Nitriding is the process of binding nitrogen to a metal in order to harden it. This is one form of case hardening, which involves creating a thin layer of a strong alloy on the exterior part of a metal object. Some of the most commonly nitrided materials are low carbon steels, though the process can also be used with other metals such as titanium or molybdenum. Nitriding can be used for a wide variety of different components, including gears, dies, and cam or crankshafts. There are three main nitriding processes, each of which uses a different nitrogen rich substance to achieve the desired level of case hardening.
The process of nitriding can be achieved through the combination of a nitrogen rich medium, metal, and heat. When subjected to intense heat, some of the nitrogen from the donor medium can become alloyed with the metal. When done properly, this may result in a very hard alloy of nitrogen and the metal being used. Some of the first studies that involved this type of case hardening took place in the 1930s. The process was discovered around the same time in both the United States and Germany, though it was not used widely until after the end of World War II.
Ammonia gas was one of the first mediums used to alloy nitrogen with steel in a process called ammonia nitriding. The procedure typically involves heating a piece of metal and then subjecting it to ammonia. Due to the heat, the gas can then break down into hydrogen and nitrogen, at which point some of the nitrogen may sublimate into the metal. This method can allow great control over how deep the nitrogen penetrates and the specific properties of the ensuing alloy. Other benefits, compared to other methods, include the ease with which large batches can be processed and inexpensive equipment.
Another process that can alloy nitrogen with metal typically involves a bath of cyanide salt. This type of nitriding requires that heated metal be placed into nitrogen rich salt. In addition to alloying with nitrogen, the metal may also pick up carbon from the salt in a process known as carbonitriding. This is a relatively fast and inexpensive process, though the salts involved are usually very toxic. The overall cost of the procedure may increase if a jurisdiction has strict regulations that govern the disposal of toxic substances.
Plasma nitriding is another process that may be used to case harden metal. In this process, pure nitrogen is typically superheated into plasma. Other gases may also be used, though the general concept involves the creation of nitrogen ions that can then bond with the metal. This type of case hardening is typically very accurate, fast, and can result in parts that are immediately ready for use.