The mechanical properties of carbon steel mainly depend on its carbon content. Generally speaking, as the carbon content increases, the hardness and strength of steel improve, while its plasticity, toughness and weldability decrease. The following is a detailed classification of the mechanical properties of carbon steel:
1. Low Carbon Steel
Carbon content is generally below 0.25% (e.g., Q235, 10#, 20# steel).
- Strength and Hardness: Relatively low strength and hardness, with a soft texture.
- Plasticity and Toughness: Excellent plasticity, outstanding cold workability (such as stamping, bending, drawing), and resistance to fracture. High toughness and strong impact resistance.
- Process Properties: Excellent weldability, with acceptable machinability (inferior to medium carbon steel).
- Application Characteristics: Mainly used for manufacturing low-strength structural parts relying on good formability and weldability, such as steel plates, steel pipes, screws, nuts and building structures.
2. Medium Carbon Steel
Carbon content is generally between 0.25% and 0.60% (e.g., 45# steel, 40Cr, Q345).
- Strength and Hardness: Significantly higher strength and hardness than low carbon steel, with strong load-bearing capacity.
- Plasticity and Toughness: Lower plasticity and toughness than low carbon steel, moderate cold deformation ability; excessive bending may cause cracking.
- Process Properties: Poor weldability (prone to cracking after welding; preheating and post-weld heat treatment are usually required). Excellent machinability.
- Heat Treatment Characteristics: This type of steel is most suitable for quenching and tempering (quenching + high-temperature tempering), which provides excellent comprehensive mechanical properties, i.e., a combination of high strength and good toughness.
- Application Characteristics: Used for manufacturing mechanical parts under medium load, such as gears, shafts, connecting rods, keys, etc.
3. High Carbon Steel
Carbon content is generally above 0.60% (e.g., 65Mn, T8, T10 tool steels).
- Strength and Hardness: Very high strength and hardness, with excellent wear resistance.
- Plasticity and Toughness: Very poor plasticity and toughness, highly brittle, almost no cold plastic deformation ability, and prone to brittle fracture under impact.
- Process Properties: Extremely poor weldability; welding is generally not performed. Poor machinability (especially in the quenched state).
- Heat Treatment Characteristics: Must be heat-treated (e.g., quenching + low-temperature tempering) to achieve its performance, mainly pursuing high hardness and high wear resistance.
- Application Characteristics: Mainly used for manufacturing tools and parts requiring wear resistance or high stress, such as springs, cutting tools, measuring tools, bearing balls, drill bits, etc.
Summary
Trend with increasing carbon content:Hardness ↑, Strength ↑, Wear resistance ↑, Plasticity ↓, Toughness ↓, Weldability ↓.
Explanation of Key Mechanical Properties
- Yield Strength: The stress at which a material begins to deform plastically (no longer returns to its original shape), determining the load-bearing limit of a structure.
- Tensile Strength: The maximum stress a material can withstand before fracture.
- Elongation: An indicator of plasticity; a higher value means less brittleness and better formability.
- Impact Toughness: The ability of a material to resist sudden impact loads. Toughness decreases significantly at low temperatures.
