Contamination of P91 Material By Carbon Steel

[pjdobson]

Can anyone give me some more detailed information on the phenomenon of carbon contamination of P91 material when it comes into contact with carbon steel (e.g. when carbon steel is used to lift P91 material, some of the carbon steel material gets embedded into the P91 material). Specifically, what happens to the microstructure of the P91, and what are the results of this kind of contamination?

 

[EdStainless]

It isn't carbon contamination, it is Fe contamination. The embedded Fe in the surface corrodes very easily. These locations can provide sites for the easy initiation of corrosion attack.

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[pjdobson]

EdStainless: Thanks for the response. It is also possible that, if the contamination takes place prior to fabrication and PWHT that the carbon can react with the Cr in the P91 forming chromium carbides during PWHT at the contamination sites and alter the P91 material property?

 

[pjdobson]

EdStainless: Sorry, my previous post was supposed to be a question and not a statement.

 

[davefitz]

I had never heard of carbon contamination of P91, but that doesn't mean it isn't possible. Sounds like the fabricator is on a fishing expediton to deflect criticism of their fab procedures and blame the erector.

What sort of microstructural changes have you detected? If you completed a hardness traverse and plastic film replication and found ferrite instead of martensite, then it is not carbon contamination, but fabricator education that is the root cause.

 

[metengr]

I concur with davefitz. Using iron contamination for an iron-base alloy steel is indeed a fishing expedition. There are no issues.

 

[stanweld]

Concur with davefitz and metengr. Surface decarburization due to the multiple thermal treatments seen in P91 fabrications is of more concern. Carbon will not be added during heat treatment in oxidizing environments; it will be subtracted.

 

[EdStainless]

The only chance to add carbon is if the parts are heavily contaminated with oil or grease going into HT.
I would be more worried about oxidation/decarb damage in this grade also.

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