1 hot die working conditions, including hot die forging die, hot extrusion die and die-casting mold categories. As previously described. Hot working conditions for the main features of the mold is in contact with hot metal, which is the working conditions with the cold mold the main difference. So will have the following two problems: (l) heated metal mold cavity surface. Usually die forging hammer work. The cavity surface temperature up to 300 ~ 400 ℃ above hot extrusion die more than 800 ℃ up to 500 a; die casting mold cavity temperature and type of material and casting temperature. Such as black metal die-casting mold cavity when the temperature can reach above 1000 ℃. This high temperature causes the cavity surface hardness and strength significantly reduced, in the use of prone play pile. This. Hot die steel for the basic performance requirements for high-resistance thermoplastic change, including the high-temperature hardness and high temperature strength, high resistance thermoplastic change, in fact, reflects the high-tempered steel stability. Thus we can find the hot die steel alloy of the first approach, by adding Cr, W, Si. Other alloying elements can improve the stability of steel tempering. (2) cavity surface metal thermal fatigue (cracking). Thermal model of the work is characterized by intermittent. Each time the hot water after each metal forming, oil, air and other media, cooling the surface of the cavity. So. Thermal model of the working condition is repeated heating and cooling, so that the cavity surface of the metal produced repeated thermal expansion and contraction, that is repeated under compressive and tensile stress. As a result, the surface of the cavity caused by cracks, the phenomenon known as thermal fatigue, which, hot-die steel made of the second basic performance requirements. That has a high thermal fatigue resistance. In general, the impact of thermal fatigue resistance of steel main factors: ① steel thermal conductivity. High thermal conductivity of steel, can minimize mold heated metal surface, thereby reducing the tendency of steel to thermal fatigue. Generally believed that the thermal conductivity of steel and cooperation on carbon, high carbon content, low thermal conductivity, it should not be hot work die steel with high carbon steel. Usually used in the production of medium carbon steel (C0.3% 5 ~ 0.6%) combined carbon is too low. Will lead to the hardness and strength decrease. Also a disadvantage. ② the critical point of impact of steel. Critical point usually steel (Acl) higher. Steel, the lower the tendency of thermal fatigue. So. Usually by adding alloying elements Cr, W, Si, the critical point lead to improve the steel. Thereby improving the thermal fatigue resistance of steel. (2) used hot work tool steel (1) hammer forging die steel. In general, there are two steel hammer forging die first, the more prominent role when working under shock loading. Therefore, the mechanical properties of steel are higher, especially for the plastic deformation resistance and toughness are higher; second forging die cross-section of a larger size (<400mm) so the steel of high hardenability requirements, to ensure the and properties of the mold evenly. Building steel forging a common 5CrNiMo, 5CrMnMo, 5CrNiW, 5CrNiTi and 5CrMnMoSiV and so on. Different types of hammer dies should be used in different materials. For large or large hammer forging die to 5CrNiMo as well. Also can be used 5CrNiTi, 5CrNiW or 5CrMnMoSi and so on. For small and medium sized hammer forging die steels are usually used 5CrMnMO. (2) hot extrusion die steel, hot extrusion die of the work is characterized by slow loading, so the cavity heating temperature is higher, typically up to 500 to 800 ℃. The use of such steel shall be high performance requirements of high-temperature strength (ie high temper resistance) and high thermal fatigue performance-based. Ak and hardenability of the requirements may be appropriate to lower. Hot extrusion die size is generally small, usually less than 70 ~ 90 mm. Commonly used hot extrusion die has 4CrW2Si, 3Cr2W8V and 5% Cr-based and other hot work tool steel. Its chemical composition as shown in Table 4.16. Which 4CrW2Si. Can do the cold-working die steel, hot die steel can be done. As for different purposes, using different heat treatment methods. Cold model used for low quenching temperature (870-900 ℃) and low or medium temperature tempering; for thermal model is used when a higher quenching temperature (usually 950 a 1000 ℃) and high-temperature tempering. (3) die-casting mold steel. Overall, the use of steel for die casting and hot extrusion die performance requirements similar to steel, that require high temper resistance and high thermal fatigue resistance based. So often the choice of steel and hot-extrusion die steel roughly the same. As usual using 4CrW2Si. And 3Cr2W8V other steel. But differ as to the low melting point Zn alloy die-casting mold. Choice of 40Cr, 30CrMnSi and 40CrMo, etc.; of Al and Mg alloy die-casting mold, can be used 4CrW2Si, 4Cr5MoSiV other on the Cu alloy die-casting mold. Use more 3Cr2W8V steel. In recent years. With the application of ferrous metal die-casting process, the use of high melting point of aluminum and nickel alloys. Or to 3Cr2W8V steels Cr-Al-SI ternary infiltration, for the manufacture of ferrous metal die-casting mold. Recent experiments at home and abroad is also high-strength copper alloy for ferrous metal die-casting mold material.