Problems brazing steel: joint keeps breaking

[jscpm]

I have a man attempting to braze a steel joint and he is repeatedly failing at it. He has some experience welding but this is his first experience brazing.

The joint is two pieces of A36 low carbon steel that are cut from a flat 3/8" thick and 4" wide and are being butt joined at an angle. So, the surface being joined is a rectangle about 1/2" x 4".

The flux is a 1600-degree black brazing flux and brazing alloy is a 15% silver alloy with no cadmium.

He carefully cuts, grinds, polishes both sides of the braze and degreases it before applying the flux. I have seen the joint surfaces and they are absolutely clean and bright.

The torch is a medium duty oxy-propane torch. He has both a cutting head and a heating head, but I think he has mostly been using the cutting head and moving it steadily to try to avoid overheating.

What happens is that he makes the braze and it appears to be correct. But then when it cools down, he grabs both ends and can just break the braze by snapping it apart with his hands or by banging it on the welding table.

Any suggestions for how to help him figure out what he is doing wrong and get the braze working?

 

[Garwood]

Braze wets into the steel when done right. It's real obvious when it's going right vs when it's not when you have experience.

My gut says not enough heat. I use a propane weed burner to preheat stuff. I use oxy acetylene for brazing and cutting.

Propane is much colder than acetylene.

 

[DDoug]

LOOK at the break, with magnification.

 

[moonlight machine]

Why are you brazing? 1/2" thick and 4"wide. This just screams for Mig, Tig or stick.

 

[SD&M]

I have a man attempting to braze a steel joint and he is repeatedly failing at it. He has some experience welding but this is his first experience brazing.

The joint is two pieces of A36 low carbon steel that are cut from a flat 3/8" thick and 4" wide and are being butt joined at an angle. So, the surface being joined is a rectangle about 1/2" x 4".

The flux is a 1600-degree black brazing flux and brazing alloy is a 15% silver alloy with no cadmium.

He carefully cuts, grinds, polishes both sides of the braze and degreases it before applying the flux. I have seen the joint surfaces and they are absolutely clean and bright.

The torch is a medium duty oxy-propane torch. He has both a cutting head and a heating head, but I think he has mostly been using the cutting head and moving it steadily to try to avoid overheating.

What happens is that he makes the braze and it appears to be correct. But then when it cools down, he grabs both ends and can just break the braze by snapping it apart with his hands or by banging it on the welding table.

Any suggestions for how to help him figure out what he is doing wrong and get the braze working?

Hi jscpm:

Long, long ago, in a galaxy far, far away I used to build bike frames (mostly lugged steel road bikes) that were usually brass or silver brazed. Without seeing a picture of the failed joint I can only speculate, but what you describe sounds like what we used to call a "dry joint" or a "cold joint", where the filler metal doesn't fully penetrate and fill the small gap between the two parts being joined. If you post a picture, I bet we can figure out what's happening. Until you've got it down, it's easy to make a joint that is *covered* with filler metal, but that exhibits poor penetration.

We generally used the 45% silver alloy (with Cd), and a 56% silver (Cd-free) filler metal, and the brazing flux was usually a white paste. The brass alloys ("low fuming bronze") used a different flux that worked at the higher working temps required with brass. I don't remember ever seeing any black fluxes for silver brazing, but I may be mistaken. It's important to use the correct flux for the appropriate filler metal, and for an appropriate joint design.

It's also helpful to know how good the fit up of the joint is before brazing, as the strength of the finished joint is greatly affected by this. Some of the silver brazing alloys lose much of their strength as joint clearance exceeds 0.005", while the brass alloys are more tolerant of larger clearances.

(Just saw Moonlight's post above, and his comments are right on the money IMO. Is the brazing process a customer requirement, or is it just what he decided to try?)

 

[CarbideBob]

Very strange indeed. We hammer hard on brazed tips and failure should be the carbide breaks.
A good bip on the bench and it breaks?
Braze flow on the outside dos not count so maybe not getting inside if a tight fit?
Rod applied or sheet?
My work much smaller but rod is 100% useless. Never works well across a batch.
No matter what it does not get in the places it needs to.
Bob

 

[johansen]

I would try using the Harris white flux, not the high temperature black flux, and make sure that 15% silver braze is NOT the phosphorous type used for brazing copper refrigerant lines. The phosphorus makes a brittle intermetallic with iron. Not to mention it isn't self fluxing on steel, and I've tried to use it because I'm cheap and had no success.

 

[jwmelvin]

I would try using the Harris white flux, not the high temperature black flux,

I’m curious about this - is the white flux better as long as you aren’t going past it’s temperature range? I’ve had good success in steel with the black flux and 45%, but maybe because I’m inexperienced and tend to overheat?

 

[Big B]

Sounds like a cold solder joint only with bronze. He needs more heat.

 

[Ries]

Is there some technical reason you are brazing, or using flux and silver solder? For 40 years now, I have been succesfully brazing mild steel in sizes like this with dc tig and a silicon bronze filler rod. Fast, strong, virtually no cleanup. Much simpler, and it works.

 

[gbent]

We need pictures of the failed joint.

The surface should not be polished. Finished with a 36 grit snag grinder would be better. Oven cleaner is the easiest cleaner for brazing. A drop of water will not bead up on a properly cleaned surface. Cleaning with oven cleaner will leave the parts very susceptible to rust, so treat appropriately.

 

[johansen]

I’m curious about this - is the white flux better as long as you aren’t going past it’s temperature range? I’ve had good success in steel with the black flux and 45%, but maybe because I’m inexperienced and tend to overheat?

Never used the black to be honest, but the 56% silver with white flux is so easy to use on 316 stainless. I couldn't find my friends flux so I found a local store selling the powdered flux for oxy torch brazing. Could not make it work. Turns out it's active temperature is 1400F. Way too hot for the stainless steel mass spectrometer part I was trying to braze with propane.

Anyhow you don't have to heat the part much warmer than the 1250F the silver braze melts at and it flows immediately.

 

[cyanidekid]

vital to start with a clean joint. as said, some roughness will actually help, however I don't think that's the problem. my first thought was also a scarf joint would be better, but the failure sounds much too easy to even be improved by that.

for those who don't know, the black flux is generally better at higher temps. and on steel and steel alloys. the jar I have calls it "handy Flux Type B-1 for low temp brazing, working range 1100 to 1700F. for brazing steel, stainless steel , nickel high temp super alloys and tungsten and chromium carbides".

I'd say make sure its not a phosphorus rod as johanson says,... and why aren't they using a 50% silver alloy? if its a high volume part, it should be furnace brazed anyway, so is the 3 to 6 cents per joint really that important? yea, sounds like its just flowing around the joint, but I think you guys would see that failure, at least id hope so, so first thing, change to a decent alloy, then inspect under magnification.

P.S. I've got 40 oz. of cadmium free 50% silver wire I'd sell for market, which is about 1/3 what it goes for.
also 24oz of Cd 50Ni3 that's a cadmium containing brazing alloy designed for joining carbide to steel id also sell at market.

 

[scsmith42]

At a minimum you don't have enough heat. It is well known in the industry that oxy-propane is only suitable for soldering and cutting - not brazing. You really need the higher heat provide by oxy-acetylene or TIG.

What brazing rods are you using? Generally for steel, using O/A I will use a flux coated bronze filler, or a flux coated silicon aluminum bronze filler. With TIG, I'll use a bar rod and no flux.

 

[michiganbuck]

A little tab of silver can make a braze a lot better.. but ant as tough as a weld.

 

[fciron]

Without pictures of the broken joint we’re all just guessing and sharing stories of past failures.

If your welder has zero experience brazing he may be going at it wrong. In welding you put the filler rod into the flame and the puddle, in silver brazing you want to heat the part so that filler flows over the surfaces and into joint.

Both use a torch and filler rod, but the processes are much different.

 

[magneticanomaly]

I read a story once about persistent failure of a brazing process, that turned out to be caused by a fixture that pressed the joint too tightly together. Silver-solder joint requires a few thousandths of an inch of space for the braze to flow.

 

[implmex]

Hi All:
Another often overlooked cause of problems is running an oxidizing flame.
With oxyacetylene it's pretty easy to tell by looking at the flame, but I have no experience with oxypropane.

All the brazing I've ever done has resulted in the braze not flowing consistently if the joint is too cold and spitting and fuming if the joint it too hot, so one would think it's pretty obvious if the temperature is wildly wrong, but who knows.
I agree, without pictures of the fractured joint it's impossible to diagnose anything.

Cheers

Marcus
www.implant-mechanix.com
www.vancouverwireedm.com

 

[jtbain]

You want hard silver solder from jeweler supply or just make some with 4 parts sterling and one part 22cal brass. Melt it down and pour it in water.

Always white Flux for silver. Harris or handy Flux is fine. Any white brazing Flux. Good luck.

 

[jellywerker]

You can definitely braze (silver solder, like what's being discussed) with propane, just clarifying that before anyone reads much into what Scssmith said above.

You can fillet braze (bronze braze) with it happily as well, but need an oxyfuel setup.

Sure acetylene is hotter, but propane is fine. I do a little blocking with firebrick to keep the heat in better.

Nothing to add to the actual problem that others haven't already said. Sounds like too little heat, maybe too tight of clearance. Your pieces will basically be glowing red at proper temp.