Carbide brazing with Mapp/Air torch?

Hello,

I need to braze a 1/2" diameter, 1/8" thick tungsten carbide disk (RNG42 - C2 or C5) to the end of a 1/2" diameter 3" long brass bar.

My local supplier can provide me with brazing ribbon, 0.005" thick, composed of 50%Ag, 20%Cu, 28%Zn, 2%N, to join the two materials (using a silver solder flux paste).

I am doing this for a hobby application, and only need to do a few of them every once in awhile.

My question is whether or not I would be able to do this with the following Mapp/Air torch:

I already have one of the Oxy/Mapp torches from Bernzomatic, but with the high cost of the oxygen cylinders I am hoping to be able to use the Mapp/Air torch instead.

I realize that the Mapp/Air torch may not be the "best" choice, but will it be reasonably practical for hobby purposes?

Thanks for any suggestions,

Jon

I have brazed carbide with a handheld propane torch. With a three inch long brass rod you are going to need to do something to keep the heat from escaping. Maybe burying most of the brass in some vermiculite or something similar. You might need to use two propane torches to get enough heat. Propane is hot enough, Mapp is probably better to get things hot in a hurry.

Dan

Thank you, Oliver (and Dan also). I had planned on holding the brass bar vertically in a vise, but I think now I'll drill a hole in a brick and pack some vermiculite around the brass bar so the heat doesn't bleed off so fast. I hadn't considered this before.

BTW, the only brazing ribbon I can get locally has a liquidus of 1305 degrees F; is this too close to the melting point of Brass (1600 degrees F)?

Jon

I would use something like a tin can full of vermiculite. The part should be somewhat above the bottom of the can so the bottom of the part is insulated also.

Dan

"Jon Danniken" wrote: (clip) but I think now I'll drill a hole in a brick and pack some vermiculite around the brass bar so the heat doesn't bleed off so fast.(clip) ^^^^^^^^^^^^^^^^^ Insulating the brass is a good idea, but I would leave an inch or so of the rod standing exposed ABOVE the brick. For what you're doing, it is better to apply the heat to the brass rod, so the whole joint gets hot. If you apply the flame from above, you will get the carbide really hot way ahead of the brass--not good.

I have used a product called "Heat Stop" when brazing brass boat shafts, gun parts etc. It is a heat barrier paste the stops heat travel. It can be used to shield parts you want to remain cool or to contain heat for parts you want to remain hot. Its also used by Rescue crews to protect people when thy use a torch to cut them out of wrecks. Small quantities can be ordered from

Thanks, Leo. Yes, that is what I had planned on doing, I should have clarified a bit more.

This it brings up another point that I have about the brazing process itself (I've never brazed - only soldered together circuits). Since I'm using a ribbon type of brazing ribbon, should I just make a "sandwich" consisting of:

brass rod - flux - ribbon alloy - flux - carbide

and then heat the assembly, trying to bring the temperature of the brass rod up at the same rate as that of the carbide? In other words, should I assemble this "sandwich" together cold, then apply head until the alloy melts? I figure I'll need one hand for the torch and another hand to hold the carbide in place, so I'm somewhat limited in my mobility.

Thanks again for any further suggestions,

Jon

what I would do would be to first heat the brass, apply the flux and the brazing alloy to get a nice even coating of the alloy on the end of the rod, then do the same with the carbide disc THEN put the peces together (with the coated sides facing obviously) and heat until the brazed sides melt together. this way you can be shore that you have a good metalic bond, and the flux is doing its job. if you just assemble a sandwidch cold and heat it up, you can't be shour its not just stuck with flux until its too late.

the brass will probably be alot easter to get a good bond to than the tungsten carbide.

Oliver :)

Thanks for that, Oliver. I had read on the carbideprocessors website that pre-tinning the carbide is a good idea, and I guess if I'm doing that I might as well pre-tin the brass as well. If both pieces are pre-tinned, would I need to still apply flux to the pre-tinned pieces prior to final heating?

Jon

"Jon Danniken" wrote: (clip) If both pieces are pre-tinned, would I need to still apply flux to the pre-tinned pieces prior to final heating? ^^^^^^^^^^^^^^^ You could probably succeed without it, but I would wipe on a thin film anyway. The surfaces will be inside a "sandwich," where you can't see them. If you got a little oxidation, you wouldn't see it. (Belt and suspenders.)

I think we would start by getting a firebrick (from a ceramics hobby store) and dig a hole to fit the final part. I would suggest you touch the side of the brass and the side of the carbide to be brazed to a bench grinder wheel until you get a nice, bright surface.

Drop the brass trod down the hole in the fire brick. Apply Black flux. Put the brazing ribbon on and apply flux. Then put the carbide on.

I agree with Leo Lichtman but we would also look at it another way as well. Typically metals expand much faster under heat than carbide. If you apply the heat through the carbide then you can help compensate for the differences in the coefficients of expansion. Also if you get the carbide hot enough then the heat will readily pass though the braze alloy and the brass. I think we would try it that way first. However we also do many parts the way Leo Lichtman describes because often that is a better way.

With a part this large we would use 0.015" braze alloy trimetal. You might try a couple layers of the ribbon you have.

Tom