[HenryAustralia]

Hi, currently practising for (Australian) pressure welding test AS 1796 certificate 7, or Tig on 90mm OD 8mm wall mild steel pipe.
The problem I'm having is porosity appearing in the capping run in the last 1/4 of the cap.

Bit of background info.
Material or coupon length 100mm
Bevel 30 degrees (included 60)
Root face; knife edge
Root gap; 1.6mm
Filler; 2.4mm S2 (could be wrong)
Tungsten; 2.4 thoriated ( point with 1/4 diameter land)
Gas lens with no. 8
Gas; Argon @ 8 litres/min
Root run 100 amps
Hot fill 100 amps
Cap... 100 amps!

Ok that out of the way, root and hot pass is all fine, doing most of the fill with the hot pass, leaving 1-1.5mm to the top of the v prep.
Allowing the weld to cool down for 5 mins between runs
Doing the cap with a weave and dip, tungsten to the edge of the prep, dip, weave to other edge, dip, is giving a good finish.

The issue happens, nearly every time, when on the last 1/4 of the cap, I change position from coming up the right hands side to come up the left, light up where I left off and instantly get porosity, first reaction is I've run out of Argon, not that, the next thought is gas coverage, nothing has changed so shouldn't be, the only thing I can figure is the pipe has gotten too hot and boiled the weld metal out?
Adding filler doesn't help much, once the gas holes are there it's almost to late.
I'll add the pipe is clean, wire brush every run, haven't been 'flapping' in and out (think I'll try) clean the inside 3-4mm with a file, for the root, then Once the hot pass heats it up I give the outside a good brush with staino wire brush, brings it up pretty glassy
Any thoughts and ideas will be appreciated, thanks In advance, Henry

[weldin mike 27]

Hey,

Welcome along.

Tig posoity, or tigrosity as i call it, is one of the most frustrating things going around. Seems to appear out of nowwhere.

I could offer suggestions for you if you had consistant porosity, but when it happens only in one specific area at a certain time.....ive got nothing.

One way out there thought....when you do this portion of the pipe, are you always in the same spot in the bay? Wouldnt happen to slightly closer to the fume extraction? It only takes the slightest breeze or draft to mess things up.

Failing that, i would stop the run at a place you know is working well, and swap to the biggest shroud you can, with the shortest possible electrode stick out.

Good luck. Mick

[Rick_H]

Make sure you are snipping off the used portion of the filler rod in between passes as well or you'll push dirt into the weld. Also make sure as you are coming around you keep the filler rod shielded by the gas coverage as well.....I went through this while practicing for my ASME IX boiler and pressure vessel always showed up in the last inch or so.....

I ran a #7 cup, 3/32 tungsten gas at 20cfh. 85 root, 100 hhot and 120 root, 3/32 gap, .030" land

[coldman]

Gday Henry,
looks like you are just about there.

Things I would do a little different (for me, maybe won't suit you or anyone else).
First I would let the coupon cool off more until I could keep my hand on it.
I would bump up the argon to 10 L/min.
The cap run I would increase amps to minimum 115amps. My sweet spot is 125 amps for 3" pipe (88.9mm OD) and have also used 140 amps OK and get motoring with the torch. This will ensure that there is no roll under that gases out as you arc over it.

It could also be mearly torch angle and arc length at this last change of position as it seems to be repetitious. Watch for this and correct if necessary.
Cheers mate.

[HenryAustralia]

Thanks for the replys and ideas, found the problem today! I'm welding at a tafe college and the argon runs on a manifold system, flow meter in each bay, there is a regulator on the cylinder outside, and that reg had its flow screw only just open, enough that it would pressurize the lines, give me enough argon to do most of the weld, then it would run low, still giving enough so it didn't appear to lose gas all together! When I tested it at the torch it would always show plenty of flow because it had time to build up pressure, problem solved!

[weldin mike 27]

Great, that issue is a frustrating animal,

Mick

[Otto Nobedder]

I'll have to file that one away for future reference. I dial back the regulator in front of a flowmeter all the time, but it's specifically to reduce line pressure and "surge" when using helium for leak detection (I usually need a very controlled amount). I would not have connected the dots on this one to guess you were running out of line pressure at the same point in the weld every time, but it makes perfect sense now that you've identified it.

"Good on ya!" for sorting it out.

Steve S

[RichardH]

Thanks for the replys and ideas, found the problem today! I'm welding at a tafe college and the argon runs on a manifold system, flow meter in each bay, there is a regulator on the cylinder outside, and that reg had its flow screw only just open, enough that it would pressurize the lines, give me enough argon to do most of the weld, then it would run low, still giving enough so it didn't appear to lose gas all together! When I tested it at the torch it would always show plenty of flow because it had time to build up pressure, problem solved!

Boy, that'd be a bugger when a class of folks stepped into the shop to weld. Suddenly your welds all go to crap. Really, with a setup like this, there's no reason the central flow meter shouldnt stay wide open (or be removed). I have a similar setup.