1. Hot extrusion mold systems and performance requirements
Figure 1-1 shows a hot extrusion mold system, mainly composed of an extrusion cylinder, a bushing, an extrusion rod, and a mold. Generally, the material of the extrusion mold system is selected based on the material of the extrusion part. The most commonly used extrusion material is aluminum alloy, whose ingots are heated to about 500 ℃ and then molded to form a surface. The copper alloy ingot needs to be heated to a higher temperature of about 800 ℃, that is, the A1 phase transformation point temperature of the mold steel is near or higher. Ferroalloys are heated to about 1200 ℃.
Generally, SKD61 steel and SKD62 steel are used as mold system materials, but for mold components that are in contact with ingots, such as molds, mandrels, extrusion pads, and linings, which are severely worn, SKD61 modified steel and SKD7 steel with high temperature and good wear resistance are used. Like mold pads, spindle seats. Auxiliary components such as extrusion cylinders that do not directly contact the ingot billet use SKT4 steel.
The key to these aluminum extrusion tools is the mold. Taking a hot extrusion mold as an example, the analysis results of its damage forms and performance requirements are shown in Figure 1-2. As the extrusion object is a high-temperature ingot in a pressurized state, the forming surface of the mold will undergo wear. In addition, its repeated bending deformation can accelerate wear and tear, while also causing cracks and damage due to fatigue. However, compared to molten state die casting, the temperature of ingots for extrusion is lower, so thermal cracking is relatively slight. In addition, in order to prevent wear and adhesion, nitriding treatment (generally gas nitriding and salt bath soft nitriding) can be carried out. Usually, after pressing a certain amount of ingots, repair them when the wear is still slight, and continue using them after nitriding treatment again. Therefore, for the same mold, nitriding with heat preservation at 570 ℃ for about 5h usually needs to be repeated 10-20 times, resulting in a gradual decline in the hardness inside the mold, resulting in a decline in strength. Therefore, materials are required to have excellent high-temperature softening resistance to withstand repeated nitridation.
In the past, aluminum extrusions were mainly used as building materials, but now, with the development of lightweight cars, trucks, and compartments, the use of high-strength aluminum alloys to make large components has an increasing trend. Therefore, mold materials are required to have better hardenability, higher resistance to high temperature softening, and high temperature strength.
2. Steel for aluminum extrusion molds
Aluminum extrusion molds are generally made of 5% Cr-1% Mo based SKD61 and SKD62 steel. In contrast, steels with better high-temperature softening resistance include 3% Cr-3% Mo based SKD7, etc. As shown in the tempering hardness curve in Figure 1-3, in the temperature range of 500-550 ℃ beyond secondary hardening, its softening resistance is the growth of chromium containing carbides (M7C3 type) and the increase of molybdenum based fine carbides (Mo2C) during tempering.
In order to meet the demand for large-scale molds in recent years, steel grades such as DHA2 (0.4% C-Ni-5.5-Cr-1.5% Mo-V) developed to improve softening resistance and high-temperature strength while maintaining hardenability have played a role. In addition, low Si-5% Cr-3% Mo steels that can be used for die casting molds have also been applied.
3. Steel for extrusion dies of copper and copper alloys
The heating temperature of copper alloys is inherently high, coupled with processing deformation heat, which can exceed the transformation point and enter the austenitic transformation zone for general mold steels. Therefore, it is necessary to use heat resistant steels such as SUH660 (≤ 0.08% C-26% Ni-15% Cr-1.25% Mo-2% Ti-V, AL). Because it is an austenitic heat resistant steel with an initial hardness of 35-37HRC, its ambient temperature strength is lower than that of SKD series mold steel, but its strength decreases slightly at high temperatures, which can be complementary to SKD series mold steel, and is particularly suitable for situations where high temperature strength needs to be maintained.
