Welding Tips and Tricks

Hey guys I am new to this page and fairly new to welding... I am a boilermaker apprentice and I have a question about a tube test coupon... is a loose 3/32 gap with 3/32 filler wire on 100 amps ok? I want to make it as easy as possible for me and I don’t wanna mess up when it comes test time... I use lay wire technique... thanks guys I would appreciate all the help!


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Amps are normally dictated by the thickness of the metal, not the wire or gap. And of course, position of the weld. Any further information you can provide to get more specific answers?

Schedule 80 2” tube and also scratch start or lift arc?


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100amps might be a little high. Try 90 amps to start with and work up from there if you need to. Lift start is better than scratch for sure.

3/32 or 1/8 tungsten? Which is better to use for root pass and what amperage for each?


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I use 3/32" for everything.

TaylorBlake wrote:3/32 or 1/8 tungsten? Which is better to use for root pass and what amperage for each?


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don't even look at 1/8 tungsten until above 200 amps or so.

I'm an old retired BM tube welder. I have taken hundreds of those test to get hired.
Your coupons are beveled with a knife edge for the TIG root, I would start at around 50 amps. If your too hot, and drop a bump inside, or too much push through, you will get looked out before getting to the stick out. I liked a 3/32" gap, and used 1/8" filler, no chance of the filler wire falling through on the inside. This is personal preference, some do the opposite.

Lay it in there, and keep your heat on the sides. Don't come up the bevel or you will get suck back. If your coupons pull together, and start to get too heavy fill, go to 3/32 filler. Don't forget a back purge, any sugaring on the inside will bust you out.

Tungsten size never mattered to me, you grind it to a sharp point anyways. Whatever the last guy in the test booth was using, I go with. No difference in amperage between them needed, because again the tungsten is a point.

They will tell the parameters, usually zero push through, but sometimes they allow up to certain amount.

I thought you wanted a little push through... a nice wedding band... I thought flush and under flush would bust you out


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50amps might be alright for boiler tube, but this coupon is schedule 80 pipe. Needs 90 amps min.
Purging is for stainless steel, this coupon I believe is carbon steel so purge not required unless on WPS.

Coldman wrote:50amps might be alright for boiler tube, but this coupon is schedule 80 pipe. Needs 90 amps min.
Purging is for stainless steel, this coupon I believe is carbon steel so purge not required unless on WPS.

Yes sir Coldman you are correct on the pipe... so would 90-100 amps be good?


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Coldman wrote:50amps might be alright for boiler tube, but this coupon is schedule 80 pipe. Needs 90 amps min.
Purging is for stainless steel, this coupon I believe is carbon steel so purge not required unless on WPS.

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Coldman you are correct on the pipe... so would 90-100 amps be good?


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I'd be on 90amps for sure.

I would be anywhere from 90 to 120 amps. Totally depends on fit up and test position and accuracy of the weld machine.. IE 5G would be 90 ish. Inside finish needs to be flush or above. How much above is determined by the WPS. Last one I took was up to 1/16 over fill on the inside. But no unbroken edges, or below flush / undercut. I use 3/32 tungsten or 1/8.. But if I’m inside a tube for the back half before I fill in the window it’s a 9 torch and 3/32 for sure.

6g position loose 3/32 gap with 3/32 filler


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If the bevel is a knife edge, and the gap is 3/32, 90 amps will blow away rather quickly unless you are stuffing wire in and traveling very quickly.

Tungsten size, amperage are all very personal preferences. I’ve used 1/8” to weld 0.030 stainless with great results and used 3/32 fo 1/2 carbon. Use what works for you. I run my welds hotter than many others but my travel speed is probably faster too as a result. Getting started, learn to weld hot is my advice. You’ll be more proficient when time matters and get better penetration when it absolutely matters.

It’s all practice, find what works for you-

As I said, the test parameters will be given. For super critical boilers, (around 3000 PSI) the wall tubes are very thick, with a small diameter, push through restricts the flow. You are probably testing on a 4 to 6 inch Arkansas Bell Hole. I've tested in refineries for header repairs where they wanted an 1/8" push through, that was very hard for me. In general, a slight push through is what they want, but wanted you to know, not always.

You go in with 90 amps on a knife edge... well, go ahead, that's how you learn. If you start colder, you can always turn it up. If your hot and blow the edge off, your done. If you drove across the country to make or break a test, your packing up and heading home with empty pockets.
I stand corrected for the back purge, that would be stainless test. But, pressure vessel welding in refineries will back purge for mild steel. One of the last jobs I was on, we replace a mud drum on a CO boiler, and I remember it taking a long time for the purge to fill.

I would thing the apprenticeship training would have you up to speed before sending you out to test to get hired.

Good luck, and tell us how it went.

Nothing like waiting for the little meter to hit the magic number. Then it’s balls out( a reference to an elevator mechanical brake) for those of an un pure mind.