GMAW/STT Weld Process

Date 05-04-2011 15:09

First of all, IMO, and it is just my opnion, though I've been involved in running a lot of STT(though admitedly its been a while), There is nothing in STT technology to inherenly improve quality or productivity other than in specialized applications.
The best argument for quality with STT is the reduced spatter argument and I will agree it is easier to achieve low spatter wth STT than it is with standard CV for inexperienced welders. But I've welded miles and miles of C-Steel and low alloy roots with conventional CV (excellent Lincoln equipment as well as others)and no internal spatter.
Also, IMO STT is waste of time on C-Steel and low alloys.
Most specfications prohibit its use beyond the root passed so its almost a who cares about sidewall fusion.
The fact that STT needs what is called a 'plasma boost' should tell you there is an issue with arc energy.
When you cut the current to reduce spatter there is a cost. Even if it is an instantaneous cut of current.
The existence of the plasma boost also should give a clue as to its ability to achieve greater penetration.
The biggest advantage I have found for STT is position welding open roots of big bore high alloys. It really kicks azz here. If your customer allows a GMAW process for high alloy roots.
There may very well be some other applications where STT really shines, but IMO it isn't production pipe assemblies.
There are some really smart people at Lincoln, and other manufactures, that are doing some really great things pertaining to arc instabilities and perhaps STT is related to that. But you will need to have a very serious interest to keep up with it.

By TimGary (****)

Date 05-04-2011 18:20

I recently completed practical testing using RMD (Miller's version of STT) in sheet metal fillet and butt welds.
The purpose was to compare this transfer with Short-Arc and Accu-Pulse.
Test results showed that while RMD was good at gap filling and bead prolfile, similar results could be achieved with both Short-Arc and Accu-Pulse when the machione settings were fine tuned appropriately.
I did notice that the root penetration on the open butts was easy to control, so I can see the benefit of RMD as a quicker-cheaper replacement for GTAW root pipe welding. However, I've not done bends and tensiles to verify.

Tim

By Shane Feder (****)

Date 05-05-2011 02:12

Hi guys,
A couple of posts from a while ago on my opinion of Lincoln STT welding

Hi all,
First used Lincoln STT in New Zealand in the late '90s for light, medium and heavy wall carbon and chromolly piping.
Absolutely brilliant machine when set up properly.
Root with STT and Fill & Cap with SAW using pipe rotators.
The great thing about the STT (apart from the speed) is that a uniform fit-up is not required. If it is GTAW you need a consistent root gap all the way around the pipe but STT is different.
The gap could be 0 - 4 mm (we considered 4 mm to be maximum gap) and we would have 3 large tacks (bridge or root tacks, it didn't matter).
Weld between 2 & 3 o'clock from one tack and stop just short of the next (unless it is a feathered root tack)
Take a 7" or 9" grinder with a cutting disc (we called them pipeliners, about 3.2 mm thick) and cut through the prep in places where the root gap is less than 3.2 mm.
The beauty of the STT is there is no need to remove the "swarf" that has been pushed into the pipe by the cutting disc. The weld burns away the swarf and you end up with a GTAW like root run at about 5 x the speed.
Have also used for vertical down on 6 mm square butt groove welds for thin walled tanks.
Regards,
Shane

Greetings to all from "Down under",
Used to work for a piping company in NZ that were very innovative and were forever looking to get an edge over the competition.
In the workshop we were using variable speed rotators that the company designed and built. We were originally "stoving" the root/hot pass runs using cellulose electrodes and then filling and capping with FCAW.
We then purchased 4 Lincoln STT welders (can't remember the models) and we were very impressed with the speed and ease of use on pipework.We put only three tacks in the root and "knifed" the tacks as we welded the root run.The brilliant thing about the process was that the "fit-up" didn't have to be spot on to enable the welder to produce top class welds.The gap could vary between 0 and 4 mm and all you had to do was "knife" the root with a 3 mm cutting disc and the resulting root run looked exactly the same no matter that the root "landing" varied greatly in places. A 12" root run was being welded in approx 1.5 minutes which is a huge improvement on the GTAW process which is usually used for refinery piping.We were then filling and capping with SAW which meant the spools were going out the shop door at a great pace.
The company had the pipe spooling so "cost-effective" that they were exporting spools to Australia and beating local companies prices.
As a CWI and former pipe-welder I can't speak highly enough of the STT process,
Regards,
Shane

By js55

Date 05-05-2011 19:19

Shane,
Why would you compare STT to GTAW and cellulosics and not conventional CV GMAW?
Virtually everything you've stated I've seen done quite easily for decades with conventional GMAW, and actually GTAW in the downhill progression.
In fact, if you want to compare GTAW with STT how about the fact that if you run downhill GTAW you don't even necessarily have to grind your tacks AT ALL, whether they are penetrated or not. You could never get away with this with any GMAW.

By fschweighardt (***)

Date 05-05-2011 19:25

I am with Jeff, put that root in with conventional Short-arc GMAW, I would probably feather the tacks right after I put them in (tack with GMAW). After the root, maybe a hot pass, maybe move straight to the SAW, depends on how hot you run the SAW.