Brazing

[lock down]

Hope I spelled it right.......Anyway I have a Triump motorcycle & the part where the kick stand mounts snapped off. Now, I looked at the piece and it looked like it was cast metal,but I tried putting a magnet to it & it stuck...so I would think it would be OK to mig it.
I was told that it HAD TO BE brazed........Can some one explain what that is,& why is it better than mig welding???
Thanks in advance.......

[MAC702]

Just because it's cast, doesn't tell you everything. Cast what? Iron, steel, stainless? Many stainless grades are magnetic, after all.

WHO told you it had to be brazed?

[Rich59]

Long ago when lots of welding was done with oxy/ace brazing was a common method of joining motorcycle and bicycle frames. Actually I have seen an old airplane frame that was brazed as well.

The brazing gave acceptable strength in the joints and was supposed to be easier to master than welding. Since the braze depends on capillary action the metal did not have to be heated to form a puddle. This was an advantage on light tube type frames.

The other advantage for brazing is that you can join dissimilar metals. Generally that is not possible with welding.

Don't know if any of this helps but it is what I can recall being told along the way.

[Tailshaft56]

I would clean it up and try your mig. worst case it won't work but you should be able to tell right away. If it doesn't you can always grind away your mistake. Be sure to run several short beads to minimize any warping.

[Rockski]

brazing is generally joining 2 metals (same kind or different) with capiliary reation, you dont need to melt the metal into a puddle but the brass will melt and flow to wherever your flux and you ground down to expose the grain

somebody correct me on this but isnt it a carbonizing flame you want when you braze with brass?

[gnm109]

You would be far better off doing a weld on it. Either TIG, MIG or Stick in no particular order. While it's true that many motorcycle frames, in particular Britsh ones, were brazed together, this had mainly to do with new construction. The frame joints were swaged, pressed and pinned together and then sweat brazed to hold them.

In your case, you have a broken lug that was originally one piece. It won't be strong enough to hold the bike if its merely brazed. There's a lot of weight on a sidestand on a bike. Brass will definitely break. A good weld will hold it permanently.

There's nothing set in stone about brazing on motorcycles, expecially with regard to repairing them. I've welded many a motorcycle part and mostly use TIG, although now that I have a MIG it would be a good choice for me as well. Even stick could do a nice job on something like that.



Good luck.

[elvergon]

Just adding...the most important feature of brazing is that you won´t put as much heat in the metal you´re welding reducing HAZ and most of the times eliminating heat treatment.

[gnm109]

Originally Posted by elvergon

Just adding...the most important feature of brazing is that you won´t put as much heat in the metal you´re welding reducing HAZ and most of the times eliminating heat treatment.

This is the best argument for doing a nice clean weld rather than brazing. Brazing will put a great deal of excess heat into that HAZ on the frame tube. This is generally a bad idea.

[76GMC1500]

Brazing is stronger, too. Use a silver braze or nickel-silver braze. Joint strengths are on the magniture of 80ksi to 120ksi. A welded joint is usually 60-70ksi. Brass brazed joints give a strength of 30-45ksi and I wouldn't recommend it on something like a kick starter. It seems like you don't know the base metal you are trying to join. With this kind of uncertainty, brazing is an excellent choice because it can join a wide range of metals, even dissimilar. It works very well for joining cast metals as well. If you were to weld it, you would have to select the proper filler rod. 45% silver brazing alloy is the most commonly used for joining cast metals. 50N is used in in high stress, high elongation applications like joining the ends of bandsaw blades.


Brazing does put more heat into the metal because it takes a long time to preheat and work the brazing metal. But, if you're good, you can work below the temeratures at which the physical properties of the metals begin to change. For steel, you can work below the eutectoid temperature and for stainless steel, the sensitized range. Using softer alloys like brass allow you to work at lower temperatures still, reducing shrinkage in autobody applications.

[pulser]

Originally Posted by Rockski

somebody correct me on this but isnt it a carbonizing flame you want when you braze with brass?

Just wanted to mention that Rockski makes of good point here, one that many people may not have ever learned. It is best to use a slightly carburizing flame for brazing. An oxidizing flame, will oxidize, duh.

[Rich59]

It occurs to me that the reason you may have been told you must braze the part is concern for other brazed joints. If the heat effected zone encompassed other brazed connections you could "burn them out" .

I know that if you are going to weld and braze in close proximity you always do the welds first, let it cool and then braze.

[elvergon]

Originally Posted by gnm109

This is the best argument for doing a nice clean weld rather than brazing. Brazing will put a great deal of excess heat into that HAZ on the frame tube. This is generally a bad idea.

whoa whoa whoa....I´ve read on other posts that brazing is better cause it won´t put as much heat in the metal when welding...if I remember correctly you can braze with 800 ºC which is lower than the around 3000 ºC from oxy-acetilene or other welding process. I recall this from 1 book and www.offroadfabnet.com forums. SO is this wrong?

[MAC702]

Originally Posted by elvergon

whoa whoa whoa....I´ve read on other posts that brazing is better cause it won´t put as much heat in the metal when welding...if I remember correctly you can braze with 800 ºC which is lower than the around 3000 ºC from oxy-acetilene or other welding process. I recall this from 1 book and www.offroadfabnet.com forums. SO is this wrong?

The definition of brazing means not melting the weldment and using a filler that melts above 800 degrees, to differentiate it from soldering which uses a filler that melts below 800 degrees.

To melt the filler would require putting MORE heat into the weldment than is required to melt the filler, however.

But I do very little brazing, and none of it on things where heat input matters, so I can't address that part of the issue.

[lock down]

Originally Posted by MAC702

Just because it's cast, doesn't tell you everything. Cast what? Iron, steel, stainless? Many stainless grades are magnetic, after all.

WHO told you it had to be brazed?

I think it's steel.........A bike builder who specializes in Triumps told me that the tab was brazed from the factory & MUST be brazed again.....

[lock down]

Originally Posted by Rich59

Long ago when lots of welding was done with oxy/ace brazing was a common method of joining motorcycle and bicycle frames. Actually I have seen an old airplane frame that was brazed as well.

The brazing gave acceptable strength in the joints and was supposed to be easier to master than welding. Since the braze depends on capillary action the metal did not have to be heated to form a puddle. This was an advantage on light tube type frames.

The other advantage for brazing is that you can join dissimilar metals. Generally that is not possible with welding.

Don't know if any of this helps but it is what I can recall being told along the way.

Thanks for the info.........I guess it makes sense since the kick stand tab must be strong to hold up a motorcycle......

[lock down]

Originally Posted by Tailshaft56

I would clean it up and try your mig. worst case it won't work but you should be able to tell right away. If it doesn't you can always grind away your mistake. Be sure to run several short beads to minimize any warping.

That's exactly what I did......seems to be fine...........I just don't want to come out one evening and find my bike down one one side......

[lock down]

Alot of useful info on brazing. I want to thank all who chimed in. I decided to mig it.......and so far it's OK.........just a little nervous now........Just have to remember not to try to kick start the bike with the weight on the stand...

[ulav8r]

About heat input, there seems to be a lot of misunderstanding. Brazing occurs at about 800-1200 degrees F, depending on the filler used. It will take a while to bring the parts being joined to this temp and the flame is usually moved around to bring the whole part up to temp. If the part was originally heat treated it may be adversly affected, depending on the original heat treatment and the filler being used. The final heat treatment to temper the part could have been at 300-1000 degrees depending on the alloy being used and the desired hardness/strength in the part. If the original tempering was done at 1000 and the braze was done at 800, brazing might not affect the strength of the part.

Welding occurs at a much higher temperature, as you all know. However, if the part was tempered in such a way that the break is in a soft area and the hardened section is protected during welding, it would be possible to weld the break without weakening the part. It is quite possible to tig weld a small break in a large part and be able to handle the part directly after if the weld is at a distance from where it is handled. If the same part is brazed, it would be possible to have the whole part at 500 degrees or more by the time the braze is completed because the torch is less efficient at transferring heat to the part.

[gnm109]

Originally Posted by ulav8r

About heat input, there seems to be a lot of misunderstanding. Brazing occurs at about 800-1200 degrees F, depending on the filler used. It will take a while to bring the parts being joined to this temp and the flame is usually moved around to bring the whole part up to temp. If the part was originally heat treated it may be adversly affected, depending on the original heat treatment and the filler being used. The final heat treatment to temper the part could have been at 300-1000 degrees depending on the alloy being used and the desired hardness/strength in the part. If the original tempering was done at 1000 and the braze was done at 800, brazing might not affect the strength of the part.

Welding occurs at a much higher temperature, as you all know. However, if the part was tempered in such a way that the break is in a soft area and the hardened section is protected during welding, it would be possible to weld the break without weakening the part. It is quite possible to tig weld a small break in a large part and be able to handle the part directly after if the weld is at a distance from where it is handled. If the same part is brazed, it would be possible to have the whole part at 500 degrees or more by the time the braze is completed because the torch is less efficient at transferring heat to the part.

Good information for sure. The heat would certainly be an issue on a motorcycle frame, not to mention the other nearby components that could be overheated.

Anything except brass would be my choice since brass really is not strong enough to hold up a 400 pound motorcycle on an angle. As a long-time motorcyclist and having broken many a frame section in my day, I would shy away from brazing where a big load is centered.

MIG, TIG or Stick would be quick and would minimize collateral damage from heat with a resulting weld that would last permanently.

[IH 392]

What type of different brazing rods are there? I know that brass, and nickel/silver are used, but are there other types of rods that are used? and in what circumstances would they be used..dissimilar metals?
- figured i'd ask while we are on the subject

[PaulG]

lock down,

If it's been brazed before, then that's why your being told to braze it again. Generally a brazed joint cannot be welded if it has already been brazed unless you remove all brazing material, which in many cases is difficult.