The plasma nitriding process began in the 1920's as an alternative to conventional gas nitriding. After World War II the plasma nitriding process received widespread acceptance in Germany, Russia, China and Japan. The process was not introduced into the United States until after 1950 and has only been used as a production process for the past 20-25 years.
As technology progressed so did the plasma nitriding process. Improved controls and, in later years, the microprocessor have allowed engineers to consistently control the metallurgical properties of the nitride layer. This control, shorter cycle times, simplified masking techniques, less product distortion and a consistent reproducible process have lead to an increase in the popularity of the process in recent years.
The nitriding cycle begins by placing the product into the vacuum chamber and evacuating the chamber to a desired vacuum pressure. Upon reaching the desired vacuum, the unit is back-filled with a process gas to begin the preheating cycle. The standard preheating cycle ranges in temperature from 850 to 1050 Fahrenheit. When the preset heating time has elapsed, the product is subjected to an ion bombardment to clean impurities from the surface. The process gas is ionized by a voltage that is applied to the product.
This ionized gas collides with the product removing impurities from the surface and preparing the product for the nitriding process to begin. When the product surface has been cleaned sufficiently, the nitriding cycle begins. A controlled flow of nitrogen, hydrogen and methane are introduced into the chamber and ionized by the voltage applied to the product. The plasma generated by the ionization envelops the surface of the product with a blue-violet glow. The combination of the heat and energy of the plasma cause the gasses to react with nitride forming elements in the steel.
As the process gasses react with the elements in the steel, a wear resistant layer is formed. This layer can consist of a gamma prime Fe4N or an epsilon Fe2-3 N composition depending on the percentage of each gas in the chamber. The choice of the particular composition would depend on the application of the product in the field. In addition to increasing the steel's abrasion-resistance, the nitride layer also improves the fatigue strength and reduces the friction coefficient. The nitriding cycle is continued for 2 to 72 hours until the desired case depth of 0.002" to 0.024" is achieved. The processing time is dependent on the composition of the steel being nitrided and the required case depth. Low alloy steels are generally processed for longer cycle times.