Part 1 - Cold working die steel
Cold working die steel includes molds for manufacturing punching and cutting (blanking and punching molds, trimming molds, punches, scissors), cold heading molds, cold extrusion molds, bending molds, and wire drawing molds, etc.
1. Working conditions and performance requirements for cold working die steel
During the operation of cold working die steel, due to the high deformation resistance of the processed material, the working part of the mold bears great pressure, bending force, impact force, and friction force. Therefore, the normal reason for scrapping cold working molds is generally due to wear and tear. There are also cases where they fail prematurely due to fracture, collapse force, and deformation exceeding the tolerance.
Compared with cutting tool steel, cold work die steel has many similarities. The mold is required to have high hardness and wear resistance, high bending strength, and sufficient toughness to ensure the smooth progress of the stamping process. The difference lies in the complex shape and processing technology of the mold, as well as the large friction area and high possibility of wear, making it difficult to repair and grind. Therefore, higher wear resistance is required. When the mold is working, it bears high punching pressure and is prone to stress concentration due to its complex shape, so it requires high toughness; The mold has a large size and complex shape, so it requires high hardenability, small deformation, and cracking tendency. In short, the requirements for hardenability, wear resistance, and toughness of cold work die steel are higher than those of cutting tool steel. However, the requirements for red hardness are relatively low or basically not required (because it is formed in a cold state), so some steel grades suitable for cold work molds have also been formed, such as the development of high wear resistance, micro deformation cold work die steel and high toughness cold work die steel.
2. Steel grade selection
Usually, according to the usage conditions of cold working molds, the selection of steel grades can be divided into the following four situations:
① Cold working mold with small size, simple shape, and light load.
For example, small punches and scissors for cutting steel plates can be made of carbon tool steels such as T7A, T8A, T10A, and T12A. The advantages of this type of steel are; Good processability, cheap price, and easy source. But its disadvantages are: low hardenability, poor wear resistance, and large quenching deformation. Therefore, it is only suitable for manufacturing tools with small dimensions, simple shapes, and light loads, as well as cold working molds that require a low hardening layer and high toughness.
② Cold working molds with large dimensions, complex shapes, and light loads.
The commonly used steel types include low alloy cutting tool steels such as 9SiCr, CrWMn, GCr15, and 9Mn2V. The quenching diameter of these steels in oil can generally reach over 40mm. Among them, 9Mn2V steel is a type of cold work die steel developed in China in recent years that does not contain Cr. It can replace or partially replace steel containing Cr.
The carbide heterogeneity and quenching cracking tendency of 9Mn2V steel are smaller than those of CrWMn steel, and the decarburization tendency is smaller than that of 9SiCr steel, while the hardenability is greater than that of carbon tool steel. Its price is only about 30% higher than the latter, so it is a steel grade worth promoting and using. However, 9Mn2V steel also has some drawbacks, such as low impact toughness and cracking phenomenon found in production and use. In addition, the tempering stability is poor, and the tempering temperature generally does not exceed 180 ℃. When tempered at 200 ℃, the bending strength and toughness begin to show low values.
9Mn2V steel can be quenched in quenching media with relatively mild cooling capacity such as nitrate and hot oil. For some molds with strict deformation requirements and low hardness requirements, austenitic isothermal quenching can be used.
③ Cold working molds with large dimensions, complex shapes, and heavy loads.
Medium alloy or high alloy steel must be used, such as Cr12Mo, Crl2MoV, Cr6WV, Cr4W2MoV, etc. In addition, high-speed steel may also be used.
In recent years, the trend of using high-speed steel as cold working molds has been increasing, but it should be pointed out that at this time, it is no longer the use of the unique red hard strength of high-speed steel, but rather its high hardenability and high wear resistance. Therefore, there should also be differences in the heat treatment process.
When using high-speed steel as a cold mold, low-temperature quenching should be used to improve toughness. For example, the commonly used quenching temperature for W18Cr4V steel cutting tools is 1280-1290 ℃. When making cold working molds, low temperature quenching at 1190 ℃ should be used. Another example is W6Mo5Cr4V2 steel. By using low-temperature quenching, the service life can be greatly improved, especially by significantly reducing the loss rate.
④ Cold working molds that are subjected to impact loads and have thin blade gaps.
As mentioned above, the performance requirements of the first three types of cold work die steels are mainly high wear resistance, so high carbon hypereutectoid steel and even ledeburite steel are used. However, for some cold working dies, such as the side tower cutting and blanking dies, which have thin butt joints and are subject to impact load when in use, high impact toughness is required. In order to solve this contradiction, the following measures can be taken:
Ⅰ- reduce the carbon content and use hypoeutectoid steel to avoid a decrease in toughness of the steel caused by primary and secondary carbides;
Ⅱ- Adding alloy elements such as Si and Cr to improve the tempering stability and temperature of the steel (tempering at 240-270 ℃) is beneficial for fully eliminating quenching stress and improving performance without reducing hardness;
Ⅲ- Add elements such as W to form refractory carbides to refine grains and improve toughness. The commonly used steels for high toughness cold working molds include 6SiCr, 4CrW2Si, 5CrW2Si, etc.
3. Ways to Fully Utilize the Performance Potential of Cold Working Die Steel
When using Cr12 type steel or high-speed steel as cold working molds, a prominent problem is the high brittleness of the steel, which is prone to cracking during use. To this end, it is necessary to refine carbides using sufficient forging methods. In addition, new steel grades should be developed. The focus of developing new steel grades should be to reduce the carbon content of steel and the number of carbides forming elements.
Cr4W2MoV steel has advantages such as high hardness, high wear resistance, and good hardenability. It also has good tempering stability and comprehensive mechanical properties. It is used for manufacturing silicon steel sheet dies, etc. It can increase the lifespan by more than 1-3 times compared to Cr12MoV steel. However, the forging temperature range of this steel is narrow, and it is prone to cracking during forging. The forging temperature and operating specifications should be strictly controlled.
Cr2Mn2SiWMoV steel has low quenching temperature, small quenching deformation, and high hardenability. It is known as air quenched micro deformation die steel.
7W7Cr4MoV steel can replace W18Cr4V and Cr12MoV steel. Its characteristic is that the non-uniformity of carbides and toughness of the steel have been greatly improved.
Part 2 - Hot working die steel
1. Working conditions of hot working molds
Hot working molds include hammer forging molds, hot extrusion molds, and die-casting molds. As mentioned earlier, the main characteristic of the working conditions of hot working molds is contact with hot metal, which is the main difference from the working conditions of cold working molds. Therefore, it will bring the following two problems:
(1) The surface metal of the mold cavity is heated. Usually, when hammering dies are working, the surface temperature of the die cavity can reach over 300-400 ℃, and the hot extrusion die can reach over 500-800 ℃; The temperature of the die-casting mold cavity is related to the type of die-casting material and pouring temperature. When die-casting black metal, the mold cavity temperature can reach over 1000 ℃. Such high usage temperatures will significantly reduce the surface hardness and strength of the mold cavity, making it prone to folding during use. The basic performance requirement for hot die steel is high thermoplastic resistance, including high-temperature hardness and strength, and high thermoplastic resistance, which actually reflects the high tempering stability of the steel. From this, the first way to alloying hot die steel can be found, that is, adding alloying elements such as Cr, W, Si can improve the tempering stability of the steel.
(2) Thermal fatigue (cracking) occurs on the surface metal of the mold cavity. The working characteristics of hot molds are intermittent. After each hot metal formation, the surface of the mold cavity needs to be cooled by media such as water, oil, and air. Therefore, the working state of the hot mold is repeatedly heated and cooled, so that the surface metal of the mold cavity will undergo repeated thermal expansion, that is, repeatedly subjected to tensile and compressive stress. As a result, the surface of the mold cavity will crack, which is called thermal fatigue. Therefore, the second basic performance requirement for the hot die steel is put forward, that is, it has high thermal fatigue resistance.
Generally speaking, the main factors affecting the thermal fatigue resistance of steel are:
① The thermal conductivity of steel. The high thermal conductivity of steel can reduce the degree of heating on the surface metal of the mold, thereby reducing the tendency of steel to thermal fatigue. It is generally believed that the thermal conductivity of steel is related to its carbon content. When the carbon content is high, the thermal conductivity is low, so it is not suitable to use high carbon steel for hot work die steel. Low carbon content of medium carbon steel (C0.3% 5-0.6%) is commonly used in production, which can lead to a decrease in the hardness and strength of the steel and is also detrimental.
② The critical point effect of steel. Usually, the higher the critical point (Acl) of steel, the lower its thermal fatigue tendency. Therefore, the critical point of steel is generally increased by adding alloying elements Cr, W, Si, and lead. Thus improving the thermal fatigue resistance of steel.
2. Steel for commonly used hot working molds
(1) Steel for hammer forging dies. Generally speaking, there are two prominent issues with the use of steel for hammer forging molds. Firstly, it is subjected to impact loads during operation. Therefore, the mechanical properties of the steel are required to be high, especially for plastic deformation resistance and toughness; The second reason is that the cross-sectional size of the hammer forging die is relatively large (<400mm), which requires high hardenability of the steel to ensure uniform microstructure and performance of the entire die.
Commonly used hammer forging die steels include 5CrNiMo, 5CrMnMo, 5CrNiW, 5CrNiTi, and 5CrMnMoSiV. Different types of hammer eye molds should use different materials. For very large or large hammer forging dies, 5CrNiMo is preferred. 5CrNiTi, 5CrNiW, or 5CrMnMoSi can also be used. 5CrMnMo steel is usually used for small and medium-sized hammer forging dies.
(2) Steel is used for hot extrusion molds, and the working characteristic of hot extrusion molds is slow loading speed. Therefore, the heating temperature of the mold cavity is relatively high, usually up to 500-800 ℃. The performance requirements for this type of steel should mainly focus on high high-temperature strength (i.e. high tempering stability) and high heat fatigue resistance. The requirements for AK and hardenability can be appropriately lowered. Generally, the size of hot extrusion molds is small, often less than 70-90 mm.
Commonly used hot extrusion molds include 4CrW2Si, 3Cr2W8V, and 5% Cr type hot work die steels. Among them, 4CrW2Si can be used as both cold work die steel and hot work die steel. Due to different uses, different heat treatment methods can be used. When making cold molds, lower quenching temperatures (870-900 ℃) and low or medium temperature tempering treatment are used; When making hot molds, a higher quenching temperature (usually 950-1000 ℃) and high-temperature tempering treatment are used.
(3) Steel for die-casting molds. Overall, the performance requirements of steel for die-casting molds are similar to those for hot extrusion molds, with high tempering stability and thermal fatigue resistance being the main requirements. So the commonly used steel type is generally the same as the steel used for hot extrusion molds. As usual, steel such as 4CrW2Si and 3Cr2W8V are used. However, there are differences, such as the use of 40Cr, 30CrMnSi, and 40CrMo for low melting point Zn alloy die-casting molds; For Al and Mg alloy die-casting molds, 4CrW2Si, 4Cr5MoSiV, etc. can be selected. For Cu alloy die-casting molds, 3Cr2W8V steel is mostly used.
Professional Die Steel Supplier – Jinbaicheng Metal
JINBAICHENG is the world’s leading supplier of cold work and hot work die steels, plastic die steels, die casting tool steels and custom open-die forgings, processing over 100,000 tons of steel each year. Our products are manufactured at 3 production facilities in shandong, jiangsu, and guangdong province. With more than 100 patents, JINBAICHENG sets worldwide standards including being the first steel manufacturer in China to receive ISO 9001 certification. Official website: www.sdjbcmetal.com Email: jinbaichengmetal@gmail.com or WhatsApp at https://wa.me/18854809715
Post time: Jun-21-2023