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Carbonitriding Passivation Problems

 

Hi,

My company has been carbonitriding a particular automotive gearbox component for a number of years. Recently we were asked to investigate why there was so much variation in the surface hardness.

After numerous trials, checking out all of the various manufacturing operations, we came to the conclusion that the problem originated during turning, where the particular cutting fluids used appeared to be affecting the surface to the extent that it was actually possible to achieve zero case depth in certain areas.

As part of this investigation we also established that where the component was subjected locally to electrochemical deburring, the case was always perfect.

We are aware of such 'passivation' effects with nitrided components but were very surprised to experience such dramatic results with components treated at 880 °C.

Based on the results of our trials, we believe that the solution to our problem lies in subjecting the components to some form of caustic pickling prior to the carbonitriding.

Has anyone had such an experience as this? If so how did you overcome it? Can anyone recommend a solution that will effectively strip this adsorbed layer?

Thanks,

Michael Gilpin
- Canterbury, Kent, England

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Passivative coverage layer formation is due to cutting fluid used in machining which create a stable thin layer and obstruct the diffusion process like carburising, carbonitriding,Nitriding,we have come across this problem,in carburising.

The solution for this problem is to pre oxidize the components at 400-450 °C in Air atmosphere for about 2 hrs, This will help to break the layer and create the surface more active and hence proper diffusion take place.

K. Kanchi Varadaiah
- Bangalore, Karnataka, INDIA