Plug welds

[upand_at_them]

I'm new to this and have a question...

I'm joining two parts with two or three plug welds. The top part (with the plug holes) is 2" wide, 3" long 3/16" flat bar. The bottom part is a 6" long 3/8" square bar. If I drill divots into the bottom part where the plug welds will be, will that make them stronger? Or is there any other prep (besides cleaning) that will make for a stronger weld?

[milomilo]

What size holes? What welding process?

[farmersammm]

Your plug welds will be no stronger than the square inches of weld contacting the 3/16 around the perimeter of the hole in that plate. Drilling deeper into the 3/8 gives no additional strength to the assembly. Welds are sized/spec'd taking into account just how much of the weld contacts the parent metal. Small contact area, less strength. The weak link is the steel itself.

[upand_at_them]

So the bigger the plug welds the better?

Holes are currently 1/4", process is flux-core.

[villageblacksmith]

So the bigger the plug welds the better?

Holes are currently 1/4", process is flux-core.

There is nothing wrong with what you are trying to do. I am only referring to your plan of utilizing plug welds to join two pieces of steel. There is, however, a "fly in the ointment". That is your 1/4 inch holes. They need to be larger. They are too small.
And it is "arc force" that will provide your divot. That's one way of describing it at least.

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[upand_at_them]

Okay, then I'll make the holes as large as the bar being joined: 3/8", or maybe just 23/64".

By the way, the plug welds won't be the only welds joining the two pieces. I'll also be laying beads on the other side. I just wanted the fullest join since I'll be stressing the 3/8" bar and don't want it to break free.

[villageblacksmith]

Okay, then I'll make the holes as large as the bar being joined: 3/8", or maybe just 23/64".

By the way, the plug welds won't be the only welds joining the two pieces. I'll also be laying beads on the other side. I just wanted the fullest join since I'll be stressing the 3/8" bar and don't want it to break free.

I seriously doubt you will achieve your goal even with a 3/8 hole. At least with the type of equipment I suspect you are using. I would recommend a 3/4 hole as the smallest size that may be effective.

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[upand_at_them]

But a 3/4" hole is better than the 3/8" material being welded to.

[Kelvin]

What are you making that requires such Godawful strength? Assuming you're using A36 mild steel (yield strength 36ksi), each 3/8" plug weld is gonna require nearly 3/4 ton of force to yield. Now multiply that by how many plug welds you're making.

In my experience, overwelding -- and the warping and distortion that go with it -- is a much more common problem than insufficient strength. Steel is stronger than you probably realize.

You might want to estimate your expected maximum loads before overwelding and then having to cut it all apart and re-do it (with fewer welds) because it twisted up like a pretzel...

[milomilo]

Okay, then I'll make the holes as large as the bar being joined: 3/8", or maybe just 23/64".

By the way, the plug welds won't be the only welds joining the two pieces. I'll also be laying beads on the other side. I just wanted the fullest join since I'll be stressing the 3/8" bar and don't want it to break free.

Then you do not need plug welds. The welds on the other side will be all you need.

[upand_at_them]

Here's what I did. No plug welds. It's part of a small metal bender, for 16 gauge bar. I wound up using a 1/2" round for the handle.

It's hard to tell due to sitting on a stone wall, but I had a little burn in at the end of the welds (bottom right and top left). I was working with my backup (handheld) shield as my auto-dark helmet died. Otherwise I'm fairly satisfied with how well they're joined.

[shortfuse]

Your biggest problem with plug welds will be slag inclusions and porosity......those small holes will be more like tack welds at best.