Induction Brazing, Carbide and Steel Response Rates

Induction Brazing, Carbide and Steel Response Rates

Got a call from an old friend who was having problems with a saw blade. The steel shoulders on the saw blade were ripping off as soon as the customer used them. In one case the steel fractured in shipment.

They had closed the plant and consolidated in a new plant. He was in charge of the new plant including equipment that had been shipped there from other plants.

The problem they were having was that the induction coils were set the heat the steel more so than the carbide. This meant that they had a heat affected zone way too far back into the steel. Ideally the heat affected zone on the steel should be no more than a quarter inch or so. This means that you should not see any discoloration of the steel, after brazing more than a quarter inch onto the steel as measured from the back of the saw tip. The way they were doing it the whole shoulder was discolored.

You really need to direct the heat into the carbide, from the carbide into the braze alloy and then into the steel.

Induction heating works by generating a magnetic field in the induction coil. The induction field just sits there and the material does all the work. As a comparison, when you use an oxy – acetylene torch you are putting about 5,000 or 6,000°F temperature into the material. The torch does all the work. The torch generates the heat. With induction heating the coil generates a magnetic field but does not generate any heat. The heat comes from the material inside the coil. When you put a piece of steel inside and induction coil the molecules in the steel are excited by the magnetic field and excited molecules are hot molecules. So the heat is actually generated by the way the steel, or other material, responds to the magnetic field.

Steel is just about all iron and most steels respond extremely well to a magnetic field. Tungsten carbide is maybe five or 10% cobalt. It is the cobalt binder in the tungsten carbide response to the magnetic field.

If you have two parts, one steel and one carbide, of equal mass the steel is going to respond much better to the magnetic field than the carbide will.

Figure the steel is about 90% iron and the carbide is 10% cobalt. So the steel will have somewhere around 9 times the material to respond to the magnetic field that the carbide will. This is one of the reasons that the steel heats up so much faster than the carbide.

This whole area of induction heating and magnetism is incredibly complex. There is a huge number of factors that can affect how a given material responds to a magnetic field.  When I have explained above is an extremely simplified version but it is, nevertheless, extremely important to remember when using an induction brazing operation to braze carbide to steel.

We did a video on this and it’s pretty good. It definitely shows the steel heating up a lot sooner and getting a lot hotter than the carbide.

 

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