[MW:20093] RE: Black spots on DSS Weld Joints.

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

pgoswami@quickclic.net

pgoswami@sympatico.ca

 

 

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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