High-speed tool steel is mainly used to manufacture high-efficiency cutting tools. When machining tool steel, due to its high red hardness, good wear resistance and high strength, it is also used to manufacture molds, rolls, high temperature bearings and high temperature springs with high performance requirements. The high-speed tool steel has a hardness of over HRC63 after heat treatment. It can still maintain high hardness at an operating temperature of about 600°C, and its toughness, wear resistance and heat resistance are good. The main alloying elements of annealed high-speed tool steel are molybdenum, molybdenum, chromium, vanadium, and some high-speed tool steels have added cobalt, aluminum and other elements. This type of steel is a high-carbon high-alloy laisenite steel, and one of its main structural characteristics is that it contains a large amount of carbides.
The carbides in the as-cast high-speed tool steel are eutectic carbides, which are broken into particles and distributed in the steel after hot pressing, and are called primary carbides; the carbides precipitated from the austenite and martensite matrix are called It is secondary carbide. These carbides have a great influence on the performance of high-speed tool steels, especially secondary carbides, which have a great influence on the austenite grain size and secondary hardening of steel. The number and type of carbides are related to the chemical composition of steel, while the particle size and distribution of carbides are related to the amount of deformation of steel. Tungsten and molybdenum are the main alloying elements of high-speed tool steel, which play an important role in the secondary hardening and other properties of the steel. Chromium plays an important role in the hardenability, oxidation resistance and wear resistance of steel, and also has a certain effect on secondary hardening. Vanadium plays an important role in the secondary hardening and wear resistance of steel, but reduces the grindability.
The quenching temperature of high-speed tool steel is very high, close to the melting point, and its purpose is to make the alloy carbide dissolve into the matrix more, so that the steel has better secondary hardening ability. The hardness of high-speed tool steel increases after quenching, which is the first hardening, but the higher the quenching temperature, the lower the strength and toughness after tempering. After quenching, the low-temperature tempering hardness decreases below 350℃. The tempering hardness gradually increases above 350℃. When tempering (with different chemical composition and different tempering temperature) occurs in the range of 520~580℃, the second hardness peak appears and exceeds Hardening hardness, this is secondary hardening. This is an important characteristic of high-speed tool steel.
In addition to high hardness, wear resistance and red hardness, high-speed tool steel also has certain technological properties such as thermoplasticity and grindability. The main alloy element of multi-system high-speed tool steel is tungsten, which contains no molybdenum or contains a small amount of molybdenum. Its main characteristics are small overheat sensitivity, low decarburization sensitivity, wide temperature range for heat treatment and hot working, but the carbide particles are coarse and the distribution uniformity is poor, which affects the toughness and plasticity of the steel.
The main alloy elements of tungsten-molybdenum high-speed tool steel are tungsten and molybdenum. Its main feature is that the particle size and distribution of carbides are better than tungsten-based high-speed tool steel, decarburization sensitivity and overheating sensitivity are lower than molybdenum-based high-speed tool steel, the use performance and process performance are better.
The main alloy element of molybdenum-based high-speed tool steel is molybdenum, which contains no tungsten or contains a small amount of tungsten. Its main characteristics are fine carbide particles, uniform distribution, good toughness, but high sensitivity to decarburization and overheating, and a narrow range of hot working and heat treatment.
Cobalt-containing high-speed tool steel is to add a certain amount of cobalt on the basis of general high-speed tool steel, which can significantly improve the hardness, wear resistance and toughness of the steel.
Powder high speed tool steel is produced by powder metallurgy method. First, the low-oxygen high-speed tool steel pre-alloy powder is prepared by the atomization method, and then the powder is compacted into a fully dense steel billet by cold and hot static presses, and then forged and rolled into a material. Powder high-speed tool steel has fine carbide, uniform distribution, good toughness, grindability and dimensional stability. It can produce super-hard high-speed tool steel that may produce higher alloy element content by ingot method.
Powder high-speed tool steel can be divided into 3 categories, the first type is cobalt-containing high-speed tool steel, which is characterized by hardness close to cemented carbide, and also has good forgeability, machinability, grindability and toughness . The second category is cobalt-free high-tungsten, molybdenum, vanadium superhard high-speed tool steel. The third category is super wear-resistant high-speed tool steel. The hardness is not too high, but the wear resistance is excellent, mainly used in working conditions that require high wear resistance and withstand impact loads.