Arc blow in G3 test plate

[dstig1]

We're doing all 7018 on this. Yes it's beveled as it is a G3 Groove test coupon setup. We've also tried stopping halfway, cooling the part in water, then restarting with a new rod. Same issue.

Like I indicated, it is a rather vexing problem that even the instructor is having issues with.

 

[Gary Fowler]

When magnetism is encountered on plate or pipe, it doesnt have to be a strong field to effect the arc. I have seen it when you would barely be able to feel it and it would be impossible to weld. If you have some small iron fillings or grinder dust, dust a little on the plate and see if it sticks. Shield arc mentioned wrapping the leads on it which works for pipe sometimes but it is a hit or miss. Above 500F a magnetic field will completely dissappear but may come back when the piece cools. There are demagnetizers that you can use also. I would bet you dollars to a donut that magnetism is your problem and it has nothing to do with rod angle, type of rod, current setting or any other technique problem. If your arc starts jumping around like a lightning storm, it is due to a magnetic field and something is causing that to occur.

 

[dstig1]

Solved! The instructor looked into it more and talked with some other folks about the issue. What he had us do was to tack the plates a lot more. We had just been tacking in 3 spots on each side of the backer plate, but he said to run a full "tack" bead up and over the backer onto the front of the backer (but outside the groove joint), mainly at the top of the joint where the problems were. That seemed to fix it. I assume it allowed the magnetic flux lines to better spread out in the metal as it was more fully connected together. No more arc blow!

I suggested wrapping the work lead around the fixture but he had the same concern I did - they are so short, it would be very hard if not impossible to do.

Thanks for all your help. Good info and I like to learn more about these things.

 

[dcyrilc]

What's the purpose/reason for the backer plate?

 

[Shield Arc]

What's the purpose/reason for the backer plate?

Easier to make the weld.

 

[dcyrilc]

Easier to make the weld.

Thanks SA. I'm hoping to start taking a welding class this summer and start learning to weld. Probably should have done this 30 years ago, but what the heck. Never too late, right?

Back the the reason for backing the weld. I've seen this done for years where Traffic Signal pole shafts or mast arms are spliced. I always assumed it had something to do with the strength of the weld.

 

[Shield Arc]

The backing plate is nothing but a crutch! Doing open root separates the men from the boys.
If you want a real challenge do open root with 7018.

 

[dstig1]

With a 1/4" root gap? I'll take the crutch please!

 

[dcyrilc]

With a 1/4" root gap? I'll take the crutch please!

This is the first that you mentioned spanning a 1/4" gap.:confused2: (I went back and reread to make sure). Don't think I'll try that for my first weld. Maybe the second.:laughing:

 

[dstig1]

Yeah, I can't say I am any expert in this, but I assume it is the standard configuration for this test. The teacher has us doing "G3 vertical up" plate tests now, which is what I have been having issues with. We take 2 plates that are 3/8" thick and about 6" long x 2-3" wide, and bevel one edge at 60 deg or so. We then take a 1/4" backer strip that is about 1.5-2" wide and tack the two plates to it so the bevels are facing each other and the point of the bevel (on the backer) is 1/4" apart. Then weld away!