Current status and trend of mold heat treatment

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 structure transformation and excessive residual stress formed by heat treatment cause deformation of the mold during processing, assembly and mold use after heat treatment, thereby reducing the accuracy of the mold, or even scrapping.

Mould strength: Improper heat treatment process formulation, irregular heat treatment operation or incomplete heat treatment equipment condition causes the strength (hardness) of the mould to be processed to fail to meet the design requirements.

The working life of the mold: the unreasonable structure and excessive grain size caused by the heat treatment lead to the decline of the main properties such as the toughness of the mold, the cold and hot fatigue performance, and the wear resistance, which affect the working life of the mold.

Mold manufacturing cost: As an intermediate or final process of the mold manufacturing process, the cracking, deformation and performance caused by heat treatment will cause the mold to be scrapped in most cases, and it will continue to be used even through repairs, which will increase the working hours. , Extend delivery time and increase mold manufacturing cost.

It is the heat treatment technology that is closely related to the mold quality that makes these two technologies mutually promote and improve together in the process of modernization. Since the 1980s, the rapid development of international mold heat treatment technology has been vacuum heat treatment technology, mold surface strengthening technology and mold material pre-hardening technology.

Mold vacuum heat treatment technology

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 heating oxidation and non-decarburization, vacuum degassing or degassing, and eliminating Hydrogen embrittlement 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 parts determine 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 quenching, vacuum water cooling and vacuum nitrate austempering. The main applications of mold vacuum heat treatment are vacuum oil quenching, vacuum quenching and vacuum tempering. In order to maintain the excellent characteristics of the vacuum heating of the workpiece (such as the mold), the selection and formulation of the coolant and the cooling process are very important. The mold quenching process mainly uses oil cooling and air cooling.

For mold working surfaces that are no longer machined after heat treatment, vacuum tempering should be used as much as possible after quenching, especially vacuum-quenched workpieces (molds), which can improve mechanical properties related to surface quality, such as fatigue performance, surface brightness, And corrosive etc.

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 (or even a single piece), multi-variety characteristics of mold production, high requirements for heat treatment performance and the characteristics of not allowing waste products, intelligent heat treatment of molds becomes necessary. The intelligent heat treatment of the mold includes: clarifying the mold structure, materials, and heat treatment performance requirements; computer simulation of the temperature field and stress field distribution of the mold heating process; computer simulation of the temperature field, phase change process and stress field distribution of the mold cooling process; heating and Simulation of cooling process; formulation of quenching process; automatic control technology of heat treatment equipment. Foreign industrialized countries, such as the United States, Japan, etc., have carried out technology research and development in this area in terms of vacuum and high-pressure gas quenching, mainly for molds.

Mold surface treatment technology

In addition to the reasonable coordination of the matrix with sufficiently high 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: wear resistance, corrosion resistance, friction coefficient, fatigue performance, etc. The improvement of these properties is very limited and uneconomical by relying solely on the improvement and improvement of the matrix material, and through surface treatment technology, you can often get twice the result with half the effort, which is the reason why surface treatment technology has developed rapidly.

The surface treatment technology of the mold is a systematic engineering of changing the morphology, chemical composition, structure and stress state of the mold surface through surface coating, surface modification or composite treatment technology to obtain the required surface properties. From the surface treatment method, it can be divided into: chemical method, physical method, physical chemical method and mechanical method. Although new processing technologies aimed at improving the surface properties of molds are emerging, the most common applications in mold manufacturing are nitriding, carburizing and hardened film deposition.

The nitriding process includes gas nitriding, ion nitriding, liquid nitriding, etc. In each nitriding method, there are several nitriding technologies, which can meet the requirements of different steel types and different workpieces. Because the nitriding technology can form a surface with excellent performance, and the nitriding process and the quenching process of the die steel have good coordination, at the same time, the nitriding temperature is low, no intense cooling is required after nitriding, and the deformation of the mold is extremely small. Surface strengthening is the earlier nitriding technology and the most widely used.

The purpose of mold carburization 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 from this is to use lower grade materials, that is, through carburizing and quenching. To replace higher-level materials, thereby reducing manufacturing costs.

The more mature hardened film deposition technologies are CVD and PVD. In order to increase the bonding strength of the surface of the film layer, a variety of enhanced CVD and PVD technologies have now been developed. Hardened film deposition technology was first applied to tools (knives, cutting tools, measuring tools, etc.), with excellent results. Many kinds of tools have adopted hardened film coating 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 relatively high, and it is still only applied to some precision and long-life molds. If a heat treatment center is established, the cost of coating hardened film will be greatly reduced. , If more molds adopt this technology, the overall level of mold manufacturing in my country can be improved.

Pre-hardening technology of mold materials

Heat treatment of molds in the manufacturing process is a process that most molds have used for a long time. Since the 1970s, the idea of ​​pre-hardening has been proposed internationally. However, due to the constraints of the rigidity of the processing machine tool and the cutting tool, the pre-hardening The hardness cannot reach the hardness of the mold, so the research and development investment 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 internationally industrialized countries that used pre-hardened modules on plastic mold materials had reached 30% (currently Above 60%). my country began to use pre-hardened modules (mainly imported products) in the mid to late 1990s.

The pre-hardening technology of mold materials is mainly developed and implemented by mold material manufacturers. By adjusting the chemical composition of the steel and equipping the corresponding heat treatment equipment, it is possible to mass-produce pre-hardened modules with stable quality. In terms of pre-hardening technology for mold materials, my country has a late start and small scale. Currently, it 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 manufacturing accuracy of the mold. It is foreseeable that with the advancement of processing technology, pre-hardened mold materials will be used in more mold types.