The main difference between the working conditions of hot work die steel and cold work die steel is that the main feature of the working conditions of hot work die steel is contact with hot metal. Hot working dies mainly include hammer forging die, hot extrusion die and die casting die. Due to the characteristics of working conditions of hot working die steel, the following problems will be brought:

（1）模腔表层金属产生热疲劳（龟裂）。热作模具的工作特点是具有间歇性．每次使热态金属成形后都要用水、油、空气等介质冷却模腔的表面。因此．热模的工作状态是反复受热和冷却，从而使模腔表层金属产生反复的热胀冷缩，即反复承受拉压应力作用．其结果引起模腔表面出现龟裂，称为热疲劳现象，由此，对热模具钢提出了二个基本使用性能要求．即具有高的热疲劳抗力。一般说来，影响钢的热疲劳抗力的因素主要有：①钢的导热性。钢的导热性高，可使模具表层金属受热程度降低，从而减小钢的热疲劳倾向性。一般认为钢的导热性与合碳量有关，含碳量高时导热性低，所以热作模具钢不宜采用高碳钢。在生产中通常采用中碳钢（C0.3%5～0.6%）合碳量过低．会导致钢的硬度和强度下降．也是不利的。②钢的临界点影响。通常钢的临界点（Acl）越高．钢的热疲劳倾向性越低。因此．一般通过加入合金元素Cr、W、Si、引来提高钢的临界点。从而提高钢的热疲劳抗。

(1) Thermal fatigue (cracking) occurs on the surface metal of the mold cavity. The working characteristic of hot working die is intermittent. After each hot metal forming, the surface of die cavity should be cooled by water, oil, air and other media. Therefore, the working state of the hot die is repeatedly heated and cooled, so that the metal on the surface of the die cavity produces repeated thermal expansion and cold contraction, that is, it repeatedly bears the action of tensile and compressive stress. As a result, the surface of the die cavity is cracked, which is called thermal fatigue. Therefore, two basic performance requirements are put forward for the hot die steel, that is, it has high thermal fatigue resistance. Generally speaking, the main factors affecting the thermal fatigue resistance of steel are: ① thermal conductivity of steel. The high thermal conductivity of the steel can reduce the heating degree of the metal on the surface of the die, so as to reduce the thermal fatigue tendency of the 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, so high carbon steel should not be used as hot work die steel. In production, medium carbon steel (c0.3% 5 ~ 0.6%) is usually used, and the carbon content is too low, which will reduce the hardness and strength of the steel. It is also unfavorable. ② The critical point of steel. Generally, the higher the critical point (ACL) of steel, the lower the thermal fatigue tendency of steel. Therefore, the critical point of steel is generally improved by adding alloying elements Cr, W, Si and lead. So as to improve the thermal fatigue resistance of steel.

（2）模腔表层金属受热。通常锤锻模工作时．其模腔表面温度可达300～400℃以上热挤压模可达500一800℃以上；压铸模模腔温度与压铸材料种类及浇注温度有关。如压铸黑色金属时模腔温度可达1000℃以上。这样高的使用温度会使模腔表面硬度和强度显着降低，在使用中易发生打垛。为此．对热作模具钢的基本使用性能要求是热塑变抗力高，包括高温硬度和高温强度、高的热塑变抗力，实际上反映了钢的高回火稳定性。由此便可以找到热模具钢合金化的一种途径，即加入Cr、W、Si．等合金元素可以提高钢的回火稳定性。

(2) The surface metal of the mold cavity is heated. Generally, when the hammer forging die is working, the surface temperature of the die cavity can reach more than 300 ~ 400 ℃, and the hot extrusion die can reach more than 500-800 ℃; The cavity temperature of die-casting die is related to the type of die-casting material and pouring temperature. For example, when casting ferrous metal, the mold cavity temperature can reach more than 1000 ℃. Such a high service temperature will significantly reduce the surface hardness and strength of the die cavity, and stacking is easy to occur in use. Therefore, the basic performance requirements for hot work die steel are high thermoplastic deformation resistance, including high temperature hardness and high temperature strength, high thermoplastic deformation resistance, which actually reflects the high tempering stability of the steel. Therefore, a way of alloying hot die steel can be found, that is, adding alloy elements such as Cr, W and Si can improve the tempering stability of steel.

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