Nitriding
What is nitriding?
Nitriding, also known as nitrogen hardening, is a surface treatment method for steel in which nitrogen—and often also carbon—is introduced into the surface of a metal part at a low enough temperature that the part remains solid throughout the entire process. This process creates a thin and durable nitride layer on the surface of the metal piece, enhancing its wear and corrosion resistance. Nitriding is commonly used in many industrial applications to improve the performance and durability of metal components.
Mechanism of the nitriding process
During nitriding, a nitride layer about 5–20 micrometers thick forms on the steel's surface, protecting the part from wear and corrosion. Beneath this protective layer, a diffusion zone develops, which can be up to a millimeter thick and improves the fatigue strength of the part.
Nitriding methods
Nitriding can be performed using several different methods:
- Gas nitriding: The part is held in a furnace for 30–90 hours, with ammonia gas supplied. The furnace temperature is 500–530 °C (932–986 °F). Ammonia decomposes into free nitrogen and hydrogen gas, with nitrogen diffusing into the metal surface to form the nitride layer.
- Carbonitriding: In this method, a gas containing both nitrogen and carbon, such as carbon dioxide, is used. The temperature is slightly higher, 570–580 °C (1,058–1,076 °F), and the treatment time is shorter, 2–6 hours. This method is now the most common.
- Plasma nitriding: Plasma nitriding uses strong electric fields to ionize nitrogen in a vacuum furnace. This enables more precise control over case depth and layer thickness.
- Salt bath nitriding: In this method, the nitrogen source is a nitrogen-containing salt, which often also adds carbon to the metal surface. The use of salt baths has decreased due to their toxicity and environmental impact.
Advantages and challenges of the nitriding process
The main advantage of nitriding is its ability to improve the wear and corrosion resistance of metal surfaces. The nitrided surface is extremely hard and durable, significantly extending the lifespan of components. Typical applications include gears, crankshafts, camshafts, and screws.
Comparison of nitriding methods
Gas nitriding:
- Advantages: Precise process control, suitable for large batches, low equipment costs.
- Disadvantages: Surface contamination can affect results, and ammonia is hazardous if inhaled.
Salt bath nitriding:
- Advantages: Short processing times, simple process.
- Disadvantages: Toxic salts, expensive and highly regulated waste disposal.
Plasma nitriding:
- Advantages: Precise microstructure control, minimal distortion, wide temperature range.
- Disadvantages: High equipment costs.
Practical applications of nitriding
Nitriding is suitable for various metal alloys, such as unalloyed and alloyed steels, titanium, aluminum, and molybdenum. Components like gears, crankshafts, and mold tools particularly benefit from the improved wear and corrosion resistance provided by the nitride layer.
Black nitriding
A special post-treatment called black nitriding adds a black oxide layer to the treated surface, improving appearance and providing extra corrosion protection. This method is usually carried out using steam or nitrous oxide gas.
Summary
Nitriding is an effective surface treatment technique that enhances the wear and corrosion resistance of metals. The various nitriding methods offer versatile options for treating different materials and meeting diverse industry needs. They ensure that metal components perform well and have a long lifespan even in the most demanding conditions.
