Brazing Mild Steel

Hi Everyone,

Thank you for all your responses. I only have relatively small parts (3"x2"x3/4" to braze.

I have a blowtorch with a MAPP gas cylinder which I hope will do the job.

My thinking now is:

• Although more expensive silver solder has a lower melting point than other brazing alloys and flows more easily through the joint.
• Ensure a clearance of about 0.0015" in the joint. Best achieved by centre popping the mating surfaces and not overtightening the clamping pressure.
• Ensure absolute cleanliness in the joint. I have some Citric acid based cleaner.
• Use the correct flux. I have some easy flow flux and some silver solder.

I attended the Silver soldering lecture at the Fosse the other week. It was stated there you should mix the flux with water and washing up liquid before applying it to the job. Will the washing up liquid not contaminate the job?

Con Nicoll

I've always added a drop of washing up liquid to the flux mixture with no problems.

For best advice see **LINK**

Russell

Only use a drop. It doesn't seem to do any harm and helps the flux 'wet' the metal.

Neil

I can vouch for that, the advice on CuP really helped me understand what was going on when soldering, why using the right type of flux is very important etc..

I do believe CuP has written a guide you can buy, which i suspect i will do myself, in good time.

Not to be sniffed at!

Michael W

I think that the need for a gap in a soldered or brazed joint is because of the nature of the melting process. Unless you have a eutectic alloy, the melting process starts with pasty rather than completely runny metal. This is because some of the metal melts first, and what is unmelted comprises small crystals. And it is these crystals that prevent or interfere with flowing into the joint. Once the whole joint, filler and all, are up to a higher temperature, the crystals dissolve and flow properly. And this is, of course, what you need, as otherwise your joint will be partly eutectic, and partly some other alloy, rather than uniformly the same.

Another cause of poor flowing is the presence of dust from the air and minute solid particles remaining in the melted flux. Some dusts (especially emery, for example) act to repel the flux so that flow is impeded, and a wide joint gap may help to allow any grains to be flushed out by molten flux or brazing alloy.

I would be keen to learn of any more scientific explanation (based on tests rather than experience etc).

And PS - silversmiths always relied on borax until fancy modern fluxes were made available. I think this was in the early part of last century - but for may, borax is still generally used, for silver and gold solders.

Regards, Tim

That's right, Tim. I mentioned my use of borax in jewellery making earlier.

The "fancy" flux I sometimes use instead (on heating it bubbles up less than borax) is auroflux. It works well on gold and silver. I'd like to know what's in it.

One other thing jewellers often use is a boric acid slurry brushed on before soldering to reduce the dreaded fire-scale you can easily get when soldering sterling silver, and even with some gold alloys. Some people say the boric acid improves solder flow. I use it a lot but I'm not sure whether it does help, nor whether its help (in addition to the flux) is really needed if your soldering discipline is good.

.

Bill,

Have a look at section 2.2 of the Safety Data Sheet

It wasn't difficult to find ...  under 'Questions & Answers' on the link that you provided .

MichaelG.

Edited By Michael Gilligan on 02/11/2018 16:23:23

Sorry, Michael, I've searched the questions and answers section of that link high and low and I can see no safety data sheet.

Bill, it took me 2 attempts to get there but here is a link to the SDS,

**LINK**

DaveD

.

Bill,

The second item on the Questions & Anwers tab is:

Do you have a safety data sheet for this product ?

... now click the little 'down arrow' [or the text] and you will see the answer.

MichaelG.

.

Note: Meunier kindly provided a direct link, but I thought you might also find 'how to' useful for future navigation of Cookson's site.

Edited By Michael Gilligan on 03/11/2018 20:33:38

Thank you, Meunier and Michael!

I've worked out why I couldn't see what you were seeing, Michael; I was viewing the page on my desktop, which has a browser extension that blocks ads, trackers etc. I had to disable the browser extension in order to be able to see the actual questions and answers lurking beneath the tab of that name.

Washing up liquid or dish detergent is a surfactant, a "compound that lowers surface tension between ... a liquid and a solid."

As indicated by Neil, adding detergent to the water used with the flux paste facilitates improves dispersion or wetting action of the flux by reducing the surface tension between the flux mixture and the surfaces it is applied to.

While I defer to CuP Alloys 1 in all matters brazing, a little detergent goes a long ways.

The 'drop' of detergent is specified in Rolls-Royce Aero literature (or used to be). They also specify surgical spirit instead of water. When a flame is applied the spirit burns and bakes on the flux (I always use Tenacity 5)

Brian

I think they work on liquids to liquids, too. Think here - those dirty dishes or oil slicks?

Yes.

The ellipsis in the quotation denotes words left out. Liquid-to-liquid and gas-to-liquid seemed superfluous to the discussion at hand.

The full phrase reads as follows:

"Surfactants are compounds that lower the surface tension (or interfacial tension) between two liquids, between a gas and a liquid, or between a liquid and a solid. Surfactants may act as detergents, wetting agents, emulsifiers, foaming agents, and dispersants."

Source: Wikipedia

No, the need for enough, but not too much gap is to facilitate: 1) capilliary movement of the braze filler metal along the length of the joint between parent metals; and 2) joint strength.

The article in the following link discusses aspects of gap size, joint tensile strength, flux voids and brazing atmosphere.

"The Famous Joint-Strength vs Joint-Clearance Chart" Dan Kay. 2013.04.07

While the conclusions hold implications for industry and obsessives, hobbiests such as myself can continue to follow age-old practice to enjoy satisfactory results.

Interesting reading there, Eddy. I always think that if the ideal gap is somewhere around half to one-and-a-half thou, depending on which chart you follow, any two pieces of metal will have about that clearance between them unless they have been blued and hand scraped to a better fit, or perhaps surface ground and micro polished.

Most pieces of metal in the home workshop cut straight from stock, or even milled to shape, and given a bit of a rough up with some coarse emery paper before brazing will have a tiny gap in this range due to surface imperfections, lack of true flatness etc.

I remember from apprentice days the weeks of painstaking filing it can take to get two pieces of steel to mate within less than one thou. It's a lot harder than many people think.

Just be careful that the mild steel is not leaded.

The joint will look good but a good smack with a hammer will show the problem.

I've used neat washing up liquid (no water) in my flux mix with no discernible drawbacks and it's easier to control how many drops of detergent you add than drops of water. Usually it's a 50/50 mix of water and detergent though.