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Cold work die steel and hot die steel composition of the difference
Nov 10, 2017

Cold work die steel and hot die steel composition of the difference

 First, cold work die steel

Cold work die steel, including the manufacture of punching die (blanking punching die, trimming die, punch, scissors), cold heading die and cold extrusion die, compression die and drawing die and so on.

1. Cold working die steel working conditions and performance requirements

Cold work die steel at work, due to deformation of the processed material resistance is relatively large, the work part of the mold to withstand great pressure, bending force, impact and friction. Therefore, the normal causes of cold die scrap is generally wear and tear, but also due to fracture, collapse and deformation of ultra-poor early failure.

Cold work die steel and cutting tool steel, there are many things in common. Require the mold has high hardness and wear resistance, high bending strength and sufficient toughness to ensure the smooth progress of the stamping process, the difference is that the mold shape and processing technology is complex, and the friction area, the possibility of wear and tear , So hard to repair it. Therefore, the requirements of higher wear-resistant mold work withstand punching force, but also because of complex shape prone to stress concentration, so require a higher toughness; mold size, complex shape, so require a higher hardenability, Smaller deformation and cracking tendency. In short, cold work die steel hardenability, wear resistance and toughness, etc. than the requirements of the tool steel higher, but in terms of red hardness is less demanding or essentially no requirement (because it is cold forming), so Also formed a number of suitable for cold work die steel, for example, the development of a high wear-resistant, micro-deformation cold work die steel and high toughness cold work die steel. The following combination of the relevant selection of steel further instructions.

2. Steel selection is usually followed by cold-working die conditions of use, the steel can be divided into the following four options: (1) small size, simple shape, light-duty cold-working die.

For example, a small punch, shears and other optional steel cutting T7A, T8A, T10A, T12A and other carbon tool steel. The advantages of this type of steel are: workability, cheap, easy to source. But the disadvantage is: low hardenability, poor wear resistance, large quenching deformation. Therefore, only suitable for the manufacture of some small size, simple shape, light-duty tools and require the hardened layer is not deep and maintain high toughness of the cold mode and so on.

(2) large size, complex shape, light load of cold work die. Commonly used steel 9SiCr, CrWMn, GCr15 and 9Mn2V low-alloy cutting tool steel. The hardenable diameter of these steels in oil is generally up to 40 mm or more. Among them, 9Mn2V steel is a non-Cr cold-working die steel developed in recent years in China, which can replace or partially replace the steel with Cr. 9Mn2V steel carbides heterogeneity and quench cracking tendency smaller than CrWMn steel, decarburization tendency 9SiCr steel smaller, and hardenability than carbon tool steel, the price is only about 30% higher than the latter and therefore is A worth promoting the use of steel. However, 9Mn2V steel also has some disadvantages such as low impact toughness, found in the production of fragmentation, tempering stability is poor, the tempering temperature is generally not more than 180 ℃ tempering at 200 ℃ flexural strength and toughness Low value began to appear. 9Mn2V steel in the salt, hot oil and other cooling capacity more moderate quenching quenching medium. For some deformation of the strict requirements and hardness is not very high mold, austenitic austempering can be used. (3) large size, complex shape of heavy-duty cold work die. To be used in the alloy or high alloy steel, such as Cr12Mo, Crl2MoV, Cr6WV Cr4W2MoV, etc., in addition to the selection of high-speed steel. In recent years, the tendency of high-speed steel to do cold work die has been increasing, but it should be pointed out that the high-speed steel is no longer the red hardness of the unique strengths, but with its high hardenability and high wear resistance . To this end, in the heat treatment process should also be different. Selection of high-speed steel to do cold mold, low temperature quenching should be used to improve the toughness. For example, W18Cr4V steel cutting tool used when the quenching temperature is 1280-1290 ℃. When doing cold work die, it should be used 1190 ℃ low temperature quenching. Another example is the W6Mo5Cr4V2 steel, the use of low-temperature quenching can greatly increase the life expectancy, especially significantly reduced the discount rate.

(4) by the impact load and knife thin cold mold. As mentioned above, the first three types of cold-working die steel performance requirements are based on high wear resistance are high carbon eutectoid steel even Rongsheng steel. However, some cold-working die plus side trim, blanking die, etc., its counterparts are thin, the role of the impact load by the impact should be the main requirements of high impact toughness. In order to solve this contradiction, the following measures can be taken: ① reduce co-carbon content, the use of sub-joint steel, in order to avoid the primary and secondary carbides caused by the decline of toughness of steel; ② Add Si, Cr and other alloying elements to improve Tempering steel stability and tempering temperature (240-270 ℃ tempering) This will help to fully eliminate the quenching stress so that grilled Ji increase, without reducing the hardness; ② W and other elements to form a refractory carbide to refine the crystal Grain, improve toughness. Commonly used high toughness cold-working die steel with 6SiCr, 4CrW2Si, 5CrW2Si and so on.

3. Give full play to the cold potential performance of die steel approach. In the use of cold-working die made of Cr12 steel or high-speed steel, a very prominent problem is the brittleness of steel, easy to crack in use. For this purpose, the carbide must be refined by means of adequate forging, in addition to which new types of steel should be developed. The focus of the development of new steel should be to reduce the amount of carbon steel and carbide forming elements. Cr4W2MoV steel with high hardness, high wear resistance and good hardenability and other advantages, and has good tempering stability and comprehensive mechanical properties, with dry-made silicon steel die, etc., can make the life of Cr12MoV steel increased by 1 ~ 3 times more but the steel forging temperature range is narrower, forging river cracking, should be strictly controlled forging temperature and operation regulation Cr2Mn2SiWMoV steel quenching temperature is low, quenching deformation, high hardenability, free quenching deformation of steel mold Said 7W7Cr4MoV steel can replace W18Cr4V and Cr12MoV steel, which is characterized by the carbide carbide unevenness and toughness has been greatly improved.

Second, hot work die steel

1. Hot working mold working conditions

 Hot die including hammer forging die, hot extrusion die and die casting mold three categories. As mentioned earlier, the main feature of hot working die working conditions is the contact with the hot metal, which is the main difference with the working conditions of cold work die. Therefore, it will bring the following two problems:

(1) the surface of the cavity metal heating. Usually hammer forging work, the cavity surface temperature up to 300 ~ 400 ℃ above the hot extrusion mold up to 500-800 ℃ above; die-casting mold cavity temperature and die-casting material types and pouring temperature. Such as die-casting black metal when the cavity temperature up to 1000 ℃. Such a high temperature will make the cavity surface hardness and strength significantly reduced, easy to use in the stack. To this end, the basic performance requirements for hot die steel are high thermoplastic resistance, including high temperature hardness and high temperature strength, high thermoplastic resistance, in fact, reflects the high tempering stability of steel. This will be the first way to find alloying hot die steel, that is, adding Cr, W, Si, and other alloying elements can improve the steel's tempering stability.

(2) Mold cavity surface thermal fatigue (crack). The working characteristics of the hot mold is intermittent, each time the hot metal forming the water, oil, air and other media cooling cavity surface. Therefore, the working status of the hot mold is repeated heating and cooling, so that the mold cavity surface metal repeated thermal expansion and contraction, that is, repeated tensile and compressive stress, the result of the cavity surface cracks, known as thermal fatigue , Thus, the second basic performance requirement for hot tool steel is proposed, that is, it has high thermal fatigue resistance.

In general, the factors that affect the thermal fatigue resistance of steel are: thermal conductivity of steel. High thermal conductivity of steel, mold surface metal can reduce the degree of heat, thereby reducing the thermal fatigue tendencies of steel. It is generally believed that the thermal conductivity of steel is related to the amount of carbon and the thermal conductivity is low when the carbon content is high, so hot-working die steel should not be made of high-carbon steel. In the production of carbon steel is usually used (C0.3% ~ 0.6%) combined carbon content is too low, will lead to the decrease of hardness and strength of steel is also unfavorable. ② the critical point of steel. Normally the higher the critical point of the steel (Ac1), the lower the thermal fatigue tendencies of the steel. Therefore, generally by adding alloying elements Cr, W, Si, lead to improve the critical point of steel. Thus improving the thermal fatigue resistance of steel.

2 commonly used hot die steel

(1) forging die steel. In general, the hammer forging die steel has two problems more prominent First, the role of impact load at work, so the mechanical properties of steel higher requirements, in particular, is more resistant to plastic deformation and toughness requirements; Second, forging Die larger cross-sectional size (<400mm), so the hardenability of the steel higher, in order to ensure uniform structure and performance of the entire mold. Common hammer forging floor steel with 5CrNiMo, 5CrMnMo, 5CrNiW, 5CrNiTi and 5CrMnMoSiV and so on. Different types of hammer eye mold should use different materials. Large or large forging die to 5CrNiMo as well, can also be used 5CrNiTi, 5CrNiW or 5CrMnMoSi and so on. For small and medium-sized hammer forging mold is usually selected 5CrMnMO steel.

(2) hot extrusion die steel, hot extrusion die is characterized by the slow loading speed, therefore, the higher temperature cavity heating, usually up to 500 a 800 ℃. The use of such steel performance requirements should be high temperature strength (ie, high tempering stability) and high thermal fatigue resistance based. Ak and hardenability requirements may be appropriate to lower. General hot extrusion die smaller size, often less than 70 ~ 90 mm. Commonly used hot extrusion die 4CrW2Si, 3Cr2W8V and 5% Cr type hot work die steel. Which 4CrW2Si, both cold work die steel, hot work die steel, due to different purposes, can be used in different heat treatment methods. For the cold mold when the lower quenching temperature (870-900 ℃) and low or medium temperature tempering; for the hot mode is the higher quenching temperature (usually 950-1000 ℃) and high temperature tempering.

(3) die-casting mold steel. Overall, the die casting mold steel performance requirements and hot extrusion die steel similar to that.

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