Hot working dies include hammer forging die, hot extrusion die and die casting die. As mentioned above, the main characteristic of hot working die working condition is contact with hot metal, which is the main difference from cold working die working condition. Therefore, there will be two problems as follows:

(l)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著降低，在使用中易发生打垛。为此.对热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。

(l) The surface metal of die cavity is heated. In general, the surface temperature of die cavity of hammer forging die can reach 300 ~ 400 ℃ or above, and hot extrusion die can reach 500 ~ 800 ℃. The cavity temperature of die casting die is related to the type of die casting material and pouring temperature. For example, the temperature of die cavity can be over 1000 ℃ when casting ferrous metal. Such a high temperature will make the surface hardness and strength of the die cavity significantly reduced, and it is easy to stack in use. Therefore, the basic performance requirement of hot die steel is high thermoplastic deformation resistance, including high temperature hardness and high temperature strength, high thermoplastic deformation resistance, which actually reflects the high tempering stability of steel. Therefore, the first way of alloying hot die steel can be found, that is, adding CR, W, Si and other alloy elements can improve the tempering stability of the steel.

(2)模腔表层金属产生热疲劳(龟裂)。热模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，对热模具钢提出了第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：

(2) Thermal fatigue (cracking) occurs on the surface metal of die cavity. The working characteristic of hot die is intermittency. The surface of die cavity must be cooled by water, oil, air and other media every time hot metal is formed. Therefore, the working state of the hot die is repeated heating and cooling, so that the surface metal of the die cavity will produce repeated thermal expansion and cold contraction, that is, repeatedly bear the tensile and compressive stress. As a result, cracks appear on the surface of the die cavity, which is called thermal fatigue phenomenon. Therefore, the second basic performance requirement for hot die steel is put forward, that is, high thermal fatigue resistance. Generally speaking, the main factors affecting the thermal fatigue resistance of steel are as follows:

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.

① Thermal conductivity of steel. The high thermal conductivity of steel can reduce the heating degree of the surface metal of the die, thus reducing the thermal fatigue tendency of the steel. It is generally considered that the thermal conductivity of steel is related to the carbon content, and the thermal conductivity is low when the carbon content is high. Therefore, high carbon steel is not suitable for hot working die steel. In production, medium carbon steel (c0.3% 5 ~ 0.6%) is usually used. If the carbon content is too low, the hardness and strength of steel will be decreased, which is also unfavorable

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力.

② Critical point effect of steel. Generally, the higher the critical point (ACL) of steel is, the lower the thermal fatigue tendency of steel is. Therefore, the critical point of steel is generally increased by adding alloying elements Cr, W, Si and lead. So as to improve the thermal fatigue resistance of steel