The Difference Between Low, Medium And High Carbon Steels

Carbon steel can be divided into three main types according to its carbon content: low carbon steel, medium carbon steel and high carbon steel. Due to the difference in carbon content, each type of carbon steel also shows significant differences in its organizational structure, mechanical properties, processing technology, etc. However, it is crucial for us to choose the right material to understand the differences between low carbon steel, medium carbon steel and high carbon steel.

What Is Carbon Steel?

Carbon steel, also known as carbon steel, is a steel material with iron as the main element, a certain amount of carbon (generally less than 2.11%) but without adding a large amount of other alloying elements. Carbon steel is the most basic and common type of steel material. According to the different carbon content, carbon steel can be divided into low carbon steel, medium carbon steel and high carbon steel. The higher the carbon content, the greater the strength and hardness of the steel, but the plasticity and toughness are correspondingly reduced. The lower the carbon content, the better the ductility and welding performance of the steel.

Classification Of Carbon Steel:

The classification of carbon steel is mainly based on its carbon content. According to international and domestic commonly used standards, carbon steel is classified as follows:

• Low carbon steel: carbon content is generally between 0.04% and 0.25%;
• Medium carbon steel: carbon content is between 0.25% and 0.60%;
• High carbon steel: carbon content is between 0.60% and 1.00%.

Although the classification is mainly based on carbon content, in specific applications, the content of other elements (such as manganese, silicon, sulfur, phosphorus, etc.) and the use and mechanical properties of steel will also be referred to.

What Is The Difference Between Low Carbon Steel, Medium Carbon Steel And High Carbon Steel?

Their differences are mainly reflected in the following 5 aspects, as shown below:

1. Chemical Composition:

The most direct difference between low carbon steel, medium carbon steel and high carbon steel is reflected in their chemical composition. As the carbon content gradually increases, the hardness and strength of the steel increase, but its ductility and weldability decrease.

• Low carbon steel: In addition to the low carbon content, it often contains a small amount of silicon (0.17~0.37%), manganese (0.35~0.65%), etc., in order to improve its strength and machinability. Due to its low carbon content, it has good plasticity, toughness and weldability.
• Medium carbon steel: The carbon content is generally between 0.25% and 0.60%, and it also contains strengthening elements such as manganese (0.50%~1.65%). Its mechanical properties are relatively good and suitable for heat treatment. Compared with low carbon steel, it has higher strength and hardness, but its toughness is reduced.
• High carbon steel: The carbon content is between 0.60% and 1.00%. Due to the high carbon content, its quenching hardness and wear resistance are significantly improved, but its weldability and machinability are poor.

2. Mechanical Properties:

In terms of mechanical properties, the differences between these three types of steel are particularly obvious. Mechanical properties include tensile strength, yield strength, elongation, impact toughness, hardness, etc.

• Low carbon steel: The tensile strength is usually between 370~500MPa, the yield strength is between 200~300MPa, and the elongation is as high as more than 25%. Its outstanding features are good toughness and strong ductility, and it is suitable for stamping, stretching and other processes.
• Medium carbon steel: After quenching and tempering, the tensile strength can reach 600~800MPa, the yield strength is between 400~600MPa, and the elongation is between 15%~20%. It has good comprehensive mechanical properties and is especially suitable for structural parts manufacturing.
• High carbon steel: The tensile strength can reach more than 900~1300MPa, but the elongation is usually less than 10%, and the impact toughness is poor. After quenching and tempering, it can obtain extremely high hardness, which is an ideal material for cutting tools and molds.

3. Heat Treatment Performance:

Heat treatment is an important means to improve the performance of steel. Low carbon steel, medium carbon steel, and high carbon steel respond differently to heat treatment.

• Low carbon steel: Due to its low carbon content, its hardness cannot be significantly improved by heat treatment. It mainly uses cold working and carburizing treatment to enhance the surface hardness. After annealing, the material can be softened for subsequent processing.
• Medium carbon steel: Suitable for quenching and tempering, that is, quenching and tempering, it controls the organization to obtain the required balance between strength and toughness, and is the preferred material for manufacturing automobile and railway parts.
• High carbon steel: The heat treatment effect is the most significant. High carbon steel can obtain high hardness through quenching, and adjust its toughness through tempering. It is used to manufacture high-strength and high-wear-resistant parts such as springs, knives, and measuring tools.

4. Weldability And Processability:

Weldability and processability directly affect the manufacturing cost and ease of use of steel.

• Low carbon steel: It has excellent weldability and cold forming properties, and can be welded by a variety of methods. It is widely used in construction and machinery manufacturing.
• Medium carbon steel: It has average weldability and is prone to cracks during welding. It usually requires preheating and post-heat treatment to improve welding quality. Its processability is also slightly inferior to low carbon steel.
• High carbon steel: poor weldability, prone to hardening cracks, usually not suitable for welding. It is also prone to cracking and chipping during processing, requiring the use of special tools and cooling methods.

5. Applications:

Due to the differences in performance, the uses of the three types of steel are also significantly different.

• Low carbon steel: mainly used for building structures (such as steel bars, steel sections), mechanical parts (such as screws, nuts), car bodies, etc.
• Medium carbon steel: mainly used to manufacture load-bearing structural parts, such as gears, shafts, connecting rods, crane arms, etc.
• High carbon steel: mainly used to manufacture wear-resistant and high-strength tools such as knives, measuring tools, springs, ball bearings, etc.

Common Grades:

In the Chinese GB standard and the American ASTM standard, steels with different carbon contents have their typical representatives:

• Low carbon steel: Q235 (China), A36 (USA), SS400 (Japan)
• Medium carbon steel: 45# steel (China), 1045 steel (USA), S45C (Japan)
• High carbon steel: T8, T10 steel (China), 1095 steel (USA), SK85 (Japan)

Corrosion Resistance And Surface Treatment:

Carbon steel itself does not have good corrosion resistance, so it usually needs to be matched with surface treatment in application.

• Low carbon steel: Hot-dip galvanizing, electrogalvanizing, plastic spraying and other processes are often used to enhance corrosion resistance, and are widely used in outdoor or humid environments.
• Medium carbon steel and high carbon steel: Due to their high strength, they are often used in industrial equipment, focusing more on oil seals, anti-rust coatings or phosphating treatments, rather than daily exposure environments.

It should be noted that steels with high carbon content are more prone to oxidation, especially during welding and heat treatment. Therefore, high carbon steel is usually coated with a protective layer or heat treated before and after use to prevent corrosion.

Conclusion:

Although low carbon steel, medium carbon steel and high carbon steel belong to the carbon steel series, due to the different carbon content, the three have their own distinct characteristics in mechanical properties, machinability, weldability, heat treatment characteristics and applicable fields. Reasonable selection and application of these three types of steel are of great significance to improving the quality of engineering products, controlling manufacturing costs and extending service life.

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