Problems welding a bolt

[chevynut]

I was assembling some parts today and I needed to weld some bolts to a plate. I countersunk the holes, dropped in flat head socket head cap screws that I ground to a smaller diameter head, but still larger than the hole. The bolts were cleaned by bead blasting so they were bare steel. I held them in place with nuts and TIG welded them. The welds looked great. Then I ground down the welds to make them smooth and some of them were full of bubbles. So, I cleaned and welded them again with more rod and it kept getting worse the more I tried to fill the bubbles. I even tried to grind out the weld and weld again, and it just kept getting worse. I even tried MIG to no avail.

I assume this has something to do with the carbon in the socket head screw, but I've done this before with no issues though I usually have some small pits after I grind the welds. What's going on? I know bolts are welded on all the time. All I need to do is keep the bolts from rotating but they need to be smoothed down too.

[Rick_L]

I don't think the amount of carbon matters - in fact a bolt should not have all that much carbon. Likewise any alloying element that helps make strength shouldn't be a problem.

So that leaves contamination. Either the bolts or a problem with the gas or filler. Since you tried the mig, it's probably not the gas or the filler.

Try two things for some hint of what's wrong. Try welding two of those bolts to each other. Then try welding two grade 8 or good socket head bolts of a different style or size together after cleaning the same way.

[chevynut]

Rick, I've welded with this same Argon bottle on other stuff with no problems. Maybe I need to wash everything off with acetone really good and try it again. I just welded some seat mounts together and didn't even clean them that well, and had no welding issues. I usually don't find contamination to be that big of a problem when I TIG weld, but I know once you get "volcanoing" it's hard to get rid of it. What was weird is the bubbles get worse and worse if I go over it again. Typically I find that contamination burns off and adding TIG rod helps flush it out.

I didn't think the carbon should be that big of an issue, because I think these bolts are medium carbon grade 8+. It's not like they're cast iron.

Thanks for the suggestions. I'll try playing around with it and see what happens.

[chevynut]

The bolts I used were from a bin of rusty socket head bolts I had. I bead blasted them, ground the heads down to about half their diameter (dropped them into water to cool them off, then dried them with air), and dropped them into the countersunk hole. The top of the head was protruding above the plate. That created a channel around the head that I melted the rest of the head into, and added filler rod to make it a little higher then level. Then I had to fill the hex hole up. Like I said, the welds looked good until I ground them down smooth.

I wonder if some remaining rust caused the bubbling. Something sure did.

[Rick_L]

What about the material you were welding the bolts to? Could it have been the problem?

I feel like the conversation is backwards here, you have more experience at tig welding steel than I do. And I'm sure you've had craters previously.

Thing is, something is wrong and you should be able to find it.

Another thought, glass beads don't cut rust like sand or other "sharp" blasting media. Is it possible the parts just weren't clean enough?

[MP&C]

How was the blasting media cleaned off?

[chevynut]

The parts I was welding to were just mild steel, pickled and oiled so there was no scale. But I probably didn't clean it very well before welding and I usually don't have to on parts like that. We just take parts made of that material off the shelf and weld them on. As far as the bolts like i said, I blasted them, ground the head down smaller, cooled them in water, and just blew them dry with air. I assumed that would remove the media, but maybe it didn't. I bought "glass beads" from HF which I have bought before, but I'm pretty sure the box was mis-labeled. This stuff is like white alumina that I have used before but quit using it. It's destroying my viewing window which the glass beads didn't seem to do, and it leaves a rougher surface than the glass beads do. When I looked at if after I bought it, it seemed like "crushed" glass instead of glass beads...but I still think it's not glass. I get a lot more dust from it too.

I've done quite of bit of TIG welding and once in a great while I'll have a "volcano" problem or pits. But I don't take a lot of pains cleaning stuff up before welding, and I don't have problems like this. When I went back over the welds to try to fill the pits after grinding them flat, the melted steel looked like swiss cheese. It just bubbled up. I'd grind it out with a cutoff wheel and try again, and it did the same thing over and over.

It was a frustrating experience and I had to discard one of my parts because I just couldn't fix it.

[markm]

Before the POS lawyers got involved when the power company replaced their towers they allowed the demolition company to sell the heavy angle iron. I built several nice car trailers using this metal under the wood floor. It was a very difficult to get the galvanized coating off the areas where you had to weld. Even when it appeared very clean it reacted to the welding process. The story was they were afraid someone would buy up this metal rebuild a tower and if it fell they would get sued.

[chevynut]

These bolts had no zinc plating they were originally black oxide but they rusted over the years. I weld zinc-plated nuts and bolts all the time, however, I just drop them in muriatic acid for a few minutes. W also have to remove zinc from the C4 k-members where we weld on them. Never have a problem with that.

[Hotroddder]

I think socket head cap screws are hardened, maybe they are too hard to weld without annealing.

Hotroddder

[chevynut]

Yes they're hardened but they are not high carbon steel. They're medium carbon content and I'm pretty sure that's not the problem. I blasted and cleaned them better this last time and they welded a lot better, but I still got some small pits. The biggest problem is when I try to weld up the socket hole and it's the hardest place to clean out thoroughly. I'm pretty sure it's related to cleanliness, not the steel. I did some research and most every source said welding a grade 8 bolt shouldn't be a problem.