Brazing advice/help

[jonbuoy]

Could anybody here give me some advice on brazing? I've cut and bent some steel to mount my seat plate onto. I have an oxy torch and silver and brass (I think) rods and powder flux. I've tried to shape the bar to match the frame although it needs a bit more work to get it 100%. Do you think this butt joint will be strong enough? - I'm having difficulty getting the capillary action that AWS website talks about on the test pieces I'm using. I can only get the brass stuff to flow when the metal is bright red - I'm a bit worried about stressing/warping the steel tubes if I do this on the bike. I could use the silver braize but I don't know about the strength. All my hammer tests are failing at the moment. I've tried a few sites and can't find a real dummies guide. Any advice appreciated.

[The Pastor]

the metal is suposed to be bright red, if im not mistaken.

[Motu]

If it's supported at the other end brazing will be strong enough.You have to get the metal into the red zone for the bronze to flow,if it's not hot enough it will glob....too hot and the zinc will burn out with a spluttering noise,and smell.What size rod are you useing? I prefer a thin 1/16th rod - heat it and dip in flux powder.You need to get both pieces of metal the same temp for it to flow over both.Thicker rods take too much to heat up.I have some 1/8th flux coated low temp rods that work on just a dull red heat.What material is that bar? Looks like stainless to me....you can braze stainless,but it's keen on rejecting all attempts.

I'd MIG it.

[jonbuoy]

Thanks Motu, brass ones are 1.5mm diameter I have been dipping them in the flux the bars plain steel and would have the same joint at the other end, MIG would be nice but I don't have one. I've been borrowing a stick welder and can do a half decent bead now but it all turns to shit when I try to stick two bits together - especially at 90 angle. I don't know how thick the frame tube is either bit paranoid about putting a hole in it.

[Ixion]

I agree with Mr Motu about thin rods for brazing . I almost always use 1/16". Brazing's quite a precision thing you don't want huge amounts of metal around. You'll definately need a local bright red heat, though the area involved can be very small. But, you'll get the frame at least that hot locally with any method - brazing oxy arc or mig. After all anything other than brazing has to actually melt the frame metal - so that's a lot hotter than red heat.

Is the frame heat treated? Do you have a specific reason for worrying about stress or distortion? If so, there are techniques to minimise them, or after treat for them.

On bike frames where the metal is usually old, I sometimes like to "tin" the surface with bronze first just to make sure that I'm going to get a good adhesion. Brazing can actually be stronger than welding (though probably not in this case).

[jonbuoy]

Thanks guys, I got it, went down a tip size and I can keep a smaller area red hot now I put some powder between the two joints before I started heating this time and a bit of weight on it and away she went - hammer test passed. Its not too purdy but I'll keep at it.

[Ixion]

Good stuff. I like as small a tip as I can use for brazing (most of the time a #8) and a hot flame. Keep the red hot area small, only a few rod widths. If you're brazing a large mass like a bike frame, it's a good idea to preheat the wider general area with a large soft flame to just under red heat. Than a small hot flame heating just a small area to red heat, let the nronze flow in under its own impetus an move quickly on.If I have to fil gaps with the bronze, I'll join the touching surfaces first without woryying about the gaps, then come back and gap fill (which adds nothing to the strnegth of the joint)

[The Pastor]

p.s learn to take some pictures that arnt blurry! (try using a "macro" button, or don't put the camera so close).

[Motu]

I normaly use a tip a size bigger than I would use welding the same material.I get in there quick with a lot of heat,this minimises heat damage to other parts.Both pieces need to be the same heat...even though the frame material is thinner it will act as a better heat sink....then again it may heat up too quickly.

[stanko]

when you are trying to get the flux onto the job, you can mix up a bit of flux with some spit (or water) and make a paste to spread around the joint, when you heat it it does the flux job so you dont need to concentrate so hard on dipping the hot rod in th flux and transferring it to the job.

[Ixion]

Yeah, though it stinks if you use spit!

Having all the joint surfaces really clean makes a big difference in brazing.