Carbon steel is an alloy composed of iron and carbon, with a carbon content typically ranging from 0.0218% to 2.11%. It is one of the most common steel grades, widely used in industry, construction, and machinery manufacturing. Besides carbon, carbon steel also contains small amounts of silicon, manganese, sulfur, phosphorus, and other elements to improve its properties.

Carbon content has a significant impact on the properties of carbon steel: higher carbon content results in higher hardness and strength, but lower plasticity and toughness; lower carbon content leads to better ductility, weldability, and machinability.

Classification of Carbon Steel

Based on carbon content, carbon steel can be divided into three categories: low-carbon steel, medium-carbon steel, and high-carbon steel.

1. Low-carbon steel: Carbon content less than 0.25%

2. Medium-carbon steel: Carbon content between 0.25% and 0.60%

3. High-carbon steel: Carbon content between 0.60% and 1.70%

These three types of carbon steel have different properties, uses, and processing methods. The selection should be based on actual application requirements.

Low-carbon steel, also known as mild steel, has a carbon content not exceeding 0.25%. Due to its low carbon content, it possesses good plasticity, toughness, weldability, and cold working properties. It has lower strength and hardness but high ductility, allowing for various forming processes such as rolling, bending, and stamping.

Low-carbon steel has excellent weldability and is not prone to cracking, therefore it is widely used in building structures, machinery manufacturing, and vehicle manufacturing.

Common low-carbon steel products include angle steel, channel steel, reinforcing bars, steel plates, and steel pipes. Low-carbon steel is generally used without heat treatment, but in applications requiring increased surface hardness, its properties can be improved through carburizing or nitriding.

Typical applications: Engineering structural components, containers, vehicle frames, agricultural machinery, steel reinforcement in construction, etc.

Medium Carbon Steel: Medium carbon steel has a carbon content between 0.25% and 0.60%, and is a type of steel with a relatively balanced strength and toughness. Its plasticity and toughness are slightly lower than low carbon steel, but its strength, hardness, and load-bearing capacity are higher.

Medium carbon steel has poorer weldability than low carbon steel; preheating and post-heat treatment are necessary during welding to prevent cracking.

Depending on its application, medium carbon steel can be divided into ordinary carbon structural steel and high-quality carbon structural steel. High-quality medium carbon steel, after heat treatment, can obtain good comprehensive mechanical properties, such as high strength and good wear resistance.

Typical applications: Used for manufacturing medium-strength mechanical parts, such as connecting rods, crankshafts, gears, shaft parts, bolts, pins, and construction steel.

High Carbon Steel: High carbon steel has a carbon content between 0.60% and 1.70%, also known as tool steel. It has very high strength, hardness, and wear resistance, but lower plasticity and toughness. High-carbon steel is commonly used in the manufacture of knives, hammers, saw blades, taps, drill bits, springs, and cutting tools. Due to its high carbon content, it has poor weldability and is prone to cracking during hot working. Therefore, preheating and tempering are usually required to relieve stress and prevent deformation.

High-carbon steel is sensitive to temperature changes; uneven heating can easily lead to changes in its microstructure and cracking. Therefore, strict control of temperature and time is necessary during heat treatment.

Principles for Selecting Carbon Steel

When selecting carbon steel, the following factors need to be considered:

1. Operating environment: Whether wear resistance, corrosion resistance, or high load-bearing capacity is required.

2. Processing technology: Whether welding, stamping, cutting, or heat treatment is required.

3. Cost and lifespan: A comprehensive consideration of economic efficiency and service life.

4. Mechanical performance requirements: A balance between strength, toughness, and hardness.

For example:

 Low-carbon steel should be used for structural components requiring high weldability and plasticity;

Medium-carbon steel is suitable for components requiring a certain level of strength and hardness but also some toughness;

 High-carbon steel should be used for components such as knives and springs requiring extremely high wear resistance and strength.

Carbon Steel Performance Comparison Table:

Type | Carbon Content | Characteristics | Common Uses

Low-carbon steel | ≤0.25% | Good plasticity, strong weldability, easy to process | Building structures, reinforcing bars, containers, machine housings

Medium-carbon steel | 0.25–0.60% | Moderate strength, good overall performance | Gears, shafts, building materials, machine parts

High-carbon steel | 0.60–1.70% | High strength, good wear resistance, poor toughness | Knives, springs, saw blades, cutting tools

Summary

Carbon steel is a widely used metallic material, and its performance depends on its carbon content and processing method.

Low-carbon steel is suitable for structural components and welded parts; medium-carbon steel is most widely used in machinery manufacturing; high-carbon steel is mainly used for tools and high-strength components.

In the engineering design and manufacturing process, the correct selection of carbon steel type can significantly improve the service life and performance of products.