Brazing Mild Steel

There was an article a year or two ago in ME about fabricating a bogie for a 5 inch gauge Princess Coronation using silver solder. I think this was using mild steel plate.

Does any one know anything about what advantages silver solder has over other brazing alloys. In fact can anyone point me at any information on brazing mild steel.

I am fabricating a bogie "casting" for a 2.5 gauge tender. I would like some guidance.

Thank you in anticipation

Silver soldering is brazing but in the UK the term brazing is generally taken to be when a higher temp alloy is used so the advantage of silver soldering is that you don't need as much heat and can do it with just a propane torch and suitably sized burner whereas the higher temp brazing really needs oxy-propane or oxy acetylene.

The other useful thing is that you can add brass or bronze parts to your steel fabrication without risk of melting them which you may do at brazing temperatures.

For all things silver solder related have a look at Cup Alloys' site, particularly the section "best brazing practice"

If you have access to oxy/propane you might be better off to consider brazing,for the cost,if nothing else.

Have you any idea how much silver solder rods cost? dont even ask ! Chronos used to sell them,many moons ago,but they dropped them,due to cost,I believe. I still have a few,but am very careful about wasting them.

The other advantage that I did not mention is that Silver Solder tends to flow far better than Braze so you will end up with just a small fillet of solder in all the internal corners, try that with braze and you will get a big fat fillet that will take forever to try and clean up on a part like a bogie. should run like this

You won't need much solder for a small bogie like that maybe a quids worth at the most, add a bit for the steel and you are still well in pocket compared to the cost of casting.

If your a skin flint like me, brass wire works ok on steel, or narrow strips cut from thin brass sheet, using borax bought from the garden shop at $NZ 1 per kg.

Ian S C

BRAZING IS A PROCESS.

It is identical to soldering. It is only an undocumented international convention that distinguishes between the two. Quite simply, if you are making your joints at a temperature below 450 deg C then you are soldering.

Above 450 deg C then you are brazing.

BRAZING HAS NOTHING TO DO WITH THE FILLER METAL USED.

IT HAS NOTHING TO DO WITH THE HEATING TECHNIQUE. Gas torch, furnace (vacuum or atmospheric), electrical resistance, induction are all viable heating techniques to produce brazed joints

Jet engines are brazed with gold/palladium brazing alloys.

Automotive components are brazed with copper and copper brazing alloys (this includes brass!)

Nuclear reactor components are brazed with nickel brazing alloys.

Steam boilers, bogies, clocks, are brazed with silver brazing alloys.

Look up the term "brazing" in any source and you will find that in the definition there are two words that repeat and those two words are "CAPILLARY ATTRACTION"

iF YOU ARE NOT USING CAPILLARY FLOW TO EFFECT THE JOINT THEN YOU ARE NOT BRAZING.

All too often silver brazing alloys and brass alloys are not used to braze two components together. They are used to stick two components together and block a hole and that is expensive. Look at some bicycle frames, office furniture and the multitude of joints made in steel using brass rod and oxy-acetylene torches.

I have supplied, in the past and for over 18 months, a company in excess of 200 kg per month of a silver brazing alloy that was used to do precisely that. The financial risk became too great and the orders were passed directly to my supplier.

The customer refused to change the joint design and to use the alloy as a brazing alloy.

In doing so they failed to take advantage of the many technical advantages of using the process. Silver solder, because of the price, is only used because it satisfies a technical requirement.

It will join a wider range of parent materials than welding.

It is carried out at lower temperatures - less distortion.

As a filler metal, it is more corrosion resistant.

It produces leak free joints.

It produces joints stronger than the parent materials.

It offers better strength at elevated temperatures.

It offers a good colour match to parent materials.

It can be carried out using relatively inexpensive equipment. Propane/air torches are fine.

If none of the above are important then save your money and use soft solder, or glue or screws!

In all cases, the use of a silver solder, despite its price, produces the cheapest cost per joint. If as an individual you can't reach this conclusion, you are are not using the product correctly. Examine your joint design (gap and length), examine your heating technique to get the alloy to flow precisely where you want it, examine the form in which the silver brazing alloy is being used. There is more than 1.5mm dia rod.

As regards, other companies ceasing to sell silver solder, I believe that was due in no small way to CuP Alloys. They passed on the benefits of their large buying power with German and Spanish suppliers to the model engineer and backed that up with good sound technical back up.

For more information, there is a book available from CuP Alloys.

"A Guide to Brazing and Soldering - everything you wanted to know about Silver Soldering but were afraid to ask"

I cannot speak too highly of it!

Keith

PS Brass strip and borax will produce excellent joints. But please "braze" with it and not simply stick the pieces of steel togerther

Edited By CuP Alloys 1 on 28/10/2018 11:30:45

Hi Con,

I build in 2.5" gauge (Gauge 3) too and have used brass and silver brazing to make various small fabrications. I've not attempted a 'bogie' as yet - but have assembled a loco building frame from laser cut parts which is a good bit larger than any of my bogies.

LBSC used to describe "SIF-bronzing" - which is simply brass brazing by another name. In fact I use 'SIF' brazing rods and 'SIF' flux for my brass brazing (SIF being the brand name). If you search for "SIF" you should find the brazing guidelines they published at one time (although I haven't checked this recently). I purchased about sixty 1metre SIF rods a few years back for about £30 - a lifetimes supply for most hobbyists - and a great deal less than silver solder would cost. It also helps if you watch for special deals - which the welding dealers sometimes have.

Whilst I trained with Oxy/A at welding classes - I couldn't justify the cost for home use. So I purchased a Bullfinch 404 brazing torch which works just off propane. This torch is good enough for small items but will need help (extra heat input) if you want to work on larger masses. It can of course also be used for silver soldering/brazing - which it does with ease.

Jason makes a good point about silver flowing more easily and not tending to 'lump' quite so much as brass but this is about getting enough heat into the part without burning off the flux. This is certainly easier to do with Oxy/A than my 404 but care with heat shielding and component support helps. Smaller parts are obviously easier to do and suggests that 'staging' a larger fabrication makes good sense. I have built assemblies mostly using SIF brazed parts, with final assembly with silver solder - which is much cheaper and a bit easier.

As an aside - I am a great fan of Kozo Hiroka's silver soldering recommendations - which I use whether silver or brass brazing. Essentially this involves holding parts in place securely (without heavy clamping), making sure there is clearance between parts (for capillary flow) and absolute cleanliness. He stage solders/brazes his parts - basically one major joint at a time with thorough cleaning in between. This approach also works well when using a brass then silver braze mix.

In summary - fabrications are good way to make custom parts. Brass brazing is useful and cheaper than silver soldering when fabricating steel components. Silver brazing is really the only way to braze brass fabrications (or brass & steel ones). Both need high heat which will require some clean-up (I use Citric acid) and brass is not quite as easy to get a neat joint with as silver. But if you think of this as making a custom casting - then you can machine it clean and true it up anyway.

Hope this helps. There are some photos in my album (in Brazing) of the engine frame which was brass brazed - and last week I made a couple of small stand-offs to fit a DRO to the front slot of my Atlas MF mill. Simply some steel angle brazed to cylindrical turned parts - with some 'thickeners' added to allow deeper tapped holes in the angle. They need a bit more work to finish them and make 'em presentable. They could easily have been silver brazed instead but this took the same time and helps to conserve my precious (e.g. expensive) silver solder stock for when I really need to use it. Once you start doing brazing (with silver or brass) it makes a lot of things so much easier and it will become a routine part of your hobby.

Hope this helps.

Regards,

IanT

Edited By IanT on 28/10/2018 13:06:58

Con Nicoll 1

If you are a novice to brazing, besides choosing filler to suit your needs, pay attention to CuP Alloys and IanT's remarks about gap/clearance along the joints. As alluded to, capilliary action of the filler metal is central to successful brazed joints. It is essential that an adequate gap be present for this action or flow to take place, if parent metals are butted tightly together, results will be less satisfactory.

Another consideration that can be non-intuitive is that braze filler metal flows toward heat. As the pieces are approaching temperature and you begin to apply filler metal, play the flame opposite the point of filler application and let capilliary action draw the filler along to the destination.

Here is one of several discussions on the subject, perhaps you may find it useful:

https://www.lucasmilhaupt.com/en-US/brazingfundamentals/properbrazingprocedure/

Yes, absolutely Eddy.

The parts above were held together with stainless steel screws, tight enough to not slip but not really wrenched up. However - the clearance was provided by simply making centre punch marks around the flat surface of the turned parts where they mated with the angle. This gives enough clearance for capillary action (a few thou) to take place. It's another of Kozo's tips, Very easy to do and makes for a tidy joint.

Regards,

IanT

Ian T, just out of interest can you tell us specifically what flux you used for brazing those mild steel parts? You say you used SIF flux but there seem to be several varieties of SIF flux such as "bronze", "stainless", "aluminium", "cast iron". Which variety will work best with mild steel using silver solder, e.g. some Silverflo 55?

Also, will straight borax (I have a cone of it at home that I use for soldering silver and gold parts when jewellery making) be just as good a flux for mild steel when using a silver solder as opposed to brass wire?

Thanks!

.

That's an excellent link, thanks Eddy.

For emphasis [and noting that they are discussing one scenario] :

[quote] Note that the strongest joint (135,000 psi/930.8 MPa) is achieved when the joint clearance is .0015" (.038mm.) When the clearance is narrower than this, it's harder for the filler metal to distribute itself adequately throughout the entire joint - and joint strength is reduced. Conversely, if the gap is wider than necessary, the strength of the joint will be reduced almost to that of the filler metal itself. Also, capillary action is reduced, so the filler metal may fail to fill the joint completely - again lowering joint strength. So the ideal clearance for a brazed joint, in the example above, is in the neighborhood of .0015" (.038mm.) [/quote]

MichaelG.

Hi Bill,

Yes, I use SIFbronze No 1 rod (1.6mm) and SIFbronze Flux.

This flux is a pink colour and tends to be a bit 'gritty' to mix up. I've just checked on SIFbronze pricing and was surprised (and pleased) to see that it still seem quite reasonable. It's showing as £26.85 inc. VAT (for 1kg/Qty 60) 1m 1.6mm rods and 1kg 2.4mm rod is even a bit less at £24.75 inc VAT. You can buy a few rods for less on eBay but it works out a lot more expensive in the longer run. See here for details:

https://www.weldequip.com/sifbronze-brazing-rods.htm

BTW - for any small scale modellers - this rod is also a cheap & useful source of brass rod as it turns well and can be threaded. So doubly useful if you have some around.

With regards Borax - I've not used it myself but if you have done so before, I don't see why it wouldn't work on steel. A quick test on some scrap would be a good idea though.

For silver brazing/soldering I use HT5 Flux powder (from CuP) as it seems to be similar to Tenacity - and I've an unfortunate habit of sometimes slightly over-cooking my parts (too much heat whilst faffing around) which can burn off the flux. So I need all the help I can get - not a problem mostly, as if I'm in any doubt, I just clean everything up (Citric bath again) and have another go with a fresh flux coating. Easier to do it right first time though...

Hope this helps.

Regards,

IanT

Well - I'm a slow typer and Keith had already answered your question about Borax Bill. As he says HT5 isn't that expensive and does the job well.

I also meant to comment on Michael's post - as 15 thou sounded like quite a gap to me, then I realised that it was 1.5 thou (I need new glasses). However, I'm not sure I'd get hung up on how much clearance there is - provided there is some. As Kozo suggests, the worse thing is to clamp the parts together too tightly...

Regards,

IanT

Edited By IanT on 31/10/2018 10:10:11

Michael's quote is 1.5 thou not 15

IanT.

Micheal's .0015" looks like 1and half thou to me. He just left off the 0 before the decimal.

.

Michael's quote was verbatim

The original author omitted the leading zero.

.. Yes, it's one and a half thou.

MichaelG.

Seems like I'm not the only one! 

Regards,

IanT

Edited By IanT on 31/10/2018 13:44:41

Many thanks, Ian T and Keith, for your helpful answers.

Bill.

I've done some 'emergency' brazing with brass rod and borax. It worked fine, but I needed a lot more effort to get that extra temperature. IMHO silver solder with the proper flux is is easier because of that alone, but also it seems to flow into the joint better.

Neil