The working characteristics of hot work die steel are not continuous. Each time the metal material is formed in the heat treatment process, it is necessary to cool the surface of the cavity with water, oil, gas and other raw materials. Therefore, the operating state of the hot mold is to repeatedly heat up and cool down, so that the metal material on the surface of the mold cavity repeatedly expands and contracts. That is, the result of repeatedly bearing the tensile and compressive stress field causes the cavity surface to crack, which is called thermal fatigue phenomenon. Therefore, hot die steel clearly points out the requirement of the second essential feature, that is, it has high thermal fatigue resistance.
(1) Thermal conductivity of steel
The high thermal conductivity of steel can reduce the heating degree of the metal on the surface of the mold, thereby reducing 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 it is not suitable to use high carbon steel for hot work die steel. In production, medium carbon steel (C0.3% 5~0.6%) is usually used with too low carbon content, which will lead to a decrease in the hardness and strength of the steel. This is also disadvantageous.
(2) Influence of critical point of steel
Generally, the higher the critical point (Acl) of the steel, the lower the thermal fatigue tendency of the steel. Therefore, generally by adding alloying elements Cr, W, Si, the critical point of the steel is increased, thereby improving the thermal fatigue resistance of the hot work die steel.
In order to make metal workpieces have the required special properties, we process metal workpieces. In many cases, processing is also a particularly important step. Not only is the raw material important, but steel is the most widely used material in the machinery industry. The microstructure of hot work die steels are complex and can be controlled by heat treatment, so the heat treatment of steel is the main content of metal heat treatment. Next, HS Steel will introduce the heat treatment process of hot work die steel.
(1) The purpose of heat treatment is to change the internal structure of the metal material to improve its properties.
(2) Unlike other processing techniques, heat treatment generally does not change the shape and overall chemical composition of the workpiece, but increases or improves the performance of the workpiece by changing the microstructure of the workpiece or changing the chemical composition of the workpiece surface.
(3) After the heat treatment of hot work die steel, such a significant change in its properties is due to the change in its internal structure after different heating and cooling processes.
(4) In order to formulate the correct heat treatment process specification and ensure the heat treatment quality, it is necessary to understand the structure change law of steel under different heating and cooling conditions. The law of microstructure transformation in steel is the principle of heat treatment of steel.
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