The difference between hot work die steel and cold work die steel
The high thermal conductivity of steel can reduce the degree of heating of the surface metal of the mold, thereby reducing the thermal fatigue tendency of the steel. It is generally believed that the thermal conductivity of steel is related to the amount of combined carbon. When the carbon content is high, the thermal conductivity is low, so high-carbon steel is not suitable for hot work die steel. In production, medium carbon steel (C0.3%5～0.6%) is usually used. The carbon content is too low. It will cause the hardness and strength of the steel to decrease. It is also unfavorable.
②The impact of the critical point of steel. Generally, the higher the critical point (Acl) of steel, the lower the thermal fatigue tendency of steel. Therefore, the critical point of steel is generally increased by adding alloying elements Cr, W, and Si. Thereby improving the thermal fatigue resistance of steel.
2. Commonly used hot work die steel
(1) Steel for hammer forging dies. Generally speaking, steel for hammer forging dies has two prominent problems. One is impact load during work. Therefore, the mechanical properties of steel are higher, especially for plastic deformation resistance and toughness; the second is hammer forging. The cross-sectional size of the mold is relatively large (<400mm), so the hardenability of steel is required to ensure uniform structure and performance of the entire mold.
Commonly used hammer forging building steels are 5CrNiMo, 5CrMnMo, 5CrNiW, 5CrNiTi and 5CrMnMoSiV. Different types of hammer molds should be made of different materials. For extra large or large hammer forging dies, 5CrNiMo is better. 5CrNiTi, 5CrNiW or 5CrMnMoSi can also be used. For small and medium-sized hammer forging dies, 5CrMnMO steel is usually used.
(2) Hot extrusion die uses steel. The working characteristic of hot extrusion die is slower loading speed. Therefore, the heating temperature of the die cavity is higher, usually up to 500-800℃. The service performance requirements of this type of steel should be based on high high temperature strength (that is, high tempering stability) and high heat fatigue resistance. The requirements for ak and hardenability can be appropriately lowered. The size of the general hot extrusion die is small, often less than 70-90mm.
Commonly used hot extrusion dies include 4CrW2Si, 3Cr2W8V and 5%Cr type hot work die steels. The chemical composition is shown in Table 4.16.
Among them, 4CrW2Si. can be used as cold work die steel and hot work die steel. Due to different uses, different heat treatment methods can be used. When used as a cold mold, use a lower quenching temperature (870-900°C) and low or medium temperature tempering treatment; when used as a hot mold, use a higher quenching temperature (generally 950-1000°C) and high temperature tempering treatment.
(3) Steel for die-casting molds. On the whole, the performance requirements of die-casting die steel are similar to those of hot-extrusion die steel, that is, it requires high tempering stability and high thermal fatigue resistance. Therefore, the steel grades usually selected are generally the same as the hot extrusion die steels. As usual, steels such as 4CrW2Si. and 3Cr2W8V are used. However, there are some differences. For example, 40Cr, 30CrMnSi and 40CrMo can be used for Zn alloy die-casting molds with lower melting point; 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.
In recent years, with the application of ferrous metal die-casting technology, high-melting aluminum alloys and nickel alloys are mostly used. Or 3Cr2W8V steel is subjected to Cr-Al-SI ternary infiltration to manufacture ferrous metal die-casting molds. Recently, high-strength copper alloys are also being tested at home and abroad as die-casting mold materials for ferrous metals.
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