This is part 2 of the series. Part 1 can be found here.
Salt-bath nitriding is a low-temperature operation utilizing cyanide-based salts, and the die is nitrided in the molten salt. The molten salt liberates free nitrogen to diffuse into the surface of the steel. The salt bath treatment has a distinct advantage, which is excellent temperature uniformity within the bath.
Plasma Nitriding (aka Ion Nitriding)
Ion nitriding is not a new subject because the procedure has been known for many years. In fact, since 1932 the phenomenon of plasma was first commercialized by Dr. Wenheldt and Dr. Berghaus in Germany.
The process technology was based on the generation of gaseous plasma using continuous DC electrical power. Ion nitriding is governed by the same laws of physics for gaseous diffusion into the steel at elevated process temperatures.
Diffusion laws are the same whether the nitrogen source is ion, salt bath or gas. The speed of the reaction of decomposition and diffusion appeared to be faster with ion nitriding than with conventional techniques, but this is an incorrect assumption. However, the speed of the process gases for diffusion are somewhat faster due to the nitrogen gas being electrically ionized during the process unlike the gaseous nitride procedure, which necessitates the use of a catalyst for the preparation of nitrogen to diffuse into the steel surface – the workpiece itself.
In addition to this, the process gases involved in plasma nitriding can be varied in terms of gas ratios (one uses molecular nitrogen and molecular hydrogen). This now means that there is initially a gaseous ratio of nitrogen to hydrogen at 1 to 1. Therefore, the gaseous ratios can be varied as required for the successful application of the forging die or extrusion die, which now gives us the ability to create the appropriate surface metallurgy that will suit the particular application and steel chemistry.
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 for ion nitriding is to make use of a phenomenon known as sputter cleaning. This phenomenon can be likened (a simple comparison) to that of atomic shot blasting.
Instead of using steel shot carried in the blast of it out to the workpiece surface, one now uses ionized-gas electrons that are carried electrically to the workpiece surface at a very high speed. This prepares the steel surface for nitriding in a more effective manner than when using conventional degreasing methods. The overall cycle times for the process using the ion-nitriding technique are generally considered to be faster than those of the more conventional nitriding methods.
The process-control parameters for the ion-nitriding process are listed (there are other process variables that are not listed here). The principle variables are as follows.
- Process temperature
- Process time
- Process gas ratios
- Process voltage
- Process duration (power and time)
- Pulse-off time
- Pulse duty cycle
- Process pressure