gtaw down hill welding

By Lawrence (*****)

Date 02-08-2011 18:00

I think for the most part the prohibition would be tied to the "essencial variables" in the applicable code.

Most codes that employ GTAW require a PQR (procedure qualification record) If the procedure was qualified with uphill travel than production welds are often limited to that direction of travel.

If the procedure was qualified with downhill travel. (You have proven by testing that it can be done) Than your production WPS's may be written with that direction of travel.

How the above is applied is *strictly* controled by the specific code.

In general terms.. Downhill GTAW may result in less penetration than uphill travel. This can lead to lack of fusion issues. On the other hand if minimum penetration on thin sections is what you need, it could be an advantage.

By js55

Date 02-08-2011 22:21

Downhill GTAW has some real advantages. Though it isn't taken advantage of very often.
Keep in mind, beyond 15deg from TDC in a pipe rolled 1G is downhill.
You can weld much faster and much hotter, both.
You can increase current as well to ensure good penetration when closing welds.
Its great for autogenous excessive penetration repairs.
Lawrence is right, there are some things you need to be cognizant of.
Gettin down around 9:00 on a pipe weld is tough and fusion can be problematic. You can't just roll over the top of the wire like you do uphill.
But the primary issue is making sure your welders are qualified to do it. Direction of travel is a performance essential variable.

By jarcher (**)

Date 02-09-2011 04:05

I've got one mate that's been welding TIG root passes for years and years and still can't seem to make the right hand bottom quarter of a 6G successfully in upward progression. Go figure. Still, AFAIK, he's never been culled out for running that one quarter downhill.

By Metarinka (****)

Date 02-15-2011 04:14

In the sheet metal world it's the preferred method for metal thinner than 16 gauge. So much so that I would be prone to say vertical welding of sheet metal under 18 gauge is very impractical if not impossible.

As mentioned it's a good method for controlling penetration and achieving higher travel speeds.

By Jim Hughes (***)

Date 02-09-2011 15:41

diverdileo,
it's been a few years ago but during my Nuke days we were using Diametrics auto welding process for the containment area and we qualified the WPS up and down from the root to the cap with the GTAW process and beat X-Ray. All the welding was in the 5G position so we welded half down hill and half uphill. 100% radiography. It was 1.250 wall and we would shoot info's every 1/2"

Jim

By Lawrence (*****)

Date 02-09-2011 18:20

Jim,

Great response!

Question related: Did you slope current or travel speed any differently on the up progression as opposed to the down?

I've seen the orbitals do those 360 degree welds and often wondered... I have no experience with that aspect.

By Jim Hughes (***)

Date 02-10-2011 13:39

Lawrence,
If my memory serves me correctly, we had to take some wire out (WFS) during the down hill side and some back-ground amperage. This was during the 80's and we were using a Gold Track III or IV with a F head. You had a pendant box that you would use to make adustments. The main problem we had, is we had to constantly be looking at the silica floating on the puddle to make sure we would not wash over the top of it. (ER70S-3) So we put a piece of tungsten in a wooden file handle and when the silica got to be about 3/16" of an inch we would reach up and touch the tungsten to it and pick it out of the puddle. It seemed to be worse on the down hill side. The technology has improved so much since then, I'm sure there are many more bells and whistles available now.

Jim

By swsweld (****)

Date 02-13-2011 23:42

Lawrence,
Like Jim said, we usually decreased wire speed and current on the downhill side when doing 360 welds. Lots of variables though, when we were welding XXwall pipes with high currents (carrying lots of metal) the downhill side, due to gravity, was too risky so I decrease wire speed and current to achieve a safer puddle. On the downhill side it is more critical to manipulate the wire entry to find the sweet spot in the puddle. It's less forgiving on the DH side compared to the UH side. Welding uphill you have the classic tear drop to feed the wire but DH doesn't provide the same "target". It was also normal to have to strip or run an extra layer or two on the DH side to flush it out evenly with the UH side before capping.

I can't remember the magic pipe diameter size (36" I think. It takes coordination to run two weld heads simultaneously on one joint) where we began using two weld heads both vertical uphill instead of one weld head doing 360's. The slower option was one weld head welding vert. UH with two wire spools and wire feed motors to where you had to rewind the head from 12 to 6 o'clock, switch the direction of travel, switch the wire feed motor and put about 3-5 degrees of lead angle in the tungsten before firing up. Of course you can use that same set-up to weld 360, down slope, switch everything to weld the opposite direction then fire back up if WPS and job specs allow 360 UH/DH welding. That is the most productive method if pipe size is too small for two machines. Arc time is boss, not rewind time.

Every job had its own requirements that dictated the weld progression, weld head set up, etc. Usually it was production but not always. PQR's, WPS's and WQTR's were always in order as most or our work was nuclear and high pressure steam pipe on fossil fuel plants. DH Machine GTAW was very safe but with inexperienced operators it can be dangerous...think avalanche :)