Welding stainless with AC

[John Stevenson]

I have a Murex Tradestig 180 AC/DC TiG welder which I use for mainly repair work, building shafts up broken castings etc but nothing critical.

Recently it has developed a fault on the DC side and has no power.
The AC side seems OK and it could be the rectifier bank.

Needing to build a run of stainless up on a worn shaft seal the other day I tried it on AC and to be honest couldn't see a difference between that and the correct DC

So the answer is can I weld stainless on AC for non critical parts ?

I may repair this welder but I'm holding fire as I may be able to get a 350 amp AC / DC set quite reasonable.

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[radkins]

If you are using the common 308 or 309 rod this should be no problem at all and you may even see less of a tendency for the rod to overheat as stainless rods are prone to do.

[macona]

DC is completely dead? Thats kind of odd. Usually one diode opens and you get partial output or one shorts and the machine just hums..

Check to see if there is a thermal circuit breaker in line someplace to protect the diodes.

[John Stevenson]

No it's not completely dead, the HF is there and when you strike you get a tiny arc which just fizzles out.

Problem is I never know in emergency repair job just when I need this and I only have the one TiG.
If I send it away it will take at least a week knowing these guys, probably two and I'm stuffed, at least it's welding in AC mode and I can work on the guy with the 350 amp set.
He's looking to upgrade but has been offered a pittance in part chop.

I'm just interested in whether it's OK to use AC.

I was down the local sheet metalworks getting some enclosures bent and I asked them but they didn't know although they had noticed the same thing and one job had been done on AC as the guy hadn't noticed that the welder had been swapped to do a small alloy job.

[radkins]

Pardon me I missed the part about this being a Tig machine. I thought you were talking about a stick welder so what I said in my first reply is totally meaningless, sorry about that!

[small.planes]

as I understand it the EN portion of the AC is just like you were welding straight DC, and 2/3 of the heat goes into the workpiece. The EP portion of the cycle will be as though you were welding reverse polarity, and 2/3 of the heat goes into the tungsten. However the gas ionization on this polarity cleans the metal of tenacious oxides, which is needed for aluminium, and until recently I thought was needed for stainless. You'll probably heat your tungsten more than ideal, and there might be a variation in penetration of the weld if you looked real close using special tools, but other than that I cant see a problem.
You might want to turn the balance (if thats available) to maximise the normal current (EN), and maybe use a tungsten with the right additive for AC (zirconiated?) or a bit larger in diameter.

Dave

[John Stevenson]

Thanks Dave,
No balance on this machine and God know what tungsten is in it anyway, I don't weld the microns :D

Most of the stainless work is building up worn seal grooves on pump shafts. When the stainless is turned off it looks decent, no porosity and pin holes so I'm taking it as OK.

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[piniongear]

Needing to build a run of stainless up on a worn shaft seal the other day I tried it on AC and to be honest couldn't see a difference between that and the correct DC

So the answer is can I weld stainless on AC for non critical parts ?

Certainly. The AC weld penetration will be somewhere between that of DC straight polarity and reverse polarity.
While recommended mainly for the lighter metals such as aluminum and magnesium, stainless will weld very well using AC.
Attached is a chart for you to review........pg

http://i12.photobucket.com/albums/a214/piniongear/Welding/AC-DC001.jpg