With the continuous improvement of science and technology, the mold industry has gradually become an indispensable product in the manufacturing and manufacturing enterprises, resulting in a large increase in the number of molds. As the number of molds increases, people gradually become more and more demanding. Among them, the casting mold is a typical representative of the mold, which uses the cavity to cast a fluid liquid, and after the liquid is cooled and solidified, the parts having the same shape and shape as the mold can be formed, and after the casting is completed, the products need to be taken out. The heat treatment is carried out in a heat treatment electric furnace, which not only affects the production efficiency, but also generates an oxide layer on the surface during the removal process, which not only affects the quality of the subsequent heat treatment, but also increases the loss of the material.
Mold manufacturing precision: Unevenness, incompleteness of the microstructure transformation and excessive residual stress caused by heat treatment cause deformation of the mold after heat treatment, assembly and mold use, thereby reducing the accuracy of the mold and even scrapping.
Mold strength: The heat treatment process is improperly formulated, the heat treatment operation is not standardized, or the state of the heat treatment equipment is not perfect, resulting in the strength (hardness) of the treated mold not meeting the design requirements.
Working life of the mold: Unreasonable structure caused by heat treatment, excessive grain size, etc., leading to the decline of main properties such as mold toughness, hot and cold fatigue performance, anti-wear performance, etc., affecting the working life of the mold.
Mold manufacturing cost: As the 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 be used through repair, it will increase the working hours. , to extend the delivery time and increase the manufacturing cost of the mold.
It is the heat treatment technology that has a very close relationship with the quality of the mold, which makes these two technologies promote each other and improve together in the process of modernization. In recent years, the field of rapid development of international mold heat treatment technology is vacuum heat treatment technology, surface strengthening technology of molds and pre-hardening technology of mold materials.
First, the vacuum heat treatment technology of the mold
Vacuum heat treatment technology is a new type of heat treatment technology developed in recent years. Its characteristics are urgently needed in mold manufacturing, such as preventing oxidation and non-decarburization, vacuum degassing or degassing, eliminating Hydrogen embrittlement, thereby improving the plasticity, toughness and fatigue strength of materials (parts). Factors such as slow vacuum heating and small temperature difference between the inside and outside of the part determine the deformation of the parts caused by the vacuum heat treatment process.
According to the different cooling medium used, vacuum quenching can be divided into vacuum oil quenching, vacuum gas quenching, vacuum water quenching and vacuum nitrification. The main application of mold vacuum heat treatment is vacuum oil quenching, vacuum gas quenching and vacuum tempering. In order to maintain the excellent characteristics of vacuum heating of workpieces (such as molds), it is very important to select and formulate coolants and cooling processes. The mold quenching process mainly uses oil cooling and air cooling.
For the working face of the mold which is no longer machined after heat treatment, vacuum tempering is used as much as possible after quenching, especially vacuum hardened workpiece (mold), 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 tissue simulation and performance prediction technology) of the heat treatment process makes the intelligent heat treatment of the mold possible. Due to the small batch (even single piece) of the mold production, the characteristics of multiple varieties, and the high requirements for heat treatment performance and the fact that waste products are not allowed, the intelligent processing of the mold becomes a necessity. The intelligent heat treatment of the mold includes: clarifying the structure, material and heat treatment performance requirements of the mold: computer simulation of the temperature field and stress field distribution of the mold heating process; computer simulation of the temperature field, phase transformation process and stress field distribution of the mold cooling process; heating and Simulation of the cooling process; formulation of the quenching process; automated control technology for the heat treatment equipment. In developed countries, such as the United States and Japan, in the field of vacuum high-pressure gas quenching, technology research and development in this area has been carried out, mainly aimed at molds.
Second, the surface treatment technology of the mold
In addition to the reasonable cooperation of the mold with sufficient strength and toughness, the surface properties of the mold are crucial to the working performance and service life of the mold. These surface properties are: abrasion resistance, corrosion resistance, friction coefficient, fatigue properties, and the like. These performance improvements, relying solely on the improvement and improvement of the matrix material, are very limited and uneconomical, and surface treatment techniques can often achieve twice the result with half the effort, which is why surface treatment technology has developed rapidly.
The surface treatment technology of the mold is to systematically change the morphology, chemical composition, microstructure and stress state of the mold surface by surface coating, surface modification or composite treatment technology to obtain the desired surface properties. From the way of surface treatment, it can be divided into: chemical methods, physical methods, physical and chemical methods, and mechanical methods. Although new treatment technologies are being developed to improve the surface properties of molds, most of the major nitriding, carburizing and hardening film deposits are used in mold manufacturing.
The nitriding process includes gas nitriding, ion nitriding, liquid nitriding and the like. In each type of nitriding method, there are several kinds of nitriding techniques, which can adapt to the requirements of different workpieces of different steel types. 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, and the nitriding temperature does not require intense cooling after low nitriding, the deformation of the mold is extremely small, so the surface strengthening of the mold The use of nitriding technology is earlier and the most widely used.
The purpose of mold carburizing is mainly to improve the overall strength and toughness of the mold, that is, the working surface of the mold has high strength and wear resistance. The technical idea introduced here is to reduce the manufacturing costs by replacing the higher-grade materials with lower-grade materials, ie by carburizing and quenching.
The most mature sclerosing film deposition technology is CVD and PVD. In order to increase the bonding strength of the surface of the film workpiece, various enhanced CVD, PVI) technologies have 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 have been coated with a cured film as a standard process. Molds 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 still 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. If more molds adopt this technology, they can improve the overall level of mold manufacturing in China.
Third, the pre-hardening technology of the mold material
The heat treatment of the mold during the manufacturing process is a process that most molds use for a long time. Since the 1970s, the idea of pre-hardening has been proposed internationally, but due to the rigidity of the machine tool and the constraints of the cutting tool, the pre-hardening The hardness of the mold cannot reach the hardness of the mold, so the research and development of the pre-hardening technology is not large. With the improvement of the performance of machine tools and cutting tools, the development of pre-hardening technology for mold materials has accelerated. By the 1980s, the proportion of pre-hardened modules used in plastic molds in industrialized countries in the world has reached 30% (currently More than 60%). In the mid-to-late 1990s, China began to use pre-hardened modules (mainly imported products).
Pre-hardening technology for mold materials is mainly developed and implemented by mold material manufacturers. By adjusting the chemical composition of the steel and equipped with the corresponding heat treatment equipment, it is possible to mass produce pre-hardened modules of stable quality. In China, the pre-hardening technology of mold materials started late and has a small scale. At present, it cannot meet the requirements of domestic mold manufacturing.
The use of pre-hardened mold materials can simplify the mold manufacturing process, shorten the manufacturing cycle of the mold, and improve the manufacturing precision of the mold. It is foreseeable that as processing technology advances, pre-hardened mold materials will be used for more mold types.