What are the differences between low carbon steel, medium carbon steel and high carbon steel?

Low carbon steel is also called soft steel. The carbon content ranges from 0.10% to 0.30%. Low carbon steel is easy to accept various processing such as forging, welding and cutting. It is often used to make chains, rivets, bolts, shafts, etc. Carbon steel with a carbon content of less than 0.25% is also called soft steel because of its low strength, low hardness and softness. It includes most ordinary carbon structural steels and some high-quality carbon structural steels. Most of them are used for engineering structural parts without heat treatment, and some are used for mechanical parts requiring wear resistance after carbonization and other heat treatments. The annealed structure of low carbon steel is ferrite and a small amount of pearlite. Its strength and hardness are low, and its plasticity and toughness are good. Therefore, it has good cold formability and can be cold formed by curling, bending, stamping and other methods. This kind of steel wing has good weldability. Low carbon steel with very low carbon content has very low hardness and poor machinability. Quenching treatment can improve its machinability. Low carbon steel is generally rolled into angle steel, channel steel, I-beam steel, steel pipe, steel strip, steel plate, and is used to make various building components, containers, boxes, furnace bodies, and agricultural machinery. High-quality low carbon steel is rolled into thin plates to make deep-drawn products such as automobile cabs and generator covers; it is also rolled into bars for making mechanical parts with different strength requirements. Low carbon steel is generally not heat-treated before use. Carbon content above 0.15% is carburized or cyanided and used for parts such as shafts, sleeves, sprockets, etc. that require high surface hardness and good wear resistance. Low carbon steel is limited in use due to its low strength. Appropriately increasing the manganese content in carbon steel and adding trace amounts of alloying elements such as vanadium, titanium, and niobium can greatly improve the strength of steel. If the carbon content in steel is reduced and a small amount of aluminum, boron, and carbide-forming elements are added, an ultra-low carbon bainite group can be obtained, which has high strength and maintains good plasticity and toughness.
Medium carbon steel is a carbon steel with a carbon content of 0.25% to 0.60%. There are many products such as killed steel, semi-killed steel, and boiling steel. In addition to carbon, it can also contain a small amount of manganese (0.70% to 1.20%). According to product quality, it is divided into ordinary carbon structural steel and high-quality carbon structural steel. Hot working and cutting performance are good, but welding performance is poor. Strength and hardness are higher than low carbon steel, but plasticity and toughness are lower than low carbon steel. Hot rolled materials and cold drawn materials can be used directly without heat treatment, or they can be used after heat treatment. Medium carbon steel has good comprehensive mechanical properties after quenching and tempering. The highest hardness that can be achieved is about HRC55 (HB538), and σb is 600 to 1100MPa. Therefore, in various uses with medium strength levels, medium carbon steel is 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 has a higher carbon content than low carbon steel, higher strength, and poor weldability. Commonly used are 35, 45, and 55 steels. The main characteristics of medium carbon steel arc welding and its casting repair are as follows: (1) Hardened structure is easy to form in the heat affected zone. The higher the carbon content and the thicker the plate, the greater this tendency. If the welding materials and process specifications are not properly selected, cold cracks are easy to form. (2) Since the carbon content of the base metal is high, the carbon content of the weld is also high, which is easy to form hot cracks. (3) Due to the increase in carbon content, the sensitivity to pores increases. Therefore, the requirements for the deoxidation of welding materials, degreasing and rust removal of base metals, and drying of welding materials are more stringent.
High carbon steel is often called tool steel, with a carbon content ranging from 0.60% to 1.70%, which can be hardened and tempered. Hammers, crowbars, etc. are made of steel with a carbon content of 0.75%; cutting tools such as drills, taps, reamers, etc. are made of steel with a carbon content of 0.90% to 1.00%.