Are you grinding your tacks?

I never had any trouble with tie ins on RMD.

 

I will put 4 bridge tack on my pipe joint. Put the root in one side then the opposite side cut the bridge tacks completely out. This way I only have 4 tie in spots. I will feather tie ins to knife edge.

I am a pipeline welder and new to the semi automatic processes and am working on this procedure for our shop. I’ve had test pieces X-rayed everything shot fine. I have qualified the welding procedures but we don’t take the samples out of the tack areas.

My starts are good coming out of my tie ins. It’s mainly when I come in to the ties.

Some will be good some will not. Just wondering if anyone one had tried bending their tie points.

 

I am using a heavy 1/8" root opening. 1/16 land. I have a tapperd nozzle tapperd tip using 75/25 flow is @ 35. Using. 035 L56. Welding at the 2-3 o'clock position with the pipe rolling away from me. WFS I tried anywhere from 220-280. 250 seems to be the best.

 

Don't bend the tie ins. They will fail most of the time, even if they look nice. That process and short arc has a tendency to trap gas in the tie in. Just a tiny little gas pocket is all it takes. Just about every time I would TIG backweld a MIG root on big pipe, it would blow out a gas pocket on the tie in.

You could try grinding down and running back over the tie in, if you want to try and get it it to bend.

I would do away with the land, but that's just me.

 

Run your root pass as you normally would. Then grind down into the tie in. Just about every time you will find a tiny little dot, that's the gas pocket. If you run over it again, you will most likely burn that out. I never did come up with a way to stop it doing that.
Most pipe codes allow for small indications in the x ray, that's why you can mostly get away with it, depending on the wall thickness. Occasionally, we would get a job that called for TIG root on big pipe, or even TIG all the way out. I always thought they wanted to eliminate the inherent gas pockets in the roots.

 

If you're still bending straps, it would be pretty cool if you would try my suggestion and report back. I'm curious to see if it works. I had to try three times before I passed the bend test at the fab shop because they kept cutting the straps on the tie in.

 

You can always call Miller support and ask this same question. They have people there who can help you figure this out.
with correct machine set up and welding procedure.

 

I have tried grinding into the starts and stops about a 1/2" back. Im running my stops up onto the bevel before I terminate the arc that gets rid of most of the gas pockets coming out of my tack. its really just tieing into the starts of my tack is where all the problems are its like when I tie in no matter what theres something there. Its not real big it would pass X-ray I just figured it wouldn't be this hard to get rid of. I have been working with the Miller reps in WI and they have been a great deal of help.

Thanks for the feed back.

 

here are some pics of prep work and bends I am also putting a FCAW fill Cap. The bend on the Right is what im getting some times i get it some times i dont.

 

What I was saying was do your tie in as normal, then grind down into it and run over it again. The gas pocket is not in the tack, it forms when you make your tie in, for some reason. Grind back down into it until you see the dot, then run some clean filler.

 

It looks like you need to feather your root back further for the tie ins. Then you can get the puddle established before the tie in. A little more grinding for the hot pass but better tie ins. Would need to see what your root looks like on the inside of the pipe before grinding. The one in the top pic. would fail and a couple others look like they are very light on fusion. Your root looks heavy enough but not quite enough penetration especially when using a flux-core hot pass that should push it through a touch more. I haven't used RMD but the same root profile should apply. Maybe a slightly wider gap would help?

 

The problem is that MIG roots trap gas on the tie in. I've come across this very same thing many times. Both on weld tests (bending straps) and production welds (backwelding MIG roots with TIG). You can have a perfectly fine looking tie in, maybe a little flat and you want to build it up a bit - as soon as you start welding over it, it explodes and blows a big glob on your tungsten. I couldn't tell you how many times I've had that happen.
I've had weld tests, where the root looked perfect, not pass the bend test. Happened three times in a row. That's why MIG root tests are always bend tests and not just x ray.

You can see the crack in the strap is right in the middle of the root, it's not a side wall tie in issue.

MIG is not the best process for running roots, it has its flaws. It's fine for most run of the mill piping systems that you might come across, low pressure water lines and drains and such, and it's very fast. But, there's a reason they use TIG instead of MIG on most critical, high pressure stuff.

 

Just so you guys don't think I'm just blowing hot air, this was some of the stuff I welded on before the latest layoff. I was welding that pipe in the foreground when I took that pic of them moving that pipe around.

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sub arc makes it easy.

 

Sub arc will work your butt off. You think they get good fits on that big pipe? I spent more time grinding than welding. I finally had to give it up, my carpal tunnel couldn't take it. They had to put me back on little pipe.

 

Big pipe looks like big pain

 

Big? I worked in a vessel shop and helped fit a head back on that fell off... The vessel was 10' diameter and 2 1/2" thick. The heads were 2 3/4" tapered to 2 1/2" where they met the shells. 10 ton crane had about all it could handle with the head. The head was tacked on with round bar used for bridge tacks about every 6" around the whole circumference. Round bar was about 1 1/2" long and welded with 7018 on each side. I was talking with some other welders and all of a sudden we started hearing these "ping" sounds. They got more frequent and we figured out what was happening pretty quick. The tacks were breaking. Then bang, the head fell off. It put a big dent in the concrete floor and destroyed a new 60 series wire feeder on a cart. The cart moved enough the XMT 304 underneath the feeder wasn't damaged. Fitting the head back on was a job. Had to use wedges and lots of pounding trying to get the best fit. Some sections required the bridge tacks about 4" apart. Then I had the job of putting in the Mig root with preheat and the hot pass prior to sub-arc. Spent more time grinding the tacks out than it took to do the root. Weld 3" grind, feather root, weld 3" grind, feather root. My wrist was pretty sore after that but I realized why welders get carpal tunnel. Not often but some customers specified Tig root on vessels or piping. Largest diameter I saw for Tig root was about 6' and 3/4" thick.

On the same vessel (about 50 tons total). They made a mistake and cut a hole for a 24" nozzle but the nozzle was only 20". The nozzle also required a 2 1/2" thick repad. They didn't want to have a new repad rolled so used the one they had. A friend and I got the 1st shift to weld it up. We just brought a box each of 1/4" 7018 and 2 tiger torches for preheat. Flux-core wasn't approved for vessels at the time. It took about 2 1/2 shifts day/night for 2 welders to weld it up. The shell has a bevel for a 24" nozzle but the nozzle is only 20" then the repad had a bevel for 24" but the nozzle is only 20". The bevel was cut to allow room to get the welding electrodes to the bottom of it. Massive amount of weld and very hot sitting up there. After welding, the repad had to be ground so it looked it was cut the right size with only a 3/8" fillet showing around the nozzle and just a 3/8" fillet around the outside of the repad. It was still fairly hot the day after the welding was finished. Only good thing was the time went by pretty fast. As far as weld size that's by far the most I've ever done on a single joint.

 

How did y'all fill that gap between the nozzle and the shell? That's a helluva screwup, anybody get fired?

 

Nobody was fired. I worked night shift and the day shift fitted the nozzle. Thinking back I can't remember if only the repad was cut too big or they put some spacers in to fill the gap. There were some bars under the flange holding the nozzle up. They did some welding from the inside and we had to gouge a bunch from the outside to get to the bottom. With a flange on the nozzle it needed to be widened enough to get in there. Grinding it out after gouging was fun too. They used ultrasound to test it before the repad went on. It was in 1997 so a little while ago. I do remember the repad was cut in two because it would have been in the way to weld the nozzle. That required beveling both sides of the repad for 100% penetration too. I think the repad was 4" wide. It was just a ton of weld.

 

That's mental.

I bet screwups are more common than we think.

I had an inspection to do last year of two of our super duplex lacquer vessels. The fab shop had (completely by accident) used 400 series stainless for the manway lid neck ring, and also the agitator neck ring. In fact, myself and a colleague went to see these vessels post-manufacture and we didn't see anything wrong - of course, it all looked shiny!

After 2 years, the neck rings were both destroyed by chlorides and leaking around the welds.
One of the vessels had this "supplementary" neck ring (also 400 stainless) put in - we figured it was because the hole had been cut too big, so they added something to the diameter.

You didn't see this on the "as-built" drawing, though :jester:

Still to sort a repair plan... at the fab shop's expense.

 

This got me thinking and going back in my memory. The hole for the nozzle was cut for 20". I recall now the person who ordered the repad made the mistake and ordered it for 24" instead of 20". There wasn't a big rush for the vessel and I think it was cheaper to fill it up with weld than to have a new repad made. That kind of makes sense beause the repad has to be cut from a large piece of plate.

My first job out of school a foreman made a huge mistake on a 750 barrel tank with studs for insulation welded on it. They filled the tank with water and hydrotested it. It passed but the foreman forget to tell the laborers to let the air pressure off before draining the water. It sucked the top of the tank in! That was an expensive mistake. In the same shop they asked a new apprentice to cut the D rings off a tank that they use for moving them around before the final lifting lugs are installed. Oh he cut them off. Only problem was instead of cutting the D rings and then grinding the weld down on the tank he cut the D rings out of the tank. It was a just a standard 400 BBL tank and they filled up the holes but the apprentice must have wanted to crawl under a rock after that.

I've had some mishaps of my own too. Was running one of the overhead cranes to move a finished tank to the door. Got distracted by something else in the shop when I was setting it down and never noticed that a nozzle got caught between the crane control cable and the support chain. Down worked and I got the tank on the ground but the crane started moving in the opposite direction. I kept going down and luckily the maintenance guy saw it and shut the breaker off. It pulled a bunch a wires out of the crane control. The maintenance guy there was really good and had it fixed pretty quick. Maybe there should be a new thread for the biggest or dumbest mistakes made?