Nitride networks will form as a result of the lack of process control during the nitriding process.
To understand what causes networks to form, let’s consider a glass of water. If we introduce a small amount of salt (sodium chloride) into the water, we have made a solution. If we continue to add salt to the water, we will change the solution to a saturated solution. We can dissolve the excess salt in the saturated solution simply by raising the water temperature. When the heating procedure is discontinued, however, the excess salt that was taken into solution will now start to precipitate out of solution and settle in the bottom of the receptacle.
This is the principle of nitride networking. The networking is simply formed as a direct result of nitrogen diffusing into the surface of the steel, particularly at corners. After the completion of the nitrogen diffusion cycle when the steel begins to cool down, the excess nitrogen will precipitate out of solution and form hard, brittle networks.
The networking will generally occur at the corners of a steel component or on the corner of a gear-tooth profile. This occurs because the nitrogen will attack the corner of the gear tooth profile from all directions. The networking is potentially a brittle condition and can cause surface exfoliation of the comound layer, even if the compound layer has been ground (but not completely ground off).
To minimize the occurrence, there are a couple of options available. The first of these is the Floe (two-stage) nitride process. The downside of the process is that it employs a much higher process temperature. This could potentially soften the core.
The next option is the control of the nitride potential of the process. This can be accomplished by process-gas dilution or by instrumented atmospheric control.
The final option is perhaps the most dramatic, and that is to change to process methodology by going over to plasma nitriding and blending the process gas ratios according to the surface metallurgy that is required.
The bottom line regarding the control of the surface metallurgy of a nitrided steel is control of the process. Plasma nitride will control the: DC voltage (or pulsed DC voltage), pulse duration (if pulsed DC voltage), amperage, process pressure, precisly metered process gas flows, time at temperature, process temperature and sputter cleaning.
However and whatever process method of nitriding is selected, it is mandatory to have control of the process nitriding potential and the auxilliary process parameters. Gone are the days that one could just load up the furnace, flood the process chamber with ammonia and walk away until the cycle time was complete.