The industrial thermal-processing industry has become very energy consciousness and demands the optimum use of whatever chosen energy utilization is selected. The furnace loading demands the optimum loading so as to maximize both the thermal energy and the appropriate process-gas consumption.
The furnace loading needs very serious consideration in relation to the nitriding process method chosen. It may be surprising for the reader to learn that the two most effective methods of thermal heat transfer and loading density of nitriding system are fluidized-bed nitriding and salt-bath nitriding. These two methods of thermal-energy transfer from the heat source to the workload are very thermally efficient because of energy conductivity and heat transfer.
Exciting developments are taking place with the fluidized-bed method of nitriding, which enables the user to not only nitride but to diffuse other elements such as aluminum (which is a nitride former) into the surface of the steel.
The selection of the steel for gear manufacture comes down to core-hardness conditions and support of the formed case. Some nitriding steels contain aluminum up to 1% in the analysis. This group of steels does not lend itself to the manufacturer of any type of gear. This is because aluminum is a strong nitride former, which will result in extremely high surface-hardness values.
The other strong consideration for the use of the nitriding process for gear heat treatment is that there is no phase change that takes place in the steel while being nitrided (providing one performs the process at a temperature below that of the steel’s final tempering temperature). Once the process cycle has been completed, the system is simply cooled down and no quench is involved. This means that quenching distortion is not an issue (e.g., carburizing heat-treatment cycle).
It cannot be said that no distortion occurs during the nitriding cycle no matter which method has been chosen. Because of the diffusion of nitrogen into the surface of the steel, some growth will take place. However, the amount of growth will be dependent on the case depth selected. Additionally, the amount of distortion that will occur will still depend on the machining practice of “feed and speed.”
Because of the minimal growth, the gear teeth can be machined to slightly under size and grown back to size because of the surface growth that occurs in the nitriding process. A lapping tolerance can be left on the gear to accommodate the final required surface finish.
It is very strongly believed that nitriding has a strong future because of the reduction of potential distortion due to low process-temperature requirements.