Hardwire Root Pass

[david339933]

I am looking for a process to substitute GTAW in welding pipe (root). What is the fastest hardwire process that will give adequate results? I was thinking STT (surface tension transfer) would work, but was wondering if there was a better process. I am looking for more speed, without sacrificing quality.

Thanks in advance.

 

[Duwe6]

Not familiar with STT. Good pipe shops have good results using Short-Circuit transfer GMAW-S, with the pipe being rolled, and the weld slightly downhill from the top-dead-center. The problem with GMAW-S is, it is VERY easy to turn the volts down enough to turn a weld into a Braze. This is why ASME has such stringent requirements on GMAW-S.

For 'Position' welds, Pulsed GMAW is fast and 'trustworthy'. No risk of your weld turning into a braze. And it can be VERY fast, assuming a good, uniform fit-up with a uniform land on the root face.

 

[unclesyd]

Here is a relatively new process that is producing very sound welds at a pretty good clip. Just recently I've seen heavy wall pipe welds that were done in comparable time to flux core but had none of the potential problems seen with flux core. I know three people that are using Tip-Tig and are quite happy. Most of the time you cause your existing power supply. If you have been in welding very long you will notice this is an automated version of the old hot wire Tig.
There is a lot of hype by Ed Craig, but he knows welding, that you will have to read through. Ed likes his soap box, the good thing is he can backup what he says.

http://www.weldreality.com/TIP-TIG-welding.htm

 

[weldtek]

I saw the STT in operation at the show in Chicago several years ago. We aren't big in pipe fabrication and I didn't see it being a good fit for us at the time, but, it may be just what you're looking for. Your local Lincoln rep would probably be happy to demo it for you. Miller has some pretty nice equipment also.

 

[Kiwi2671]

David,
We were doing heavy wall carbon and chromolly pipe welds with Lincoln STT machines in rotators 15 odd years ago and they were brilliant.
Unlike GTAW set up where a good clean, uniform root setup was critical the STT was very forgiving.
If you had a bad fit-up - 1 mm root in places up to 4 mm in others (definitely 4 mm maximum) we would just knife it with a 3.2 mm cutting disc and then weld. No need to clean the swarf out of the inside of the pipe - it was picked up by the molten root.
GTAW quality root runs at about 1/4 the time.
Regards,
Kiwi

 

[vanci]

I would look into either Lincoln's STT or Miller's RMD process. I have heard good things about both and have not used either.I would definatly get a blue and a red machine to demo. Are you using GTAW all the way out (root to cap)? If so a definate time saver would be to GTAW the root and hot pass and GMAW fill and cap passes. Also, machine GTAW will increase production over manual GTAW.

 

[racookpe1978]

There is another problem with with wire-fed welding processes on the root pass on pipe-pipe welds in particular, and piping systems in general that has not been brought up yet.

GTAW manually feeds a stiff wire into the melted puddle, so there is (seldom) any wire "stub" left sticking out of the weld. Those that do occur (wire stub or electrode sticking) are very visible, are ground off, and the welder restarts - usually with a freshly ground electrode.

When the root gap on a pipe-pipe weld prep or pipe-fitting weld prep is larger than the wire dia - and this will almost almost always be the case - then the initial "feed" of the wire can get aimed into that root gap. The wire goes into the gap, the welder slightly moves has hand or position just a little bit, and the arc starts up. But the first 1/2, 1, or 1-1/2 inches of wire are already into the inside of the pipe before the arc melts off the wire and the puddle starts up back in the prep area between the two pipes. The stub length of wire is left inside as the start of a plug, or flow trap, or OFD material inside the pipe invisible to the inspectors and cleaners later on.

If EVERY weld and EVERY arc start point is begun by "aiming" the wire directly into the angled wall of the pipe prep area, then this wire stub is immediately melted when the air starts and there is no remnant left inside the pipe. But that degree of care in starting of every weld arc is very hard to verify.

If you begin this weld process, boroscope every pipe after fabrication of the next piece and before flushing and QA to look the stubs and wire traps inside every pipe. Removal of the stubs inside, and of the broken off stubs and grinding grit after they get ground off inside can be very difficult.

If you do the root pass with manual GTAW, then fill out the welds with wire feed, no wire stubs can be created inside the pipe, but then you need a slower root pass.

 

[vanci]

racookpe1978, I am pro GTAW root pass. However, if a GMAW process were to be used for the root pass, what if the welder started on a tack? That should reduce the likely hood of wire traps occuring.

 

[racookpe1978]

True. There are several ways of ensuring that the first arc always begins melting the wire - so the wire cannot feed through that gap between the pipe and fitting - but they all begin by simply recognizing the eventual certainty that the wire will get through and go into the pipe as a stub.

Once recognized, the problem is half-solved.