The subject of ion nitriding is not new. In fact, the procedure has been known since 1932. The phenomenon of plasma was first commercialized by Dr. Weynheldt and Dr. Berghaus in Germany. The technology was based on the generation of the gaseous plasma using continuous DC electrical power. The process of ion nitriding is governed by the same laws of physics for gaseous diffusion into steel at elevated temperatures. These also govern diffusion of nitrogen derived from salt bath or gaseous nitride systems.
The speed of the reaction of decomposition and diffusion appear to be faster with ion nitriding than with conventional techniques. This is an incorrect assumption, however. The speed all of gaseous preparation is considerably faster due to the immediate gaseous ionization without the need for time for catalytic reaction as is observed with salt bath and gaseous nitriding techniques. In addition, the process gases involved in plasma nitriding can be varied in terms of gas ratios, thus giving the process the ability to create both the necessary and appropriate surface metallurgy that will best suit the working environment of the die.
The surface of the steel is prepared for the nitriding procedure in a very different manner than is seen for conventional surface-treatment preparation. The surface preparation method for ion nitriding is to make use of a phenomenon known as a “sputter cleaning,” which can be likened to atomic shot blasting. Instead of using steel shot carried in a blast of air to the workpiece surface, the process now uses ionized gas electrons that are carried electrically to the workpiece surface at a very high speed. The steel surface is prepared for nitriding in a more efficient manner than when using conventional degreasing methods. The overall cycle times for the process using ion nitriding are generally faster than those of more conventional nitriding methods.
The main process-control parameters for ion nitriding are as follows:
- Process temperature
- Process time
- Process gas ratios
- Process voltage
- Pulse duration (power on time)
- Pulse off time
- Pulse duty cycle
- Process pressure