The metal on the surface of the cavity is heated. Usually when the hammer forging die works. The surface temperature of the mold cavity can reach 300-400 \u2103 or more. The hot extrusion die can reach 500-800 \u2103 or more. For example, when die-casting ferrous metal, the cavity temperature can reach above 1000 \u2103. Such a high use temperature will significantly reduce the surface hardness and strength of the mold cavity, and it is easy to stack during use. to this end. The basic performance requirements for hot work die steel are high thermoplastic resistance, including high temperature hardness and high temperature strength, and high thermoplastic resistance, which actually reflects the high tempering stability of steel. From this we can find the first way of hot die steel alloying, namely adding Cr, W, Si. Other alloying elements can improve the tempering stability of steel.The metal on the surface of the cavity produces thermal fatigue (cracking). The working characteristics of hot work molds are intermittent. Every time the hot metal is formed, the surface of the mold cavity must be cooled with water, oil, air and other media. therefore. The working state of the hot mold is repeated heating and cooling, so that the metal on the surface of the mold cavity undergoes repeated thermal expansion and contraction, that is, repeated tensile and compressive stress. As a result, the surface of the mold cavity is cracked, which is called thermal fatigue phenomenon. Therefore, the second basic performance requirement for hot mold steel is put forward. That is, it has high resistance to thermal fatigue. Generally speaking, the main factors that affect the thermal fatigue resistance of steel are: the thermal conductivity of steel. The high thermal conductivity of steel can reduce the heating of the metal on the surface of the mold, thereby reducing the tendency of steel to thermal fatigue. It is generally believed that the thermal conductivity of steel is related to the amount of carbon, and when the carbon content is high, the thermal conductivity is low, so it is not appropriate to use high carbon steel for hot work die steel. Medium carbon steel (C0.3% 5 \uff5e 0.6%) is usually used in production with too low carbon content. Will cause the hardness and strength of steel to drop. It is also unfavorable. The impact of the critical point of steel. Generally, the higher the critical point (Acl) of steel. The lower the tendency of steel to thermal fatigue. therefore. Generally by adding alloying elements Cr, W, Si and lead to raise the critical point of steel. Thereby improving the thermal fatigue resistance of steel.