CF8M Sensitization

[mrfailure]

Does it make sense for cast CF8M (i.e. 316 chemistry) to sensitize in the HAZ adjacent to the weld if the casting is post-cast weld repaired and is used in water service? Selective attack is occurring in the ferrite in the austenite matrix below selective corrosion adjacent to the weld. Carbon would be in the sensitization range if this were a wrought steel (actually, carbon is excessive at 0.13%). I can use feedback on my theory regarding sensitization of ferrite (theory being carbides precipitate on the ferrite interface with the austentite) in this instance.

 

[metengr]

Yes, indeed.

 

[moltenmetal]

If you need low carbon to avoid sensitization, you need CF3M rather than CF8M.

 

[mrfailure]

Agree about CF3M (my favorite option). The other route would be to post-weld heat treat the casting to put chrome carbides back in solution.

 

[EdStainless]

Attack of the ferrite is not sensitization, that is normal for these grades.
residual ferrite has a very lean chemistry making it much less corrosion resistant.
Add the sensitization to this and you are assured of corrosion.
What FN did this material have? what was the actual FN of the welds?

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[jwhit]

I'm on board too with the CF3M suggestion. I would be concerned about the base and weld metal chemistry if you're seeing a 0.13 Carbon. Could the wrong filler have been used or if a GTAW or GMAW process could the shielding gas have been incorrect? Another possibility is carbon pick up from some type of mold metal reaction.
My personal opinion is that you always solution anneal CF8M after welding, unless there is a real good reason not to. Even CF3M, with it's reduced degree of sensitization, is better off with a solution anneal after welding. A lot of the corrosion issues with these materials are due to chromium depletion at the ferrite austenite interface caused by the carbon and chrome reacting preferentially to form a chromium carbide.

 

[mrfailure]

Good point on solution annealing, jwhit. There are many interesting problems involved with chemistry in this failure but I wanted to focus solely on the question of whether sensitization could occur in such castings as a sanity check. I have confirmed the mechanism since my last posting. Evidence of the mechanism includes complete depletion of chromium found in the corrosion product (normally would just expect Cr content to be lower but carbon was extra plentiful in this casting to allow such complete depletion) and selective intergranular ditching when etched electrolytically with 10% oxalic acid.

Thanks for all of the helpful comments!

 

[arunmrao]

Post weld repair,solutionizing the casting is a good suggestion,if it could be possible. In my experience weld repair of CF8M is not very common,if the foundry has good foundry practices. We very rarely experienced any major repairs.

CF3M again is a good alternative,if the cost permits. Unlike 316 and 316L the cost difference for CF8M and CF3M is significant. Of course those practising secondary refining processes are competitive in their pricing.

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[EdStainless]

But then 0.13C isn't CF8M, it is trash casting.
No, CF3M is no more expensive from a good foundry.

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