How to prevent decarburization of seamless carbon steel pipes

Surface decarburization in seamless pipes is primarily caused by oxidizing gases such as oxygen, water vapor, and carbon dioxide. When these gases come into contact with the heated surface of steel wires, simultaneous oxidation and decarburization occur. The solid solution carbon in iron has a high affinity for these gases, leading to the carbon on the surface being decarbonized.

Effects of Decarburization on Carbon Steel Pipes:

- Structural Integrity:

Decarburization results in the oxidation and reduction of surface carbon elements to CO and CO2, making the metallographic structure on the surface lighter. This weakens the overall performance and service life of the steel pipe.

- Strength and Hardness:

For steel that requires quenching, decarburization lowers the surface carbon content, preventing complete martensitic transformation. Consequently, the desired hardness and strength are not achieved.

- Toughness and Ductility:

Decarburization alters the steel structure, reducing its toughness and ductility, which makes the steel more prone to fractures.

- Magnetism:

The magnetism of decarburized steel significantly decreases.

- Plasticity and Wear Resistance:

The plasticity of the forging surface diminishes, leading to potential cracking during die forging. Decarburization also affects wear resistance and can cause cracks due to uneven deformation.

- Fatigue Strength:

Decarburization lowers the fatigue strength of steel, causing premature fatigue damage in parts. Improper forging processes can leave decarburized layers on parts, degrading performance and potentially leading to scrapped parts.

Preventive Measures for Decarburization

1. Control the Heating Process:

Properly control heating temperature, duration, and cooling rate to avoid excessive heat and prolonged exposure. Maintain the heating temperature below the steel’s critical temperature and ensure minimal heating time with moderate cooling rates. Ensure steel parts are smooth and not exposed to oxygen during heating.

2. Use Protective Gas:

Employ protective gases like hydrogen, nitrogen, or argon during heating to isolate steel from air contact. This prevents surface carbon oxidation and decarburization. Production workers can use nitrogen to maintain furnace pressure, reducing the presence of oxidizing gases.

3. Minimize Heating Time:

Shorten heating time to reduce surface carbon oxidation risk. Utilize rapid heating methods such as high-frequency or induction heating to minimize exposure duration.

4. Post-Processing Treatment:

Improve decarburized steel through carbonitriding and thermal diffusion treatments. Carbonitriding involves simultaneous carbon and nitrogen infiltration into the surface, enhancing hardness, wear resistance, and fatigue strength. Low-temperature gas nitrocarburizing treats steel at lower temperatures, improving toughness and reducing brittleness.

5. Remove Surface Attachments:

Keep steel surfaces clean to minimize reactions with oxidizing substances. Methods include:

Cleaning with solvents and emulsions to remove grease and dust.

Using wire brushes for manual or power tool rust removal.

Shot blasting to remove rust and create a rough surface.

Chemical pickling for cleaning and preparing surfaces for further processing.

6. Detection and Monitoring:

Conduct regular inspections to detect and address decarburization issues promptly.

7. Control Carbon Dioxide Content in Furnace:

Maintain the carbon dioxide content below the equilibrium point to prevent decarburization. This involves calculating the equilibrium point based on temperature and carbon content and adjusting furnace gas composition accordingly.

8. Use Coating Protection:

Apply high-temperature resistant, anti-oxidation, and anti-decarburization coatings like RLHY-31 to protect steel surfaces during heating. This coating forms a dense protective layer, preventing oxidation and extending the steel’s service life.

By implementing these preventive measures, decarburization can be controlled, thereby maintaining the integrity and performance of seamless steel pipes.