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Composition and selection method of hot work die steel

Working conditions of hot work die

Hot work dies include hammer forging dies, hot extrusion dies and die casting dies. As mentioned above, the main feature of the working conditions of the hot working die is the contact with the hot metal, which is the main difference from the working conditions of the cold working die. Therefore, it will bring about the following two problems:


(l) The surface layer metal of the cavity is heated. Generally, when the hammer forging die works, the surface temperature of the cavity can reach 300~400 °C or higher, and the hot extrusion die can reach more than 500-800 °C; the cavity temperature of the die-casting die is related to the type of die-casting material and the pouring temperature. For example, when die casting ferrous metal, the cavity temperature can reach 1000 °C or above. Such a high use temperature causes a significant decrease in the hardness and strength of the cavity surface, and snoring is likely to occur during use. 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, which actually reflects the high tempering stability of steel. Therefore, the first way to alloy the hot die steel can be found, that is, the addition of alloying elements such as Cr, W, Si. can improve the tempering stability of the steel.


(2) Thermal fatigue (cracking) occurs in the surface layer of the cavity. The working characteristics of the hot mold are intermittent. Each time the hot metal is formed, the surface of the cavity is cooled by a medium such as water, oil or air. Therefore, the working state of the hot mold is repeated heat and cooling, so that the surface metal of the cavity is repeatedly subjected to thermal expansion and contraction, that is, repeatedly subjected to tensile and compressive stress. The result is cracking on the surface of the cavity, which is called thermal fatigue phenomenon. Thus, a second basic performance requirement for hot die steel is proposed. That is, it has high thermal fatigue resistance. In general, the main factors affecting the thermal fatigue resistance of steel are:


1 The thermal conductivity of steel. The high thermal conductivity of the steel can reduce the degree of heat 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 carbon combined. When the carbon content is high, the thermal conductivity is low. Therefore, hot-working die steel should not be made of high carbon steel. In the production, medium carbon steel (C0.35%~0.6%) is usually used, and the carbon content is too low. It will lead to the decrease of hardness and strength of steel. It is also unfavorable.


2 The critical point effect of steel. Generally, the higher the critical point (Acl) of the steel, the lower the thermal fatigue tendency of the steel. Therefore, the critical point of steel is generally increased by adding alloying elements Cr, W, Si. Thereby improving the thermal fatigue resistance of steel.


Commonly used hot work die steel

(1) Steel for hammer forging die. Generally speaking, there are two problems in the steel for hammer forging die. One is that it is affected by the impact load during work. Therefore, the mechanical properties of steel are required to be high, especially for plastic deformation resistance and toughness. Second, hammer forging The cross-section of the mold is large (<400mm), so the hardenability of the steel is required to ensure uniform microstructure and performance of the entire mold.


Commonly used hammer forging steels are 5CrNiMo, 5CrMnMo, 5CrNiW, 5CrNiTi and 5CrMnMoSiV. Different types of hammer eye molds should use different materials. For the extra large or large hammer forging die, 5CrNiMo is preferred. 5CrNiTi, 5CrNiW or 5CrMnMoSi may also be used. For small and medium-sized hammer forging dies, 5CrMnMO steel is usually used.


(2) Steel for hot extrusion die. The working characteristic of the hot extrusion die is that the loading speed is slow. Therefore, the cavity is heated at a high temperature, usually up to 500-800 °C. The performance requirements for such steels should be based on high temperature strength (ie high tempering stability) and high thermal fatigue resistance. The requirements for ak and hardenability can be appropriately lowered. The general hot extrusion die size is small, often less than 70~90 mm.


Commonly used hot extrusion molds include 4CrW2Si, 3Cr2W8V and 5%Cr type hot work die steel. Among them, 4CrW2Si. can be used as both cold work die steel and hot work die steel. Different heat treatment methods can be used due to different uses. For the cold mold, the lower quenching temperature (870-900 ° C) and low temperature or medium temperature tempering treatment are used; when the hot mold is used, the higher quenching temperature (generally 950-1000 ° C) and high temperature tempering treatment are adopted.


(3) Steel for die-casting molds. Generally speaking, the performance requirements of steel for die-casting molds are similar to those of steel for hot extrusion die, that is, high tempering stability and high thermal fatigue resistance are required. Therefore, the steel grades usually selected are generally the same as those for hot extrusion die. Steels such as 4CrW2Si. and 3Cr2W8V are often used. But it is different, such as the lower melting point of Zn alloy die-casting mold. 40Cr, 30CrMnSi and 40CrMo can be selected. For Al and Mg alloy die-casting molds, 4CrW2Si, 4Cr5MoSiV and other Cu alloy die-casting molds can be used. 3Cr2W8V steel is used.


In recent years, with the application of the ferrous metal die casting process, high melting point aluminum alloy and nickel alloy are used. Or Cr-Al-SI ternary co-infiltration of 3Cr2W8V steel is used to manufacture ferrous metal die casting mold. Recently, a high-strength copper alloy has been experimentally used as a die-casting material for ferrous metals.