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Nitriding of steel
Introduction of nitrogen in the steel is performed at temperatures 500 to 550 °C (930 to 1020 °F) range. Nitriding is altering the composition of the surface like carburization does. The difference between nitriding and carburization is that in nitridization nitrogen enters the ferrite phase, and that the nitridization process does not involve heating into the austenite phase field, thus eliminating the need to quench a part. As a result it provides excellent dimensional control.
In nitriding, nitrogen is introduced into the surface of a steel component by heating it in a fused salt bath containing nitrogen-bearing salts (typically, sodium cyanide, NaCN) or in a gas stream containing cracked ammonia (NH3). Steels suitable for nitriding contain aluminium, vanadium, tungsten or molybdenum; these form stable nitride precipitates that harden the surface to a depth of about 500 micrometres. The temperature — 495 to 565 °C — is lower than that for carburizing, giving less distortion, and the surface does not require later heat treatment (as carburizing does (to acquire its hardness)). Nitriding provides a very high surface hardness (almost 70 HRC), increased wear resistance, improved fatigue life, and enhanced corrosion resistance.
Nitriding is most effective when applied to steels containing nitride-forming elements such as chromium, molybdenum, vanadium, aluminium. Examples are members of the AISI 4100, 4300, 5100, 6100, 8600, 8700, 9300 and 9800 series. The process can also be 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.
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, aluminium-extrusion dies, injectors and plastic-mould tools.
Nitriding is of immense economic significance. The equipment can be expensive, but tooling costs are generally low, and many parts can be treated simultaneously, keeping labor cost low.
Since nitriding by salt bath requires cyanide salts, this process requires stringent safety precautions and environmental regulation. Disposal of spent baths must follow proper procedures. A single nitriding furnace cycle is energy-intensive (e.g. maintaining a 900 Fahrenheit temperature for as long as 72 hours in one such cycle), but the ability to enhance the performance of cheap, low-energy, recyclable steels more than compensates.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Nitridization". A list of authors is available in Wikipedia.