After adding alloying elements to the steel, a certain amount of alloy carbides can be formed, the grains can be refined, the hardenability can be improved, and the tempering stability can be increased to meet the requirements of increasing wear resistance and improving toughness.
1. The role of Mn. Manganese strongly increases the hardenability of steel, greatly reduces the martensitic transformation temperature of steel, and increases the amount of retained austenite after quenching, which is beneficial to preventing workpiece quenching deformation, quenching cracking, and stabilizing dimensions. However, reducing the thermal conductivity of steel has greater overheating sensitivity and aggravates the second type of temper brittleness. Mn should be added in combination with Mo, V, Cr, W, etc. It is limited in impact-resistant and high-strength toughness die steels.
2. The role of Si. Silicon increases the hardenability and tempering stability of steel, significantly deformed resistance and impact fatigue resistance; it can also improve the oxidation resistance and corrosion resistance of steel. However, silicon promotes the precipitation of carbon in the steel in the form of graphite, resulting in a more serious decarburization tendency, and increases the overheating sensitivity and the second type of temper brittleness of the steel.
3. The role of Cr. Chromium significantly increases the hardenability of steel and effectively improves the tempering stability of steel. With the increase of carbon content in steel, carbides such as (Fe·Cr)3C and (Fe·Cr)23C are formed sequentially. These carbides have better stability, thereby reducing the overheating sensitivity of steel and improving the resistance of steel Abrasive. Chromium has a passivating effect on the steel surface, making the steel resistant to oxidation. However, higher chromium content will increase the carbide inhomogeneity and the amount of retained austenite. Generally, in low alloy steel, the mass fraction of chromium is 0.5%~1.5%; in high toughness die steel, the mass fraction of chromium is 4%~5%; 6%~12%.
4. The role of Mo. Molybdenum can improve hardenability and high-temperature creep strength, and its tempering stability and secondary hardening effect are also stronger than chromium; it can also inhibit the second type of temper brittleness caused by Cr, Mn, and Si. But molybdenum increases the decarburization tendency. The mass fraction of molybdenum in commonly used die steel is 0.5%~5%.
5. The role of W. A major advantage of tungsten is that it causes secondary hardening and significantly improves the thermal hardness of steel; it improves wear resistance and reduces the overheating sensitivity of steel better than molybdenum. However, tungsten can strongly reduce the thermal conductivity of steel. Excessive tungsten makes the carbides of tungsten uneven and reduces the strength and toughness of steel. In the cold working die steel with high bearing capacity, the mass fraction of tungsten is less than 18%, and there is a tendency to replace W with Mo and V and reduce the W content.
6. The role of V. Vanadium mainly exists in steel in the form of V4C3. Because V4C3 is stable and insoluble, and has extremely high hardness, vanadium can significantly improve the wear resistance and thermal hardness of steel; at the same time, vanadium can also refine grains and reduce overheating sensitivity. But too much vanadium will reduce forgeability and grindability. Therefore, the mass fraction of vanadium is generally controlled at 0.2%~2%.
7. The role of Co. The main function of cobalt is to improve the red hardness of high-speed steel and increase the secondary hardening effect. In cemented carbide materials, cobalt is an important binder.
8.The role of Ni. Nickel can not only increase the strength of steel, but also improve the toughness of steel, and at the same time improve the hardenability of steel; when the content is high, it can significantly improve the corrosion resistance of steel. But nickel has a tendency to increase the second type of temper brittleness.
