Mold materials and mold thermal operation methods

Mold material selection is a very important link in the entire mold making process. Mold material selection needs to meet three principles. The mold should meet work requirements such as wear resistance and strength and toughness. The mold should meet process requirements. At the same time, the mold should meet economic applicability.

(1) The mold meets the working conditions requirements

1. Wear resistance When the blank plastically degenerates in the mold cavity, it both flows and slides along the cavity surface, causing severe friction between the cavity surface and the blank, causing the mold to fail due to wear. Therefore, the wear resistance of materials is one of the most basic and important properties of molds. Hardness is the main factor affecting wear resistance. Generally speaking, the higher the hardness of the mold parts, the smaller the wear amount and the better the wear resistance. In addition, wear resistance is also related to the type, quantity, shape, size and distribution of carbides in the material. ​

2. Strong toughness Most of the working conditions of molds are very harsh, and some often bear large impact loads, resulting in brittle fracture. In order to prevent mold parts from suddenly breaking during operation, the mold must have high strength and toughness. The toughness of the mold mainly depends on the carbon content, grain size and organizational state of the material. ​

3. Fatigue fracture performance During the working process of the mold, fatigue fracture is often caused under the long-term action of cyclic stress. Its forms include small energy multiple impact fatigue fracture, tensile fatigue fracture, contact fatigue fracture and bending fatigue fracture. The fatigue fracture performance of the mold mainly depends on its strength, toughness, hardness, and the content of inclusions in the material. ​

4. High temperature performance: When the working temperature of the mold is higher, the hardness and strength will decrease, leading to early wear or plastic deformation of the mold and failure. Therefore, the mold material should have high anti-tempering stability to ensure that the mold has high hardness and strength at the working temperature. ​

5. Heat and cold fatigue resistance: Some molds are in a state of repeated heating and cooling during work, which causes the surface of the cavity to undergo tension and pressure changes, causing surface cracks and peeling, increasing friction, and hindering plastic deformation. Dimensional accuracy is reduced, resulting in mold failure. Hot and cold fatigue is one of the main forms of failure of hot work molds. This type of mold should have high resistance to cold and hot fatigue. ​

6. Corrosion resistance: When some molds, such as plastic molds, are in operation, due to the presence of chlorine, fluorine and other elements in the plastic, highly corrosive gases such as HCI and HF are decomposed after heating, eroding the surface of the mold cavity and increasing its surface roughness. , aggravating wear and failure. ​

(2) The mold meets process performance requirements. The manufacturing of molds generally goes through several processes such as forging, laser cutting titanium, and heat treatment. In order to ensure the manufacturing quality of the mold and reduce the production cost, its material should have good forgeability, machinability, hardenability, hardenability and grindability; it should also have small oxidation, decarburization sensitivity and quenching Tendency to deform and crack. ​

1. Forgeability: It has low hot forging deformation resistance, good plasticity, wide forging temperature range, and low tendency of forging cold cracking and network carbide precipitation.

2. Annealing processability: The spheroidizing annealing temperature range is wide, the annealing hardness is low and the fluctuation range is small, and the spheroidizing rate is high. ​

3. Cutting processability: Large cutting amount, low tool loss, and low processing surface roughness.

4. Sensitivity to oxidation and decarburization: It has good antioxidant capacity when heated at high temperatures, has slow decarburization speed, is not sensitive to heating media, and has a small tendency to produce pitting. ​

5. Hardenability: It has uniform and high surface hardness after quenching. ​

6. Hardenability: A deeper hardened layer can be obtained after quenching, and it can be hardened using a gentle quenching medium. ​

7. Quenching deformation and cracking tendency: Conventional quenching has small volume changes, slight shape warping and distortion, and low abnormal deformation tendency. Conventional quenching has low cracking sensitivity and is not sensitive to quenching temperature and workpiece shape. ​

8. Grindability: The relative loss of the grinding wheel is small, the limit grinding amount without burns is large, it is not sensitive to the quality of the grinding wheel and cooling conditions, and it is not prone to wear and grinding cracks. ​

(3) The mold meets economic requirements. When selecting materials for the mold, the principle of economy must be considered to reduce manufacturing costs as much as possible. Therefore, on the premise of satisfying the performance, first choose the one with lower price. If you can use carbon steel, you will not need alloy steel. If you can use domestic materials, you will not need imported materials. In addition, the production and supply situation of the market should also be considered when selecting materials. The selected steel types should be as small as possible, concentrated and easy to purchase.
In the 1950s, all the steel used for molds in our country followed foreign steel grades. In the 1960s, in order to save raw materials and improve blank accuracy, cutting technology and precision forming technology developed rapidly. In order to improve production efficiency, many efficient pressure processing equipment were used, and die forging hammers were gradually replaced by presses. The performance of the original mold steel often cannot meet the high performance requirements of service conditions, which affects the service life of the mold and the promotion and application of new pressure processing techniques and equipment. In the late 1970s, precision and large-scale engineering plastic products were increasingly used, and the demand for steel for plastic molds increased sharply, which also put forward new requirements for the performance of plastic mold steel. However, there was no special steel for plastic molds in my country at that time. ​

Since the 1960s, with the support of relevant national ministries and commissions, Chinese scientific and technological workers have developed many new mold steels based on national conditions. Some of these new steel types have excellent performance and relatively good process performance and have been favored by mold manufacturing and user units. welcome. During this period, some commonly used foreign steel grades were also introduced, some of which passed production trials and achieved good results. For some cold work die steels and hot work die steels with better use effects, relevant departments also organized comparative performance test studies and put forward suggestions for selection and application. In order to meet the needs of high wear-resistant and long-life molds, my country’s cemented carbide developed rapidly in the late 1950s. At the same time, a variety of steel-bonded cemented carbide was also developed, which have achieved good results when used as molds. ​

This article is divided into six parts: cold work mold steel, hot work mold steel, plastic mold steel, cemented carbide and steel-bonded cemented carbide, mold heat treatment, prospects and suggestions. ​

Cold work die steel Currently, the commonly used cold work die steels in my country are still old steel grades such as low alloy tool steel CrWMn and high carbon high chromium tool steel Cr12MoV and Cr12. CrWMn steel has appropriate hardenability and wear resistance, and has small heat treatment deformation. However, after drop forging, CrWMn steel needs to strictly control the cooling rate and use appropriate heat treatment to make the carbides uniform and fine granular and distributed on the matrix. Otherwise, it is easy to form network carbides, causing chipping and cracking of the mold during use. High carbon and high chromium tool steel has high wear resistance, but its carbide segregation is serious, resulting in a decrease in the directionality of deformation and strength and toughness. The degree of segregation can be improved to a certain extent by repeated upsetting. ​

In 1981, my country introduced the internationally accepted high-carbon, high-chromium tool steel D2 (Cr12Mo1V1). Compared with Cr12MoV steel, the carbide segregation of D2 steel is slightly improved compared to Cr12MoV, and the strength and toughness are slightly improved. The service life of molds made of D2 steel is also improved to varying degrees. High-speed steel (mainly W6Mo5Cr4V2 and W18Cr4V) has higher wear resistance and strength and is often used to make molds, but its toughness cannot meet the needs of complex, large-scale and impact-loaded molds. ​

In order to improve the strength and toughness of this type of steel, our country has developed some new cold work die steels.

Such as: 1 Low alloy cold work die steel

The main features of this type of steel are good craftsmanship, low quenching temperature, small heat treatment deformation, good strength and toughness, and appropriate wear resistance. Such as GD (6CrMnNiMoVSi), 7CrSiMnMoV (CH for short), DS steel, etc. GD steel is used to make cold stamping molds that are prone to chipping and breakage and have a long service life. The composition of CH steel is the same as Japan’s SX105V steel. It is a flame-quenched steel that is often used to make mold parts for production lines such as automobiles. During flame quenching, the cutting surface of the mold edge is heated, and there is a high toughness under the hardened layer. The base body is used as a liner, so that the mold has a higher service life. DS steel is an impact cold work die steel whose impact toughness is significantly better than the commonly used tool steel 6CrW2Si for high-toughness blades. ​

2 Matrix steel Matrix steel generally refers to steel whose composition is the same as the chemical composition of the matrix in the quenching structure of high-speed steel. In the early 1970s, the United States and Japan studied matrix steels with grades VascoMA, VascoMatrixI and MOD2, which are equivalent to the matrix of M2 and M36 high-speed steel, but they have not been widely used. Our country has developed some base steels, such as 65Cr4W3Mo2VNb (65Nb for short), 65W8Cr4VTi (LM1 for short), 65Cr5Mo3W2VSiTi (LM2 for short) steel, etc. The main feature of these matrix steels is that their carbon content is slightly higher than that of the matrix to increase the amount of primary carbides and improve wear resistance. A small amount of strong carbide-forming elements niobium or titanium is also added to form a relatively stable carbide. It prevents the growth of grains during quenching and heating and improves the process performance of steel. This type of base steel has been widely used to make cold extrusion, thick plate cold punching, cold heading and other molds. It is especially suitable for large and complex molds for difficult-to-deform materials. It can also be used as warm extrusion molds for ferrous metals. 3. Wear-resistant cold work die steel with high toughness

In order to improve the carbide segregation of Cr12 cold work die steel, improve its toughness, and further increase the wear resistance of the steel, my country has done a lot of research work and developed many new steel types, such as LD, ER5 and GM steel. wait. In these steels, the chromium content is appropriately reduced to improve carbide segregation, and the tungsten, molybdenum and vanadium content are increased to increase the secondary hardening ability and improve wear resistance. Compared with Cr12 cold work die steel, this type of steel has improved carbide segregation and has higher toughness. This type of steel has better wear resistance than Cr12 cold work die steel, so the molds produced have a longer life and are more suitable for use in high-speed punches and multi-station punches. Hot work die steel Commonly used hot work die steels in my country are 5CrMnMo, 5CrNiMo and 3Cr2W8V steel. ​

5CrNiMo steel is mainly used for large and medium-sized hot forging dies. However, its hardenability is not high enough, its tempering stability is not high, and its performance cannot meet the performance requirements of large-section forging dies. 3Cr2W8V steel is widely used as hot extrusion dies for ferrous and non-ferrous metals and die-casting dies for Cu and Al alloys. This kind of steel has high thermal stability and can be used at temperatures up to 650°C. However, tungsten hot work die steel has low thermal conductivity and poor thermal fatigue resistance. ​

In the early 1980s, my country introduced H13 (4Cr5MoSiV1), a commonly used foreign chromium-based hot work die steel. H13 steel has good thermal and cold fatigue properties. When the operating temperature does not exceed 600°C, it can replace 3Cr2W8V steel and the die life has been greatly improved. , so H13 steel has been rapidly promoted and applied, and its output has exceeded 3Cr2W8V steel. In order to adapt to the higher requirements for mold steel in terms of strength, toughness and thermal stability of new pressure processing technologies and new equipment, my country has developed a number of new hot work mold steels, mainly including:

1 Hot forging die steel

In China, in the 1980s, a large number of analyzes, comparisons and studies were done on domestic and foreign steels in view of the problem that the hardenability of 5CrNiMo steel could not meet the needs of large-section hammer forging dies and the operating temperature did not exceed 500°C. Research work shows that the Cr, Ni, and Mo contents in similar foreign steel 5CrNiMoV are higher than domestic 5CrNiMo steel, and contain a small amount of V. Therefore, its hardenability and tempering stability are higher than domestic 5CrNiMo steel, and it is recommended to use 5CrNiMoV Steel, used to make large, complex, heavy-duty hammer forging dies. ​

Large-section hot forging die steels 5Cr2NiMoVSi and 45Cr2NiMoVSi have also been developed in China, which have been widely used. Compared with 5CrNiMo steel, the carbon content in these steels is slightly lower, the Cr and Mo contents are increased and appropriate V and Si are added, so they have high hardenability and thermal stability. The carbon and silicon in 45Cr2NiMoVSi steel are slightly lower than those in 5Cr2NiMoVSi steel, making it more suitable for hammer forging dies. This kind of steel is used to manufacture forging dies for mechanical presses above 4000t and forging hammer dies above 3t. Its service life is 0.5~1.5 times longer than that of 5CrNiMo and 5CrNiMoV. 3Cr2MoWVNi steel is also a hot forging die steel developed in my country and has a long service life. ​

2Die steel for hot extrusion

H13 is a widely used hot work die steel at home and abroad. It has good thermal and cold fatigue properties when the operating temperature does not exceed 600°C. It is used as hot extrusion dies and aluminum alloy die-casting dies and has a relatively long service life. However, H13 steel has a large size effect. In foreign countries, processes such as out-of-furnace refining, high-temperature diffusion annealing, and 60w led lighting metal forging parts are used to improve its size effect and reduce the composition segregation of Cr and Mo. In China, processes such as electroslag remelting are mostly used. .

Our country has developed many hot work die steels with good strength, toughness and high thermal stability for hot extrusion. Some steels are developed on the basis of foreign molybdenum series 3Cr3Mo3V steel and chromium series H13 steel, and have certain characteristics in alloying, such as HMI (3Cr3Mo3W2V), TM (4Cr3Mo2WMnVNb), Y4 (4Cr3Mo2MnVB), Y10 (4Cr5Mo2SiV1), HD2 (4Cr3Mo2VNiNbB), 012Al (5Cr4Mo3SiMnVAl) and other steels. These steels have high thermal stability while maintaining good strength and toughness, and are used to make hot extrusion dies, precision

Forging dies, non-ferrous metal die-casting dies, etc., have good use results. ​

Relevant departments in our country have organized some research units and users to select 27 domestic and foreign applied and newly developed hot work die steels, test and compare their basic mechanical properties, process properties and service performance, and put forward various types of hot work die steels. Material selection criteria for hot work molds. ​

Plastic Mold Steel The output value of plastic molding molds has occupied the first place in the total output value of the mold industry. China used to have no special 1008 steel for plastic molds. In recent years, while introducing foreign steel for plastic molds, we have independently researched and developed some new special steels for plastic molds.

1Pre-hardened plastic mold steel

This type of steel is made into modules after being fully forged in the steel factory, and is pre-heated to the required hardness (generally pre-hardened to 30~35RHC) before being used for mold making by the user. P20 (ie 3Cr2Mo) is the most widely used pre-hardened plastic mold steel abroad. It has been included in my country’s alloy tool steel standards and has been widely used in some factories in my country since the 1980s. 718 is a modified P20 steel produced in Sweden. It has higher hardenability than P20. After quenching and tempering, it can maintain uniform hardness in large cross-sections. It is also widely used in our country. ​

2. Easy cutting pre-hardened steel

In order to improve the cutting performance of pre-hardened plastic mold steel, easy-cutting elements can be added. The United States, Japan, and Germany have all developed some free-cutting pre-hardened steels. Foreign free-cutting pre-hardened steels are mainly S series, but also include S-Se series and Ca series. But Se is more expensive. The anisotropy of S-series free-cutting steel is relatively large, and as the cross-section increases, the segregation of sulfides becomes more serious. ​

China has developed some sulfur-containing easy-cutting pre-hardened plastic mold steels, such as 8Cr2MnWMoVS (8Cr2S) and S-Ca composite easy-cutting plastic mold steel 5CrNiMnMoVSCa (5NiSCa). 5NiSCa steel uses S-Ca composite free-cutting system and spray metallurgy technology to improve the morphology and distribution of sulfides and the anisotropy of the steel. The distribution of sulfides is still relatively uniform in large cross-sections. 5NiSCa steel has high hardenability and mirror polishability. When the mold hardness is 35~45HRC, various processing can be carried out smoothly. ​

3Non-quenched and tempered plastic mold steel

This kind of steel can reach pre-hardened hardness after titanium forging and rolling without scheduling treatment, which is beneficial to saving energy, reducing costs and shortening the production cycle. This type of steel developed in our country includes: medium carbon manganese boron air-cooled bainitic steel, which can be used to make plastic molds and rubber molds; non-quenched and tempered plastic mold steel 2Mn2CrVTiSCaRe (FT), S, Ca, and Re are added to the steel as easy Cutting elements, better cutting performance than S-Ca composite free-cutting steel; low-carbon MnMoVB series non-quenched and tempered bainitic large-section plastic mold steel (B30), S and Ca are added to the steel as free-cutting elements, industrial Trial production shows that the hardness of the 400mm thick slab is air-cooled after hot rolling and is relatively uniform along the cross-section. ​

4Age hardened steel

Our country has developed several low-nickel age-hardened steels. These steels are mechanically processed after quenching and tempering, and then aged to increase the hardness by precipitating intermetallic compounds. The deformation after heat treatment is very small. Age-hardened steel is suitable for making high-precision plastic molds, transparent plastic molds, etc. ​

This type of steel includes 25CrNi3MoAl, 10Ni3Mn2AlCu (PMS) and 06Ni6CrMoVTiAl steel. After quenching and tempering, these steels have a hardness of 20~30HRC and can be machined. After aging, the hardness can reach 38~42HRC. ​

5 Corrosion-resistant plastic mold steel

When plastic products are made from chemically corrosive plastics, the mold must have anti-corrosion properties. Corrosion-resistant 1018 steel is generally used to make the mold. In this case, good wear resistance is also required. Commonly used steel grades are 4Cr13 (420), 9Cr18, and 17-4PH. PCR (0Cr16Ni4Cu3Nb) is a corrosion-resistant plastic mold steel developed in my country. It has good comprehensive mechanical properties and good corrosion resistance.

Cemented carbide and steel-bonded cemented carbide Cemented carbide is a type of composite material manufactured by powder metallurgy. Carbide has high hardness, good wear resistance, high elastic modulus and high operating temperature. Used to make certain molds, the service life of the mold can be increased several times or dozens of times. However, cemented carbide is brittle, has poor bending strength and toughness, and cannot be machined. As a mold material, cemented carbide is mainly used in wire drawing dies, cold extrusion and cold punching dies that are not subject to high impact force. At present, our country can produce various grades of cemented carbide, which can basically meet the needs of the domestic market. ​

In order to meet the needs of manufacturing micro drills for drilling integrated circuit boards, dot matrix printing needles for computers, precision tools and molds, etc., in recent years, various countries have developed some microcrystals (WC grains less than 1 micron) and ultra-fine grains. Grain cemented carbide (WC grain size is less than 0.6 microns). In traditional cemented carbide, the WC grain size is 1.3 to 1.5 microns. Ultra-fine grained cemented carbide makes up for many shortcomings of conventional cemented carbide, expands its application scope, and has achieved good results in manufacturing wear-resistant and impact-resistant molds. Some research units and cemented carbide factories in my country have developed various brands of microcrystalline cemented carbide and ultra-fine grained cemented carbide. The development of high-performance ultra-fine grained cemented carbide is still a hot spot in cemented carbide research. Steel-bonded cemented carbide is a composite material composed of carbide as the hard phase and steel as the bonding phase. Steel-bonded cemented carbide has good wear resistance, its strength and toughness are generally higher than cemented carbide, and it has process properties such as heat treatability, machinability, forgeability and weldability. Molds are the main application areas of 1045 steel-bonded carbide. Our country began to develop this material in the 1960s, and has developed various grades of steel-bonded cemented carbide. It is used as steel-bonded cemented carbide for molds. The hard phase mainly uses TiC and WC, and the matrix of steel mainly uses low-alloy chromium. Molybdenum steel, medium and high alloy tool steel or high speed steel, such as TiC series GT35, R5, D1, T1 and WC series TLMW50, GW50, GJW50. Steel-bonded carbide has been used to make cold heading dies, extrusion dies, drawing dies, blanking dies, wire drawing dies, hot heading dies, etc. ​

The development of powder metallurgy technology and the application of hot isostatic pressing led to the production and use of segregation-free powder high-speed steel in the 1970s. Its main features are strength, toughness, grindability, isotropy, and heat treatment processability that are better than ordinary ones. High speed steel and has a relatively long service life. In the future, this technology will be used to produce high-carbon, high-vanadium, high-wear-resistant cold mold steel that cannot be produced by conventional processes. This type of steel has better cutting processability and grinding performance, and has better toughness. The service life of the mold made is as long as Some carbide alloys are similar. Various brands of powder metallurgy high wear-resistant cold die steel have been produced abroad, but there is still little research in China. ​

Mold heat treatment

The cost of mold manufacturing is high, especially some sophisticated and complex cold stamping dies, plastic molds, die-casting molds, etc. Using heat treatment technology to improve the performance of molds can greatly increase the life of the mold and have significant economic benefits. my country’s mold technicians attach great importance to the development of mold heat treatment technology. ​

1 Vacuum heat treatment

The mold steel has good surface condition and small deformation after vacuum heat treatment. Compared with quenching in the atmosphere, the surface hardening of the mold after vacuum oil quenching is relatively uniform and slightly higher. The main reason is that during vacuum heating, the surface of the mold steel is active, does not decarburize, 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 flexural strength. After vacuum quenching, the fracture toughness of steel is improved, and the mold life is generally increased by 40% to 400% or even higher than that of conventional processes. Cold work mold vacuum quenching technology has been widely used. ​

2Cryogenic treatment

Research work in recent years has shown that cryogenic treatment (-196°C) of mold steel can improve its mechanical properties. Some molds have significantly improved their service life after cryogenic treatment. The cryogenic treatment of mold steel can be carried out between the quenching and tempering processes, or it can be cryogenically treated after quenching and tempering. If residual austenite still remains in the steel after quenching and tempering, it will still need to be tempered again after 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 structural changes of different steel types during cryogenic cooling, their microscopic mechanisms and their impact on mechanical properties require further research.

3. High-temperature quenching and cooling quenching of the mold. Some hot-work mold steels, such as 3Cr2W8V, H13, 5CrNiMo, 5CrMnMo, etc., are heated and quenched at a temperature higher than conventional quenching, which can reduce the number of carbides in the steel, improve their shape and distribution, and make the solid The distribution of carbon dissolved in austenite is uniformized, and more lath martensite can be obtained in the steel after quenching, improving its fracture toughness and hot and cold fatigue resistance, thereby extending the service life of the mold. For example, for 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 mold steels such as W6Mo5Cr4V2, W18Cr4V high-speed steel and Cr12MoV, the quenching temperature can be appropriately lowered to improve its plastic toughness, reduce the tendency of brittle cracking, and thereby increase the mold life. For example, the quenching temperature of W6Mo5Cr4V2 can be 1140~1160℃. 4. Chemical heat treatment Chemical heat treatment can effectively improve the wear resistance, corrosion resistance, anti-seize, oxidation resistance and other properties of the mold surface. Almost all chemical heat treatment processes can be used for surface treatment of mold steel.

Research work shows that both high-carbon and low-alloy tool steels and medium-high carbon and high-alloy steels can be carburized or carbonitrided. When carburizing or carbonitriding high-carbon low-alloy steel, a lower heating temperature and a shorter holding time should be selected as much as possible. This can ensure that there are more undissolved carbide cores on the surface, and carburizing and carbonitriding After co-penetration, the surface carbides become granular, and the total volume of carbides also increases significantly, which can increase the wear resistance of the1137  steel. After carburizing W6Mo5Cr4V2 and 65Nb steel molds and vacuum carburizing of 65Nb steel molds, the life of the molds is significantly improved. ​

For alloy steel molds tempered at high temperatures of 500~650℃, surface nitriding or nitrocarburizing can be carried out in a range lower than the tempering temperature or while tempering. ​

The nitriding process currently uses processes such as ion nitriding and high-frequency nitriding. Ion nitriding can shorten the nitriding time and obtain a high-quality nitrided layer. Ion nitriding can improve the corrosion resistance, wear resistance, thermal fatigue resistance and anti-adhesion properties of the die-casting mold. ​

Nitrocarburizing can be carried out in gas medium or liquid medium. The brittleness of the carburized layer is small, and the co-carburizing time is much shorter than the nitriding time. The thermal fatigue performance of die-casting molds and hot extrusion molds can be significantly improved after nitrocarburizing. Nitrocarburizing has good application effects on cold heading dies, cold extrusion dies, cold punching dies, drawing dies, etc. ​

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 infiltration-promoting effects, thus developing new processes such as rare earth nitriding and rare earth nitrocarburizing. ​

5. Boronizing and metalizing

Boronizing can be solid boronizing, liquid boronizing, paste boronizing, etc. The most commonly used method is solid boronizing, and solid boronizing agents are already available on the market. After solid boronization, the hardness of the surface layer is as high as 1400`2800HV, with high wear resistance, good corrosion resistance and oxidation resistance. ​

The boronizing process is commonly used on various cold work molds. Due to the improvement in wear resistance, the mold life can be increased several times or more than ten times. Boronizing medium carbon 11141 steel can sometimes replace high alloy steel to make molds. Boriding can also be applied to hot working dies, such as hot extrusion dies. ​

The boronized layer is relatively brittle, the diffusion layer is relatively thin, and the supporting force for the boronized layer is weak. For this reason, boron nitriding or boron carbonitriding can be used to strengthen the transition zone and make the hardness change smoothly. In order 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, titanium infiltration and other processes can be used to treat cold work and hot work molds. Among them, the TD method (molten salt metal infiltration) has been used in some applications, which can increase the life of the mold several times or even more than ten times. ​

6 Vapor phase deposition

Vapor deposition is divided into physical vapor deposition (PVD) and chemical vapor deposition (CVD) according to the basic principles of formation. ​

PVD is divided into vacuum evaporation, sputtering and ion plating. Ion plating is a technology that combines evaporation and sputtering. The ion plating film has the advantages of strong adhesion, good throwing ability, and a wide range of base materials and coating materials to be plated, so it has been widely used. In recent years, multi-arc ion plating has attracted people’s attention. At present, ion plating TiN is widely used in molds. This film not only has high hardness, but also has good toughness, strong bonding force, and high temperature resistance. Multi-component films developed on the basis of TiN, such as (TiAl)N, (TiCr)N, etc., have better performance than TiN and are a more promising new type of film.

CVD is a method that uses chemical methods to cause the reactive gas to chemically react on the surface of the base material to form a covering layer (TiC, TiN). There are many methods of CVD. Generally, the reaction temperature of CVD is above 900°C, and the hardness of the coating reaches above 2000HV. However, high temperatures can easily deform the workpiece, and the interface of the deposited layer can easily react. The development trend is to lower the temperature and develop new coating ingredients. For example, metal organic compound CVD (MOCVD), laser CVD (LCVD), plasma CVD (PCVD), etc. ​

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 supply material is at least 103W/㎝2. Their common characteristics are: fast heating speed, heating area can be selected according to needs, small deformation of the workpiece, no need for cooling medium, clean processing environment, good controllability, and easy to realize automated processing. Domestic and foreign research has invested heavily in the principles and processes of high-energy beam heat treatment. The more mature ones are laser phase change hardening, small-size electron beam treatment and medium-power ion implantation, and have been applied in improving mold life. ​

Outlook and Suggestions

It can be considered that our country has established a relatively complete series of mold materials, some of which have excellent performance and have reached the international advanced level. The research and development of mold heat treatment in my country can also keep pace with the international situation, and some new mold heat treatment technologies have been promoted and applied to varying degrees. ​

In view of the existing problems, the following suggestions are made for the future development of mold materials and mold heat treatment technology in my country:

1. Accelerate the commercialization and refinement of mold steel production and the commercialization of mold steel distribution.

my country’s annual mold steel consumption exceeds 200,000 tons, and it is increasing year by year. In recent years, the import volume of foreign mold steel accounts for about 1/3 of the demand for mold steel, showing a gradual increase trend. The main problem is that there are few varieties and specifications of 12114 steel in my country, and the degree of commercialization of mold steel production, refinement and distribution is low. In some industrially developed countries, metallurgical enterprises supply 50-60% of machined mold steel products, while more than 80% of China’s mold steel is still supplied as black round bars. More and more mold manufacturing plants require mold steel suppliers to quickly provide the required steel after the mold design is completed, reducing the amount of steel in stock and shortening the molding cycle. Chinese steel manufacturers have not adapted to this commodity market mechanism, which is an important reason for the increasing expansion of imported mold steel in China. ​

2. Vigorously promote the application of new mold steels with excellent performance and continuously improve the series of mold steel grades.

Our country has developed many new mold steels with certain characteristics, some of which have excellent properties, reaching or exceeding the level of similar foreign steels. However, the promotion quantity and application scope of these new steels are not large enough. The main reason is that the production of mold steel in China has not yet taken the road of productization and refined materials, and the distribution method does not meet the requirements of the commodity market. To solve these problems, these properties are good. The new mold steel has broad promotion prospects and will produce huge economic benefits. ​

China already has a relatively complete series of mold steels, but it still needs to continuously improve its quality, expand its applications, and further preserve the good and eliminate the bad in the application. At the same time, we selectively develop advanced mold steel and improve China’s mold steel series, such as developing powder metallurgy mold steel, multi-element easy-cutting plastic mold steel, and establishing steel series for forming molds such as glass, ceramics, refractory bricks, and floor tiles.

3. Further improve the quality of mold steel

Some special steel plants in my country have adopted new metallurgical equipment and processes to produce mold steel, such as out-of-furnace refining, vacuum smelting, rapid electric cable car rocker support metal forging parts machines and precision forging machines. The quality of some mold steels has been greatly improved, such as D2, P20, etc. Steel has been exported in bulk, exported