Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. These processes are most commonly used on low-alloy steels. They are also used on titanium, aluminium and molybdenum. Typical applications include gears, crankshafts, camshafts, cam followers, valve parts, extruder screws, die-casting tools, forging dies, extrusion dies, firearm components, injectors and plastic mold tools.
Processes 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.
Additional improvement of the durability of a nitrided surface is possible using the ZeroFlow method. Developed by SECO/Warwick and Poznan University of Technology, the method also allows several-fold reductions in process gas consumption, thereby lowering nitriding costs, and reduces emissions of post-process gases to the environment. 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.
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