Difference Between Low, Medium & High Carbon Steel

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 mild steel, the percentage content of carbon is 0.04 (0.05)%-0.25 (or 0.29)%.
• 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 (0.05)% – 0.25% and the other with 0.04 (0.05)% – 0.29%.

Low Carbon vs Medium Carbon vs High Carbon Steel

Steel Types	Carbon Content (C) %
Low Carbon Steel	0.04 ≤ C ≤ 0.29
Medium Carbon Steel	0.29 < 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 0.29%). Due to its low strength, low hardness and softness, it’s also called mild steel. It includes most of the plain carbon steel and a part of high-quality carbon steel, mostly without heat treatment, used for engineering structural parts.

Typical low carbon steels include:

• US ASTM A36, SAE AISI 1008, 1012, 1015, 1018, 1020, 1022, 1025, 1029, etc.
• European EN S185, S235, S275, S355, S450, DC01 (St12), etc.
• Chinese GB Q195, Q215, Q235, Q275, 08# (08 steel), 10#, 15#, 20#, 25#, etc.

The low carbon 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 low hardness and poor machinability, and normalizing treatment can improve the machinability.

Applications of Low Carbon Steel

Low carbon 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 a carbon steel with a carbon percentage content of 0.25% (or 0.29%) to 0.60%. It includes most of the quality carbon steel and a portion of the plain carbon 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, so preheating is required before welding. 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.

Applications of Medium Carbon Steel

The 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 (Rockwell C hardness can be up to 60-65 HRC), high elastic limit and fatigue limit after proper heat treatment or cold drawing hardening, and the cutting performance is acceptable, compared to low carbon steel and medium carbon steel, its weldability and cold plastic deformation is the worst. 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 carbon steel is mainly used in the manufacture of springs, wear parts and high hardness tools.