Mr. Rao,
Please see my answers to your queries.
Thanks
Pradip Goswami, P.Eng,IWE
Welding & Metallurgical Specialist
Ontario, Canada
From: materials-welding@googlegroups.com [mailto:materials-welding@googlegroups.com] On Behalf Of Vasantharao
Sent: Saturday, February 15, 2014 1:46 AM
To: materials-welding@googlegroups.com
Subject: Re: [MW:20108] RE: Black spots on DSS Weld Joints.
Dear Sir,
We use Pure Argan - 99.998% with moisture <5PPM, Oxygen <5 PPM as per Gas MTC.-- That's fine.
We use separate shop for DSS welding for avoiding contamination from CS welds/ grinding. --That's fine
Welders aware of Acetone cleaning of joint, filler wires etc. -- That's fine, and should be done.
Purging gas O2 level continuously monitored with instruments till completion of welding (maintained <0.03%).--This is good but still try to keep lower, (<0.02%).
Random checks during welding observed inter pass temp. some time more than 150 Deg.C but <200 Deg.C -- That's fine.
Some time we preheat to 50 Deg.C to remove moisture. - That's fine.
This problem typically we observe in 6" & 8" Pipes, others are satisfactory (lower dia). --This tells me that there're issue with some pipes, assuming that same welding filler (brand, batch and through cleaning as mentioned above) is used for welding.
Is it dark spots are any inter metallics? ( As per leterature it forms at 300-700 Deg.C, generally during welding, this range not encountered). -- Intermetallics are detected only under microscope, they can not be interpreted by visual inspection alone. 2209 is a relatively lean filler metal (wrt ER 2593 and 2594 filler). Hence I would not suspect much intermetallic. However as a cross check you may take replicas or put one small section of the welds with dark indications under the microscope to see if it contains intermetallics. Note to get to the intermetallics, you have to do the grinding , polishing and micro etching, which will remove the dark spots. A better approach is to see the section under SEM & EDAX, which may be done on as it is condition. EDAX will tell what the contaminant is , if metallic or hydrocarbon
Pickling & Passivation will be costlier, management may reluctant to go for it. --There's no other way if the steel is grossly contaminated with any chemicals
Is there any effect of TIG nozzle sizes, Moisture condensation at regulators?. --I do not think so.
Some of the Pipes are received from China, is there any material chance?. --I do not want to be prejudiced.But where's there's bulk purchase/ trading, such mishandling would happen and would produce bad results while welding. If the products are directly purchased from the mill, post this problem directly to the manufacturer. Otherwise if it's purchased through dealers( with many sub-agents) you may have a hard time to get the correct answer.
This problem typically we obseve in 6" & 8" Pipes. --If the quantity is not much get them replaced with products from some reputed mill
Your suggestions noted, i will work on those.
Thank you for your valuable suggestions.
regards
P.Rao.
+968 93883351
On Friday, 14 February 2014 7:57 AM, pgoswami <pgoswami@quickclic.net> wrote:
Hi Mr.. Rao,
These black spots are essentially the products of contamination in the weld metal.There may not be a single reason towards this .It may happen as follows:-
- Welding Fillers :- The drawing lubricants e.g graphite could contaminate the welds and cause these spots to be formed. Through acetone cleaning of all the fillers should be done before welding. Also fillers with oxidized tips should be used following removal of the oxidized tip(by cutting).
- Shielding gas: check the purity of the gas, it should be 99.95% or more. Premixed gases say Ar+N2 may contain traces of moisture , which may increase the oxygen potential and the resultant oxidation of the puddle. As you said earlier the "O2" content was well below 0.2%.. Assuming the fact that for shielding gas everything was under control it would be always advisable to check the right purity of the shielding and purging gas, prior to welding.
- The condition of the pipes:- Duplex pipes should always be pickled and cleaned at the mill. If not done at the mill, it's preferred to pickle the pipes and remove the excess scale or oxides in the fabrication shop. Usually such oxides could be very microscopic and very difficult to remove by grinding. Excess oxides may introduce contamination in the weld.Also cross contamination during grinding could harm the welds.
The dark spots are bad , with potential for pitting during procedure qualification testing or while put in service.
Let me know if it helps.
Thanks
Pradip Goswami, P.Eng,IWE
Welding & Metallurgical Specialist
Ontario, Canada
From: Vasantharao [mailto:vasantharao@yahoo.in]
Sent: Wednesday, February 12, 2014 11:57 PM
To: pgoswami
Subject: Fw: Black spots on DSS Weld Joints.
Dear sir,
Can you give you suggestions on DSS dark spots as group member are not responding.
Is there any problem with dark spots. Why it forms.
regards
P.Rao
On Wednesday, 12 February 2014 1:38 PM, Vasantharao <vasantharao@yahoo.in> wrote:
Dear Experts,
We observe Dark spots on Welding of Duplex Stainless Welds.
Material is : UNS S31803 , Filler Wire for root - ER2594, rest ER2209.
Sample pictures are attached for reference.
Oxygen level in shielding / Purging maintained well below 0.2%.
Interpass <150 Deg.C
Pl. share your valuable information.
Thank you.
P.Rao
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