Nitridization

Nitridization, also known as nitriding, is a process which introduces nitrogen in the surface of a material. It is used in metallurgy, for example, for case hardening treatment of the steel surface.

Nitriding is widely used in automotive, mechanical and aeronautical engineering. Typical components are gears, crankshafts, camshafts, cam followers, valve parts, extruder screws, die-casting tools, forging dies, extrusion dies, injectors and plastic-mould tools.

Process

Nitriding alters the composition of the surface, much like carburization does, by adding nitrogen to the ferrite phase. Nitriding steel is performed at 500 to 550 °C (930 to 1020 °F). The greatest advantage nitriding has over carburization is that it does not need additional heat treatments to harden, therefore nitriding maintains better dimensional control.

In nitriding, nitrogen is introduced into the surface of a steel component by heating it in salt bath containing nitrogen-bearing organic compounds (known as "pack nitriding") or in a gas stream containing nitrogenous gas, usually ammonia (NH₃) (known as "gas nitriding"). Steels suitable for nitriding contain aluminium, chromium, vanadium, tungsten or molybdenum; these form stable nitride precipitates that harden the surface to a depth of about 500 micrometres. Nitriding provides a very high surface hardness (almost 70 HRC), increased wear resistance, improved fatigue life, and enhanced corrosion resistance.

Examples of steels that are easily nitrided include the SAE 4100, 4300, 5100, 6100, 8600, 8700, 9300 and 9800 series, stainless steels, some tool steels and certain cast irons. Ideally, steels for nitriding should be in the hardened and tempered condition, requiring that the tempering temperature be higher than nitriding temperature. A fine-turned or ground surface finish is best.

The equipment can be expensive, but tooling costs are generally low, and many parts can be treated simultaneously, keeping labor cost low. Since pack nitriding may requires the use of "cyanide" salts, this process requires stringent safety precautions and environmental regulation. Disposal of spent baths must follow proper procedures.

ee also

*Carbonitriding
*Case hardening
*Surface engineering
*Surface finishing
*HEF Groupe, international nitriding company

External links

* [http://www.key-to-steel.com/Articles/Art117.htm Key to Steel - Nitriding]