How effective is flux at oxide removal? 2

[tomwalz]

Our brazing is production brazing so we want to avoid rework. We know that borax based flux (such as the White and Black fluxes mention that are Potassium salts of Boron and Fluorine) will remove oxides well. We also use refluxing and reheating to correct poor flow. However sometimes refluxing does not correct the issue. We have also seen where brazing to an oxidized surface makes a weak joint.

We operate on the assumption that the surface has to be oxide free and we have to keep it oxide free however we know that this is more conservative than it needs to be.

We braze carbide to steel and finds that this is much more a problem with tungsten carbide than steel.

Would anyone have guidelines as to how much oxide flux can handle. All I have gotten from the experts is that it can handle a little but that it is primarily designed to prevent oxide formation.

Any help would be appreciated.

Thanks,
tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[deanc]

I would always say start bright and shiny.

http://www.handyharmancanada.com/

Good handbook

http://www.brazingandsoldering.org/

Phil Roberts book is one of the best I have seen.

http://www.brazetec.de

Has excellent info. in English

 

[tomwalz]

Those are excellent references and I thank you for them.

I have also found the following helpful in the past.

AWS Brazing Handbook
AWS Brazing
ASM Sourcebook on Brazing and Brazing Technology
ASM Principles of Soldering and Brazing
ASM Handbook - Volume 6 Welding, Brazing & Soldering
Schwartz - Ceramic Joining
Schwartz – Joining of composite Matrix materials

However none of them seem to address the issue of flux in the removal of oxides in a quantifiable manner. .

We do start bright and shiny. When I write and lecture I take the position that flux can clean a surface but that its primary function is to prevent oxide formation. I recommend starting with a clean, prepared surface and using flux solely as an oxide preventer.

Recently I was challenged by someone who said that he never cleaned the surface and that Boron based flux always worked well for him.

What I am searching for is some sort of a guideline as to how well flux does remove oxides.


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[metengr]

tomwalz;
Flux does not clean the surface of metals to be joined or prevents oxidation, it serves to remove existing oxides to enable wetting of the braze filler metal. In one of my older ASM Handbooks (Volume 6, 8th Edition) on Welding and Brazing, on page 611;

Under Fluxes, "the flux serves to reduce surface oxides, ensuring that filler metal will flow and that the base metals will be satisfactorily wetted by it."

..."They (fluxes) temporarily reduce protective oxides, until their chemical activity is exhausted, and then oxides re-form."

The application of flux is really based on experience - having just enough to remove existing surface oxides to ensure wetting. Cleaning of the base metals PRIOR to fluxing is absolutely essential to maximize the oxide reducing effects of the flux (these are my words).

 

[deanc]

P. Roberts addresses this in his book,including carbides.

Industrial Brazing Practice which you can find on Amazon.

Its about $100,but worth every penny if you are looking at the practical side of this process.

 

[tomwalz]

I guess it is very highly situational. I think we will continue our practice of cleaning thoroughly and using flux to keep it clean. Operating on the principle that anything is cheaper than rework. (A bit broad, perhaps, but a pretty good rule of thumb.)

Tom

From the AWS Handbook
Fluxes are not designed or intended for the primary removal of oxides, coatings, oil, grease, dirt, or other foreign materials from the parts to be brazed.

And 2 paragraphs later

Oxides are the principal source of surface contamination. The dissolution and removal of oxides during brazing are the most common functions of a flux.

AWS Brazing Handbook 4th edition
FLUXES AND ATMOSPHERES

The purpose of a brazing flux' is to promote the formation of a brazed joint by protecting the base metal and filler metal from oxidation. The brazing flux may also serve to remove surface oxides and therefore reduce surface tension to promote freer flow of filler metal.

When metals are exposed to air, chemical reactions occur. The rate of these reactions generally is accelerated as the temperature increases. The most prevalent reaction leads to formation of oxides, though nitrides and even carbides are formed in some instances. The rate of oxide formation varies with each metal composition and the nature of the oxide. Oxide tenacity, structure, thickness, and resistance to removal or further oxidation are all factors that need consideration. Oxide formation in air on some metals such as aluminum occurs instantaneously, even at, or below, room temperature. In almost all conditions, such surface oxides or other compounds create barriers to the formation of brazed joints.

FLUXES
Fluxes are not designed or intended for the primary removal of oxides, coatings, oil, grease, dirt, or other foreign materials from the parts to be brazed. All parts prior to brazing must be subjected to appropriate cleaning operations as dictated by the particular metals (Chapter 7).

In making a braze, the flux is often needed to combine with, remove, or dissolve those unwanted residual compounds or products of the brazing operation which would otherwise impair filler metal flow.

Oxides are the principal source of surface contamination. The dissolution and removal of oxides during brazing are the most common functions of a flux. So as not to impair metal flow, the flux must also be fluid and displaced readily by the molten brazing filler metal. Wetting ability and viscosity of a flux at brazing temperature, therefore, are important properties.

Under some circumstances, flux also may suppress volatilization of high vapor pressure constituents and the formation of oxide fume condensate in a filler metal.

Brazing by Mel Schwartz ASM International
One of the functions of a flux is to remove the oxide layer on the joint area and expose clean base metal.

ASM Vol. 6 Welding. Brazing and Soldering 1993

Definitions
Flux - Material used to prevent, dissolve or facilitate removal of oxides and other undesirable surface substances.

ASM Vol. 6 Welding. Brazing and Soldering 1993

Selection Criteria for Brazing and Soldering Consumables by A. Rabinkin, AlIiedSignal Amorphous Metals

Cleaning the base-material components being joined and protecting them from oxidation during heating are essential in both brazing and soldering processes. Chemically active substances, called fluxes, are commonly used to provide such cleaning and shielding. The use of nonmetallic agents in fluxes is generally undesirable, because of the increased propensity for their entrapment and the resultant formation of voids in the finished joint. However, the use of certain active elements that can be added to filler-metal alloys to act as a flux can be advantageous. However, this concept usually only works with brazing filler metals. For solders, working temperatures are generally too low for the reduction reactions to take place with adequate speed, although in some cases these reactions do indeed occur.


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[deanc]

http://www.brazingandsoldering.org/PDF/semext.pdf

Please read this paper from Roberts. It is for Aluminum but the basics are the same. I think you will find your answer here.

Having spent hours and hours on some brazing problems I can only say this is one of the best.

 

[tomwalz]

You are correct. That is quite a piece of work. The part about flux and oxide removal wasn’t in this introduction however.

Since you truly have an interest in brazing, you might wish to view my modest efforts. I am afraid my language lacks the elegance of the book you recommended but, in my audiences, I find that everyone understands a phrase such as “broke when we hit it” while something more elegant may obfuscate.

Our newsletters are at:

http://www.cs4u.org/

Our books are at: (free, old version is at the bottom)

http://www.carbideprocessors.com/Brazing/book/index.htm

These pages are being revised. If you can’t find them, let me know and I will be happy to send you PDF versions.

Again, thanks to all for providing the new sources of information.

tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

 

[deanc]

Nice Site! You should get a gold star for your work.

Never searched for it as I am not really into tools....but brazing is brazing.

I have already downloaded some of your work and stored it.

Thanks
deanc

 

[tomwalz]

Thank you, Sir.

I am frequently amazed at how often I run into people using the same words and terms correctly to mean different things.

In my world the client has already determined to braze and is brazing. They just want a problem solved. I find a picture of good and bad parts is much more relevant to their situation than flow charts.

Thus I really appreciate information of the “flowchart “ type.

Again, my thanks.

Tom


Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com