Mr. David Pye (View Profile)
Pye Metallurgical International Consulting.
When two gears are meshed together, they are in point contact with each other on what is known as the pressure face, the success or failure of either of the two gears is dependent on:
- Surface hardness
- Core hardness
- Case depth
- Grain size
- Physical size of the gear
- Gear Material
- Quality of formed case
The failure of either gear will be (more than likely at the point of contact of the two gears. The root cause of the failure may not be obviously apparent but it will manifest itself generally at the point of contact of the two gear teeth.
Courtesy Gear Tech USA (Robert Errichello)
The damage will be observable due to the mechanical failure of the immediate surface. However, the root cause of the failure may well be (but not limited to) the core hardness value. If the core hardness is such that the gear face loading exceeds the tensile strength and core hardness of the gear, the case will fail. This is simply because the loading on formed carburized case exceeded the core’s ability to support the case.
The failure may manifest itself in the form of pitting on the gear pressure point surface. This is known as contact failure.
The carburizing process is what will ‘make or break’ the gear tooth. Therefore the better the control of the carburizing process, the better the surface metallurgy of the gear. This does not necessarily follow that one needs the most sophisticated equipment to control the process. Not at all!!
The atmosphere process control can be accomplished very simply with any of the following methods:
- Shim analysis
- Dew point in conjunction with the graph ‘Saturation of Carbon in Iron (Austenite)
- Test bar analysis
- Magnetic change
The problem with shim analysis is that the result is usually about 45 minutes old and it is labor intensive. However it is (if managed correctly) very accurate.
The dew-point method is of course the tried and tested method. However it will be a magnitude more accurate if it is run in conjunction with the Saturation of Carbon in Iron graph.
Test bar analysis can only evaluate the process conditions after the carburizing process.
The Magnetic change method of atmosphere control is based on the change in magnetic properties of a known wire length. When the wire is carburized, there will be a change in the magnetic measurement of the wire. There is a Japanese unit out in the market place for this type of atmosphere carbon potential measurement.
The surface metallurgy of the gear is very critical to the performance of the gear.
In order for the surface metallurgy to be successful, the core metallurgy and mechanical properties demand equal consideration. It is the core hardness and tensile strength that will give support to the formed surface carburized case. If the core hardness is too soft, then it is likely that the core will collapse under the loading the takes place on the formed carburized case. A recommended core hardness is usually around 38Hrc to 42Hrc.
The complaint then is, the material is too hard to machine. Remember, that the core hardness is determined during the carburize, austenitize and quench. It is the austenitize temperature that will harden the case surface, but it will also manipulate the core hardness of the steel.
The selection of the material will be made from the analysis of the material in relation to carbon and other alloying elements such as the carbide formers of:
- And others
In order to accomplish toughness in the steel, there should be a consideration for the amount of nickel present in the analysis. However, too much nickel will begin to promote the problem of retained austenite.
It goes without saying that process temperature control is critical. Remember that both:
- Time at temperature
will promote grain growth. All carburizing procedures necessitate time at temperature for the diffusion of the surface carbon content. Thus, good temperature control and in particular good temperature uniformity within the process is a mandatory requirement.
Process gas generation
Process gas generation is also critical to the success of the carburizing process. The choices of process gas generation will depend on:
- Process economics
- Equipment availability
- Natural gas and air generation with enrichment gas additives
- Synthesized atmospheres
- Low pressure carburizing with enrichment gases
The carburizing process has reached the level of a well controlled scientific method of surface treatment. This level of science is necessary for the success of the gear design to be both functional and ensure the life of the gear under its performance environment.
The reduction of contact damage to the tooth profile is dependent on three things:
- Gear design
- Control of the thermal process
- Material selection
What is the answer then to reduce the risk of Contact Damage? There is not one single answer that will eliminate the risk of contact damage. The reduction of the risk on contact damage will be dependent on a better understanding of the subject engineering science of contact damage in relation to the material selection and finally the metallurgical process control and evaluation.
Contact and scuffing damage microphotograph (Courtesy Fairfield Manufacturing USA)