Introduction to Surface Treatment for Low-Carbon Steel Parts

Low-carbon steel, typically containing less than 0.25% carbon, offers good ductility and toughness but suffers from low hardness, poor wear resistance, and susceptibility to corrosion. To overcome these limitations, surface treatment technologies are widely applied to “complement” the steel—maintaining a tough core while providing a hard, wear-resistant, and corrosion-resistant surface. These treatments fall into two main categories: surface heat treatment (altering the surface microstructure) and surface protection (applying a protective layer).

1. Surface Heat Treatment: Case Hardening

These methods enhance surface hardness by modifying the chemical composition or microstructure of the outer layer. They are primarily used for structural components requiring high wear resistance.

• Carburizing: This is the most essential strengthening method for low-carbon steel. The workpiece is heated to 900–950°C in a carbon-rich atmosphere, allowing carbon atoms to diffuse into the surface, raising the surface carbon content to 0.8%–1.2% while the core remains unchanged. After quenching and low-temperature tempering, surface hardness can reach 58–63 HRC while the core retains good toughness. Widely used for gears, shafts, and other parts subjected to impact and wear.
• Nitriding and Carbonitriding: Nitriding introduces nitrogen atoms into the surface at 500–600°C, forming an extremely hard nitride layer with minimal distortion. Carbonitriding combines the advantages of both carburizing and nitriding, offering good performance at lower processing temperatures.

2. Surface Protection: Coatings and Platings

These methods focus on isolating the steel from corrosive environments to improve weather resistance.

• Coating Protection: This is the most common method. According to standards such as ISO 12944 or GB/T 30790, the surface must be thoroughly cleaned (removing rust and grease) to achieve specific cleanliness and roughness requirements, ensuring coating adhesion. Typical coating systems include primer, intermediate, and topcoat layers.
• Metal Plating: For example, zinc plating (galvanizing). Through electroplating or hot-dip galvanizing, a sacrificial zinc layer is formed on the surface. Even if scratched, the zinc layer protects the underlying steel. Hot-dip galvanizing offers superior corrosion resistance compared to electroplating.
• Functional Surfaces: For specialized needs, advanced techniques may be applied. For instance, chemical vapor deposition (CVD) can grow a graphene coating, leveraging its excellent impermeability for corrosion protection. Alternatively, superhydrophobic surfaces created through low-surface-energy modification enable self-cleaning and anti-fouling properties.

Conclusion

Selecting the appropriate surface treatment requires considering the service conditions (wear, impact, corrosive environment) and cost. For transmission components subjected to heavy loads and wear, carburizing is the preferred choice. For outdoor structural parts, hot-dip galvanizing or high-performance coatings are more economical and efficient.