39. Flux

Flux covers the area being brazed and keeps oxygen away from the materials. A good braze joint is both

physically and chemically bound. If oxygen gets in then the steel or carbide will oxidize instead of bonding

to the braze alloy.  Flux has mild cleaning properties but the parts must be clean before starting.  Flux is

not designed to clean off oil, greases, dirts or anything else

White Flux- Standard high temperature brazing flux. Preferred by many because you can see the work

through the flux while heating.

Black Flux - Black flux gives maybe 50 to 100% more protection and stays good for about 400 degrees F

higher than white flux.   It is White flux with extra Boron added. Provided for greater protection at higher

temperatures and / or extended heating times.

Purified Flux – Typically gives 30% To 100% Better Braze Joints

This is Black flux with several extra steps to give a creamier, more consistent product. The extra steps

remove impurities that can get trapped in closed joints. This is an especially good product for use in close

joints subjected to high stresses such as carbide tipped saws.

The critical part of saw and tool brazing is what goes on inside the braze joint.   Ordinary flux is inexpensively made and has up to 10% odd size particles and non-active minerals in it.   These odd size particles and non-active minerals get lodged in the braze area and

Contaminate the joint.  Purified flux is black flux that has had extra processing steps.  These processing steps take the black article out and leave the flux a rich, creamy brown color.   If you take a little of each flux and rub it between your fingers you can feel that Purified Flux is not only smoother but the particles are smaller and there are no extra large particles. 

Use lots of Flux

Many people make the mistake of thinking of flux as expensive so they don’t us enough.  While it is expensive it is an absolutely essential part of good brazing.  Flux is cheap compared to making bad tools.  Flux heavily both sides of the saw plate and the notch.  You can save a penny’s worth of flux and ruin a $100 saw blade. 

Remixing Flux Is Like Remixing Concrete

It is theoretically possible but a lot harder than most folks think it should be.   When flux dies out it goes through a chemical change and you can just add water to bring it back.  Using dried flux is sure to cause you problems. 

There are many types of fluxes.  We will discuss two types; White flux and Black flux.  These are the most commonly used fluxes for tungsten carbide and both are excellent products.  There are special fluxes for ceramics.  Special ceramic fluxes and alloys are generally too expensive for tools and wear parts.  Properly preparing the ceramic surface allows you to braze very successfully with these two fluxes.

Flux covers the area being brazed and keeps oxygen away from the materials.  A good braze joint is both physically and chemically bound.  If oxygen gets in then the steel or carbide will oxidize instead of bonding to the braze alloy.

Flux has mild cleaning properties but the parts must be clean before starting.  Flux is an essential part of the brazing operation.  It contributes to material cleanliness.  It may clean off some existing oxides but it mostly prevents the formation of new oxides.  

Flux is not designed to clean off oil, greases, dirts or anything else.  The surface must be clean before it is fluxed or the flux will not work properly. Flux keeps oxygen from getting to the parts and ruining the braze joint.  The suppliers do not think that flux has any cleaning properties and do not advertise it as such.  There is an opinion in the industry that flux is a cleaner and that the action of the flux also serves to float up and off scale and loosen dirt.

In any case the joint must be thoroughly clean before you begin brazing.  Flux is certainly not primarily a cleaner or degreaser.  However, because flux reacts with oxygen, flux can and will remove oxides occurring on the braze joint area.  Once again the braze joint should be thoroughly cleaned before applying flux.

There is a chapter on cleanliness that applies as well as a chapter on surface treatments.

Use lots of flux.   Metals will not flow if there is not enough flux used.  As a general rule coat both sides of the plate and the notch with a thick enough layer of flux so that you cannot see through it.   

There are two primary objectives that all fluxes must meet.  They are:

1.  Remove oxygen from the surface of the material.

2.  Prevent further oxidation during the braze operation.

Both the white and black meet these objectives.

A common saying is that braze alloy follows the flux.  The alloy tends to flow where the flux has cleaned and prepared the joint.  The alloy can also flow beyond that if the surface is clean enough and does not oxidize too badly during heating.

All metals and alloys have oxygen on the surface, primarily as metal oxides - CuO, FeO, etc.  These metal oxides must be reduced (oxygen pulled away from the metal) before the braze alloy can make a metallurgical bond.  Both the white and Black flux perform this function.

Once the oxygen atom is removed from the metal surface, it must be contained within the flux.  The containment is performed by the boron atom in the formation of borate compounds.  Flux is a protective sponge for the braze joint.  As with a sponge, flux will only hold so much oxygen.  How long flux will absorb oxygen depends both on the length of the heating cycle and temperatures used.  A long, low-temperature cycle can be equivalent to a short, high-temperature cycle.  The flux will quit working at somewhere around 1600-1800 F depending on the length of the cycle and the amount of oxygen available.  In tool brazing with the more common silver alloys the flow point is somewhere below 1400 F depending on the alloy.

Flux application

Different fluxes can be applied many ways such as dipping, spraying, brushing and automatic paste dispensers.  Brushing may be the best way since the brushing action helps clean the surface physically.  For all practical purposes the various ways of applying flux can be considered roughly equivalent if they deposit the same amount of flux in the proper locations.

As you can see up through this point, both fluxes appear to be about the same.  The primary difference between white flux and black flux is that the black flux contains more boron.  Most of this additional boron is added in the elemental form.  The elemental boron is chocolate brown, not black.  It turns the white flux dark brown.  I have no idea why, thirty years ago, they decided to call it Black Flux and not Brown Flux.

Flux can be thinned with water for application.  Tap water can work but distilled or deionized water is probably better.

Once flux has dried to a solid it is very difficult to add liquid and turn it back into a successful paste.  Flux is pretty inexpensive and there is very little need to use flux that has dried.

Flux life

The life of both the white and black flux is related to the oxidizing environment to which they are exposed.  The oxidizing environment (or source of oxygen) normally is the metal surface, oxygen in the air, and oxygen in the flame.  A change of oxygen from any of these sources affects the life of the flux.  The life of both the white and black flux is directly related to the amount of boron atoms left in the flux.  When all of the boron atoms are tied to oxygen atoms, the flux becomes “spent” or “expended”.

An example of the oxidizing environment would be two customers working out of the same flux container.  One customer cleans his parts immediately before brazing and uses a neutral flame.  The other customer has a “rusty” surface and turns up the oxygen to where it approaches a cutting torch.  These are two obvious differences in oxidizing environment.

White and Black fluxes

In our experience we have seen saw brazing problems with White flux that were eliminated by switching to Black Flux.  This is what we saw and this is what has been reported to us from other sources.  This is a disputed opinion.  Scientifically, there should not be that much difference between the two flux formulations.  There are many people who are brazing successfully with white flux.  Both are excellent products and both have unique qualities.  If you are having brazing problems and using white flux then switching to Black Flux is something worth trying.

Both fluxes perform as follows.

A.  At 212 F water boils off

B.  At 850 / 900 F flux turns liquid

C.  At 950 / 1000 F flux becomes active

The main difference:  At 850 / 900 F when both fluxes turn liquid, the white flux is more transparent, making it easier to see the assembly.  With time (oxidation), the black flux also becomes transparent.

White flux is a combination of Potassium salts of Boron and Fluorine.  It consists of Potassium Tetraborate, Boric Acid, Potassium Pentaborate, Potassium Bifluoride, Sodium Dodecyl sulfate, Boron and water.  Black flux is white flux with extra boron.

Black flux is white flux to which elemental Boron has been added.  The way flux works is that it captures and holds oxygen to keep it away from the braze area.  It is the boron and the boron compounds that trap the oxygen.  More boron means that the flux can trap more oxygen.

Black flux with extra Boron does two things.  The extra Boron means that the flux can absorb more oxygen.  Since flux prevents oxidation by absorbing the oxygen this means greater protection.  Black flux also works up to 2200 F while white flux is rated up to 1600 F and my source gave the figure of 1300 F as a recommended figure for maximum temperature.

Black flux is different than white flux in two ways from a scientific standpoint.  Black flux gives more protection for a longer time because it traps more oxygen and it takes a higher heat.  In a torch brazing operation black flux is more forgiving and is more tolerant than white flux.

Black flux sort of gives maybe 50 to 100% more protection and stays good for about 400 degrees F higher than white flux. 

There is also a problem with brazing with a torch in that white flux and black flux react differently and a brazer who is used to dealing with black flux will have to relearn his techniques somewhat.  One of the clues we use here is the amount of flux coating during and after brazing and differences between the way they look. 

It is obviously possible to get excellent resulting by using either flux for brazing since a lot of people are doing it.  The problem is that the two fluxes seem to be sold as being replacements one for the other without sufficient explanation or training on the differences between the two.

White flux- Standard high temperature brazing flux.  Preferred by many because you can see the work through the flux while heating.

Black flux – White flux with extra Boron added.  Provided for greater protection at higher temperatures and / or extended heating times. 

Purified flux – Black flux with several extra steps to give a creamier, more consistent product.  The extra steps remove impurities that can get trapped in closed joints.  This is an especially good product for use in close joints subjected to high stresses such as carbide tipped saws. 

Purified Flux

30% To 100% Better Braze Joints

                          Purified flux at 50x magnification                          Standard flux at 50x magnification

                           Smooth, even and consistent                                 Large grains of foreign material

You can see and feel the difference immediately.

Purified flux is black flux that has had extra processing steps.  These processing steps take the black article out and leave the flux a rich, creamy brown color.   If you take a little of each flux and rub it between your fingers you can feel that Purified Flux is not only smoother but the particles are smaller and there are no extra large particles. 

Flux is made to prevent oxygen from getting to the parts as they are heated.   Steel and especially tungsten carbide oxidize at room temperature.  The hotter they get the more they oxidize.  Above 1,000 F tungsten carbide oxidizes extremely rapidly and forms an unbrazeable surface.    Purified flux is good for more time at higher temperatures, up to 1,700 F.  

The original idea with flux was that it was to be applied on top of the braze area.  No special effort was made to purify the flux because it worked well and they wanted to keep the cost down. 

However the critical part of saw and tool brazing is what goes on inside the braze joint.   Ordinary flux is inexpensively made and has up to 10% odd size particles and non-active minerals in it.   These odd size particles and non-active minerals get lodged in the braze area and can seriously effect the strength of the braze joint.