Hot work die steel includes three types: hammer forging die, cold extrusion die and die casting die. The main feature of hot work die work is contact with hot metal materials, which is the key difference from cold work die work standards. As a result, the following two-level problems arise:
Generally, when the hammer forging die is working, the surface temperature of the die cavity can reach above 300~490℃, and the cold extrusion die can reach above 503-800℃; the cavity temperature of the die-casting die is related to the raw material type and pouring temperature of aluminum die-casting.
For example, when aluminum die-casting light metal, the cavity temperature can reach above 1000 ℃. Such a high application temperature will significantly reduce the surface strength and compressive strength of the cavity, which is prone to stacking during application. Therefore, the basic performance index for mold steel is high thermoplastic deformation resistance, including high temperature strength and high temperature compressive strength, high thermoplastic deformation resistance, which in fact reflects the high tempering reliability of steel. Therefore, it is possible to find the first way of fine-grain strengthening of hot mold steel, that is, adding aluminum alloy elements such as C, W, Si, etc., can improve the tempering reliability of steel.
The working characteristics of the hot mold are intermittent, and each time the hot metal material is formed, tap water, oil, gas and other substances are required to cool the surface of the mold cavity.
Therefore, the operating state of the hot mold is constantly exposed to heat and cooling, which in turn causes the metal material on the surface of the mold cavity to cause continuous thermal expansion and contraction, that is, it continuously bears the effect of tensile and compressive stress. As a result, the surface of the mold cavity is cracked, called thermal fatigue condition. Therefore, the second basic performance index is clearly defined for hot work die steel: high thermal fatigue resistance.
In general, the key elements that compromise the thermal fatigue resistance of steel are:
1. Heat transfer of steel
The high heat transfer property of steel can reduce the heat level of the metal material on the surface of the mold, thereby reducing the thermal fatigue selectivity of the steel. It is generally believed that the heat transfer property of steel is related to the carbon content. When the carbon content is high, the heat transfer property is low, so the hot work die steel is not suitable for medium carbon steel. Low carbon steel is generally used in production, and the carbon content is too low, which will reduce the hardness and compressive strength of the steel, which is also not good.
2. The zero-point hazard of steel
The higher the zero boundary point (Acl) of the steel, the lower the thermal fatigue selectivity of the steel. Therefore, the zero boundary point of steel is generally improved by adding aluminum alloy elements Cr, W, and Si. This improves the thermal fatigue resistance of the steel.
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