Mr. David Pye (View Profile)
Pye Metallurgical International Consulting.
Gears have nitrided for many years, yet the subject of nitriding of gears is still very controversial, and continues to be so. The process of nitriding is in reality, process technology. Meaning that it is becoming very a favored thermal process technique due to the low process temperature required and no quenching. However this does not mean no distortion.
When considering the process of nitriding of gears, the thermal treatment of gears requires a very careful selection of the choice of steel in relation to the gear application such as;
- Core hardness requirement
- Surface hardness requirement
- Case depth requirement
- Gear operating conditions
When making the choice for the steel for a gear to be nitrided, it will be necessary to consider at the initial design stage of the gear, the working environmental conditions that the gear will operate under. Some design considerations would be as follows:
- Tooth Pitch
- Tooth design loading
- Fatigue bending performance
- Case depth requirement
- Operating temperature
- Gear type (helical, spiral bevel, spur gear, pinion driveshaft)
- Core hardness required
- Core tensile strength
- Allowable surface growth
- Grinding tolerance
- Surface metallurgy (Compound layer, type of case to be formed in relation to the steel chemistry)
Traditionally gear types have been carburized, quenched and tempered in today’s manufacturing environment. This necessitates high-temperature processing conditions, for the diffusion of carbon into the steel surface. This must be followed by austenitizing and quenching. The obvious net result is the potential for distortion. The distortion aspect will also be influenced by the machining conditions prior to the carburizing. The distortion will be influenced by;
- Process temperature selection
- Time at the process temperature for the carburizing procedure
- Grain growth
- Potential for grain boundary oxidation
- Potential for retained austenite
- Preheat treatment conditions if applied, such as normalize and anneal
- If forged, what are the start and finish temperatures of the forge procedure
- Pre-machining residual stresses
The process of nitriding on the other hand can be accomplished by any of the following nitride process methods;
- Gas nitriding
- Salt bath nitriding
- Low Pre nitriding
- Plasma nitriding
These processes offer to the engineer, the ability to pre-treat the steel in order to develop the required core hardness and tensile strength. The appropriate core hardness is necessary to support the formed metallurgical case when the gear is nitrided. During the nitride process (and provided that the nitride process temperature is approximately 30° C below the steels core tempering temperature) the core hardness is not affected. There is absolutely no opportunity for grain boundary oxidation during the nitride procedure, nor is there any opportunity for retained austenite conditions, simply because the nitride process temperature is well below any transformation temperatures.
The thermal processing industry has become energy consciousness and demands the optimum use of whatever chosen energy utilization is selected.
The loading of the furnace (for whatever process method is chosen) necessitates that the optimum loading is utilized so as to maximize both the thermal energy and the appropriate process gas consumption.
One needs also to consider the available surface area of work to be nitrided
The furnace loading needs very serious consideration in relation to the nitride process method 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
- Salt bath nitriding
These two methods of thermal energy transfer from the heat source to the work load are very thermally efficient because of energy conductivity and heat transfer.
There are exciting developments 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 and thus increasing the surface hardness. (It must be said at this point, that it is not always a good thing to achieve very high hardness values on gears, simply because if thing here is subject to impact starting, the pressure the gear tooth may well begin to fracture and chip, thus causing serious premature failure)
The selection of the steel for gear manufacture comes down to core hardness conditions and support of the formed case. Some nitriding steels contain aluminium up to 1% in the analysis of the steel. This group of steels does not lend itself to the manufacturer of any type of gear, simply because of the aluminum being 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 heat-treated during the nitriding process.(Providing one nitrides at a selected nitride process temperature which is below that of the steels final tempering temperature).
Because of the diffusion of nitrogen into the surface of the steel, some growth will take place. Once the process cycle has been completed the system is simply cooled down, and no quench is involved.
Further, this means that distortion is not an issue as a result of quenching such as takes place during the carburizing heat treatment cycle.
It cannot be said that no distortion occurs during the nitriding cycle no matter which method has been chosen. Growth will occur as a result of nitrogen diffusion into the steel. However the amount of growth will be dependent on the case depth selected.
Distortion will occur. The amount of distortion that will occur will still depend on the induced residual stress caused by machining practices of ‘high feed and speeds’.
The writer feels very strongly that nitriding has a tremendous growth potential because of the reduction of potential distortion due to low process temperature processing conditions.
Due to the potential of minimal growth occurrence, gear teeth can be machined to an under size tolerance, and grown into size because of the surface growth that will occur during the nitriding process. A lapping tolerance can be left on the gear to accommodate the final required surface finish.