What is Carbon Steel?
Carbon steel is an iron-carbon alloy with a mass carbon percentage content ≤ 2.0% and containing a small amount of impurity elements such as manganese (Mn), silicon (Si), sulfur (S), phosphorus (P), and oxygen (O), etc.
What is Low Carbon Steel, Medium Carbon Steel, and High Carbon Steel
The key factor distinguishing low, medium and high carbon steel is the percentage content of carbon, and according to the different carbon percentage content, it’s divided into the following types:
- Low Carbon Steel: also know as iron or mild steel, the percentage content of carbon is 0.04 (0.05)%-0.25 (or 0.30)%.
- Medium Carbon Steel: the carbon percentage content is 0.25 (0.30)%-0.60%.
- High Carbon Steel: the carbon percentage content is 0.60%-2.0%.
Notes: There are two versions that define mild steel, one with a carbon content 0.04 (or 0.05)% – 0.25% and the other with 0.04 (or 0.05)% – 0.30%.
Low Carbon vs Medium Carbon vs High Carbon Steel
|Steel Types||Carbon Content (C) %|
|Low Carbon Steel||0.04 ≤ C < 0.25|
|Medium Carbon Steel||0.25 ≤ C < 0.60|
|High Carbon Steel||0.60 ≤ C < 2.0|
1. Low Carbon Steel
Low carbon steel is a carbon steel with a carbon content of less than 0.25% (or 0.30%). Due to its low strength, low hardness and softness, it’s also called mild steel. It includes most of the ordinary carbon structural steel and a part of high-quality carbon structural steel, mostly without heat treatment, used for engineering structural parts.
Typical mild steels include:
- US ASTM A36, SAE AISI 1008, 1012, 1015, 1018, 1020, 1022, 1025, 1029, etc.
- European EN S185, S235, S275, S355, S450, etc.
- Chinese GB Q195, Q215, Q235, Q275, 08# (08 steel), 10#, 15#, 20#, 25#, etc.
The mild steel annealed structure is ferrite and a small amount of pearlite, which has low strength and hardness, and good plasticity and toughness. Therefore, the cold formability is good, and cold forming can be performed by a method such as crimping, bending, or press. Low carbon steel with a very low carbon content has a low hardness and poor machinability, and normalizing treatment can improve the machinability.
Mild steel has a large tendency to aging, both quenching and aging tendencies, as well as deformation and aging tendencies. When steel is cooled from high temperature, the carbon and nitrogen in the ferrite are supersaturated, and it can slowly form iron carbon and nitrogen at normal temperature, so the strength and hardness of the steel are improved, and the ductility and toughness are lowered. This phenomenon is called quenching aging. Low carbon steel can be aged even if it is not quenched. Deformation of low carbon steel produces a large number of dislocations. The carbon and nitrogen atoms in the ferrite interact elastically with dislocations, and carbon and nitrogen atoms gather around the dislocation lines. This combination of carbon and nitrogen atoms and dislocation lines is called Cottrell atmosphere. It increases the strength and hardness of steel and reduces the ductility and toughness. This phenomenon is called deformation aging.
Applications of Mild Steel
Mild steel is generally not heat treated before use, and is generally rolled into angle steel, channel steel, I-beam, steel pipe, steel strip or steel plate for making various building components, containers, boxes, furnace bodies and agricultural machinery. High-quality low-carbon steel is rolled into a thin plate to make deep-drawn products such as automobile cabs and engine covers; it is also rolled into bars for the production of mechanical parts with low strength requirements.
2. Medium Carbon Steel
Medium carbon steel is carbon steel with a carbon percentage content of 0.25% (or 0.30%) to 0.60%. It includes most of the high quality carbon structural steel and a portion of the ordinary carbon structural steel.
Typical medium carbon steels include:
- US ASTM SAE AISI 1030, 1034, 1035, 1038, 1040, 1042, 1043, 1045, 1050, 1055, etc.
- European EN C35 (1.0501), C40 (1.0511), C45 (1.0503), C55 (1.0535), C60 (1.0601), etc.
- Chinese GB 35# (35 steel), 40#, 45#, 50#, 55#, 60#, etc.
Medium carbon steel has good thermal processing and cutting performance, but its welding performance is poor. Strength and hardness are higher than low carbon steel, and plasticity and toughness are lower than mild steel. The cold-rolled or cold drawn material can be directly used without heat treatment, and can also be used after heat treatment. The medium carbon steel after quenching and tempering has good comprehensive mechanical properties.
The final heat treatment methods for medium carbon steel are as follows:
- Tempering. The organization is tempered sorbite. This kind of tissue has good comprehensive mechanical properties, high strength, good plasticity and toughness. The quenched and tempered steel should have good hardenability to ensure uniform microstructure and performance throughout the entire section of the tempering 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 cross-section.
- Low temperature tempering after quenching. The structure is tempered martensite, which has high strength and proper plasticity and toughness.
- Low temperature tempering after induction hardening. The high-frequency quenching layer is organized into a very fine needle martensite, and tempered martensite is obtained after low-temperature tempering, and a similar effect to the carburizing treatment can be obtained by this treatment. Tempering or normalizing is usually carried out before high frequency quenching. Therefore, after high-frequency quenching and tempering, the strength of the core of the part is high, the plasticity and toughness are good, and the surface hardness is high and the wear resistance is good. In addition, the surface layer of the high-frequency quenching part generates compressive stress, which has high fatigue limit and long service life.
- Austempering. The structure is bainite, which has high strength and good ductility and toughness.
- Tempering and tempering after quenching. The organization is tempered sorbite.
Applications of Medium Carbon Steel
Medium carbon steel is mainly used to manufacture high-strength moving parts, such as air compressors, pump pistons, steam turbine impellers, heavy machinery shafts, worms, gears, etc., surface wear parts, crankshafts, machine tools spindles, rollers, bench tools, and more.
3. High Carbon Steel
High carbon steel has a carbon content of 0.60% to 1.70% (maximum 2.0%), which can be quenched and tempered.
Typical high carbon steels include:
- US ASTM SAE AISI 1059, 1060, 1065, 1070, 1075, 1080, 1085, 1090, 1095, etc.
- European EN C62D (1.0611), C66D (1.0612), C68D (1.0613), C70D (1.0615), C72D (1.0617), C80D (1.0622), C86D (1.0616), C92D (1.0618), etc.
- Chinese GB 65# (65 steel), 65Mn, 70#, 70Mn, 75#, 80#, 85#, 60#, T7, T8, T10, etc.
High carbon steel has high strength and hardness, high elastic limit and fatigue limit after proper heat treatment or cold drawing hardening, and the cutting performance is acceptable, but the welding performance and cold plastic deformation ability are very poor. Due to the high carbon content, cracks are easily generated during water quenching, so two-liquid quenching is often used, and oil-hardening is often used for small-section parts. Such steels are generally used after quenching by medium temperature tempering or normalizing or surface hardening.
- High hardness (HRC 60 to 65) and good wear resistance can be obtained after heat treatment.
- The hardness is moderate under annealing and has good machinability.
- The raw materials are easy to get and the production cost is low.
- Poor thermosetting. When the working temperature of the tool is greater than 200 °C, its hardness and wear resistance drop sharply.
- Low hardenability. The diameter of the completely hardened water quenching is generally only 15-18mm; the maximum diameter or thickness of the completely hardened during oil quenching is only about 6mm, and it is easy to deform and crack.
Weldability of high carbon steel
- Due to the high hardness and wear resistance of high carbon steel parts, the material itself needs to be heat treated, so it should be annealed before welding.
- The weldment should be preheated before welding. The preheating temperature is generally above 250 or 350 °C. During the welding process, the interlayer temperature must be kept higher than the preheating temperature.
- After welding, the weldment must be insulated and slowly cooled, and immediately sent to the furnace for stress relief heat treatment at 650 °C.
High carbon steels are mainly used in the manufacture of springs, wear parts and high hardness tools.