Classification, use and grades of alloy steel
Alloy steel is a special purpose steel by adding a certain amount of alloying elements to Fe-C alloy steel. The classification of alloy steel is usually classified according to its use, alloying element content, metallurgical quality and so on. According to its use, metallurgical quality, etc., it is similar to carbon steel. Here only introduces the classification according to the content of alloying elements.
Classification of alloy steel
1. According to the total amount of alloying elements, the total amount of alloys <5 is called low alloy steel, 5% ≤ total alloy <10% is called medium alloy steel, and the total amount of alloy ≥ 10% is called high alloy steel.
2. According to the main alloying elements, such as: 3Cr3MoW2V steel is called chromium tungsten silicon steel.
Grades of alloy steel
The first digit indicates the carbon content wc. If it is a single digit, it is expressed in thousand fractions. If it is two digits, it is expressed in ten thousand fractions. If there is no number, it means wc≈1%. ); When the average carbon content wc≤0.08%, it is represented by "0", and when the average carbon content wc≤0.03%, it is represented by "00". The number after the element indicates the average percentage content of the element. No number indicates that the content of this element is less than 1.5%.
Taking 8Cr2MnWMoVS as an example, 8 means wc≈0.8%, Cr2 means wc≈2%, and the contents of Mn, W, Mo, V, and S are all less than 1.5%.
The role of commonly used alloying elements
After alloying elements are added to steel, a certain amount of alloy carbides can be formed to refine grains, improve hardenability, and increase tempering stability to meet the requirements of increasing wear resistance and improving toughness. The main alloying elements added and their functions are as follows:
1. The role of Mn Manganese strongly increases the hardenability of steel, greatly reduces the martensite transformation temperature of steel, and increases the amount of retained austenite after quenching. This is beneficial to preventing quenching deformation, quenching cracking, and stabilizing the size of the workpiece. But reducing the thermal conductivity of steel has greater sensitivity to overheating, and aggravates the second type of temper brittleness. Mn should be added in combination with Mo, V, Cr, W, etc. Limited in impact resistance and high strength and toughness die steel.
2. The role of Si Silicon increases the hardenability and tempering stability of steel, and significantly improves the resistance after deformation 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 tendency to decarburization, and increasing the overheating sensitivity of steel and the second type of temper brittleness.
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 sequentially formed. These carbides have better stability, thereby reducing the overheating sensitivity of steel and improving the resistance of steel. Abrasiveness. Chromium has a passivation effect on the surface of steel, which makes the steel anti-oxidant. However, higher chromium content will increase 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%; in high wear-resistant micro-deformation die steel, the mass fraction of chromium It is 6%~12%.
4. The role of Mo Molybdenum can improve hardenability and high temperature creep strength, tempering stability and secondary hardening effect is also stronger than chromium; and can inhibit the second type of temper brittleness caused by Cr, Mn, and Si. But molybdenum increases the tendency of decarburization. The mass fraction of molybdenum in common 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 is better than molybdenum in improving wear resistance and reducing the overheating sensitivity of steel. But tungsten can strongly reduce the thermal conductivity of steel, and excessive tungsten makes the carbides of tungsten uneven, and the strength and toughness of steel are reduced. In the cold work die steel with high load-bearing capacity, the mass fraction of tungsten is less than 18%, and there is a trend of replacing W with Mo and V and reducing the content of W.
6. The role of V Vanadium mainly exists in steel in the form of V4C3. Because V4C3 is stable and difficult to dissolve 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. However, too much vanadium will reduce the forgeability and grindability. Therefore, the mass fraction of vanadium is generally controlled at 0.2%~2%.
7. The role of Co The main role 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 bonding agent.
8. The role of Ni Nickel can not only increase the strength of steel, but also increase the toughness of steel, and at the same time improve the hardenability of steel; when the content is higher, it can significantly improve the corrosion resistance of steel. However, nickel tends to increase the second type of temper brittleness.
Each grade of alloy steel is one of the best combinations of alloying element content and alloying elements. When analyzing the performance and characteristics of alloy steel, analysis can be conducted from the analysis of alloying element content and alloying element combination of alloy steel.