[MosquitoMoto]

Sorry if this gets a bit lengthy.

I'm welding steel tube to steel tube. My fit up isn't great because I haven't done a lot of coping. Anyhow, I've been using Jody's 'rule of 33' lately with some success, so I thought "Why don't I just pulse along the edge of one tube at the gap so that I can start to close up the gap?"

(Yes...the gaps were that big!)

So I pulsed along the edge of the cut tubes. Perfect! In next to no time I had all but closed my gaps. Then...as I started to close the gap, disaster. Sparklers, fizzing, what looks like contamination and a weld that started to look like crusty garbage. Went from neat and stacked to shameful, instantly. Brown stains, too, like there was a sudden gas problem.

It did this every time I came close to bridging the gap.

I began to think that there must be some difference in welding on the 'cut edge' of the coped tube as compared to the outer surface of the other tube. Both had been cleaned the same way...ground to bright, shiny metal, then acetone.

Switched from pulse to conventional and ... Problem gone. So I've been closing my coping gaps with pulse, then when I come to bridging them, I switch to conventional DC.

Any clues as to why this might be happening?


Kym

[ignatz200]

I ran into a similar situation when I was putting together my own welding table using sections of square steel tubing.

I decided to close up everything real nice and neat and then... bang! ... just as I'm finishing it up the weld kind of puffs and explodes on me.

So what is happening? The answer is simple. As soon as you bring the weld pool around to where you are about to seal up the tubing it has now become in effect a closed container... with your very last weld point as the only means of entry or - in this case - exit. The very welding you are doing is heating up the air beneath the weld... which is IN the container. Seeking a way out it can only exit through this last weld point. At very least that pushes a little stream of air which will upset your sheilding gas flow. In my case it was strong enough to 'pop' the weld out.

The solution to same is also simple. Drill a tiny relief hole through your metal somewhere else on the piece. That equalizes the pressure from within and without. Now finish your welding as usual and then go back and hit that tiny hole with a quick spot weld to close it up.

[MosquitoMoto]

Thanks ignatz...but this is actually happening well before I seal the joint. It happens the first time I try to bridge the gap.


Kym

[Braehill]

Kym,
First off being a good fabricator is more dependent on being a good fitter than the actual welding part. Large gaps should be eliminated before the welder is turned on. If you can't close a gap in one pass you should change the way you're doing things.
We've gone past that point though so now we're into a whole new problem, oxidation.

If you have to build the cope out of weld, do it during fit up and stop and clean it just like it was a new piece, shiny bright inside and out. The part you built up has oxidized on the inside with every pass and when you get to the other tube it takes more heat to tie into it than it does to melt the edge of the tube you built up so it melts through and brings that oxidation to the top. This where your porosity is more than likely coming from. It will follow the heat and in this case it's pulled through the weld puddle causing porosity.

Len

[MosquitoMoto]

Kym,
First off being a good fabricator is more dependent on being a good fitter than the actual welding part. Large gaps should be eliminated before the welder is turned on. If you can't close a gap in one pass you should change the way you're doing things.
We've gone past that point though so now we're into a whole new problem, oxidation.

If you have to build the cope out of weld, do it during fit up and stop and clean it just like it was a new piece, shiny bright inside and out. The part you built up has oxidized on the inside with every pass and when you get to the other tube it takes more heat to tie into it than it does to melt the edge of the tube you built up so it melts through and brings that oxidation to the top. This where your porosity is more than likely coming from. It will follow the heat and in this case it's pulled through the weld puddle causing porosity.

Len

Len-

You have an extraordinary talent for explaining things clearly and succinctly. Thanks so much for your advice.

Yes...fit up. I discovered (via Jody) an online coping calculator...after I'd done the tubes for this project by eye. Dang. But in future I'll do any building up (correction) of my coping before I fit the tubes together!

I'm going to cut this project up and re-start my fit up.

It's not all a loss though. Not by far. In this project I have (successfully!) used pulse, welded out of position, standing using the pedal...all sorts. Lots of learning.

Again, Len, I really appreciate your advice. I feel like I have the best possible mentors from all over the world on this forum.


Kym

[cbfinn_99]

Moto, I can't offer any advise on the pulse/no pulse, but on the coping fit ups I use Pipemaster tools. Perfect every time.

http://www.trick-tools.com/Pipemaster_C ... Gauges_150

[MosquitoMoto]

Nice.

More tools? For me?

Good tip, but for the time being I reckon I'll just keep using the online coping calculator, it's a recent discovery I made via the forum and seems to work a treat.

Cheers!


Kym