One of the major problems with gas nitriding is the understanding of surface preparation in terms of surface cleaning. It cannot be over-emphasized how important the precleaning of the surface of the steel is. Surface cleanliness is mandatory and a primary requirement to the success of the procedure. Surface contamination can be seen in many forms (Fig. 4). Once the surface contamination has been dealt with prior to the nitriding process, we can then deal with the problem occurring at the nitride procedure.
If there is no dissociation occurring at the process temperature, check the content of the ammonia storage system and change over to the fullest storage tank. If it is seen that the dissociation is occurring but not at the appropriate requirement, there is probably something occurring to reduce flow within the system hardware. This could be caused simply by a restriction in the flow line. However, the restriction could be caused by internal oxidation of the pipework if using plain-steel tubing. The other potential cause of internal oxidation could be located within the process chamber itself. This means that the process vessel itself could be oxidized or contaminated.
An often-overlooked item is the load-support fixturing, such as baskets, trays and load-support furniture. A very simple remedy is to either shot blast or at least glass-bead blast the surfaces. The recommendation is not to use any low-alloy or plain-carbon steel as the load fixturing or support furniture. This material will act in the same manner as a sponge and take the dissociated ammonia away from the work being processed. It is the degree of gas dissociation that will determine the quality of the nitrided surface metallurgy, so it is most important to ensure that the desired dissociation is being accomplished in order to produce the nitrided surface metallurgy required.
Surface discoloration is usually attributed to ingress of oxygen or air or a surface contaminant being carried into the process on the surface of the workpiece or the load-support furniture. If oxygen is present in the process chamber, it will usually occur on the cool-down portion of the process cycle. Thus, the sealing arrangement of the process vessel will be suspect. If the part is discolored, there will be no adverse effect on the surface metallurgy. Quite the contrary, there will be an improvement in the corrosion resistance of the steel at that point of the contamination.
Some nitriding procedures are now calling for the deliberate oxidation of the nitrided surface as a corrosion-resistant barrier. Some of the trade names for this procedure are oxy-nitride, nitrox, niox and many others.