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Technical Discussions
Nigel
Engineering, - Specialist services
United Kingdom, Joined Oct 2000, 1096

Nigel

Engineering, - Specialist services
United Kingdom,
Joined Oct 2000
1096
00:14 Oct-12-2006
Looking for root erosion in thin-walled duplex stainless steel welds

We have air coolers on our plant (oil & gas) part of which consists of a row of 1 inch dia, 1,6 mm to 3,5 mm wall thickness duplex (22% Cr, UNS31803) tubes TIG welded with matching filler metal to an 8 inch duplex header (wt 11 - 18mm). Weld ferrite content range 35% - 65%.

I am no corrosion engineer but it seems that duplex is susceptible to weld root erosion or cracking in case of wet-upset in an H2S environment. So we wish to check the weld roots for such whilst new equipment fabricated from material more-suited to the potential environment are procured.

A simulation test rig of one header with 4 or 5 tubes welded in place is being made for performance demonstration purposes. Do anybody have experience of such configurations? Is low-frequency eddy current a possibility? Or DC MPI - knowing that the permeability variations may produce non-relevant indications. As the material/configuration set-up is difficult to inspect I expect signal interpretation and evaluation will be a problem. Any ideas?


 
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Sayed H. El-Nekhaly
R & D
Delta Steel Mill Company, Egypt, Joined Jun 2006, 7

Sayed H. El-Nekhaly

R & D
Delta Steel Mill Company,
Egypt,
Joined Jun 2006
7
05:52 Oct-20-2006
Re: Looking for root erosion in thin-walled duplex stainless steel welds
This type of corrosion known as intergranular corrosion. It result due to depletion of chromium to be concentrated on grain boundaries during welding of high chromium stailess steel. Traces of nitrogen within range (0.1 - 0.5%) added as alloying element in duplex stainless steel can prevent this type of corrosion. The mechanism occured by catching chromium as chromium nitrid which prevent depletion then corrosion cracking.


 
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