Inconel 625 weld overlay Fe content 1

[Biss73]

Hi,
We want to buy a new vessel with inconel 625 weld overlay. The client specification says that the production weld chemistry of Ni alloy such as 625 corrosion resistant weld overlay on the carbon steel base material shall be equal to or better than their respective Ni alloy pipe/plate material chemistry. The Fe content for inconel 625 is 5% max.

In order to reach this 5% Fe content in the weld overlay (0.1" below the surface) the vessel has to have 2 layers of overlay (0.275" thick). One fabricator suggested to us only 1 layer (0.197" thick) of weld overlay with a Fe content of 7% maximum.

The service of the vessel is Hydrocarbon + Hydrogen (H2) + Hydrogen Sulfide (H2S) with designs conditions of 2500psi @ 550°F.

Considering the cost of a second overlay, I was wondering what would be the consequence for the corrosion (or else) if we were to have a 7% Fe content inconel 625 overlay.

Thank you for your collaboration,
Biss

 

[EdStainless]

It isn't the higher Fe that hurts, it is the lower Cr and Mo that reduce corrosion resistance.
Two layers of overlay are common and considered good practice.

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Plymouth Tube

 

[metengr]

Iron increases susceptibility to halogen attack on CRO's of Inconel 625, so iron should be kept at amounts typically below 10% by mass. If your client requires maximum corrosion protection, I would agree with limiting the iron to 5% or less in mass iron content.

 

[SJones]

The age old arguments: is it 5% or 10%, where to take the analysis, etc etc. The client wants 5%; the client pays for 5%. A deviation should really only be approved on the basis of corrosion testing, and then it starts getting messy. ThyssenKrupp have done some very basic corrosion testing of various dilutions reported in NACE Corrosion 2010, Paper 10262.

Steve Jones
Materials & Corrosion Engineer

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[pradipgoswami]

Hi Biss73,

There are tons of reasearch papers and articles on the effect of "Fe" dilution in 625 alloys. 625 overlay contains both "Fe" (diluted from base metal) and Nb added through the wires. Excess of these alloying elements leads to formation of a deleterious phase, "Laves Phase", which reduces both corrosion resistance and ductility. Corrosion is of specific problem in Oil and Gas Industry, whlie high temperature ductility if a common problem in "High Sulphur Coal" Fired boliers. In 625 overlay "Nb" is required for a number of good causes(strength, corrosion resistance), while excess of iron in the deposit leads to bad causes as stated above.

For 625 overlay , in order to keep the "Fe" dilution within the specified linits(5% or below), some good points to enforce would be :-
- a minimum of 2 layer overlay, with a controlled heat input
-single pass technique to get the required overlay deposit height would mean higher heat input,i.e excess base metal dilution and more "Fe" in the weld.

-ask the filler wire manufacturer, what's the extent of "Fe" in the undiluted weld deposit. Usually as per AWS Sec II,Part-C, "Fe" in undiluted 625 deposit should be , SFA 5.11(SMAW)- 7% maximum, and SFA 5.14(GTAW)-5% maximum. Note these values are in undiluted chemistry.

-In order to restrict "Fe" to maximum 5% in the weld overlay PQR, you may have to buy weld consumables with undiluted weld "Fe" level between 2-4%, as a safe bet. This may be a bit expensive, however better discuss with consumable suppliers.

Last of all the following article provides a nice analysis of this problem, though there may not be major emphasis on "Fe dilution Alone".

Thanks

http://www.lehigh.edu/~inemg/assets/Publications/DuPont 1996 Solidification of 625 Overlay.pdf

Pradip Goswami,P.Eng.IWE
Welding & Metallurgical Specialist
Ontario, Canada.
ca.linkedin.com/pub/pradip-goswami/5/985/299