Many companies are gas nitriding, and the process is 100 years old from the patent date. So, the process is not a new process. It is, however one of the “youngest” processes.
Just as carburizing requires precision atmosphere control, so does the process of nitriding. Ammonia cannot flow without control. Precision control is a must.
The reason for the precise control is that the surface metallurgy is governed by the limit of solubility of nitrogen in iron. This solubility is determined by the graph developed by Donald T. Hawkins. Based on this graph, the critical area of solubility in terms of “weight percentage nitrogen” occurs between 5.5% and 6.1%. This means that if the percentage value in iron is exceeded during the process, there is a very strong probability that nitride networking can occur, particularly at corners, which can result in premature fracturing of the corner.
Control of the process temperature is also critical to the flow of process gas because temperature affects the ammonia decomposition. The higher the process temperature, the greater the ammonia gas-flow requirement. Of course, the greater the gas flow and the higher the process temperature, the greater the thickness of the compound layer. This phenomenon was discovered in the early days of evaluating the nitriding process.
The early work in gas nitriding process flow control showed that when the ammonia decomposes by the application of temperature to nitrogen and hydrogen, on cooling the process gas will recombine by the following equation:
Due to the catalytic effect of the processed steel, the ammonia gas is partially broken up into its elemental form of nitrogen and hydrogen, which are the insoluble gases coming from the furnace exhaust. Of the three gases that exhaust, only ammonia is soluble in water.
The simple (but tried and tested) method of control is the graduated burette and reservoir of water. The general point of control is to have the burette filled with 70 cubic centimeters of water. This means that the void above the water is insoluble nitrogen and insoluble hydrogen. In this case, there is 30% dissociation occurring.
This rate of dissociation will produce a compound layer (in relation to process temperature) of approximately 10% of the total formed nitrided case. It is a simple method that has stood the test of time for almost 100 years. In order to have some control over the gas nitride process, the above-described method is as simple as it can get. Some control is most certainly better than no control at all.
There are newer methods of gas-decomposition control. The principles of the newer control systems monitor the insoluble gases. One can now have better control of not only the thickness of the compound layer but also the composition – Epsilon Nitride to Gamma Prime within the compound layer.
In this day and age when engineering is looking to produce repeatable and consistent metallurgical results, gas flow and gas-decomposition control are key components to producing a good nitrided surface metallurgy. The control of the gaseous decomposition will determine if there is a brittle surface metallurgy or an impact-resistant metallurgy.
The core metallurgy of the processed steel should be in a hardened-and-tempered condition. The hardness value of the core will be determined by the load-carrying requirement of the formed surface case. It is the core that will support the nitrided case and will result in the success of the surface-metallurgy performance. So, it is necessary to transform the core into tempered martensite. However, the final tempering temperature of the harden, quench and temper sequence must be approximately 75ºF (25ºC) above the nitriding process temperature.
The form of the hardness profile of the nitrided case will depend on the strength of the interaction between the particular alloying elements and nitrogen as well as the ease of nucleation of the nitrided case. So it follows that these items are both monitored and controlled:
- Ammonia gas flow rate in relation to work surface area
- The measurement of the ratio of insoluble nitrogen and insoluble hydrogen
- Process temperature