Welding broken parts

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3/7/2016 11:11 PM

Does anyone know why everyone says you cannot re-weld part?
Reasons backed with actual facts not just because your buddy broke his welded frame. I've welded parts cut them apart re welded them and they still pass all testing at my work. Any ideas?

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3/8/2016 1:28 AM

I'm not a metallurgy engineer, but I will do my best at explaining what I understand on this subject:

Parts, like frames or bars for example, are, after welding, heat-treated several times. Putting it into a heat oven, going up to 500 degrees (celcius i think), bring it down progressively, re-heat, shock cool, etc, etc: there are many variations.

The main goal of the process is to increase the strength of the welded parts to be the same overall. When welding, you heat up that zone, and that zone only becomes strong, able to resist many more PSIs that the other zones. No heat treatment can yield cracked frames, and not on their welds, but on the welded zone-normal zone interface because of the sudden change of metal properties.

Your bike is strong, but it is also brittle. If it encounters a force that is more important than what it was designed for, it would snap.

If you ride smoothly, then sure, weld it back and go out there. But this russian-type of ''repair'' isn't recommended as that weld would surely break again (if not on the weld, around it as you also changed the molecular composition of the metal), and at a much lower force than it's original spec. This could mean riding your usual drop, and after a few days doing so, WHAM! You'll end up on your ass.

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It doesn't matter where a train goes. It's decidin' to get on that does.

3/8/2016 3:43 AM

There's only a couple of companies that post weld heat treat. And 500 degree wouldn't make any difference. The reason not all companies post weld heat treat is because it's expensive, and the potential of something going wrong (frame twisting etc) outweigh the possible benefits (there's no proof it will make the material stronger, as it's so thin).

A good weld is a much more important factor than anything else, and a well welded frame will be stronger than an OK welded frame that's been heat treated.

If the re weld is of good quality, and it's your own part. Why not? My old pbr is still going after being cracked and re welded 13 odd years ago.

I think the only real issue is being honest if re selling, and a lot of people won't trust a weld not done by the manufacturer because there's so many people that say they can weld. You never know how good quality it is.

There's companies dotted about all over the place that specialise in re welding and fixing parts and frames. They obviously know what they're doing, and are confident enough to put their name to it. I personally wouldn't trust some random guy that tells me he can re weld frames, but if I had a part I could part with, I'd trust one of those companies

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3/8/2016 3:05 PM

Basically most kids will take a class in highschool and MIG weld a frame back together, and do a crap job but it kinda holds, and they think they are Gods as a result.

Also as stated above, if not done WELL, you risk it breaking again, potentially badly at that. It can take the welded area and the area next to the weld (usually considered the weakest part) and MOVE it further out, where more stress could get to it, causing faster failure. Also depending on what breaks, you won't get it back exactly as it was angles etc, and sometimes if a TT or DT snap and the other tube (DT or TT) doesn't, it can cause added stresses, and maybe even microcracks that will fail very soon.

Personally I have welded a set of cranks that lasted me another year and change, and went to a buddy who ran them for like a year before getting his own. Still hanging out in a garage somewhere I'm sure. I would do it ONLY for my own stuff. If a friend asked me to, we would have a decent conversation about how IF I was to weld it up, it would be TEMPORARY, and if it crapped out down the line, they wouldn't get pissed at me.

I likely would not put a headtube back on, maybe weld up a dropout, or if I had access to a tig and noticed a CRACK, I might go over it to prolong the frame a bit for MYSELF.

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"Hey anybody ever make that mistake like right when you wake up in the morning and you believe in yourself?" -Kyle Kinane

"BIKES!" -Tom Segura

3/9/2016 5:48 AM

It's like anything else people say you can't do.

Your average jackoff can't weld parts back together and expect it to work.

If you have the experience/training and access to equipment (it sound like you do if you're testing parts at work), then there is no reason you can't repair parts.

I keep up with a blog written by a small frame builder in Texas, and he had a post awhile back about repairing a popped head tube. It seems to me like that bike was stronger when he was done fixing it.

Having said all that, unless a part broke due to a material/craftsmanship flaw, it might be time to move on once a part breaks. Steel does eventually work harden, and stuff will start breaking. I was a mechanic at a go-kart track when I was in school, and I would weld cracked frames, but it was a given that once I had to weld a frame, we could get a few more months out of it, but something else was going to crack soon, then something else and on and on until we replaced the frame.

I'm just pulling this out of my ass here (based on non bike experience), but if a crack or break is in the weld filler material, it is probably OK to repair. If it's in the base metal, it's probably time to replace that part.

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