Pipe welding SMAW or GMAW or GTAW

[StoneCold]

I am curious about what others are doing in this area.
Piping code applied is B31.3.

First lets look at the carbon steel application. Say 4" sch 40 A106 pipe. Butt weld joints. Because I like a clean inside on my pipe I will only allow a GTAW root pass and then I will allow the welder to either stick the rest of the pipe out to the cap or fill it up with GMAW.

Do others allow GMAW on the root? (still going with clean pipe) I worry about porosity and I won't let them use it. Am I wrong on that?

Second application is 316ss 4" sch 40. I will only allow GTAW all the way out? Would you allow GMAW on a 316ss pipe? Root and filler? Filler only?


Thanks
StoneCold

 

[racookpe1978]

Thank you for asking.
I'm listening (eagerly!) for more responses. 8<)

 

[metengr]

For carbon and low alloy steel in applications where pressure is above 100 psig and temperature is above 150 deg F, GTAW root and SMAW fill. For BOP low temperature and lower pressure service, GMAW is permitted, GTAW root/SMAW fill or an open root butt using SMAW is permitted.

For austenitic stainless steel same conditions would apply as above with an exception - corrosion service only and oil clean application. In corrosion service and oil clean applications - GTAW is only permitted.

 

[Ballbearing1]

metengr,
I agree with your recommendations but who decides whether it is permitted or not ?
I am unaware of anything in B31.3 that governs what processes can be welded where.
There are numerous postings on the AWS website regarding the use of cellulose electrodes on steam piping and it seems to be acceptable practice in America.I always thought it was a big no-no.
If you are following a qualified WPS is there any governing regulation as to where and when it can be used ?
Regards,
BB

 

[metengr]

I agree with your recommendations but who decides whether it is permitted or not ?

I do, as an Engineer for the company.

 

[stanweld]

StoneCold,
Are you shop or field welding? If field, I would generally agree with metengr. Making the root with E6010/E6011 electrodes makes sense as well depending on service of pipe, availability of skilled welders, etc. From my experience with today's welding skills and the B31.3 radiographic testing requirements, metengr's advice is recommended for the pipe size specified.

If shop welding, GMAW root passes and fill passes made in the 1G rotated position are more economical than the manual GTAW/SMAW approach. For out of position welds, the GTAW/SMAW or GTAW/GMAW is recommended.

 

[CMConsulting]

Good Question,

First off, keep in mind welding is a learned skill, so depending on the process, your "resources" may be limited. Also, the ASME codes are the minimum guidelines provided, and should never take the place of sound engineering judgement. So as you mentioned based on ASME B31.3 and the application there are NDE requirements, and/or the need for PWHT. For A106 GR. B S/40 I found that GMAW or GTAW will defintely provide the cleanest root pass. I would suggest either a GTAW root, with SMAW 7018 fill and cap or 6010 root, 7018 fill and cap. Using a cellulosic electrode for the root provides many advantages and some of those desired are weld metal and HAZ hardness. Since we are only talking S/40 there won't be a need for PWHT, but the need for the low hydrogen electrodes to be kept properly is VERY important, especially for steam piping, which in this case, your code of construction will be B31.1. Stainless is a whole other ball game. Again GTAW root may be the best bet, depending on resources and economics. GMAW is a little trickier for a welder, but like I said it is a learned skill. What I have seen quite a bit is GTAW root and SMAW fill and cap.

Most of the time, porosity shouldn't be a concern unless the weather conditions/poor electrode storage are in effect. One thing I would be concerned about with stainless is lack of fusion and/or incomplete penetration. There really isn't anything in B31.3 that would provide restrictions on what you can and cannot do when it comes to welding. I would consult with a qualified welding engineer or authorized inspector as to what would be your best options, considering fluid service, temperature, and pressure.

Hope that helps.

 

[rodofgod]

Stonecold,

From my experience with european steam generation, still using B31.3, It's nearly 100% GTAW for the root deposit now-a-days! As for the filler, well that depends on the thickness of the pipe! GMAW is just as good as GTAW on thicker section pipe and quite a bit quicker! This would apply to S/S welds as well.
Your fear of porosity is unfounded as long as the welder's have enough skill level and the consumables are correctly monitored!

Regards