Usually, the conditions for the use of cold-working dies can be divided into the following four cases:
(1) Cold-working molds with small size, simple shape and light load. For example, small punches, scissors for cutting steel plates, etc. can be made of carbon tool steel such as T7A, T8A, T10A, T12A. The advantages of this type of steel are: good processability, low price, and easy source. However, its disadvantages are low hardenability, poor wear resistance and large quenching deformation. Therefore, it is only suitable for manufacturing some small-sized, simple-shaped, light-loaded tools, and cold-working dies that require the hardened layer to be deep and maintain high toughness.
(2) Cold-working dies with large size, complicated shape and light load. Commonly used steel grades are low alloy cutting steels such as 9SiCr, CrWMn, GCr15 and 9Mn2V. The hardened diameter of these steels in oil is generally up to 40 mm or more. Among them, 9Mn2V steel is a kind of steel for cold working die which does not contain Cr in recent years. It can replace or partially replace steel containing Cr.
The carbide non-uniformity and quenching cracking tendency of 9Mn2V steel is smaller than that of CrWMn steel, and the decarburization tendency is smaller than that of 9SiCr steel, and the hardenability is larger than that of carbon tool steel. Its price is only about 30% higher than the latter. A steel grade worth promoting.
However, 9Mn2V steel also has some shortcomings such as low impact toughness, and cracking phenomenon is found in production and use. In addition, the tempering stability is poor, and the tempering temperature generally does not exceed 180 °C. The bending strength and toughness at 200 °C tempering. A low value starts to appear.
9Mn2V steel can be quenched in quenching medium with moderate cooling capacity such as nitrate salt and hot oil. For some molds with strict deformation requirements and low hardness requirements, austenitic austempering can be used.
3) Cold working die with large size, complex shape and heavy load. It is necessary to use medium alloy or high alloy steel, such as Cr12Mo, Crl2MoV, Cr6WV ​​Cr4W2MoV, etc., in addition to high speed steel.
In recent years, the tendency to use high-speed steel as a cold-working mold has increased, but it should be pointed out that at this time, the red hard strength unique to high-speed steel is no longer utilized, and its high hardenability and high wear resistance are utilized. For this reason, there should be a difference in the heat treatment process.
When high-speed steel is used as the cold mold, low-temperature quenching should be used to improve the toughness. For example, the quenching temperature commonly used for W18Cr4V steel as a cutting tool is 1280-1290 °C. For cold working molds, low temperature quenching at 1190 °C should be used. Another example is W6Mo5Cr4V2 steel. After low temperature quenching, the life can be greatly improved, especially the loss rate is significantly reduced.
[4] A cold working die that is subjected to an impact load and has a thin knife. As mentioned above, the performance requirements of the first three types of cold working die steels are all based on high wear resistance and high carbon hypereutectoid steel or even Rongshi steel. For some cold-working molds, trimming floor, punching die, etc. The counterpart is thin. When used, it should be subjected to impact load and should be based on high impact toughness. In order to solve this contradiction, the following measures can be taken. 1 Reduce the carbon content. Use the sub-combined steel to avoid the toughness of the steel due to primary and secondary carbides; 2 Add alloying elements such as Si., Cr. Improve the tempering stability and tempering temperature of steel (240-270 °C tempering), which is beneficial to fully eliminate the quenching stress and increase the enthalpy without reducing the hardness; 2 adding W and other elements forming refractory carbides to refine Grain, improve toughness. Commonly used high-toughness cold work die steels are 6SiCr, 4CrW2Si; 5CrW2Si.
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