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Up Topic Welding Industry / Technical Discussions / Electrode for root pass welding without using backing gas
- - By QUOCVUONG (*) Date 01-14-2008 03:59 Edited 01-16-2008 16:04
       I am going to run PQR with material A312 TP316L OD.6"x18.26mm.As I know, people usually use combination of welding process TIG for root-hot and SMAW for fill-cap but you know that  troubles  the production by backing gas requirement. So I want to use a single welding process SMAW only from root to cap.
As your experience, Could you please suggest me what kind of electrode classification is suitable for root pass welding without using backing gas. Have you ever succeeded in it?
Thanks a lot,
Parent - - By toddler (**) Date 01-14-2008 09:19
Try using TGX-XXX by kobelco. It's a flux-cored filler wire meant for that purpose. Done by GTAW but works like an electrode (no purging required).
Parent - - By js55 (*****) Date 01-14-2008 15:13
You need to be cognizant of your service environment. If you use a flux core wire keep in mind this will do nothing for your HAZ.
If its a critical application purge it.
What does critical mean?
Consult engineering.
Parent - - By 803056 (*****) Date 01-14-2008 17:20
I agree with js55. It is important to understand the service environment to select the welding process best suited for your conditions. If the service environment will permit a flux shielded welding process that will leave slag on the pipe ID, either SMAW or FCAW are possible choices. However, if the operating system includes pumps or valves or other equipment that can be damaged by detached slag, then either GTAW or GMAW are better choices.

The optimum root surface condition will be best achieved with GTAW using a purge on the pipe ID. Consider using a U-groove with a zero root opening, 1/32 inch root face, 1/4 bevel radius, and 30 degree groove angle. As for the purge, dams consisting of rubber bladders or water soluable paper will minimize the volume that has to be purged and the time required to attain a proper purge. The purge time should be sufficient to displace the volume (of the pipe being purged) 5 to 6 times before the weld is initiated. Once the initial purge is completed, the flow of the purge gas can be reduced considerably to maintain an inert environment on the ID.

Best regards - Al 
Parent - - By QUOCVUONG (*) Date 01-15-2008 08:46 Edited 01-16-2008 15:55
Thank for your advice
I also know the TIG process with 100% Ar gas for backing is bester. But I want to try looking another way. As you post the service environment is very important I fully agree with you. Because that is piping process including valves, pumps...
How about with GMAW process without using backing gas? Is it good quality?             
Parent - - By 803056 (*****) Date 01-15-2008 14:23
I would not advocate using semiautomatic GMAW or manual GTAW on stainless without a root purge. Without the purge, oxidation of the root surface will result and it will adversely affect corrosion resistance. The mechanical properties will most likely meet the bend and tensile requirements once the face and root reinforcement are removed. Still, the root surface will be rough and oxidized, not the best conditions for a piping system where corrosion resistance is required. Why would you specify stainless steel if not for corrosion resistance if the operating conditions are at or near ambient.

Best regards - Al
Parent - - By Kix (****) Date 01-15-2008 15:01
Some of those new ethanol plants that are going up are using the GTAW process for the root on stainless pipe with a rod covered in some sort of flux (NO PURGE).  My bro's are telling me it runs like crap and the roots are really inconsistent though.  He told me that they are not x-raying the welds because most of them wouldn't shoot.  They have to leave a window at the top of the pipe and let the inspector look the root in before they can finish.  I also saw in an AWS Journal awhile back about some sort of stainless rod that does not require a purge and it has no flux on it.  It was really high in silicone content or something.  The roots weren't all colorful and such, but they had no sugar.
Parent - - By Superflux (****) Date 01-15-2008 22:56
We recently used a product by the trade name "Solar Flux". It's been around for decades and is a silicon based and according to the MSDS is inert and poses no health hazards other than dust inhalation. It is mixed with alcohol and the paste applied to the inside of the pipe. It provides adequate protection from oxidation and root penetration was OK but a little bit rough (ie no wedding band root beads), looks like sugaring (can be mechanically removed), but will work in "non critical applications". We eventually abandoned its use just as a precaution so the fitters wouldn't be tempted to use it on the medical oxygen system.
Parent - - By Kix (****) Date 01-16-2008 15:01
Yeah i remember that stuff.  As soon as you welded one quarter of the pipe and took a grinder to feather starts and stops the rest of the flux would flake off.  It worked when it stayed on though.
Parent - By Superflux (****) Date 01-17-2008 01:06
Yeah, we ran into that flaking problem too. Found that a thin mixture was the way to go. Always want to fall into the old "if a little is good, a lot's gotta be gooder" until it messes something up!
Parent - - By weldgault (**) Date 01-17-2008 17:17
You can try 316L-SI and it might work.  The SI will help the corosion on the back side.  I have known some that have had success, doing this, but not recommended.   John
Parent - - By nse_joshua Date 05-16-2008 05:23
As what we've practised for SMAW root pass, we used 6011...
Parent - By Fredspoppy (**) Date 05-17-2008 11:09
Unless I missed something in the thread above, 6011 wouldn't be a good choice for stainless steel.
Parent - - By Kix (****) Date 05-16-2008 13:28
I remember reading an article in the AWS journal about a stainless rod that had no flux on it and required no purge to put a root in.  I think it was some kind of hi silicone wire.
Parent - By labib (*) Date 05-21-2008 09:42
It is an innovative pipe welding solution as a flux-cored TIG rod for the most efficient, reliable and economical single-sided welding. MAGNA 39 rod helps welders in eliminating-the need for back shielding or purging using inert gas, excessive downtime, and all related set-up costs in maintenance pipe welding. The backside of the root pass can be shielded by MAGNA 39 against the destructive impact of atmospheric nitrogen & oxygen.
Up Topic Welding Industry / Technical Discussions / Electrode for root pass welding without using backing gas

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