Features of hot work mold

Hot work molds mainly include hammer forging molds, hot extrusion molds and die-casting molds. The main feature of the working conditions of the hot work mold is the contact with the hot metal, which is the main difference between the working conditions of the cold work mold. Therefore, it will bring the following two problems.

(1) When the surface metal of the die cavity is heated and the hammer forging die is working, the surface temperature of the die cavity can reach 300～400℃, the hot extrusion die can reach 500～800℃, the temperature of the die-casting die cavity and the die-casting material The type is related to the pouring temperature. For example, the mold cavity temperature can reach ≥1000℃ when die-casting ferrous metals. Such a high use temperature will significantly reduce the hardness and strength of the mold cavity surface. Therefore, the basic performance requirements for hot work die steel are high thermal plastic deformation resistance. , Including high high temperature hardness and high temperature strength, high thermoplastic resistance, which is the tempering stability of steel. The addition of Cr, W, Si and other alloying elements can improve the tempering stability of steel through alloying.

(2) Thermal fatigue (cracking) on u200bu200bthe surface metal of the mold cavity. Hot work molds are generally intermittent during work. Each time the hot metal is formed, the surface of the mold cavity must be cooled with water, oil, air and other media. . Therefore, its working state is repeated heating and repeated cooling, so that the surface metal of the mold cavity has repeated thermal expansion and contraction, that is, it repeatedly bears the action of alternating tension and compression stress. As a result, cracks appear on the surface of the mold cavity, which is called a thermal fatigue phenomenon. For this reason, the hot work die steel is required to have high thermal fatigue resistance. The main factors affecting the thermal fatigue resistance of steel are as follows.

①The thermal conductivity of steel: higher thermal conductivity can reduce the degree of heating of the surface metal of the mold, thereby reducing the thermal fatigue tendency of steel. It is generally believed that the thermal conductivity of steel is related to the carbon content. When the carbon content is high, the thermal conductivity is low, and when the carbon content is too low, the hardness and strength of the steel will decrease. Therefore, medium carbon steel is usually used as hot work mold in production.

②The influence of the critical point of steel: Generally, the higher the critical point (Ac1) of steel, the lower the thermal fatigue tendency of steel. Therefore, the alloying elements Cr, W, and Si are generally added to increase the critical point of the steel to achieve the purpose of improving the thermal fatigue resistance of the steel.