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The requirements and composition characteristics of the material properties of the hot mold. |
1. Performance requirements.
According to the working conditions, hot mold can be divided into hot forging die, hot extrusion die, die casting mould and hot blanking die.
The hot working tool has the effect of both powerful and temperature in the work, which complicates the working conditions of the mould, and the characteristics of the mould processing materials are more strict. In order to meet the requirements of the use of hot mold, the following basic characteristics are required for the hot mold processing materials:
1) high temperature strength and good toughness. The thermal mold, especially the hot forging die, bears a great impact on the work, and the impact frequency is very high. If the mold does not have high strength and good toughness, it is easy to crack.
2) good wear resistance. Because of hot-work die work besides is generated when the blank deformation and friction and wear, but also by high temperature corrosion and oxidation iron oxide abrasive, so need to hot mould processing material with high hardness and resistance to become attached.
3) high thermal stability. Thermal stability refers to the ability of steel to maintain its normal temperature and mechanical properties at high temperature. When working in the mold, the contact is hot metal, even liquid metal, so the surface temperature of the mold is very high, usually 400, 70090. This requires that the hot mold processing materials do not soften at high temperature and have high thermal stability, otherwise the plastic deformation will occur, resulting in collapse and failure.
4) excellent thermal fatigue resistance. The working characteristics of hot-work die is repeatedly heated enough, mould thermal expansion, and cooling contraction, forming a large thermal stress, thermal stress and this is in the opposite direction, alternate. In again and again under the action of thermal stress, the mold surface can form the reticular crack (crack), the phenomenon is called thermal fatigue, die prematurely due to thermal fatigue fracture, is one of the main reasons for hot-work die failure. Therefore, the hot working materials must have good thermal fatigue.
5) high hardenability. The general size of the hot mold is large, and the hot forging die is especially so. In order to make the mechanical property of the entire die section uniform, it is required that the hot die steel has high hardenability.
6) good thermal conductivity. In order to make the mould not excessive heat, resulting in a decline in mechanical properties, as far as possible to reduce the mold surface temperature, reduce the mold internal temperature, this requires hot-work die processing materials should have good thermal performance.
7) good forming process performance to meet the need of machining.
Ii. Composition characteristics.
The composition of hot die steel is similar to that of alloy tempered steel, and the mass fraction of carbon is less than 0.5%(the mass fraction of individual steel carbon can reach 0.6% and 0.7%), and contains Cr, Mn, Ni, Si and other alloy elements. Low carbon content ensures sufficient toughness. The role of alloying elements is to strengthen ferrite and increase hardenability. In order to prevent the temper brittleness, also add human Mo,w and other elements; In order to improve the high temperature strength and thermal fatigue resistance, a considerable number of Cr, W and Si are needed. These elements raise the temperature of phase transition, so that the surface changes in the heat and cooling process without the occurrence of phase change, which can improve the thermal fatigue resistance. In addition, W, Mo, V, etc to form carbide precipitation during tempering and produce secondary hardening, make steel at high temperature remains quite high hardness, this is the normal work of the steel are one of the important conditions.
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