5 Mold heat treatment
The cost of mold manufacturing is high, especially for some precision and complex cold die, plastic mold, die-casting mold, etc. The use of heat treatment technology to improve the performance of the mold can greatly increase the life of the mold and has significant economic benefits. my country’s mold technicians attach great importance to the development of mold heat treatment technology.
5.1 Vacuum heat treatment [19, 20]
After vacuum heat treatment, the die steel has a good surface condition and small deformation. Compared with quenching in the atmosphere, the surface hardening of the mold after vacuum oil quenching is more uniform and slightly higher. The main reason is that the surface of the mold steel is in an active state during vacuum heating, without decarburization, and does not produce an oxide film that hinders cooling. When heated under vacuum, the surface of the steel has a degassing effect, so it has higher mechanical properties. The higher the vacuum in the furnace, the higher the bending strength. After vacuum quenching, the fracture toughness of steel is improved, and the life of the mold is generally increased by 40%”400% or even higher than that of the conventional process. Cold work mold vacuum quenching technology has been widely used.
5.2 Cryogenic treatment 
Research work in recent years has shown that cryogenic treatment (-196°C) of die steel can improve its mechanical properties, and some molds can significantly increase their service life after cryogenic treatment. The deep cooling of the die steel can be carried out between the quenching and tempering processes, or the deep cooling treatment can be carried out after quenching and tempering. If there is retained austenite in the steel after quenching and tempering, it is necessary to perform another tempering after the cryogenic treatment. Cryogenic treatment can improve the wear resistance and tempering stability of steel. Cryogenic treatment is not only used for cold work molds, but also for hot work molds and cemented carbide. Cryogenic treatment technology has attracted more and more attention from mold heat treatment workers, and special cryogenic treatment equipment has been developed. The microstructure changes of different steel grades in the cryogenic process and their micromechanisms and their effects on mechanical properties need further study.
5.3 High temperature quenching and cooling quenching of molds
Some hot work die steels, such as 3Cr2W8V, H13, 5CrNiMo, 5CrMnMo, etc., can be quenched by heating at a temperature higher than the conventional quenching temperature, which can reduce the number of carbides in the steel, improve its shape and distribution, and make the carbon solid-soluble in austenite The distribution is uniform, and more lath martensite can be obtained in the steel after quenching, and its fracture toughness and cold and hot fatigue resistance can be improved, thereby extending the service life of the mold. For example, a hot extrusion die made of 3Cr2W8V steel, the conventional quenching temperature is 1080″1120℃, and the tempering temperature is 560″580℃. When the quenching temperature is increased to 1200°C and the tempering temperature is 680°C (2 times), the mold life is increased several times.
For high-alloy cold-work die steels such as W6Mo5Cr4V2, W18Cr4V high-speed steel and Cr12MoV, its quenching temperature can be appropriately reduced to improve its plastic toughness, reduce brittle cracking tendency, and increase die life. For example, the quenching temperature of W6Mo5Cr4V2 can be 1140″1160℃.
5.4 Chemical heat treatment [23, 24]
Chemical heat treatment can effectively improve the wear resistance, corrosion resistance, seizure resistance and oxidation resistance of the mold surface. Almost all chemical heat treatment processes can be used for the surface treatment of die steel.
Research work shows that high-carbon and low-alloy tool steels and medium-high-carbon high-alloy steels can be carburized or carbonitrided. When high-carbon low-alloy steel is carburized or carbonitrided, lower heating temperature and shorter holding time should be selected as much as possible. At this time, it can ensure that the surface layer has more undissolved carbide cores, carburizing and carbonitriding After co-infiltration, the surface carbides are granular, and the total volume of carbides also increases significantly, which can increase the wear resistance of steel. After carburizing of W6Mo5Cr4V2 and 65Nb steel molds and vacuum carburizing of 65Nb steel molds, the life of the molds has been significantly improved.
The alloy steel molds with high temperature tempering of 500″650℃ can be used for surface nitriding or nitrocarburizing in the range below the tempering temperature or at the same time of tempering.
The nitriding process currently uses ion nitriding, high-frequency nitriding and other processes. Ion nitriding can shorten the nitriding time and obtain a high-quality nitriding layer. Ion nitriding can improve the corrosion resistance, wear resistance, thermal fatigue resistance and adhesion resistance of the die-casting mold.
Nitrocarburizing can be carried out in gas or liquid media, the brittleness of the infiltration layer is small, and the time of infiltration is much shorter than that of nitriding. The thermal fatigue performance of die-casting mold and hot extrusion mold can be significantly improved after nitrocarburizing. Nitrocarburizing has good application effects on cold heading dies, cold extrusion dies, cold punching dies, and drawing dies.
Cold work molds and hot work molds can also be sulfur-nitrogen or sulfur-nitrocarburizing. In recent years, many research works have shown that rare earths have obvious catalyzing effects, and new technologies such as rare earth nitriding and rare earth nitrocarburizing have been developed.
5.5 Boronizing and metalizing[23, 24, 25]
Boronizing can be solid boronizing, liquid boronizing, paste boronizing, etc. The most widely used is solid boronizing, and solid boronizing agents are already available on the market. After solid boronizing, the hardness of the surface layer is as high as 1400`2800HV, with high wear resistance, corrosion resistance and oxidation resistance.
Boronizing process is often used in various cold work molds. Due to the improvement of wear resistance, the life of the mold can be increased several or more than ten times. Boronizing with medium carbon steel can sometimes replace high-alloy steel to make molds. Boronizing can also be applied to hot work molds, such as hot extrusion molds.
The boronized layer is brittle, the diffusion layer is relatively thin, and the supporting force for the infiltrated layer is weak. For this reason, boron-nitriding or boron-carbonitriding can be used to strengthen the transition zone and make its hardness change smoothly. To improve the brittleness of the boronized layer, boron-vanadium and boron-aluminum co-infiltration can be used.
Metal infiltration, including chromium infiltration, vanadium infiltration, and titanium infiltration, can be used to treat cold work and hot work molds. Among them, the TD method (molten salt infiltration) has been used in some applications, which can increase the life of the mold several times or even more than ten times.
5.6 Vapor deposition [23, 25]
According to the basic principle of formation, vapor deposition is divided into physical vapor deposition (PVD) and chemical vapor deposition (CVD).
PVD is divided into vacuum evaporation, sputtering and ion plating. Ion plating is a technology that combines vapor deposition and sputtering. Ion plating has the advantages of strong adhesion, good throwing power, and wide matching of the base material to be plated and the plating material, so it is widely used. In recent years, multi-arc ion plating has attracted people’s attention. At present, ion plating TiN is widely used on molds. This kind of film not only has high hardness, but also has good toughness, strong bonding force and high temperature resistance. Multi-element films developed on the basis of TiN, such as (TiAl)N, (TiCr)N, etc., have better performance than TiN and are a new type of more promising film.
CVD is a method that chemically reacts the reaction gas on the surface of the base material to form a covering layer (TiC, TiN). There are many methods for CVD. Generally, the reaction temperature of CVD is above 900°C, and the hardness of the coating reaches above 2000HV. However, the high temperature easily deforms the workpiece and the interface of the deposited layer is prone to reaction. The development trend is to lower the temperature and develop new coating components. For example, metal organic compound CVD (MOCVD), laser CVD (LCVD), plasma CVD (PCVD), etc.
5.7 High-energy beam heat treatment
The heat source of high-energy beam heat treatment usually refers to laser, electron beam, ion beam, etc. Their common feature is: the surface power density of the supplied material is at least 103W/cm2. Their common characteristics are: fast heating speed, heating area can be selected according to needs, workpiece deformation is small, no cooling medium is needed, processing environment is clean, controllable performance is good, and it is easy to realize automatic processing. There has been a lot of research on the principle and technology of high-energy beam heat treatment at home and abroad. The more mature ones are laser phase change hardening, small-size electron beam treatment and medium-power ion implantation, and they have been applied in improving mold life.