Medium
carbon steel pipe with seamless is a carbon steel pipe with a carbon content of 0.25% to 0.60%. It includes most of the high-quality carbon structural steel and a part of ordinary carbon structural steel. Most of this type of steel is used to make various mechanical parts, and some are used to make engineering structural parts.
According to the chemical composition of steel, it can be divided into two categories: carbon steel and
alloy steel.
Carbon steel is divided into: ① low carbon steel, carbon content is less than 0.25%; ② medium carbon steel, carbon content is 0.25% - 0.60%; ③ high carbon steel, carbon content is greater than 0.6%;
Medium carbon steel has good hot working and cutting performance, but poor welding performance. The strength and hardness are higher than those of low carbon steel, while the plasticity and toughness are lower than those of low carbon steel. Cold-rolled and cold-drawn materials can be used directly without heat treatment, or they can be used after heat treatment. The quenched and tempered medium carbon steel has good comprehensive mechanical properties. The highest hardness that can be achieved is about HRC55 (HB538), and σb is 600-1100MPa. Therefore, among various uses of medium strength level, medium carbon steel is the most widely used, in addition to being used as a building material, it is also widely used in the manufacture of various mechanical parts.
Medium carbon steel pipe belongs to hypoeutectoid steel, and its annealed structure is pearlite and ferrite. As the carbon content in the steel increases, the amount of pearlite in the microstructure increases, while the amount of ferrite decreases. The quenched structure of steel with carbon content greater than 0.40% is martensite; when carbon content is greater than 0.40%, there is a small amount of retained austenite in addition to martensite, and the amount of retained austenite increases with the increase of carbon content in the steel.
Heat treatment of medium carbon steel:
The final heat treatment methods of medium carbon steel include quenching and tempering, low temperature tempering after quenching, low temperature tempering after high frequency quenching, isothermal quenching and medium temperature tempering after quenching.
(1) Quenching and tempering. The organization is tempered sorbite. This structure has good comprehensive mechanical properties, high strength, good plasticity and toughness. The quenched and tempered steel should have good hardenability to ensure uniform structure and performance on the entire section of the quenched and tempered part. Compared with alloy steel, carbon steel has poor hardenability, so it is only suitable for quenching and tempering of medium carbon steel parts with small section size.
(2) Low temperature tempering after quenching. The organization is tempered martensite, which has high strength and appropriate plasticity and toughness.
(3) Low temperature tempering after high frequency quenching. The microstructure of the high-frequency quenching layer is extremely fine hidden needle martensite, and tempered martensite is obtained after low-temperature tempering. This treatment can obtain similar effects to carburizing treatment. Quenching and tempering or normalizing are generally carried out before high-frequency quenching. Therefore, after high-frequency quenching and tempering, the core of the part has high strength, good plasticity and toughness, and its surface layer has high hardness and good wear resistance. In addition, the surface layer of high-frequency quenched parts produces compressive stress, which has a high fatigue limit and a long service life.
(4) isothermal quenching. The organization is bainite, which has high strength and good plasticity and toughness.
(5) Temper at medium temperature after quenching. The organization is tempered sorbite.
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