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How to Braze

June 13, 2012 | Comments: 0 | Views: 218

For successful brazing the quality of the materials used is very important and can have a significant effect on reducing the amount of tip loss and breakage experienced. One of the most essential things to successful brazing is choosing a quality saw plate. A steel saw plate is laser cut. The laser cutting causes a heat affected and burnt and oxidized edge on the saw plate. This edge must be ground back to clean steel. Probably the safest figure to use is 0.010". If the edge on the saw plate is not ground back, the tips may still stick to the burnt steel but it will be a very weak bond.

Steel saw plate typically comes with some sort of a protectant to prevent rust. This must be removed from the saw tip seat as well as the adjoining shoulders. The best way to do this is with some sort of a caustic solution (sodium hydroxide, NaOH). A strong caustic solution breaks the oils and greases down into soaps which are easily removed. A solvent will dilute the oils and greases but will not remove them completely. Either method for cleaning saw plate can work successfully, but the caustic method usually delivers better results.

Broken and ripped shoulders are two separate things.

What is a broken shoulder? During brazing it is possible to get the steel saw plate so hot that a change in structure takes place. This change in structure can cause a change of hardness of as much as 20 points Rockwell C. This makes the saw plate very hard and very brittle. If the saw plate is not correctly tempered back to the correct Rockwell and the correct toughness then it can snap during use. This is typically a very clean break.

There have been instances, with an improperly maintained automatic brazer, where the heat affected zone continued into the saw plate as much as an inch below the bottom of the gullets. In one case a customer returned a saw blade that had a whole section missing. A section of five tips, gullets and all, had come off as a single piece to a depth of about an inch below the bottom of the gullets.

Ripped shoulders are different than snapped or broken shoulders. With ripped shoulders the steel is tempered correctly but the saw tip gets hit so hard that it actually pulls the steel apart. These are typically not clean breaks. They will often leave deformed steel.

Another essential part of the brazing process is choosing a good brazing flux:

There are three types of brazing flux: White Flux, Black Flux And Purified Black Flux.

White Flux is potassium salts of boron and fluorine. It has some cleaning effect to remove oils and greases. It has some ability to remove existing oxides. Do not count on either one of those. Start with a clean saw plate that is oxide free. Flux, any flux, is designed to keep oxygen away from the braze alloy so that the braze alloy melts and flows instead of burning up.

Black Flux is white flux with extra boron added. Black flux is clearly superior to white flux for brazing carbide saw tips to saws. The simple switch from white flux to black flux can prevent or entirely eliminate tip loss and breakage.

Purified Black Flux is exactly what the name says. It is black flux that has gone through a separate, final, cleaning, filtering process. Ordinary black flux has some impurities and inert elements in it to keep the cost down. This is just fine if it is used entirely outside of the braze joint. In the case of brazing carbide saw tips to saw blades you also have flux between the braze alloy and the steel. If you are using ordinary black flux you will have these inert elements trapped in the joint. Purified black flux will create a more even braze joint that is typically 20 to 30% stronger than a braze joint created with ordinary black flux.

The application of the brazing flux is almost as important as the type of flux being used. The notches and both sides of the shoulders should have a layer of flux painted on them. It is very important that flux is covering the whole shoulder as the flux helps protect the shoulder from the heat to a certain extent. The big thing flux does is prevent the alloys in the steel from oxidizing. Many saw plates have nickel and chromium in the steel. If the plate is not flux these alloying elements can be turned into nickel oxides and chromium oxides or chromium carbides and thus affect the strength of the steel. If you see color on your saw plate after brazing then you need to flux to prevent that coloration.

Using a carbide saw tip that is wettable is very important. It is essential that the carbide saw tip be tested for wettability. The simplest way to do this is to put a very tiny piece of braze alloy wire in the middle of the tip with flux. Then heat the tip and see how far the braze alloy flows. If the braze alloy balls up there is something wrong with the tip. If the braze alloy flows out into a wide, flat puddle then the tip wets well.

Steel heats up much faster than carbide saw tips. In tests it looks like it is about a ratio of 3:1 to 5:1. If you heat the steel and the carbide saw tip equally the steel will get much hotter, much faster than the carbide.

Whether you are using induction or torch you should apply your heat to and then through the carbide saw tip. Ideally the carbide saw tip will come up to temperature which will heat the braze alloy which will then heat the adjacent steel to a depth of no more than 0.200". Temperature the steel reached can be determined by how far the braze alloy flows onto it. The outer limit of the flow should be no more than 0.200" from the carbide tip and 0.100" is better.

Tip loss occurs when the braze alloy does not stick to the steel or to the carbide.

If the laser cut saw plate is not gummed (ground) back enough the braze alloy may not adhere fully. The tip will come off. On the back of the saw tip will be a series of horizontal lines stretching from side to side of the saw tip. These lines are marks left by the notches from the laser cutting. Laser cutting does not really cut. Instead it melts a series of connected holes. This is what creates the lines.

If the saw tip is not treated properly for wetting and brazing the tip will come off and leave the braze alloy in the notch. Often there will be a thin gray film on top of the braze alloy in the notch. This is caused by the surface treatment on the carbide coming off. So the braze alloy stuck to the surface treatment but the surface treatment did not stick to the carbide

Braze alloy acts as both a bonding agent and a cushion between the carbide and the steel. The steel saw plate must be clean and oxide free. The carbide saw tip must be clean and readily wettability. If this is the case then the braze alloy forms two kinds of bonds. One is a pure physical bond. The other is a chemical bond. This gives you a bond strength that is greater than the strength of either the steel or the carbide.

The steel grows about 3 to 5 times as much as the carbide does during brazing. When you take the torch away the parts lock together and then they start shrinking. Because the steel will shrink 3 to 5 times as much as the carbide it wants to bend the carbide. Carbide does not bend except in extremely special circumstances. What you end up with is a highly stressed joint.

The braze alloy between the steel and the carbide must be in the range of at least 0.003" to 0.005" to keep the carbide from breaking.

If the tip gets pushed in and shoves all the molten braze alloy out of the joint then the saw tips will break.

There are four kinds of braze alloy most commonly used to braze carbide to steel.

1. 50% Silver with Cadmium

This is the traditional braze alloy and still widely used. It is easy to use and works very well. Unfortunately the government has drastically tightened the restrictions on cadmium exposure. You can use this braze alloy safely with proper ventilation. However the cadmium fumes out and deposits on its surroundings. Cadmium is also very easy to detect. Stories of companies that have been in a lot of trouble for cadmium exposure have made this alloy much less popular than it used to be.

2. 50% Silver without Cadmium (cad - free)

This is probably the most popular alternative to the cadmium alloy. It is about as easy to use as the cadmium alloy. Unfortunately it is about 30% weaker than the cadmium alloy in terms of tip loss and tip breakage.

3. 49% Silver with Manganese

This is a cadmium free braze alloy that is as strong as the cadmium alloy but it flows differently. Many people won't use it because it requires a change in procedure.

4. 56% Silver with Tin

This alloy is very easy to use but also very weak. It is an excellent alloy for hobbyists and has a great color match with stainless steel. It can be used for brazed tools but is generally not successful in medium to severe applications.

Emily Whitman Carbide Processors, Inc Exceptional Tools for the toughest applications

Source: EzineArticles
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How To Braze




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