The nitriding process is perhaps one of the most misunderstood thermo-chemical surface treatment processes that are practiced today. So the process is not as old as, for example carburising. However, it is perhaps (as far as chemistry is concerned) one of the most simple of all of the thermo-chemical surface treatments are concerned. Below are some of the problems that can occur as a result of and during the nitriding process. There are many problems that can occur with the nitride procedure, and it will be necessary to evaluate the process technique.
Gas Nitriding
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 pre-cleaning 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. 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.
Gas Dissociation
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 pipe work 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 Discolouration
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 work piece 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 discoloured, 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.
The process of forming black iron oxide to ferrous metals and other types of materials is often called black oxide. The black oxide process is a chemical conversion coating for ferrous materials, stainless steel, copper and copper-based alloys, zinc, powdered metals, and silver solder. This process is done to add corrosion resistance, enhance appearance, and minimise light reflection.
Unlike nickel or zinc electroplating, the black oxide is not deposited on the surface of the substrate. Instead, a chemical reaction is generated between the iron on the ferrous metal surface and the oxidizing salts that are present in the black oxide solution. The generated chemical reaction results in the formation of magnetite, a black iron oxide, on the surface of the coated material.
The Black Oxide Processes
The black oxide chemical conversion can be done through the hot process or cold process.
The hot process starts by fixturing, which is the hanging of individual parts from a wire, placing them in baskets, or putting them in a tumbler. After fixturing, the parts will be cleaned by submerging them in an alkaline bath and then rinsing them in clean water. For blackening ferrous materials, they must be placed in a bath of propriety blackening solution that contains sodium hydroxide, nitrates, and nitrites. This solution converts the material surface into magnetite. The proprietary blackening bath is not applied when applying hot black oxide for stainless steel parts.
The cleaning and rinsing of materials must be done in warm baths, up to 160°F. The blackening, on the other hand, must have temperatures ranging from 265°F to 285°F. The final step in the hot process is the application of oil to heated parts. To avoid corrosion, the oil seals the black finish by sinking into the porous layer of the black oxide.
Cold black oxide is only introduced since it reduces hazards and saves energy. With the cold process, a compound called copper selenium is applied to the materials at room temperature. This process is contrary to the hot process wherein the parts are subjected to numerous chemicals at high temperatures. The final step in the cold oxide process also deals with the application of oil to parts for added corrosion resistance.
While cold black oxide can offer better productivity and convenience for in-hose blackening, its coating property tends to rub off easier and offers less abrasion resistance. Hot blackening, on the other hand, is known to be cost-effective, provides superior coverage, and offers enhanced corrosion resistance.
Applications of Black Oxide
The blackening process of materials is typically used in industries that revolve around retail, automotive, electrical, home, garden, gearing, and firearms. Some products and tools that are processed with black oxide include store displays, fixtures, oil filter cans, hood fasteners, nuts, washers, wire strippers, cutters, clipping tools, small gears, electrical switches, gun components, and shotgun shell magazines. With the black oxide process, you can easily get products and tools that have a remarkable surface look and have enhanced corrosion resistance property. To know more about black oxide, feel free to contact us now at Alpha Detroit Heat Treatment.