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PLASMA SURFACE TECHNIQUE
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Posted by David on August 27, 2003 at 14:31:24:
In Reply to: White Layer posted by Aranguren on August 14, 2003 at 12:19:26:
Dear Sir,
The decision to remove white layer or compound layer will be decided by the application of the gear. Some techniques require it to be removed, others do not. Transmission gears are usually nitrided with a minimum amount of compound layer. The amount or thickness of the compound layer will also be determined by the process technique (gas, ion, salt, fluid bed) in relation to the time at temperature, the selected process temperature, the process gas chemistry and the steel chemistry. One must also accuratly control the process very precisly in order that no nitride networks are formed at the corners etc.( This will cause corners to chip and break) The layer is usually very brittle, and can, if used under fast start fast stop conditions, high impact gears, generate surface cracks, leading to exfoliation of the compound layer. The result is that the small particulant matter from the cracked compound layer, then mixes with the gearbox oil and causes further aggrevated surface failure. To remove the compound layer by shot peening is a 'guestimate' at the very best. One does not know how much surfaqce material has been removed or when you are below the compound layer. With Plasma nitriding one can control the process gas mixture very accuratly insomuch asd one can manipulate the nitriding potential of the gas. In gas nitriding ammonia is 1 part N2 and 3 parts H2 (A ration of 1:3). With this gas ratio , the compound layer will form. Usually about (depending on the steel) at a ratio of 10:1 (Case thickness to compound layer thickness) Therefore if a case of say 0.25mm has formed , then the compound layer is likely to be approximatly 0.025mm thick.(This cannot be accuratly determined, but the ratio is close. With the plasma, and because you are able to vary the ration of nitrogen to hydrogen, the compound layer thickness can be controlled, to the point of no compoubnd layer. Another method of compound layer removal was a patented process by Bell Helicopters for disolving the compound layer by immersion into a a cyanide based bath. Altough it works, it has similar limitations to the shot blast procedure, as well as effluent disposal problems
We have a book being published by ASM in Cleveland Ohio, USA called Practical Nitriding and Ferritic NitroCarburizing. All of the above is discussed in various chapters of the book. The publication release date is scheduled for 13th Dec, 2003. Hope the above will assit you.
Best Regards,
David Pye