Iron dilution in Incoloy 625 cladding on CS pipe

[pipexp]

I have observed a unique problem with respect to dilution of Fe content while welding over laying 3mm Incoloy 625 on API -5L pipe. After over laying it was observed that Fe conten has gone up to 8% in the over laid layer. Fe content as indicated in MR 01-75 FOR Incoloy 625 is 5% max. Now can any body suggest whether this can be accepted as material shall have to meet as per client's requirement.

 

[EdStainless]

Was this a single pass overlay? Sub arc strip fusion?
What was the result in the qualification trial?
What was the Fe in the starting 625?
The real issue is corrosion resistance. The overlay is not being relyed on for mechanical properties. Unless overlay with this level of Fe has been tested then it may not be acceptable. Fe dilution is why most overlay work is done either in multiple passes or with odd alloys for the overlay.
In this case I would also be concerned with the C content of the overlay.

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[deanc]

http://www.specialmetalswelding.com/publica/joining.pdf

See page 16

 

[SMF1964]

This is not really a unique problem. My experience with overlayed waterwall tubes shows that this occurs with a whole range of nickel-based alloys - 625, 622, 52...

There is a clear iron enrichment zones in nickel based overlays in boiler tubes that shows up clearly when the overlay is etched using a strong acid etch (rather than an electrolytic etch). The higher iron region etches first in the metallographic sample. Does this indicate that these areas are more susceptible to corrosion? I would say that, qualitatively, the answer is yes. It is if your environment is Etchant #89 (mixed acids - Nitric-Hydrochloric-Acetic-Glycerin).

 

[SJones]

This is the perennial question: how much Fe is too much? You will see any number you like between 5 and 25 wt % for allowable Fe content. You will even see it expressed in terms of dilution % even though the specifier has no idea how the dilution will be determined. First of all, you need to ask yourself where, exactly, in the overlay was the 8% measured? Secondly, you need to ask yourself what is the location of this Fe measurement in relation to the minimum overlay thickness (and the surface that will be exposed to the environment from which the substrate must be protected). Thirdly, ask what is the lifetime of the equipment and what is the maximum wastage rate of the overlay (yes - even CRAs react with the environment). Finally, assess whether, in the specified lifetime, enough CRA will be lost to expose the high Fe content material to the environment.

Steve Jones
Materials & Corrosion Engineer

http://www.pdo.co.om/pdo/