Solving Brazing Problems- Part 3
In our final blog in our series Solving Brazing Problems we will discuss how improper brazing temperatures and gas entrapment can affect the braze joint. We have already gone over how switching to a different type of brazing flux, or changing the type of braze alloy you use can solve a lot of brazing problems in the last blog solving brazing problems part 2, and discussed the importance of cleaning the steel plate and carbide before brazing in Part of Solving brazing problems. Please feel free to refer back to these or get more information on how to braze in the brazing section on our website or in our book Braze Failure Analysis.
Brazing carbide at the wrong temperature:
It is possible to weaken the braze joint by under heating the silver solder or braze alloy. There is often a temptation to under heat the braze joint in an effort to protect the steel saw plate. A way to avoid this is to heat the braze alloy through the carbide. This will allow you to get the braze joint to the proper temperature, without overheating the steel saw plate. If you are getting good shoulders or fillets and some feathering, then you are probably not under heating the braze joint.
It is also possible to go in the opposite direction and over heat the braze alloy. Cadmium and Zinc both have low boiling points. Zinc boils at 1664F and Cadmium boils at 1409. Braze alloy with Cadmium should be brazed between temperatures of 1170 and 1270. That is not too far below the 1409 boiling point of cadmium. Cadmium free braze alloy should be brazed between temperatures of 1250 and 1305F which is not too far below the zinc boiling point. If the brazing temperature is too high, and the zinc or Cadmium in the braze alloy begins to boil then it changes the chemistry of the braze alloy and adversely affects the braze joint. If tip loss occurs, you can examine the sides of the braze joint or the surface where the tip was and may see gas bubbles. If you do see gas bubbles then the wrong brazing temperature may have led to the brazing problem.
Colors in the Solder
The silver solder should be kind of a rich gold color. If there is any kind of another color it is a sign that something is wrong. The different colors all mean something. Blue-green is a sign that the tungsten carbide is being heated enough to bring the Cobalt out. Dark pink comes from overheating both the tungsten carbide and the flux. Copper from the solder and Carbon from the tungsten carbide makes Copper Carbonate which is either yellow or dark green copper and Tungsten is light green. Severely burnt flux can yield an orange solid. In any case if there is any evidence of another color at all it means the braze alloy was not treated properly.
Overheated Tungsten Carbide
If the heat is applied entirely through the tungsten carbide it can cause overheating of the tungsten carbide. This is sometimes done to protect the steel plate from overheating. The plate, braze alloy and tungsten carbide should be brought up to temperature together and then the torch should be drawn away from the joint over the tungsten carbide. A slow draw has proven to be best. A slow draw is one where the tungsten carbide is heated for an extra quarter to half second or so. This apparently gives the tungsten carbide and the whole joint a little more time to adjust to each other.
A Combination of Things
Sometimes a brazing problem cannot be traced to a single cause. In this case the best thing to do is to examine the whole operation. You should make sure everything is working right and being done correctly. Often this will cause the problems to disappear without identifying a specific cause.
Brazing is a complex process. Most problems are a bit of this and a bit of that. You can make a 5 % difference five times and you get a 25% improvement. It is a lot easier to make lots of little, simple, easy changes than it is to try to make one, big change that will solve all your problems.
Here is a partial list of things to check if you are having trouble with a brazing operation.
Cleanliness of the plate
Cleanliness of the tip
Flux clean and stirred
Flux on the sides of the plate.
Lots of flux inside the joint
Acetylene
Oxygen
Regulators
Hoses
Handles
Handle controls
Saw Tips
Tightness of joints
Temperature of anvils
Temperature of the shop
New furnaces
New fans
New locations
New doors
New brazer
Proper pretinning
Right kind of solder
Temperature of the tips
Kind of flux
Condition of flux
Amount of flux used
Flux on plate in notch
Flux in contact and protecting the tips
Brazing temperature
Brazing time
Color of braze joint
Sound of braze joint
Feel of braze joint
Where is heat being applied?
How is heat being applied?
Does everybody have the same problems?
Does the problem occur at a certain time of day?
Does the problem occur with a certain kind of plate?
Does the tip manufacturer have the same problem?
Does anyone else have the problem?
What tips failed?
You may never know the answer. Quite often the problem goes away while you are looking for it. When you tune up a car you can’t point to one thing and say this made the car run better and everything else was a waste of time and money.
For more tips on brazing or for Braze failure analysis visit our Brazing section.
Tags: Braze alloy, braze joint, brazing problems
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