Metallographic diagram of 50 steel (intermediate frequency quenching, low temperature tempering)

The microstructure of 50 steel (medium frequency quenching, low temperature tempering) is mainly composed of eutectoid structure, the matrix is composed of ferrite and cementite, and the retained austenite converted from martensite ( M + A) is present in the quenched steel. Whenlow temperature tempering is carried out, retained austenite is decomposed, precipitation occurs, and a small amount of carbide is formed. Figure 1 shows the microstructure of 50# steel (medium frequency quenching and low temperature tempering).

Even if the steel has been quenched and tempered, there are still defects in the microstructure, such as internal shrinkage cracks and wear marks such as bumps, scratches, and grinding burns. In order to reduce such defects, the tempering temperature and tempering time should be properly determined during quenching and tempering. To prevent thermal cracking, it is better to temper at a lower temperature for a long period of time. For example, when 50# steel is quenched in oil, the quenching temperature should be controlled at 860 ± 5 ℃, and the tempering temperature should be in the range of 510-660 ℃. When the tempering temperature is too low, the retained austenite will not be decomposed into the eutectic state, too high, the pearlite structure will decompose and produce coarse carbide, and the wear resistance will be poor.

The martensite transformation temperature of 50 steel is 727-817 ℃, and the tempering temperature should also be selected within this temperature range. The eutectoid temperature of 50 steel is 723-756 ℃. When the tempering temperature is lower than the eutectoid temperature and higher than the martensite transformation temperature, a small amount of cementite confluence occurs in the grain boundary. If the tempering temperature is in this temperature range, the compressive residual stress can be formed on the top surface of the steel and wear resistance can be improved.

The oxide layer of 50# steel after quenching and annealing is mainly aluminum oxide, chromium oxide, ferrite oxide and silicate. The aluminum oxide and silicate are fluffy and coarse, and the chromium oxide and ferrite are fine and dense. The wear resistance of the steel is related to the hardness and thickness of the oxide layer on the surface. The higher the hardness and the thicker the oxide layer, the better the wear resistance.

In general, the microstructure of 50 steel (medium frequency quenching and low temperature tempering) is mainly composed of eutectoid structure, with ferrite and cementite matrix. There are wear marks such as bumps, scratches and grinding burns in the steel. To prevent thermal cracking, the tempering temperature and tempering time should be properly determined during quenching and tempering, and it is better to temper at a lower temperature for a long period of time. The impurities and oxide layer on the surface of the steel have a great influence on the hardness and wear resistance of the steel.

Steel 20Cr50 is a commonly used alloy steel, which is mainly composed of carbon (C) and chromium (Cr). It is of relatively high strength and wear resisting ability due to its appropriate content of chromium. 20Cr50 is a hardenable steel. After heat treatment, including high-frequency quenching and low temperature tempering, the hardness and durability of 20Cr50 steel can reach excellent levels. The chemical composition of 20Cr50 steel is as follows: 20Cr50 steel contains about 0.17-0.22% of carbon, 0.50-0.80% of manganese, 0.17-0.37% of phosphorus, 0.17-0.37% of sulfur, 0.90-1.20% of chromium, and 0.50-1.00% of silicon.

The metallographic structure of 20Cr50 steel is mainly ferrite and carbide, which is a pearlitic structure. With medium-frequency quenching, the hardenability of 20Cr50 steel can achieve better results. After the quenching of medium frequency, the organization of 20Cr50 steel mainly consists of granular or rod-shaped cementite particles and medium-sized ferrite grains and uneven grain. The cementite particles present an irregular shape, and the ferrite grain size and distribution are uniform. After low temperature tempering, the ferrite grain is further distributed, and some carbide particles are also broken. The 20Cr50 steel has a lower hardness but higher strength and better ductility.

In summary, 20Cr50 steel is a commonly used alloy steel. Its excellent wear resistance, toughness and high strength attributes make it widely used in many applications, from power transmission components to aerospace structures. By heat treatment, such as medium frequency quenching and low temperature tempering, 20Cr50 steel can be further improved its mechanical properties, so that it can better meet the requirements of different industries.