Oxygen level in purging gas during welding of duplex stainless steel

[Martin Prad]

Hi friends,
 
I have 2 different specification which the requirements are different for the oxygen level in purging gas during the welding of super duplex pipe.
 
One of them calls for 25 ppm (0.0025%) another one calls for 5000 ppm (0.5%). Could you please shed some light on this as which one is up to the standard with references?
 
Thanks,
Martin.

[pgoswami]

Martin,

 

Nice to hear from you after quite a while..I have come across this question in the past while qualifying procedures for duplex S.S, on the strict requirements of oxygen, in purge gas. Please see below the extracts/recommendations of consumable manufacturers, and the general design practices  from API and as well as the technical specification of an oil major. The API recommended practice and duplex/super duplex S.S product manufacturers recommendations is between 2500-5000 ppm of oxygen in the purge gas. Note the general guidelines do not always address the most stringent requirements.

 

For the clients in the oil and gas production industries the  requirement is  typically 50 ppm or below for the (typically offshore applications)..My feel is that, it's difficult to pass the pitting corrosion test (G-48) without such strict control on the level of oxygen. However literatures suggests that a proper purging gas  can be effective at higher level of oxygen in the root pass. Depending on the residual level of oxygen one can get various levels of discolorations in the weld root (see the attached document, welding.pdf). However for offshore applications I would feel the requirements are very cut throat due to the reasons mentioned above.

 

I've attached an extract from Avesta Welding Manual , it also calls for 50 ppm max of O2.

 

If both the specification applies for the same project, then you need to discuss with the client about discrepancies. Could you please specify:-

• From O2 level of 5000 ppm, what're the additional testing requirements over and above ASME Sec-IX
• For 25ppm allowable oxygen h=what're the  weld testing requirements in the coupon?

 

If you could post some updates later it would be appreciated.

 

Thanks.

 

Pradip Goswami,P.Eng.IWE

Welding & Metallurgical Specialist & Consultant

Ontario,Canada.

Email-pgoswami@sympatico.ca,

pgos...@quickclic.net

 

Examples:-

 

1. API 938C-C.2.4 All single-sided welds shall be made with a GTAW root pass and back purged until at least 1/4 in. (6 mm) of weld metal thickness has been deposited. Backing gas shall be welding grade 90 % nitrogen/10 % hydrogen mixture, pure nitrogen or argon/nitrogen mixture with not less than 5 % nitrogen. The oxygen content of the back purged volume shall be less than 0.25 % (2500 ppm). The seller shall submit details of the method and equipment to be used for monitoring oxygen content as part of the WPS. Shielding gasses can be argon or argon with 2 % maximum nitrogen.

 

2.WEIR MATERIALS, Guidelines for Welding, ZERON 100

Back Purging:- When welding Zeron 100, it is recommended that commercial purity argon is used to displace the air behind the joint. The oxygen content of the resulting argon/air mixture must be monitored and controlled to ensure that sufficient nitrogen is retained in the backing gas “mixture’ in order to inhibit loss of nitrogen from the weld pool. In practice the oxygen level of this mixture should be controlled at approx 0.5% (5000ppm) oxygen monitored at the start of the welding sequence. In this way, a positive partial pressure of nitrogen is maintained behind the joint thus preventing nitrogen loss from the root bead, whilst the oxidation produced remains acceptable. Passes subsequent to the root pass can be made with minimum oxygen contents.

The backing gas composition should be monitored at the joint line using a portable oxygen monitor immediately prior to starting or re-starting welding in order that consistency can be maintained. Adhesive tape low in sulphur and chloride is used around the open joint seam, and the tape should be removed progressively during the welding sequence.

 

3. Metrode:-Duplex & superduplex ferritic-austenitic stainless steels

Purging

A gas purge must be used for root runs deposited using the TIG process and will normally be maintained for the first three layers or approximately 10mm of deposit. Commercially pure argon is generally used as the purge gas. Purge flow rates are determined by the pipe size but it is important that following the removal of tacks, grinding etc that the purge is allowed to stabilize again before welding. The efficiency of the purge should be monitored with an oxygen monitor to ensure the oxygen content is maintained below 0.5% (5000 pap) oxygen.

 

4.0 Worldwide:- Oil and Gas Major

During welding, DSS shall be protected from oxidation by providing a shielding purge on the inside with the same composition as the shielding gas for welding. The purge gas shall replace all air. No welding shall start until the oxygen content has dropped to below 50 ml/m3(i.e 50 ppm). During welding the root run, there may be a surge when welding is started and welding shall be stopped if this surge goes higher than 500 ml/m3. This shall be measured with an oxygen analyzer, capable of measuring oxygen contents between 0 and 1000 ml/m3.

 

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From: material...@googlegroups.com [mailto:material...@googlegroups.com] On Behalf Of Martin Prad
Sent: Wednesday, December 28, 2011 2:56 AM
To: material...@googlegroups.com
Subject: [MW:13349] Oxygen level in purging gas during welding of duplex stainless steel

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[Madhusudan Rout]

Dear Mr Martin,

As we done here SDSS fabrication we are
following client Spec, Only up to completion of root pass, Oxygen
level shall be below -100 ML/Ml3, During welding if exceed 500 Ml/Ml3
then welding has to stop,this process follow up to three layer.

Depending upon you thickness of deposit. After three layer heat input
cannot oxidize the root, It optional u can maintain up to 10 mm thick
welding ,if pipe thickness is more than 10mm u can stop backing gas .

Regards

Madhu

[Martin Prad]

Dear Goswami,
 
Thanks for your detail email as always. A pitting corrosion test to G48 shall be carried out. I am afraid a test reslut can not pass in the root part. Just looking a best practical solution what % of gas mixes is best to use as purging. There is an Oxy Monitor from Weld-Tech (Finland Company) to monitor the O2 level in purge gas. I will try to summirize your below reccommendation to use on this occasion. 
 
Very kind regards,
Martin.
 

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From: pgos...@quickclic.net
To: material...@googlegroups.com
Subject: [MW:13352] RE: 13349] Oxygen level in purging gas during welding of duplex stainless steel
Date: Wed, 28 Dec 2011 17:44:09 -0500

[Sundaram Elumalai]

Martin,

 

The only way if control the O2 level. Less than 0.05%

 

The back purge for stainless steel and nickel alloys shall reduce the oxygen level below 0.05%. An oxygen analyzer should be used to determine the oxygen content inside the pipe during purging.

 

Sure G48 will pass.

 

With Kind Regards,

 

S.Elumalai.

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[sudhakar kuppuswamy]

Dear Mr Martin,

 I am glad to write about DSS on this group,It is always good to understand the metallurgy of DSS before  welding.

           As we all know DSS are two phase alloys i.e 50% ferrite (BCC structure)& 50 % Austenite (FCC structure). The weld joint (weld metal +HAZ) shall also contain the same proportions after welding, but achieving these proportions is a nightmare..!!!   

       

         Usage of  DSS materials is particularly meant where Chloride stress corrosion cracking (CI SCC)& pitting corrosion cracking resistance are required.

         If 50:50 phase balance is not achieved during welding the loss of properties can be acute.

         If ferrite content goes high on the weld joint, it may loss the toughness properties  and if austenite content goes high weld joint will loss CI SCC resistance.

         During welding ,the DSS weld metal solidify  essentially as 100 % ferrite ,at high temperatures around 1300°C the Austenite nucleates and grows with the help of Austenite –stabilizing elements(Carbon,Nickel,Nitrogen,&Copper).The ferrite –stabilizing elements are Cr, Mo, W. (It is advised to have a clear idea about De-Long Diagram&WRC diagram)

       By selecting proper filler wires, welding parameters and shielding gases, the near 50:50 phase equilibrium can be achieved. 

                  i.      Filler wire.  It is always advised to weld with over alloyed filler wires (usually Ni & Cr content shall be high than base material).

                ii.       Welding Parameters.

                    Heat Input should not be too low or too high.

      If heat Input is too low rapid cooling occurs and ends up with more ferrite content.

      If heat input is too high chance for intermetallic compound formation are high.        

               iii.      Shielding gases.

        In order to get good pitting corrosion resistance Ar+2% N max mixer combination can be used for SDSS..

     Since nitrogen is good Austenite stabilizer it helps to form a thin layer of Austenite at root surface, which will the increase corrosion resistance..

     Oxygen content shall be closely monitored during root welding,(max 0.2% is highly recommended  ). 

Convey the following Tips to Welders..

     Advice welder to avoid too many  start &stops during welding,as it leads to rapid cooling..

                  Avoid autogenous welding. (I.e. without addition of filler )..usually welders use to do autogenous welding while tack weld fro fitup purpose& while arc termination on capping  pass welding.

If possible,kindly mail me the  design and  material details  like UNS number ,design temperature etc..if time  permits I will write more on this   topic..

--

 regards,

K.Sudhakar

IWE, Welding Engineer,

Larsen and Toubro Electromech,

Muscat, Sultanate of Oman.

[Ramin Kondori]

Dear Martin:

Attached file will be helpful...

Regards

Ramin  Kondori

   QC/Welding Engineer 

         IWE AT 0070

  

[Dino]

dear Frend,

I would like to know why welding duplex required two purging one is from mix gas and the another one is analyzer oxygen.

thanks. 

Dino 

[james gerald]

I suppose the stringent requirement applies before start of weld and the other one while the welding progresses. Normally Duplex & Superduplex are continuously monitored during welding and there are chances thet weld fumes may mix with purging gas.

Check with the Client who had written the specification

Thanks & Regards

J.Gerald Jayakumar

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