Mold heat treatment is an important process to ensure mold performance. It has a direct impact on the following properties of the mold. Mold manufacturing accuracy: uneven and incomplete structural transformation and excessive residual stress caused by heat treatment cause the mold to deform during processing, assembly and use of the mold after heat treatment, thereby reducing the accuracy of the mold and even scrapping it.Strength of the mold: The heat treatment process is not properly formulated, the heat treatment operation is not standardized, or the heat treatment equipment is in an incomplete state, which causes the strength (hardness) of the processed mold to meet the design requirements. The working life of the mold: The irrational microstructure and excessive grain size caused by heat treatment, etc., lead to the decline of the main properties such as the toughness of the mold, the hot and cold fatigue performance, and the wear resistance, which affect the operating life of the mold. Mold manufacturing cost: As an intermediate link or final process in the mold manufacturing process, cracks, deformation distortion and performance degradation caused by heat treatment will cause the mold to be scrapped in most cases. It can continue to be used even after repairing, and it will increase working hours, Extend the delivery time and increase the manufacturing cost of the mold. Heat treatment technology is very closely related to the mold quality so that these two technologies in the process of modernization, promote each other and improve jointly. In recent years, the rapid development of international mold heat treatment technology is vacuum heat treatment technology, mold surface strengthening technology and mold material pre-hardening technology.

I The vacuum heat treatment technology of the mold

Vacuum heat treatment technology is a new type of heat treatment technology developed in recent years. It has the characteristics that are urgently needed in mold manufacturing, such as preventing oxidation and non-decarburization, vacuum degassing or degassing, eliminating Hydrogen embrittlement, which improves the plasticity, toughness and fatigue strength of materials (parts). The slow vacuum heating and the small temperature difference between the inside and outside of the part determine the small deformation of the part caused by the vacuum heat-treatment process. According to the cooling medium used, vacuum quenching can be divided into vacuum oil cooling quenching, vacuum air cooling quenching, vacuum water cooling quenching and vacuum nitrate isothermal quenching. The main applications of mold vacuum heat treatment are vacuum oil cooling quenching, vacuum air cooling quenching, and vacuum tempering. In order to maintain the excellent characteristics of vacuum heating of workpieces (such as molds), the choice and formulation of coolants and cooling processes are very important. The mold quenching process mainly uses oil cooling and air cooling. For the working surface of the mold that is no longer mechanically processed after heat treatment, vacuum tempering used as much as possible after quenching, especially for the workpiece (mold) that is vacuum quenched, which can improve the mechanical properties related to surface quality. Such as fatigue performance, surface brightness, corrosion and so on. The successful development and application of computer simulation technology (including organization simulation and performance prediction technology) of the heat treatment process make intelligent heat treatment of mold possible. Due to the small batch size (or even a single piece) of the mold production, the characteristics of multiple varieties, and the high requirements for heat treatment performance and the fact that scraps are not allowed, intelligent processing of the mold becomes necessary. The intelligent heat treatment of the mold includes: clarifying the structure, materials, and heat treatment performance requirements of the mold: computer simulation of temperature field and stress field distribution during mold heating process; computer simulation of temperature field, phase transition process, and stress field distribution during mold cooling process; heating and Simulation of the cooling process; formulation of quenching process; automatic control technology of heat treatment equipment. Developed industrial countries abroad, Such as the United States, Japan, etc., in the vacuum high-pressure gas quenching, has carried out technical research and development in this area, mainly targeted at the mold

II  The surface treatment technology of the mold

In addition to the reasonable cooperation of the matrix with sufficient strength and toughness, the surface properties of the mold are critical to the working performance and service life of the mold. These surface properties refer to abrasion resistance, corrosion resistance, friction coefficient, fatigue performance, etc. The improvement of these properties is very limited and it is not economical to rely solely on the improvement and improvement of the base material. The surface treatment technology can often get more results with less effort, which is why the surface treatment technology has developed rapidly. The surface treatment technology of the mold is a system engineering that changes the morphology, chemical composition, microstructure, and stress state of the mold surface through the surface coating, surface modification, or composite treatment technology to obtain the required surface properties. From the surface treatment methods, they can be divided into chemical methods, physical methods, physical and chemical methods, and mechanical methods. Although new treatment technologies are constantly emerging to improve the surface properties of molds, many of the main nitridings, carburizing, and hardening film depositions are applied in mold manufacturing. Nitriding processes include gas nitriding, ion nitriding, and liquid nitriding. In each type of nitriding, there are several kinds of nitriding technologies, which can meet the requirements of different steel types and different workpieces. Nitriding technology can form a surface with excellent performance, and the nitriding process is well coordinated with the quenching process of the mold steel. At the same time, the nitriding temperature does not require intense cooling after Nitriding, and the mold deformation is minimal, so the mold surface is strengthened Nitriding technology was adopted earlier and is also the most widely used. The purpose of carburizing of the mold is mainly to improve the overall toughness of the mold, that is, the working surface of the mold with high strength and wear resistance. The technical idea introduced thereby is to replace the higher-level materials with lower-level materials, that is, through carburizing and quenching, thereby reducing manufacturing costs. At present, the hardened film deposition technology is more mature is CVD, PVD. In order to increase the bonding strength of the film workpiece surface, a variety of enhanced CVD, 'PVI) technologies have now been developed. The hardened film deposition technology was first applied to tools (knives, cutting tools, measuring tools, etc.), with excellent results. A variety of tools have adopted hardened film coating as a standard process. Since the 1980s, the mold has adopted the coating hardened film technology. Under the current technical conditions, the cost of hardened film deposition technology (mainly equipment) is relatively high, and it is still only applied to some precision, long-life molds. If a heat treatment center is established, the cost of hardened film coating will be greatly reduced If more molds adopt this technology, the overall level of China's mold manufacturing can be improved.

III Pre-hardening technology of mold materials

The heat treatment of molds in the manufacturing process is a process that most molds have been used for a long time. Since the 1970s, the idea of pre-hardening has been proposed internationally. The hardness cannot reach the hardness of the mold, so the investment in research and development of pre-hardening technology is not large. With the improvement of the performance of processing machine tools and cutting tools, the development of pre-hardening technology for mold materials has accelerated. By the 1980s, the proportion of industrially developed countries using pre-hardened modules on plastic mold materials had reached 30% (currently Over 60%). China began to use pre-hardened modules (mainly imported products from abroad) in the mid-to-late 1990s. The pre-hardening technology of mold materials is mainly developed and implemented in mold material manufacturers. By adjusting the chemical composition of steel and equipped with corresponding heat treatment equipment, it is possible to produce pre-hardened modules with stable quality in large quantities. China's mold material pre-hardening technology has a late start and small scale and currently cannot meet the requirements of domestic mold manufacturing. The use of pre-hardened mold materials can simplify the mold manufacturing process, shorten the mold manufacturing cycle, and improve the mold manufacturing accuracy. It is foreseeable that with the advancement of processing technology, pre-hardened mold materials will be used for more mold types.