What Are Carburising and Carbonitriding?

Carburising and carbonitriding are advanced thermochemical treatments used in the initial phase of case hardening. Typically performed at temperatures between 800°C and 940°C, these processes alter the chemical composition of the surface of low-carbon steel components. This modification enables fast cooling through quenching, producing a hard, wear-resistant outer layer (case) and a softer, tougher inner core.

After quenching, a low-temperature tempering or stress-relieving treatment is usually performed to stabilize the material.

• Carburising: Involves introducing controlled amounts of carbon to the surface, allowing it to diffuse to a specific depth.
• Carbonitriding: Adds both nitrogen and carbon to the surface, improving case hardenability. The process often utilizes controlled gaseous atmospheres or molten salt (cyaniding) as the treatment medium.

Benefits of Carburising and Carbonitriding?

These treatments enhance the performance of components by:

1. Improved Wear Resistance: Producing a high-hardness surface layer.
2. Increased Strength: Providing fatigue resistance for durable applications.
3. Cost-Effectiveness: Enabling the use of low-cost, easily machined materials.

Applications range from mild steel pressings to heavy-duty transmission parts, making them ideal for industries like automotive and manufacturing.

Types of Steels Suitable for Case Hardening?

• Mild Steels: Suitable for increased wear resistance but have limited core strength.
• Low-Carbon Alloy Steels: Provide high surface hardness and significant core toughness after carburising. These steels are generally not carbonitrided.
• Recommended Steels: BS970 lists suitable case-hardening steels with corresponding mechanical properties.

Common Issues

1. Distortion:
   High-temperature processes can cause shape changes, especially in poorly designed components or those with pre-existing stresses. Preliminary normalizing may help mitigate this.
2. Post-Treatment Processing:
   • Close tolerance components often require careful grinding.
   • Carbonitrided surfaces may pose challenges for subsequent plating (e.g., zinc plating).

How to Specify Case Hardening

To ensure optimal results, include the following details when specifying the treatment:

• Type of treatment: Carburising or carbonitriding.
• Steel specification: Include the exact material grade.
• Required case depth: Specify acceptable ranges, indicating total or effective depth.
• Hardness requirements: Surface and core hardness levels, with preferred testing methods.
• Grinding allowance: State whether depth accounts for post-treatment grinding.
• Selective treatment: Define areas requiring hard or soft surfaces.

Providing detailed drawings and general standards (e.g., Ford specifications) further ensures accurate processing.