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Tell me how Brazing a crack on a steel road frame worked for you.

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Tell me how Brazing a crack on a steel road frame worked for you.

Old 09-03-12, 04:53 PM
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sbskates
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Tell me how Brazing a crack on a steel road frame worked for you.

I have a crack above a weld I am going to have repaired. Gonna have it Brazed. Anyone done this and how did the repair workout?
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Old 09-03-12, 05:09 PM
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Can't help you from bike frame experience, but it is standard practice in crack repair to drill a hole at both ends of the crack to keep it from propagating. Generally one drills as small a hole as practical.

You might want this moved to the framebuilding section.
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Old 09-03-12, 05:20 PM
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Poor long term results. Only brazing a crack won't change the causes. Even drilling the ends to prevent properation isn't the best solution. The faces of the crack offer very minimal surface contact for the braze to connect. A much better solution is a patch. The rule of at least 3x the wall thickness of a fillet or sleeve (or patch) applies. Still, knowing the cause might guide you to the best fix. Impact, overheated weld and embrittlement, undercutting all have their long term concerns. Andy.
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Old 09-03-12, 05:30 PM
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Originally Posted by Andrew R Stewart View Post
Poor long term results. Only brazing a crack won't change the causes. Even drilling the ends to prevent properation isn't the best solution. The faces of the crack offer very minimal surface contact for the braze to connect. A much better solution is a patch. The rule of at least 3x the wall thickness of a fillet or sleeve (or patch) applies. Still, knowing the cause might guide you to the best fix. Impact, overheated weld and embrittlement, undercutting all have their long term concerns. Andy.
+1, since there's no structure to replace the strength lost to the crack the braze is the only support bridging the joint. If enough of a fillet is built up it can do the job, but it's a dicey proposition. I've seen this done successfully when the crack is caught very early on, but once it grows, there's too much lost strength, and a re-weld is called for, and even this can be problematic.

You also have to consider what caused the original failure, if it was an accident, the braze might hold up, but if it's something inherrent in the frame, then the same cause is still there, and the crack will propagate through the braze fairly quickly.
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Old 09-03-12, 05:35 PM
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Not quite sure if brazing is nearly as strong as welding a crack on a frame. I see brazing material as sort of a solder that can fill in cracks and gaps and bond fittings and lugs to the tubes, but I don't think it approaches the stregnth of actual welding in which the material deposited on the crack would be pretty much the same as the tubing it will be fused to. You pretty much make the crack dissapear all together with a good weld. I guess it depends where the brazing is done on the frame and what type of stressed it will be subjected to, but I suspect that brazing can be adequate in most areas of the bike to repair minor cracks ans tears on the lugs or tubing.

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Old 09-03-12, 05:47 PM
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I had a post front end impact under the down tube crack develop,
my cure was saw out a section from a similar tube ,
clean it and the frame up well, flux, both surfaces ,
snap the patch gusset over the crack and sweat braze the piece
to draw the brass in between the 2 layers of steel.
it worked out well enough.

there was another layer of un-cracked metal over the crack when I was done..
the brass was the bond the steel was the structural support.

Last edited by fietsbob; 09-03-12 at 05:51 PM.
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Old 09-03-12, 06:29 PM
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crack is just above the weld matter of fact right along it where seat tube meets top tube.
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Old 09-03-12, 06:39 PM
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Originally Posted by sbskates View Post
crack is just above the weld matter of fact right along it where seat tube meets top tube.
Well, you're in luck. If there's anyplace you can do a repair this way, this is it.

Before you start, understand why it cracked. Your seatpost was a bit short (or raised to high) so saddle flex was working the tube where it exits above the top tube. The cantilevered section of seat tube above the top tube isn't nearly strong enough to take this kind of stress. The fix is to use a tight fitting seatpost long enough to end at least 1" below the bottom of the top tube, so the seat mast extension benefits from the added strength of the post.

Had you done this all along it shouldn't have cracked, so you can get by with a braze repair, but the post is now more critical than before.
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Old 09-03-12, 06:53 PM
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thanks
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Old 09-03-12, 09:11 PM
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Originally Posted by fietsbob View Post
the brass was the bond the steel was the structural support.
I think of solder/braze as analogous to glue.
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Old 09-03-12, 09:28 PM
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Originally Posted by Kimmo View Post
I think of solder/braze as analogous to glue.
You have to give braze some credit for structural strength. Filet brazing of steel tubes is a proven technique with some beautiful examples of lugless brazed construction in the bicycle and motorcycle world. In fact, when joining thinwall tubing filet brazing can make a stronger joint than welding, because welds have too abrupt a transition and tend to have stress risers causing the tubes to crack at the weld, while a well done braze filet makes a smooth transition with a smaller (or no) stress riser.

The problem with repairing a welded joint isn't hat braze is too weak, but that it's difficult to build a proper filet that transfers the stress through the joint properly.

In short, braze isn't like glue, but it isn't like steel either. Like most things mechanical it isn't a question of superior or worse properties, but proper use to capitalize on the various properties.
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Old 09-04-12, 03:47 AM
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I'll add my experience with motorcycle frames.

Brazing wont cut it for anyhting "structural." It will crack because as noted previously its tensile strength is low.

So welding is the next option. A good weld will hold together, BUT... it will crack in anpother spot not too far from the first one. It even does it if you do some annealing.

Note: This is from oxy/acetylene welding. Newer tig welding may be superior to the old gas welding and it may last forever.

The OP can probably have it welded, and if it cracked because of the seat post, and he gets a longer one it may last the life of the bike.

-SP
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Old 09-04-12, 04:01 AM
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Oxy-acetylene welding will result in poor results for bicycle frames. The chromoly and other high-strength heat-treated steels used in bicycles are easily stronger than anything used on motorcycles. And the gauges used are very, very thin, 0.3-0.4mm on some frames. You have to be careful not to heat up the tubing too much or else you'll destroy its temper and strength (the highest-strength tubing can go from +200kpsi to around 50kpsi when welded). These highest-strength steels can't even be brazed with brass, but only with silver-solder at lower temps to maintain strength. Brass has strength of 60-65kpsi and when thickness is double that of medium-strength tubing, it can handle the exact same loads that will break the tubing before the brazed joint fails. The right-angle joint at the seat/top-tube is also provides excellent bracing for a fillet.

The HAZ from welding with carbide precipitates and hydrogen embrittlement will be a common causes of cracks in thin tubing. That very well could be the cause of the cracks on the OP's frame. The added stress of mis-matched seatposts sizes & lengths may just be the final-straw.

In this case, we need to have additional data:

1. close-up picture of frame area with crack
2. history of seatpost sizing
3. picture from rear of top of seat-tube to verify pinching or no-pinching of tube
4. material of frame-tubing used
5. type of welding used, gas, MIG, TIG, etc. along with shielding-gas and filler-material used

Only with these pieces of data can an appropriate long-term and permanent solution be suggested.

Last edited by DannoXYZ; 09-04-12 at 04:07 AM.
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Old 09-04-12, 08:52 AM
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Originally Posted by Chombi View Post
Not quite sure if brazing is nearly as strong as welding a crack on a frame. I see brazing material as sort of a solder that can fill in cracks and gaps and bond fittings and lugs to the tubes, but I don't think it approaches the stregnth of actual welding in which the material deposited on the crack would be pretty much the same as the tubing it will be fused to.
Brazing is not the same thing as soldering, and a properly done brazed joint is as strong as the steel it joins. The problem with brazing a crack to repair it (as noted by others above) is that 1) you haven't addressed the root cause of the crack in the first place, and 2) it's difficult to ensure full penetration of the brazing material into the crack. Voids in the braze will be stress risers and eventually cause the crack to re-open.

If I understand the OP's situation, the crack has opened up above a welded joint. This suggests that the tube was overheated or undercut or otherwise damaged in the original welding process and no further welding or brazing in that area is going to correct that.
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Old 09-04-12, 12:38 PM
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Originally Posted by JohnDThompson View Post
it's difficult to ensure full penetration of the brazing material into the crack. Voids in the braze will be stress risers and eventually cause the crack to re-open.
Prolly why I thought of it like glue... one of these I made came apart like a poorly-applied patch off a tube.

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Old 09-04-12, 10:43 PM
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Wow, lots of stuff flying out in the wind of this thread.

There is no bad material, only bad design. Bubble gum could fix this crack, if enough could be crammed in to the space. The millions of bikes with brass brazing prove that this joining method is good enough if done properly. One of the reasons that brazing is so popular is that the fudge factor is larger then welding. That the frame was welded and there is a crack might suggest as much. Welding requires that the tubes get hot enough to melt and then cool down. This much heat has many effects on the steel (and other tube materials), as mentioned. If not done well the changed steel's metalurgical qualities might not be up to the task. Add a stress not designed for (too little post insertion) and no wonder the tube cracked near the weld.

As for the motorcycle comparison bike tubing (531 and 4130) has been used before. Not in bike diameters and gauges but with brazing. The problem with lump claims of failures is that one rarely gets the chance to examine the failed joint. How well was the joint designed, preped (mitered, cleaned), how much flow of filler, how much fillet, any finishing undercutting. Of course the higher stresses and vibrations of motorcycling makes the need for proper joints greater.

I still say that a patch of suffecient size, brazed over the drilled at each end crack, is the most likely repair that will have fewest after effects.
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Old 09-04-12, 11:03 PM
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Originally Posted by Andrew R Stewart View Post
Wow, lots of stuff flying out in the wind of this thread.
You're right, but unfortunately none is necessarily germane. Before debating what is the best way to fix it, it's worthwhile to consider why it cracked in the first place.

The OP says the crack is at the top of the top tube/seat tube joint. When all is good, this is possibly the least stressed joint on a frame, and if the crack is in the seat tube itself, it's a good clue as to the cause.

The typical steel seat tube is not strong enough to support the post or rider in cantilever. It requires that the post pass beyond the top tube joing carrying it's torque into the triangle. When that's the case, the seat post buttresses the seat tube through and beyond the top tube joint, and a failure of this kind could only occur if the welded area was over hardened and brittle, and couldn't flex with the post within (IMO, not likely).

In an earlier post, I mentioned this, and suggested that a properly sized (fit and length) seatpost would adequately support the joint, and just about any repair option would do. However, an improper seat post will lead to a repeat performance no matter how it's fixed, because regardless of how the joint is made the seat tube above still won't be strong enough.

We can debate filet braze, vs. weld till the cows come home, but unless the cause of the failure is addressed, repairing it is an exercise in futility.
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Old 09-05-12, 12:21 AM
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Thanks for adding to this thread. I've learnes some interesting bits of info.

But the OP might be hopelessly confused by now!

-SP
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Old 09-05-12, 07:24 AM
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From what I've read I know:

The crack is located along the fusion line of the weld joining the seat tube and the top tube, and is on the seat tube side.

Without more info it is hard to be certain what has happened with just that bit of info. If I had to make assumptions I would tend toward assuming that the crack is the result of fatigue, in which case FBinNY has a good theory. If the crack actually curves along the edge of the weld and there isn't any deformation associated with the crack than this is a sound assumption. Of course as others have pointed out, an unexpected metallurgical stress riser may have existed in the weld, but even given a severe stress riser there would have to be some cyclical stress. I like the seat post as a culprit for the cyclical stress. Unless the OP regularly stands on his top tube...

So sbskates should get a proper seat post, and stop standing on his top tube so that this doesn't happen again.
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Old 09-05-12, 02:35 PM
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For what its worth ,,, Sorry it was left out. I inherited this crack lets say that. My seat post is usually only 6 inches off the TT with tons of post in the tube. So too high a post will be no factor in the future.
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Old 09-05-12, 03:15 PM
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I'm familiar with that fix and can be done thats not a problem, have to agree with andrew and others anyways. Generally for fixes like that nobody will warranty the work because there is a design problem involved but probably the bike will live for a lot years w/o any problem.

Would be nice to see the crack you are talking about too.

As for the fix, be sure the guy that is brazing is familiar with the material and is good at fillet brazing because he needs to reinforce the whole area not only in the crack area, my old master builder had filled small cracks in lugged bikes w/o any problem and the only you see after the repair is dont is just a hairline of bronze, but the guy also had been brazing since he was 12 y/o so he knew what he was doing. Have seen him fixing cracked fillets aswell in the same area that the OP says and in those cases he just besides filling the crack he was going all around the area to add more filler to the whole joint to reinforce it. Have seen the guy fixing tubes cut in the middle using sleeves when there was no way to find tubes around like for example in the middle of a stage race for example. Same results, a hairline, sadly the guy died like 4 months ago and pretty much after like 65 years building bikes there was almost no repair impossible for him.

No idea what bike are you talking about either... you mention braze over weld?? you mean tig/mig weld??? Not idea if you can actually braze over a tig/mig weld, anybody knows? If you can't big chance the guy has to take all that ugliness out and fillet braze the joint back together.

In the case the fillets get like holes is because the guy had problems controlling the fillet braze temperature.

Good luck with the fix and post pictures.
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