Hot work die steel
Hot work die steel hardness requirements of appropriate, focusing on red hardness, thermal conductivity, wear resistance. Therefore, low carbon content, alloying elements to increase the hardenability, improve wear resistance, red hardness-based.
Hot work die steel
1 hot working mold working conditions Hot working mold, including forging die, hot extrusion die and die casting mold three categories. As mentioned before. The main feature of hot working die working conditions is contact with hot metal, which is the main difference with the working conditions of cold working die.
Therefore, it will bring the following two problems:
(L) Metal surface of the cavity is heated. Usually hammer forging work. The cavity surface temperature up to 300 ~ 400 ℃ above the hot extrusion mold up to 500 a 800 ℃ above; die-casting mold cavity temperature and die-casting material types and pouring temperature. Such as die-casting black metal when the cavity temperature up to 1000 ℃. Such a high temperature will make the cavity surface hardness and strength significantly reduced, easy to use in the stack. to this end. The basic performance requirements for hot tool steel are high thermoplastic resistance, including high temperature hardness and high temperature strength, high thermoplastic resistance, in fact, reflects the high tempering stability of steel. From this we can find the first way of hot die steel alloying, that is, adding Cr, W, Si. Other alloying elements can improve the tempering stability of steel.
(2) Mold cavity surface thermal fatigue (crack). The hot mold is characterized by intermittent work. After each hot metal forming the water, oil, air and other media cooling cavity surface. therefore. Hot work mode is repeatedly heated and cooled, so that the surface of the mold cavity metal repeated thermal expansion and contraction, that is, repeated tensile and compressive stress. The result of the cavity surface cracks, known as the thermal fatigue phenomenon, thus, the hot mold steel made the second basic performance requirements. That is, have high thermal fatigue resistance. In general, the factors that affect the thermal fatigue resistance of steel are: thermal conductivity of steel. High thermal conductivity of steel, mold surface metal can reduce the degree of heat, thereby reducing the thermal fatigue tendencies of steel. It is generally believed that the thermal conductivity of steel is related to the amount of carbon and the thermal conductivity is low when the carbon content is high, so hot-working die steel should not be made of high-carbon steel. In the production of carbon steel is usually used (C0.3% 5 ~ 0.6%) together carbon content is too low. Will lead to steel hardness and strength decreased. It is also unfavorable. ② the critical point of steel. Usually the critical point of the steel (Acl) the higher. The lower the thermal fatigue tendencies of the steel. therefore. Generally by adding alloying elements Cr, W, Si, lead to improve the critical point of steel. Thus improving the thermal fatigue resistance of steel.
2 commonly used hot die steel
(1) forging die steel. In general, the hammer forging die steel has two problems more prominent one is affected by the role of impact load. Therefore, the mechanical properties of steel demanding higher, especially for the plastic deformation resistance and toughness higher; Second, the larger cross-section hammer size (<400mm) so the hardenability of the steel higher to ensure that the entire Die organization and uniform performance. Common hammer forging floor steel with 5CrNiMo, 5CrMnMo, 5CrNiW, 5CrNiTi and 5CrMnMoSiV and so on. Different types of hammer eye mold should use different materials. 5CrNiMo for large or large hammer forging die as well. Can also be used 5CrNiTi, 5CrNiW or 5CrMnMoSi and so on. For small and medium-sized hammer forging mold is usually selected 5CrMnMO steel.
(2) hot extrusion die steel, hot extrusion die is characterized by the slow loading speed, therefore, the higher temperature cavity heating, usually up to 500 a 800 ℃. The use of such steel performance requirements should be high temperature strength (ie, high tempering stability) and high thermal fatigue resistance based. Ak and hardenability requirements may be appropriate to lower. General hot extrusion die smaller size, often less than 70 ~ 90 mm. Commonly used hot extrusion die 4CrW2Si, 3Cr2W8V and 5% Cr type hot work die steel. The chemical composition shown in Table 4.16. Among them 4CrW2Si. Can do cold work die steel, but also do hot work die steel. Due to different uses, different heat treatment methods can be used. For the cooling mode, the lower quenching temperature (870-900 ℃) and the tempering at low or medium temperature are adopted. For the thermal mode, the higher quenching temperature (950-1000 ℃) and the tempering at high temperature are adopted.
(3) die-casting mold steel. Overall, the use of die-casting mold steel performance requirements and hot extrusion die steel similar, that is, the requirements of high tempering stability and high thermal fatigue resistance based. Therefore, the commonly used steel is generally the same as hot extrusion die steel. As usual 4CrW2Si. And 3Cr2W8V other steel. But different as the lower melting point Zn alloy die-casting mold. Optional 40Cr, 30CrMnSi and 40CrMo, etc .; for Al and Mg alloy die-casting mold, optional 4CrW2Si, 4Cr5MoSiV, such as Cu alloy die-casting mold. Use more 3Cr2W8V steel. In recent years. With the application of black metal die-casting process, the use of high-melting aluminum and nickel alloys. Or 3Cr2W8V steel Cr-Al-SI ternary seepage, used to create black metal die casting. Recently at home and abroad are also testing the use of high strength copper alloy for black metal die casting mold material.
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