industry, die casting industry has ushered in a new era of development. At the same time, higher requirements are put forward for the comprehensive mechanical properties and service life of the die-casting die. It is still difficult to meet the increasing demand of service performance only by the application of new mold materials. It is necessary to apply various surface treatment technologies to the surface treatment of die-casting mold to meet the requirements of high efficiency, high precision and high life of die-casting mold. Pressure casting is to make the molten metal fill the mold cavity under high pressure and high speed and die-casting forming. In the working process, it repeatedly contacts with the hot metal. Therefore, the die-casting mold is required to have high heat-resistant fatigue, thermal conductivity and wear resistance, corrosion resistance, impact toughness, red hardness, good demoulding performance, etc. Therefore, in recent years, the surface treatment technology of die-casting die is required to be high, and various new surface treatment technologies of die-casting die are emerging, but in general, they can be divided into the following three categories: (1) improvement technology of traditional heat treatment process; (2) surface modification technology, including surface heat diffusion treatment, surface transformation strengthening, electric spark strengthening technology; (3) coating technology, including chemical plating And so on.
1. Improvement technology of traditional heat treatment process
The traditional heat treatment process of die casting die is quenching tempering, and the surface treatment technology has been developed. Due to the variety of materials that can be used as die-casting dies, the same surface treatment technology and process application in different materials will produce different effects. Shikov * * recently proposed the substrate pretreatment technology for mold substrate and surface treatment technology. Based on the traditional technology, he proposed suitable processing technology for different mold materials, so as to improve mold performance and improve mold life. Another development direction of heat treatment technology improvement is to combine traditional heat treatment technology with advanced surface treatment technology to improve the service life of die casting die. For example, the method of chemical heat treatment, combined with conventional quenching and tempering process, is NQN (Carbonitriding- The compound strengthening of quenching and carbonitriding can not only obtain high surface hardness, but also increase the effective hardening depth, reasonable hardness gradient distribution, tempering stability and corrosion resistance of the carburizing layer, which can improve the surface quality and performance of the die casting die while obtaining good core performance.
2 Surface Modification Technology
2.1 surface heat diffusion technology
This type includes carburizing, nitriding, boronizing, carbonitriding, sulfur carbonitriding, etc.
2.1.1 carburizing and carbonitriding
The application of carburizing technology in cold, hot work and surface strengthening of plastic mould can improve the service life of mould. For example, the die-casting die made of 3Cr2W8V steel is carburized first, then quenched at 1140-1150 ℃ and tempered twice at 550 ℃, the surface hardness can reach hrc56-61, which increases the die-casting life of nonferrous metals and their alloys by 1.8-3.0 times. In the process of carburizing, the main processes are solid powder carburizing, gas carburizing, vacuum carburizing, ion carburizing and carbonitriding formed by adding nitrogen element in carburizing atmosphere. Among them, vacuum carburizing and ion carburizing are the technologies developed in recent 20 years. This technology has the characteristics of fast carburizing speed, uniform carburizing layer, gentle carbon concentration gradient and small workpiece deformation. It will play an increasingly important role in the surface treatment of mold, especially in the surface treatment of precision mold.
2.1.2 nitriding and related low temperature heat diffusion technology
This type includes nitriding, ion nitriding, carbonitriding, oxo nitriding, sulfo nitriding, and sulfo carbonitriding, oxo nitrogen sulphurizing. These methods are simple, adaptable, low diffusion temperature (generally 480-600 ℃), small deformation of workpiece, especially suitable for surface strengthening of precision die, high hardness of nitriding layer, good wear resistance and good adhesion resistance. The service life of 3Cr2W8V steel die casting die after tempering and nitriding at 520-540 ℃ is 2-3 times longer than that of non nitriding die. Many die-casting dies made of H13 steel in the United States need nitriding treatment, and nitriding instead of one time tempering. The surface hardness is as high as hrc65-70, while the core hardness of the die is low and the toughness is good, thus obtaining excellent comprehensive mechanical properties. Nitriding process is commonly used in surface treatment of die-casting die, but when the nitriding layer appears thin and brittle white bright layer, it can not resist the effect of alternating thermal stress, and it is very easy to produce microcracks and reduce the thermal fatigue resistance. Therefore, in the nitriding process, the process should be strictly controlled to avoid the formation of brittle layer. **Recently, secondary and multiple nitriding processes have been proposed abroad. By using the method of repeated nitriding, the nitride white bright layer which is easy to produce microcracks in service can be decomposed, the thickness of nitriding layer can be increased, and at the same time, there is a very thick residual stress layer on the surface of the die, so that the life of the die can be significantly improved. In addition, salt bath carbonitriding and salt bath sulfur nitrogen carbonitriding are also used. These processes are widely used abroad and rarely seen in China. For example, TFI + ABI process is to soak in alkaline oxidizing salt bath after nitrocarburizing in salt bath. The surface of the workpiece is oxidized and black, and its wear resistance, corrosion resistance and heat resistance are improved. The service life of die casting die of aluminum alloy treated by this method is increased by hundreds of hours. Another example is the oxynit process developed in France, which is nitriding after nitrocarburizing. It has more characteristics when applied to non-ferrous die-casting dies.
Due to the high hardness of boronized layer (Feb: hv1800-2300, Fe2B: hv1300-1500), the resistance to