The choice of the process method using plasma technology has really come down to the use of the Pulsed DC system in today’s process technology. Most of the traditional plasma nitride furnace manufacturers have now opted for the pulsed technology path. There still remain a small number of pulsed DC plasma-generation systems, but the process technology of Pulsed DC has many advantages over the continuous systems.
The continuous DC technology was a pioneering technology for the continuous DC process method, and many lessons were learned from it. Recent developments have been in the plasma-generation systems as well as the program control systems with the use of PC/PLC combinations for process control.
However, there seems still to be a singular lack of understanding as to the benefits of nitriding. The secret of nitriding is the pre-heat treatment of the substrate material for the nitrided case support. It is usually assumed that if the nitrided case collapses, it is caused by poor nitriding process control. But what (in reality) has happened is that the core has failed to support the formed case by being too low in supportive hardness. If one thinks of the ferritic nitrocarburizing process, it is generally applied to low-carbon steels and used primarily for wear resistance and not load resistance.
The Pulsed DC generation system was developed in Germany, and its foundations were derived from the concept of continuous DC process technique. The continuous DC system was developed because of the need to separate the use of the generated plasma glow seam for the heating of the workload up to its process temperature. By the separation, it allowed supplementary heating to be incorporated into the furnace system and used only the voltage/power to generate the plasma glow seam and reduced the serious risk of arcing. In other words, the new technology had developed its own simple arc-suppression system that reduced the serious risk of localized overheating. Typically, overheating occurred at sharp corners due to arc discharges in the continuous DC system. All of this development took place in the early 1980s.
Another misunderstanding of the nitride process is the ratio of diffusion gas (nitrogen) to hydrogen ratio. Many still use (even with the pulsed technology) the ratio of three volumes of hydrogen and one volume of nitrogen. In essence, one might as well utilize gas nitriding technology and save the cost of the plasma generation.
- 2NH₃⇄2N +3H₂(1 volume nitrogen to 3 volumes of Hydrogen)