[MW:13352] RE: 13349] Oxygen level in purging gas during welding of duplex stainless steel

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,

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

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.

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