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）模腔表層金屬產(chǎn)生熱疲勞（龜裂）。熱作模具的工作特點是具有間歇性．每次使熱態(tài)金屬成形后都要用水、油、空氣等介質(zhì)冷卻模腔的表面。因此．熱模的工作狀態(tài)是反復(fù)受熱和冷卻，從而使模腔表層金屬產(chǎn)生反復(fù)的熱脹冷縮，即反復(fù)承受拉壓應(yīng)力作用．其結(jié)果引起模腔表面出現(xiàn)龜裂，稱為熱疲勞現(xiàn)象，由此，對熱模具鋼提出了二個基本使用性能要求．即具有高的熱疲勞抗力。一般說來，影響鋼的熱疲勞抗力的因素主要有：①鋼的導(dǎo)熱性。鋼的導(dǎo)熱性高，可使模具表層金屬受熱程度降低，從而減小鋼的熱疲勞傾向性。一般認為鋼的導(dǎo)熱性與合碳量有關(guān)，含碳量高時導(dǎo)熱性低，所以熱作模具鋼不宜采用高碳鋼。在生產(chǎn)中通常采用中碳鋼（C0.3%5～0.6%）合碳量過低．會導(dǎo)致鋼的硬度和強度下降．也是不利的。②鋼的臨界點影響。通常鋼的臨界點（Acl）越高．鋼的熱疲勞傾向性越低。因此．一般通過加入合金元素Cr、W、Si、引來提高鋼的臨界點。從而提高鋼的熱疲勞抗。

(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）模腔表層金屬受熱。通常錘鍛模工作時．其模腔表面溫度可達300～400℃以上熱擠壓模可達500一800℃以上；壓鑄模模腔溫度與壓鑄材料種類及澆注溫度有關(guān)。如壓鑄黑色金屬時模腔溫度可達1000℃以上。這樣高的使用溫度會使模腔表面硬度和強度顯著降低，在使用中易發(fā)生打垛。為此．對熱作模具鋼的基本使用性能要求是熱塑變抗力高，包括高溫硬度和高溫強度、高的熱塑變抗力，實際上反映了鋼的高回火穩(wěn)定性。由此便可以找到熱模具鋼合金化的一種途徑，即加入Cr、W、Si．等合金元素可以提高鋼的回火穩(wěn)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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