Penetration???

[Ben]

I am still pretty new to welding and have some penetration questions ... because I seem to get different answers from people I have asked in the past. I mainly weld steel (mostly mild) using TIG. First, on thin sheetmetal, I assume penetration is good because I can see the weld peek through on the back side of the seam. On thicker material, however, it is a bit less obvious for me. I follow the amperage recommendations I have found in books, but I know it takes many more factors than running the right amperage to make a good weld. At what point (thickness of material) will/should you not see the weld pop through the material on a butt weld for instance? Can you ruin a weld by overheating it without punching through the material (on thicker material)? Sometimes on the thicker material, I see a hollow area in front of the arc as I am running a bead (almost like the arc is hollowing out the metal), but the filler takes care of it. Is this too hot or is this right? What is a good way to test a bead for strength, or is it best to cut one open? Ok, sorry for the sporatic questions, but any advice would be appreciated. Thanks in advance.

[JoeHobart]

At what point (thickness of material) will/should you not see the weld pop through the material on a butt weld for instance?
You will need to bevel thicker materials to ensure full penetration. I like to bevel anything larger than 1/8th, and its a requirement for 1/4" or larger when using hobby-class gear. See below picture for what a good bevel looks like.


Can you ruin a weld by overheating it without punching through the material (on thicker material)?
The underlying phenomenon driving this question is improper joint preperation cannot be overcome by extra heat.

Sometimes on the thicker material, I see a hollow area in front of the arc as I am running a bead (almost like the arc is hollowing out the metal), but the filler takes care of it. Is this too hot or is this right?
I'm not a tig guy, ill defer this to one of our tig guys.

What is a good way to test a bead for strength, or is it best to cut one open?
Lots of different ways. The most common way to test is destructive bending. Weld your coupons together, then clamp it in your vise such that you can hammer it to bend it as far as your rig allows. You will want to try both a face bend, and a root bend (top and bottom). Another way is to cut the weld apart (wheel, torch, to inspect penetration and bead profile, and investigtate for other defects like lamination, inclusions and porosity. You can use acid to etch the weld to make it stand out better. Heres a few pics from an experiment we tried.

[Ben]

Joe - thanks for the responses. Awesome pictures ... I appreciate that - it makes it a whole lot easier to envision. A picture is definitely worth a thousand words. Thanks again. Any more advice would be appreciated. Thanks.

[hankj]

Ben,

That hole you see is called a "keyhole", and it's a good thing. It means you are getting good penetration. If you've chased a keyhole all the way accross your weld, you can pretty much figure it's a good weld.

Like Joe said, joint preparation is necessary for joints over 1/8" as a rule. This may not be the case if the welder has the power to penetrate without prep. Here's a picture of a fillet weld on 3/8" made with a MM210 without any prep other than cleaning.



It was cut with a bandsaw, but NOT etched, so there COULD be a bad guy hiding in there, but it doesn't look like it!

Hank

[Mike W]

Great pics guys. Hank, the one on the right looks like it is glowing.

[Ben]

Thanks for the pic Hank ... once again, it sure makes it easier to envision. Ok, the keyhole now makes sense to me. When I first started TIG, I was a bit afraid of the pedal, but now I am trying to stay on it, and I just started to see the keyhole lately. Thanks again.

[OldSparks]

I'm a lurker who's gotten hooked on reading this site, Just thought this would be a good time to try some input. I don't know about tigging aluminum but to weld a mild steel tig root I've always bevelled the material to a point. Likewise I don't know about stepping on a foot pedal. Sounds interesting though. I've tried to throw together a little demo. The pictures show 3/16th plate with a gap just tight enough that a 1/8th rod won't fall through. Done on the flat. Numbers are amp settings I experimented at. Probably recommend 95 or so depending on how much the plate heats up on you. Lay your rod in the groove and strike your arc. Keep your torch in motion, a few slow weaves across the end of the rod from edge to edge and you'll see it flatten as it melts. Now continue weaving along your rod at a speed that is just fast enough to keep the rod flattening out. With a proper fit up you shouldn't have to worry about intentionally keyholing but you are bound to blow through as you learn the proper travel speed. When that happens, momentarily aim your torch a little higher up one of the bevelled edges while you move your rod closer to butt it up with the existing root. If you're wondering why the hot molten metal doesn't immediately fall through to your feet it has something to do with the magic that holds a water drop on the ceiling. Just keep moving the torch from side to side across the rod and science will keep sucking the filler rod up. One other thing. Tig is a multiple pass process. On this plate an inexperienced welder should lay down a fill pass and then a cover pass to finish off the weld.

[Rocky D]

Let me present another school of thought....In TIG welding I never gap....the reason is that The gap will create problems with the shrinkage you will get. I must know how much heat to input and be able to read the puddle to tell when I'm penetrating, with a metal to metal fit.Then I can gauge my shrinkage consistently. 3/16" steel is better welded with MIG...less heat input...less shrinkage. I believe TIG is not a multi-pass process,...MIG is. And I also don't believe in dipping the rod, is the only way to add filler metal to the puddle. Folks who do that end up with under-fill and a weak weld....I learned to push the rod into the puddle and watch the build up of the weld as I travel along. It is accomplished by learning to run the rod through your fingers and feeling it melt off in the puddle, and not poking through and have "arrows" sticking out the backside of your weld.

[Mike W]

If you use some backup, you won't get the arrows. I tend to agree with you about using the mig.

[OldSparks]

I totally agree that that there are other ways to weld 3/16th plate together, mig certainly being one of them. I just thought Ben's original question dealt with learning to 'tig in' a mild steel root. The point I was trying to show is that full penetration tig welds can be made on basically any thickness of material. Quite commonly on thicker material just the root and maybe the first fill pass are tigged and the remainder of the weld then stick welded in. The picture shows a jig set-up for tacking a heavy-wall boiler tube. Sorry I have no record of where it came from.

[Mike W]

Now that is heavy wall tube. Interesting picture.