The processes are named after the medium used to donate. The three main methods used are:
gas nitriding,
salt bath nitriding, and
plasma nitriding.
Gas nitriding In gas nitriding the donor is a nitrogen-rich gas, usually
ammonia (NH3), which is why it is sometimes known as
ammonia nitriding. When ammonia comes into contact with the heated work piece it dissociates into nitrogen and hydrogen. The nitrogen then diffuses onto the surface of the material creating a nitride layer. This process has existed for nearly a century, though only in the last few decades has there been a concentrated effort to investigate the thermodynamics and kinetics involved. Recent developments have led to a process that can be accurately controlled. The thickness and phase constitution of the resulting nitriding layers can be selected and the process optimized for the particular properties required. The advantages of gas nitriding over other variants are: • Precise control of chemical potential of nitrogen in the nitriding atmosphere by controlling gas flow rate of nitrogen and oxygen • All round nitriding effect (can be a disadvantage in some cases, compared with plasma nitriding) • Large batch sizes possible – the limiting factor being furnace size and gas flow • With modern computer control of the atmosphere the nitriding results can be closely controlled • Relatively low equipment cost – especially compared with plasma The disadvantages of gas nitriding are: • Reaction kinetics heavily influenced by surface condition – an oily surface or one contaminated with cutting fluids will deliver poor results • Surface activation is sometimes required to treat steels with a high chromium content – compare sputtering during plasma nitriding • Ammonia as nitriding medium – though not especially toxic it is harmful when inhaled at a high concentration. Also, care must be taken when heating in the presence of oxygen to reduce the risk of explosion
Salt bath nitriding In salt bath nitriding the nitrogen donating medium is a nitrogen-containing salt such as
cyanide salt. The salts used also donate carbon to the workpiece surface making salt bath a nitrocarburizing process. The temperature used is typical of all nitrocarburizing processes: 550 to 570 °C. Since the salts used are extremely toxic, modern environmental and safety regulation have caused this process to fall out of favor. The advantages of salt nitriding are: • Quick processing time – Usually in the order of 4 hours or so to achieve desired diffusion, where other methods take longer. • Simple operation – The salt bath and workpieces are heated to the desired temperature, and the workpieces are submerged for a given period of time. The disadvantages are: • The salts used are highly toxic – Disposal is controlled by stringent environmental laws in western countries. • Cost – These regulations have increased the costs involved in using salt baths. • Only one process possible with a particular salt type – since the nitrogen potential is set by the salt, only one type of process is possible.
Plasma nitriding Plasma nitriding, also known as
ion nitriding,
plasma ion nitriding or
glow-discharge nitriding, is an industrial surface hardening treatment for metallic materials. In plasma nitriding, the reactivity of the nitriding media is not due to the temperature but to the gas ionized state. In this technique intense electric fields are used to generate ionized molecules of the gas around the surface to be nitrided. Such highly active gas with ionized molecules is called
plasma, naming the technique. The gas used for plasma nitriding is usually pure
nitrogen, since no spontaneous decomposition is needed (as is the case of nitriding with ammonia). There are hot plasmas typified by plasma jets used for metal cutting,
welding,
cladding or spraying. There are also cold plasmas, usually generated inside
vacuum chambers, at low
pressure regimes. Usually steels are beneficially treated with plasma nitriding. This process permits the close control of the nitrided microstructure, allowing nitriding with or without compound layer formation. Not only is the performance of metal parts enhanced, but working lifespans also increase, and so do the
strain limit and the
fatigue strength of the metals being treated. For instance, mechanical properties of austenitic stainless steel like resistance to wear can be significantly augmented and the surface hardness of tool steels can be doubled. For instance, at moderate temperatures (like 420 °C), stainless steels can be nitrided without the formation of
chromium nitride precipitates and hence maintaining their corrosion resistance properties. In the plasma nitriding processes, nitrogen gas (N2) is usually the nitrogen carrying gas. Other gasses like
hydrogen or
argon are also used. Indeed, argon and hydrogen can be used before the nitriding process during the heating of the parts to clean the surfaces to be nitrided. This cleaning procedure effectively removes the oxide layer from surfaces and may remove fine layers of solvents that could remain. This also helps the thermal stability of the plasma plant, since the heat added by the plasma is already present during the warm up and hence once the process temperature is reached the actual nitriding begins with minor heating changes. For the nitriding process hydrogen gas is also added to keep the surface clear of oxides. This effect can be observed by analysing the surface of the part under nitriding (see for instance). ==Materials for nitriding==