Characteristics of tool steel

High speed tool steel is mainly used to manufacture high efficiency cutting tools. Due to its high red hardness, good wear resistance and high strength characteristics, it is also used in the manufacture of molds, rolls, high temperature bearings and high temperature springs with high performance requirements. The hardness of high-speed tool steel after heat treatment can reach HRC63 or more, and it can still maintain high hardness at the working temperature of about 600 ° C, and its toughness, wear resistance and heat resistance are good. The main alloying elements of high-speed tool steel in annealed state are many, molybdenum, chromium, vanadium, and some high-speed tool steel has added cobalt, aluminum and other elements. This kind of steel belongs to high-carbon high-alloy letensite steel, one of its main structural characteristics is that it contains a large number 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 pressure processing, and are called primary carbides. Carbides precipitated from austenite and martensite matrix are called secondary carbides. These carbides have a great impact on the performance of high-speed tool steel, especially secondary carbides, which have a great impact on the Austenitic grain size and secondary hardening properties of steel. The quantity and type of carbides are related to the chemical composition of the steel, while the particle size and distribution of carbides are related to the deformation of the 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 steel. Chromium plays an important role in the hardenability, oxidation resistance and wear resistance of steel, and also plays a certain role in 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 more dissolved into the matrix, so that the steel has a 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 tempering hardness decreases at low temperature below 350 ° C. The tempering hardness gradually increases at temperatures above 350 ° C. The second hardness peak occurs in the tempering range of 520~580 ° C (different chemical composition, different tempering temperature), and exceeds the quenching hardness, which is the secondary hardening. This is an important characteristic of high speed tool steel.
In addition to high hardness, wear resistance, red hardness and other performance, high-speed tool steel also has a certain thermoplasticity, grindability and other process properties. The main alloying element of multi-series high-speed tool steel is tungsten, which does not contain molybdenum or contains a small amount of molybdenum. Its main characteristics are small overheating sensitivity, small decarbonization sensitivity, wide temperature range of heat treatment and hot processing, but the carbide particles are coarse, the distribution of uniformity is poor, affecting the toughness and plasticity of steel.
Tungsten and molybdenum are the main alloying elements of tungsten and molybdenum high speed tool steel. Its main characteristics are that the particle size and distribution of carbides are better than tungsten high-speed tool steel, decarbonization sensitivity and overheating sensitivity are lower than molybdenum high-speed tool steel, and the service performance and process performance are better.
The main alloying element of molybdenum high-speed tool steel is molybdenum, which does not contain tungsten or contains a small amount of tungsten. Its main characteristics are fine carbide particles, uniform distribution, good toughness, but large sensitivity to decarbonization and overheating, and narrow range of hot processing and heat treatment.