First, the correlation between heat treatment and mold quality
1, the mold manufacturing accuracy
Non-uniform organization, incomplete and incomplete thermal stress caused by residual stress, resulting in the mold after the heat treatment of the processing, assembly and deformation during the use of the mold, thereby reducing the accuracy of the mold, and even scrapped.
2, the strength of the mold
Inappropriate heat treatment process, non-standard heat treatment operation, or incomplete state of heat treatment equipment cause the strength (hardness) of the treated mold to fail to meet the design requirements.
3, the working life of the mold
The irrational microstructure and excessive grain size caused by the heat treatment cause the main properties such as toughness of the mold, cold and hot fatigue performance, and anti-wear performance to decline, affecting the working life of the mold.
4, mold manufacturing costs
As an intermediate or final process of the mold manufacturing process, the cracking, deformation and poor performance caused by heat treatment will cause the mold to be scrapped in most cases. Even if it can continue to be used through repair, it will also increase the working hours and extend the delivery time. , increase the manufacturing cost of the mold.
It is the heat treatment technology and the quality of the mold that are closely related to each other, making these two technologies mutually promote and improve together in the process of modernization. In recent years, the development of mold heat treatment technology in the world has been rapidly in vacuum heat treatment technology, mold surface strengthening technology and pre-hardening technology of mold materials.
Second, the vacuum heat treatment technology
Vacuum heat treatment technology is a new type of heat treatment technology developed in recent years. Its characteristics are exactly what is needed in the manufacture of molds, such as preventing oxidation and non-decarburization, vacuum degassing or degassing. Eliminates hydrogen embrittlement, thereby increasing the plasticity, toughness, and fatigue strength of materials (parts). Vacuum heating is slow, and the temperature difference between the inside and outside of the part is small, which determines the small deformation of the parts caused by the vacuum heat treatment process.
According to the different cooling media used, vacuum quenching can be divided into: vacuum oil quenching, vacuum gas-cooled quenching, vacuum water quenching and vacuum nitrate salt isothermal quenching. In vacuum heat treatment of molds, vacuum oil quenching, vacuum air quenching and vacuum tempering are mainly used. In order to maintain the excellent characteristics of the vacuum heating of the workpiece (such as the mold), the choice and formulation of the coolant and the cooling process are very important. The oil quenching and air cooling are used for the mold quenching process.
For the working surface of the mold, which is no longer machined after heat treatment, vacuum tempering is used as much as possible after quenching, especially vacuum-hardened workpieces (molds), which can improve the mechanical properties related to the surface quality, such as: fatigue properties, surface brightness, , corrosion resistance and so on.
The successful development and application of computer simulation techniques (including tissue simulation and performance prediction techniques) in the heat treatment process has made intelligent heat treatment of molds possible. Due to the characteristics of small batches (even single pieces) and multi-types of mold production, as well as the high requirements on heat treatment performance and the fact that no rejects are allowed, the intelligent processing of molds becomes necessary.
The intelligent heat treatment of the mold includes:
1, clear mold structure, material, heat treatment performance requirements;
2. Computer simulation of temperature field and stress field distribution during mold heating process;
3. Computer simulation of temperature field, phase change process and stress field distribution during mold cooling process;
4, heating and cooling process simulation;
5, the formulation of the quenching process;
6, automatic heat treatment equipment control technology.
Foreign industrial developed countries, such as the United States, Japan, etc., have carried out technical research and development in the area of ​​vacuum high-pressure gas quenching, mainly aiming at the target as a mold.
Third, the mold surface treatment technology
In the work of the mold, in addition to requiring the substrate to have a reasonable combination of high enough strength and toughness, its surface properties are critical to the work performance and service life of the mold. These surface properties refer to: wear resistance, corrosion resistance, friction coefficient, fatigue properties, and the like. The improvement of these performances depends on the improvement and improvement of the base material. It is very limited and uneconomical. However, surface treatment technology can often achieve a multiplier effect, which is why the surface treatment technology has been rapidly developed.
The surface treatment technology of the mold is a systematic project to change the shape, chemical composition, structure and stress state of the mold surface by surface coating, surface modification or composite treatment technology to obtain the required surface properties. From the surface treatment methods, it can be divided into: chemical methods, physical methods, physical and chemical methods and mechanical methods. Although new processing techniques aiming at improving the surface properties of the molds are emerging, the major nitriding, carburizing and hardened film depositions are applied in the mold manufacturing.
1, nitriding
The nitriding process includes gas nitriding, ion nitriding and liquid nitriding. In each type of nitriding, there are several kinds of nitriding techniques that can adapt to the requirements of different steel types and different workpieces. Because the nitriding technology can form a surface with excellent performance, and the nitriding process has a good coordination with the quenching process of the die steel, at the same time, the nitriding temperature is low, the nitriding needs no intense cooling, and the deformation of the die is minimal, therefore, the die The surface strengthening is the use of nitriding technology earlier, but also the most widely used.
2. Carburizing
The purpose of carburizing the mold is mainly to improve the overall toughness of the mold, that is, the working surface of the mold has high strength and wear resistance. The technical idea introduced hereby is to use lower grade materials, ie, carburizing and quenching to replace higher grade materials, thereby reducing manufacturing costs.
3, hardened film deposition
Hardened film deposition technology, which is currently more mature, is CVD and PVD. In order to increase the bonding strength between the film and the workpiece surface, various enhanced CVD and PVD technologies have now been developed.
The hardened film deposition technology was first applied to tools (tools, cutting tools, measuring tools, etc.), and the effect was excellent. A variety of tools had applied a cured film as a standard process.
Dies have been coated with hardened film technology since the 1980s. Under the current technical conditions, the cost of hardened film deposition technology (mainly equipment) is high, and it is still only applied to some precision and long-life molds. If the method of establishing a heat treatment center is adopted, the cost of coating the cured film will be greatly reduced. . More molds using this technology can improve the overall level of mold manufacturing in China.
Fourth, the pre-hardening technology of mold materials
In the manufacturing process of molds, heat treatment is a process that most molds use for a long time. Since the 70s of the last century, the idea of ​​pre-hardening has been proposed internationally. However, due to the limitations of machine tool stiffness and cutting tools, the hardness of the pre-hardened cannot reach the hardness of the mold. Therefore, the research and development of the pre-hardening technology has not invested much.
With the improvement of the performance of processing machine tools and cutting tools, the development speed of pre-hardening technology for mold materials has accelerated. By the 1980s, the proportion of pre-hardened modules used in plastic mold materials in industrial countries in the world had reached 30% (currently At over 60%, China began to use pre-hardened modules (mainly imported products) in the mid to late 1990s.
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