Nitriding

Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. It is a form of surface hardening that is used to enhance the mechanical properties of metals, such as wear resistance, fatigue strength, and corrosion resistance. Nitriding is commonly applied to steel, but it can also be used on aluminum, molybdenum, and titanium. The process involves heating the metal in a furnace and then introducing nitrogen-rich gases, such as ammonia (NH3), which decompose at the surface and release nitrogen atoms to diffuse into the metal.

Process

The nitriding process can be performed using different methods, including gas nitriding, salt bath nitriding, and plasma nitriding, each with its own set of advantages and applications.

Gas Nitriding

Gas nitriding is the most common method, where the metal is exposed to an ammonia gas atmosphere at temperatures between 500°C and 550°C. The process duration can range from a few hours to more than 70 hours, depending on the desired case depth and material.

Salt Bath Nitriding

Salt bath nitriding involves immersing the metal in a molten salt bath that contains nitrogen-donating chemicals. It operates at temperatures of 550°C to 570°C and offers faster diffusion rates compared to gas nitriding.

Plasma Nitriding

Plasma nitriding, also known as ion nitriding, uses a glow discharge technology to ionize nitrogen gas, which then diffuses onto the surface of the metal. This method allows for precise control over the case depth and nitride layer composition.

Applications

Nitriding is used in various industries, including automotive, aerospace, and tool & die, for components that require high surface hardness and wear resistance. Common applications include gears, crankshafts, camshafts, valve parts, die-casting tools, and injection molding machine parts.

Advantages

The main advantages of nitriding over other surface hardening methods include:

• Lower processing temperatures, which reduce the risk of distortion.
• Enhanced surface hardness and wear resistance.
• Improved fatigue strength and corrosion resistance.
• The ability to nitride complex shapes and internal surfaces.

Limitations

Despite its benefits, nitriding also has some limitations:

• Not all steels are suitable for nitriding; the material must have an adequate alloy content.
• The process can introduce surface stresses, leading to cracking if not properly managed.
• The depth of the hardened layer is limited, typically ranging from 0.1 to 0.7 mm.

See Also

• Case hardening
• Surface engineering
• Heat treatment
• Steel