Solving Brazing Problems

We often get a lot of questions about brazing problems and why a braze joint failed.  Here are a few inquiries that were posted on a forum about brazing problems and some general questions about brazing.

“1 We have often used brazing materials for tungsten carbide brazing to steel that are basically just brass, sometimes with some nickel added.  These materials have performed well in field and we have not had a high percentage of items drop off.  The items we have brazed are for earth moving applications such as plough tips, mining drill bits.   Can you please tell me why all the videos say to use brazing consumables that are around 50% Ag and never mention brass type consumables?

2 In relation to your video on tungsten carbide quality, I would really be interested in tests that you have for some easier checks of tungsten carbide quality and what is good and what not.

3 I see some people use vegetable oil to mix their flux into, and paint it onto the items to be brazed. What do you normally use? Is there any problem with using vegetable oil?”

Excellent questions…

 1.  I talk about a high silver braze alloy because that is what I use.

 Most of my work is tool brazing such as saws, router bits, drill bits etc.

We have two issues that you do not. Basically we are brazing relatively small, thin pieces of tungsten carbide to narrow pieces of steel.  We use the high silver braze alloy because it holds very well and flows at a much lower temperature. We are very concerned about embrittlement of the steel if it gets too hot.   We are also very concerned about thermal stress cracking the carbide.  Because your operation uses a much larger pieces of carbide with a more robust shape and because you braze to massive pieces of steel you can very successfully use brass braze alloys. In both cases the nickel adds strength and encourages wetting.

2.  If you’re interested in a test for carbide wetting, you can see that on our website or in the brazing videos.   As a test for carbide quality I think I would first go to porosity.  You should be able to get certifications from your supplier showing how much A, B and C porosity you have.  This will give you some idea of how well the carbide is made. 

Unfortunately I have never found a simple, easy way to test carbide. Our solution is to be extremely careful about where we buy. We also watch our suppliers like a hawk.

 In 30 years in the business I have seen companies cycle back and forth from very bad to very good and so on.  

We do have a drop weight impact tester that helps determine how good carbide is.   We will also braze a piece of carbide onto a steel arm. We clamp the carbide and provide force to the arm until the carbide breaks.   It is not uncommon to see C-2 carbide from three different suppliers perform in such a manner that one has several times the strength as another.

We thin our brazing flux with water.  Vegetable oil would have the advantage in that it would not evaporate the way water does.  However vegetable oil is carbon-based and I do not like to introduce carbon into a braze joint.  Carbon does not wet at all.  Introducing a carbon containing material into a braze joint is going to make a weaker braze joint.

Having said all that I don’t think it makes that much difference for your application, but for our application where we sometimes braze saws thinner than a dime, I would not like to take a chance.

 

Sincerely,

Tom Walz

 

Brazing Failure Example:

 

 Taken from a forum post: “So, you know how I always say don’t rough with carbide bars… I didn’t feel like switching bars for 1 part and being a shallow bore I knew I wouldn’t hit the bottom and snap the bar.
Problem is being C-276, it got a bit overloaded about 80% done roughing and the head just came off at the solder joint, 3/4″ bar. I’m thinking all the heat it was starting to soak in might have helped.

Anyway, are these fixable? Looks like a copper based solder but does anyone have any idea which type is used and flux?
I’m also wondering if they’re induction heated to weld them in production, so I’m not sure if oxy-ace would do it.
Thankfully its a fairly cheap bar, wouldn’t wanna do that with the 1″ sandvik.

Thanks for any input on this.”

 

Tom’s Solutions:

I just took another look at it and there was no solder at all on the top half other than a little blob at the top surface. The bottom half split right in the middle of the solder.  Kinda surprised it lasted this long now.

It looks like copper, probably with a lot of zinc to suppress the temperature.  They used probably a fraction of a penny’s worth of material.

 

What I would recommend is:

The braze alloy I recommend is about half silver.  Silver today is $32 aTroyounce. So using the correct amount of a good braze alloy would add may be a nickel or so to the cost of making the part.

I would recommend using a  49% or a 50% silver braze alloy. The 56% silver braze alloy is much more common but it is also much weaker. The 49% is the strongest but not readily available. The 50% silver is the most common tool alloy.

I also recommend using a black or white brazing flux. I would suggest black as it is much more forgiving.

 

Method 1

1.  Take a wire brush, a small file or travel and clean out both sides of the joint.  Ideally you would want clean, gray metal on both sides. (Tungsten carbide is mostly ceramic but it looks like gray metal when it’s clean and ground.)   Do not change the shape of either part.  If you can’t grind down to bare metal than just clean it up well. All you really need to do is remove the oxide layer from the last braze alloy.

2.  If you PM me I will send you a small piece of braze alloy ribbon. It is 50% silver braze alloy which should work very well.

3.  Cut a small piece so that it is slightly oversize on the joint. You can cut the material with ordinary desk scissors. Then bend the material in a V-shape.   You will probably have a larger radius in the braze material than you do in the notch. That is all right.

4.  I would then dip the piece of braze alloy into the flux so that it gets a pretty good coating on both sides of the braze alloy. 

5.  Put the fluxed braze alloy V-shape into the steel notch and then insert the carbide part into the V.

6.  They will not fit together very well.  As the parts heat up the braze alloy will soften and the parts will fit together.

7.  You probably need to heat the parts up to about 1400 or 1500 F.  This is a little higher than the scientific flow point of the braze alloy but it will give you enough heat and enough time so that the whole joint comes up to temperature.

 Dull red or dark cherry red are the way the proper temperature is usually described.  Because of the lighting in our shop the color looks more like a fairly bright red. Ideally you would heat the part so that the braze alloy started to flow out from the joint.

8.  Remove the heat slowly and let the part air cool to room temperature. Do not move the part in any way, at any time while it is cooling.  I do not know why but I know that quick cooling the parts in water from even 200 or 300° F will cause them to fail.

 

Method 2

It may be easier to tin (or pretin) one part or the other.

In this case you will follow the procedure above that you will melt the braze alloy onto one of the parts first.

You let the pretinned part cool then you flux the surface of the other part, put the two parts together and reheat. 

The reflow temperature of the braze alloy will be about 50° F higher than the original melting point but you will still be well within the same temperature range.

Method two is about the same as method one except you only have two parts to deal with instead of three which may make your fixturing easier.

 

Fixturing

I would not suggest using fixturing that involves holding one of the parts.  The first thought that comes to mind is to get a fire brick or similar, cut a trough in it and use that to fixture the parts.

It is very important that the parts not be moved as they are cooling.

Likely causes of failure

  1. Part cleanliness – after you grind, brush or file the parts clean make sure that they are free of all greases and oils.
  2. Under fluxing – people are much more likely to use too little flux than too much flux.  For an operation the size I would suggest using somewhere around a teaspoon.  Flux is primarily an oxygen interceptor.  You have a nice, closed joint so it is going to be hard for oxygen to get into it anyway.
  3. Under heating – again the recommended braze temperature for this braze alloy is up to 1500 F. The official liquidus point is 1350 F. 1350 F looks like a dull red on the charts. I would recommend 1500 F which is going to be a pretty bright red. When I braze something I really want to hold I like to see the braze alloy flow out of the joint and over both the steel and the carbide a little bit.

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