B. Edenhofer, Ipsen Inc., Cherry Valley, IL

Summary: About 75 to 80 years ago, the positive effects of diffusing nitrogen atoms into steel components at temperatures around 500°C on the components' mechanical properties were realised and an industrial gaseous nitriding process using ammonia gas evolved. The first step on the evolution ladder after the basic process development was the effort to modify the nitriding power of the ammonia atmosphere by varying its amount of gas flow through the furnace, resulting in different degrees of ammonia dissociation and multiple process steps. In the 1960's, activities started to further increase the flexibility of the gaseous nitriding process by adding gases like nitrogen, hydrogen, endothermic gas and others to ammonia, leading to new process variations, the major one being the so-called "ferritic nitrocarburising" process Parallel to the gaseous nitriding process at normal gas pressure, a low-pressure version called ion-nitriding making use of the electric field of a high voltage glow discharge started to be first used in the industry as early as in the 1940's with a break-through in the middle of the 1970's. Thus, for the last 30 years, the pure gaseous technology and the plasma-supported gaseous technology, called ion-nitriding or plasma-nitriding, have been in fierce competition with none of the two being able to suppress the other. Newer developments on either part, like nitriding potential control, oxi-nitriding, post-oxidation, high-frequency pulsed plasma or active screen plasma, only to mention a few, have led to steadily increased utilization of both technologies, making gaseous nitring the thermo-chemical heat treatment process with the largest growth rate.