1 cold mold material
1.1 Cold Forging Die
Cold forging is a method of forging a billet at room temperature without heating the billet. Therefore, cold forging can produce products with high dimensional accuracy. On the other hand, since the cold forging processes the forging billet at normal temperature and the deformation strength is high, the load of the cold forging die is large. In recent years, the output of forged products in emerging Southeast Asian countries has continued to increase. In addition, the forging technology of high-precision forging and complex-shaped forging without cutting has also developed.
1.2 Cold forging die timber
The main material for cold work molds is high-speed tool steel. These include cold tool steels and hot work tool steels. Mass production molds use superhard alloys. Forging equipment accessories use low alloy steel and carbon steel.
1.2.1 Comparison of Properties of Cold Forging Dies
Superhard alloys have high compressive strength and good wear resistance, but they have the disadvantage of low tensile strength and toughness. On the contrary, carbon steel for mechanical structures has high toughness but poor wear resistance. Abrasion resistance and toughness are mutually opposite properties. It is very difficult to choose the best mold material.
1.2.2 Characteristics of various mold materials
1) Superhard alloy
The superhard alloy used for the forging tool is a high Co material WC-Co with a Co content of more than 10%. Cold forging tools should be based on the need to distinguish between the use of super hard alloy and tool steel. The main characteristics of cemented carbide and tool steel differ as follows.
(1) The advantages of cemented carbide
1 high hardness, good wear resistance; 2 high compressive strength; 3 longitudinal elastic coefficient 2.5-3.0 times that of tool steel; 4 small thermal expansion coefficient (about 1/2 of tool steel); 5 high thermal conductivity (approximate tool 3 times of steel).
(2) The advantages of tool steel
1 High flexural strength and tensile strength (about 2-3 times that of cemented carbide); 2 Good toughness and large elastic-plastic deformation; 3 Good machinability;
2) High speed tool steel
The materials used for cold forging, warm forging tools, and molds are high-speed tool steels that combine wear resistance, toughness, and heat resistance. In recent years, the mainstream high speed tool steel used is the Mo high speed tool steel SKH51. In addition, SKH55 and SKH57 are also used. In SKH55 and SKH57 steels, W, V, and Co were added to improve wear resistance and heat resistance.
(1) Abrasion resistance of high-speed tool steel: Base steel high-speed steel <SKH51 <SKH55 <SKH57.
(2) High-speed tool steel toughness: SKH57 <SKH55 <SKH51 <base steel high-speed steel.
(3) Heat resistance of high-speed tool steel: Base steel high-speed steel <SKH51 <SKH55 <SKH57.
3) Cold working tool steel
Cold forging die generally use SKD11. SKD11 contains more carbides and its composition is close to that of SKD61. It has practical wear resistance and toughness. Other cold work tool steels include SKD12 and SKD1. Large cold forging dies generally use cold tool steels. Compared with high-speed tool steels, hardened tool steels have less hardenability, heat treatment distortion, and dimensional changes. Cold working tool steels are used for cold forging punches and dies.
4) Low alloy tool steel
In the past, when forging products were produced in small batches, the forging tools used low-alloy tool steels SKS2, SKS3, and SKS31. The material for the stress punch is: SKS2 (850°C oil-cooled, 60HRC or more) for small-diameter punches with φ30mm or less, and SKS3 (slightly insufficient yield strength) for large-diameter punches with φ30mm or more. Due to the limitations of the heat treatment and hardness of low-alloy tool steels, low-alloy tool steels are used only for auxiliary components that are not subjected to heavy loads in forging equipment.
1.3 Selection of Cold Forging Die Material
Sometimes, the use of advanced tool materials does not increase the useful life of the tool. The reason is that tool life is determined by many factors such as design, production, technology, mold making, materials, and heat treatment. In order to increase the life of the mold through the mold material, it is necessary to know the required characteristics of the mold and select the mold material that matches it.
When selecting a cold forging die material, the hardness and the steel grade of the material must be selected after clarifying the cause of the die damage. The actual state of the mold is very complex, so it is difficult to understand the mold damage. The following is a rough standard for the selection of cold forging die materials.
1) Selection Criteria for Cold Forging Die Materials with Special Requirements for Wear Resistance
The more carbides in the steel, the higher the carbide hardness, the greater the hardness of the steel, the better the wear resistance of the mold material. Material wear resistance from bad to good is: SK5, SK4 → SKS94, SKS3 → SKD11, base steel high speed steel → SKD1, SKD51 → SKH55 → SKH57 → superhard alloy.
2) Selection criteria for tough forging die materials
The greater the toughness of the steel matrix and the less carbides, the better the toughness of the mold material. The order from which material toughness changes from bad to good is: super hard alloy → SKH57 → SKD1 → SKD11 → SKS3 → SKH55 → SKH51 → matrix steel high speed steel → SKD61.
3) Selection criteria for cold forging die materials that specifically require tensile strength
Selecting a high hardness steel can meet the requirements of the cold forging die against tensile strength. It should be noted that although the tensile strength of the material is directly proportional to the hardness, the hardness is too high and the tensile strength decreases rapidly. The hardness at which the tensile strength of each steel begins to drastically decline is different. The order of the material tensile strength from low to high is: cemented carbide → SKD1 → SKD11, SKS3 → SKH51.
4) Selection Criteria for Cold Forging Die Materials with Special Compressive Strength Requirements
The greater the elastic modulus, the higher the critical compressive load of the material. Therefore, materials with high carbide content have high compressive strength. The order of the compressive tensile strength from low to high is: SKS3 → SKD11 → SKH51 → SKD1 → SKH57 → powder high speed steel → cemented carbide.
5) Selection Criteria for Cold Forging Die Materials with Special Requirements for Fatigue Strength
Materials with good strength and toughness can improve the fatigue life of the mold. The order of fatigue resistance from low to high is: SKS3→SKD11→SKH51→base steel high speed steel→powder high speed steel.
6) Selection criteria for large size cold forging die materials
Larger molds limit the use of mold materials. Improper use of materials can cause large cracks in the mold. Each steel grade has its own limit size, and when it exceeds the limit size, it will cause the performance of the material to decline. Low-alloy steels containing a small amount of Cr and Mn can be used in a small size. The size of the mold determines the type of steel that can be used. The order of the material's extreme use size from small to big is: SKS3 (φ70mm) → SKD1 (φ150mm) → powder high speed steel (φ180mm) → SKH51 (φ18mm) → SKD1 2 (φ250mm) → base steel high speed steel, SKD11 (φ550mm ).
2 Hot work mold
2.1 hot forging die timber
Hot forging is a processing method in which a heated billet is forged and a high pressure is applied to deform it. Since the hot forging is the formation of a high temperature heated billet, the hot forging die has a shorter service life. Cold forging can produce high-precision, difficult-to-shape parts with complex shapes. However, the surface quality and dimensional accuracy of hot forged forgings are not very good. Therefore, in the production of complex-shaped forgings, closed forging or blocking forging methods that are not prone to generate fly spurs and excess materials are used, and multiple forgings are performed.
2.2 Structure and Application of Hot Forging Dies
Hot Forging Hammer Model Forging requires multiple forgings to complete the final forging product. During this process, excess material of the blank is generated, so the hot forging hammer model has a portion for containing excess material. Plastic deformation of excess material occurs in this area.
Closing forging is a forging method in which a forging blank is enclosed in a cavity of an upper die and a lower die, and the forging blank is deformed in a plurality of directions using a die and a punch, and the blank is filled in the die cavity.
2.3 hot forging die timber
2.3.1 Comparison of hot forging die material characteristics
Hot forging dies require high-temperature strength, good thermal shock, and toughness can be slightly lower. Hammering forging dies requires high toughness and is less prone to cracking, but the wear resistance can be slightly lower.
2.3.2 Warm forging die
Warm forging die materials are mostly high-speed steel SKH51 with wear resistance, toughness and heat resistance. In the case where the forging load is large and the mold is strongly cooled, SKD61 and SKD62, which have good toughness and heat softening resistance, are often used. In the case of higher requirements for high temperature strength and better requirements for thermal cracking resistance, H19 in the SKD6 and AISI standards are used.
2.3.3 Hammering hot forging die
Hammering hot forging dies and forging billet heated to high temperature contact time is short, the mold temperature is small. Therefore, the mold material should have good impact resistance and wear resistance. Therefore, a high-toughness SKT material (hardened state hardness HB352-415) is used. The most commonly used SKT-type material is SKT4. The larger the mold, the lower the hardness of the material in order to improve the toughness. The empirical criterion for preventing early cracking of the mold is that the Charpy impact value of the material is greater than 30 J/cm2. When the Charpy impact value is 5-10 J/cm2 or less, early cracks are likely to occur in large molds.
2.3.4 Hot Stamping Forging Mould
Hot stamping forging molds have a long contact time with the high-temperature billet, and the impact is small, so the high temperature hardness is required to be large enough. The materials used in general hot stamping forging dies are grades such as SKD6, SKD62, and SKD61, which have moderate high-temperature hardness, high crack resistance, good hardenability, and low heat-treatment distortion. SKD7 is used for high-life molds, and special-purpose molds that require high strength and high wear resistance are specially designed using AISI standard grades such as SKD8, SKD4 and SKD5. In addition, precipitation hardened mold steels are used for molds with high operating temperatures. The high temperature hardness of this steel is great.
2.4 Selection method of hot forging die materials
In order to increase the service life of a mold through the use of a mold material, it is necessary to understand the characteristics required for the mold and select a material that conforms to the characteristics of the mold. The general selection criteria for the hot forging die material is based on the actual die life and the determined hardness standard.
1) Selection criteria for hot forging die materials requiring wear resistance
The greater the hardness of the material, the higher the high temperature strength (hardness), the better the wear resistance. The order of material wear resistance from small to large is: SKT3, SKT4 → SKD6 (H11) → SKD61 (H13) → SKD62 (H12) → SKD7 (H10) → SKD4, SKD5 (H21) → SKD8 (H19) → Matrix Steel high-speed steel → SKH51 (M2) → powder high-speed steel.
2) Selection Criteria for Hot Toughness Die Material Requirements
The greater the toughness of the steel matrix and the less carbides, the better the toughness of the mold material. The order from which material toughness changes from poor to good is: SKH51, SKD8(H19), SKD5(H21), SKD4→SKD7(H10)→SKD62(H12)→SKD61(H13)→SKD6(H11)→SKT3→SKT4→SKT6.
Hammered forging dies are subjected to impact loads and may cause cracks. Therefore, SKT6 and SKT4 with good toughness are used. For small hammered forging dies, SKT3 with less Ni content and less toughness can be used. The toughness of SKD6, SKD61, SKD62 is second only to SKT, but it has high high temperature strength and good wear resistance, so it is often used in large-scale hammer forging dies.
3) Selection criteria for hot forging die materials requiring thermal cracking resistance
The material with high hardness, good toughness and certain high temperature strength is excellent in thermal cracking resistance. The order in which the resistance to thermal cracking of the material changes from bad to good is: SKT3, SKT4 → SKD6 (H11) → SKD61 (H13) → SKD62 (H12) → SKD7 (H10) → SKD4 → SKD5 → SKD8 (H19).
When the die thickness exceeds 230 mm, large cracks often occur suddenly. The use of high toughness, low hardness, and high hardenability materials helps prevent large cracks. The order from bad to good for material resistance to cracking is: high speed steel → powder high speed steel → high speed steel of base steel → SKD8 → SKD7 → SKT3, SKT4 → SKD62 → SKD61 → SKD6.
4) Selection criteria for hot forging die materials requiring high temperature strength
The order of the high temperature strength of the material from poor to good is the same as the order of resistance to hot cracking from poor to good. That is, SKT3, SKT4 → SKD6 (H11) → SKD61 (H13) → SKD62 (H12) → SKD7 (H10) → SKD4 → SKD5 → SKD8 (H19).
SKD62 can be used when high temperature strength and abrasion resistance are slightly better than SKD61. To increase the high temperature strength and wear resistance, use SKD7 and SKD8. However, SKD7 and SKD8 have poor hardenability. Large-size molds cannot be hardened to the center of the mold. After tempering, the toughness is poor, so it cannot be used for large molds.
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