View Full Version : Brazing . . . . . .
03-16-2004, 10:21 PM
OK boy's & girls....another stupid question from your favorite undertaker......
Am I right in assuming that brazing flux lowers the melting point of brazing rod ? . . . (kind of like the salt they put on the streets in Detroit "lowers" the melting point of water, and thus causes the ice to melt).
Well, i'm not trying to melt ice....i'm melting brazing rod.....and it seems to me that i can get a piece of steel cherry-red and touch the rod to it, and nothing much happens....BUT ! . . .when i dip the rod in some flux and then try....as soon as the flux get's onto the metal, it flows like good music !
I KNOW you need flux to do the job, I don't doubt that....BUT does the flux actually change the melting point of the rod ?
03-16-2004, 10:39 PM
Flux doesn't lower the melting point. It cleams the metal and protects it, most significantly of oxides.
Most materials won't transfer eat wel when oxidized, and won't wet when oxidized. The flux melts at a slightly lower temp than the brazing rod, cleans the metal and the rod, and lets the filer wet and fuse.
The braze filler is actually bonding to the base metal by forming a diffusion alloy at the surface, several atomic layers thick. Any contamination prevents this.
Note that you generally don;t want the base metal to be too hot. If the flux is matched to the filler, once the flux melts, you are near the melting temp. Overheating will degrade the filler alloy. Only want to be hot enough for the filler to flow. Also note that unlike electrical soldering, the heat to melt the filler doesn't all come from the base metal. You need to provide some direct heat to the filler. NOT melt it outright and dribble it in, as the base metal won't be hot enough for the braze metal to flow and fill, but enough it needs to be heated to near the melting point. This takes practice. Also, too hot mkes the filler hard to contol.
03-16-2004, 10:46 PM
03-16-2004, 11:11 PM
enlpck.....Thanks ! . . . .That bit about providing a little extra heat towards the filler rod.....I've done that, and have gotten better results with....but have always felt guilty about doing it 'cause i've always been admonished to: "LET THE WORK MELT THE ROD"
03-16-2004, 11:40 PM
I'v watched some really good guys actualy melt the filler, and then flow the glob, but the reading I'v done recommends against it, and I could never get good results that way.
Think about furness brazing, one of the best processes: the filer is heated with the base material, and both reach the filler liquidus temp at the same time. If the base metal melts the rod outright, then either a) it is cooled below liquidus, slushing the filler, or b) it was too hot to begin with.
Also: the best joints come about with a very slight gap between the parts for the filler to flow into by capiary action-depends on the filler but a few thou is typical. A large fillet doesn't add much strength, and may even weaken the joint. (can't find the references right now-- one of the recent AWS journals, I think, had an artical on the fillet issue and strength)
03-17-2004, 12:08 AM
Yes...I understand completely "how" brazing works. Fillets don't do any major work.....the idea is the "surface area" that is brazed...and a few thousandth's is very desirable in the joint.
I've been playing around with a little brazing in the workshop tonight, and I love it !
I'll tell you this: I believe that, if it's done correctly, it will have damn-near as much strength as a welded joint !
03-17-2004, 12:20 AM
ALSO...enlpck.....Do you know of any web-site's that you can buy All-State products on ?
03-17-2004, 03:08 AM
Try some 40-50 % silver solder. You can drill a 1/4" hole 3/8" deep in a twist drill. Put in a length of 1/4" music wire (spring steel) and you can make a long drill bit.
Silver solder also works good for making your own bandsaw blades. :)
03-17-2004, 08:53 AM
The strength can be phenominal. We braze and silver solder a lot of things in my shop, ranging from cable ends (don't have a swaging setup) to bearings, to ...
Don't know who distributes on the net-local ESAB distributor is our source.
03-17-2004, 05:02 PM
What kind of flux is used for brazing?
It wouldn't be the same as what you would use for soldering your plumbing - would it?
03-17-2004, 07:19 PM
Snidley....you use "Brazing Flux"....it's a white powder and comes in cans. You should be able to get it at the bigger hardware stores...and, of course, welding supply dealers.
03-17-2004, 07:43 PM
OR, you can swing by the grocery store, and buy a box of 20 Mule Team Borax.
Powder Brazing flux and the flux you use for faggot welding steel are both primarily BORAX.
NOW, quit buyin them overpriced coated brazing rods. Heat the brass rod, dip in BORAX, and go to it. On long passes, you often don't need to continue fluxing cause the flux travels ahead of the pool.
When brazing cast iron, make a paste with water, and brush it into the cast for best results.
03-18-2004, 10:15 AM
Note should be made that borax is a dandy flux for most applications with bronze rod on steel and CI, but there are other fluxes. It really depends on the application. The temperature the rod melts at has a lot to do with it (the flux must flow at a slightly lower temp and not cook down at the rod melting temp) the flux must be compatable with the filler and the base metal, etc.
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