If one is joining two tubes of identical metal, how does on choose the welding rod or wire? Assume I'm building a roll cage out of mild DOM steel. I notch the tube, fit it up, and grab my MIG/TIG welder. Do I choose a wire that has the strength of the tubing, weaker, or stronger? Is it different if I'm butt-joining two tubes, using a tube on the inside to rosette weld in place, then running a couple beads to join the exterior tubes to the interior one? Is the joint, after construction, weaker or stronger than the metals I welded together? Assume I'm a competent welder who has a full-time job assembling roll cages (or airframes, etc); of course, I don't have such a job, and I'm not a professional welder.

If I take two plates (or two sections of tubing) and do a butt joint (assume I grind down the plate edge/tubing edge to allow for full penetration of the weld), then section off a couple coupons, I'm aware of two tests for strength: one is where I bend the coupon to ensure it doesn't fail at the joint, the other is putting the coupon in tension until it fails. My understanding is any coupon failing at the weld is bad; is this wrong? Again, assuming the plates or tubing are the same, I was welding in a warm shop, etc.

This comes about because someone I know of was told in a class on welding roll cages that the welds will be the weakest part, and that you always use a weaker metal as filler. This seems contrary to what I've read and heard elsewhere, so if one of us is off base, I'd like to know. I assume in the case of a roll cage or airframe, you would take the entire unit and heat treat it to help the HAZ.

I see the American Welding Society has a publication, D1.1, which probably has all the answers; they want a couple hundred dollars for it, though.

I can't answer your question about roll cage welds being weaker than the base metal but you are correct in that welds are generally stronger than the base metal.  The material should fail before the weld.





 

Thanks, I posted it over there. A lot of the welding forums I was looking at earlier didn't have much participation; this one seems active.

ER70S6 or equivalent will be fine for DOM steel.  Wire size will depend on tubing wall thickness, but .024 or .030 is probably what you are going to need.

You are over thinking this.

The quality and penetration of your weld is going to be the most important.  If you can't throw a good quality bead, you shouldn't be welding on something your life depends on.  It took me a year or three before I had the correct equipment and skills to do this.


For those who don't know:  DOM = Drawn over mandrel   (This is not chrome moly)

Done correctly the weld is at least as strong as the base material.

Done incorrectly (including wrong filler material, inadequate penetration, bad technique, etc.) it can be weaker (and often is).

Betting your life on your weld is one thing, betting someone else's life can put you in a world of hurt.

Your example excepted, sometimes a mild steel filler rod is better. When welding 4130 CrMo steel, a mild steel rod (such as ER70S2, ESAB Spoolarc 65) will produce a more practical weld. The reason is that the alloys from the 4130 mix in with the mild steel rod a bit and form a ductile weld joint that is not prone to cracking.

Also, if you are chrome plating your weldment, the filler rod choice is important, to prevent discoloration of the chrome over the weld.

Some other welding weirdness. If welding a mild steel aluminized exhaust pipe, stainless filler rod will result in much longer joint life. As it won't rust out rapidly. Properly welded this way, it's likely to last a lifetime.

Thank you for the replies. Just to be clear (and I thought I was, guess not), I'm not welding a roll cage. Someone was told that welds are weaker in a seminar on making roll cages for NASCAR, and that didn't square with my (admittedly very small amount of) experience or anything I've read (or videos I've watched) about welding.

All of the replies, both here and over at Miller, were that this isn't the case. It's clear to me someone was handing out bad info or the person relaying the contents of the seminar was confused.

Again, thank you. I wish I had a garage so I could weld; the apartment manage takes a dim view on welding here.

the issue isn't so much that the welds are weaker as much as the lack of QC and joint design, and in particular non-destructive testing.  You really can't just look at a weld and be 100% certain it's good.  You can use various forms of testing, but that won't give you x-ray vision - you need to take x-rays for that.

That and there is the issue of stress points right next to welds.  Socket weld fittings are a great example.  Right at the end of the weld onto the main pipe, will be the place it cracks as that sudden change in cross section makes a stress concentration that will fatigue and fail.



 

And important welds are routinely x-ray inspected at start of life and sometimes even periodically in service.

It kinda depends. When welding the heat of welding will change the heat treatment of your metals. So from that standpoint you may be weakening the actual METAL. On the other side the weld will generally be thicker at the joint and the extra (weaker) thickness of metal may make up for it's lower strength.

I'm not a welder but this is what an engineering instructor told me in a materials class decades ago.

True.  I have seen perfect looking welds with zero strength.  This happens on improperly prepared hot-roll steel.