drying out of flux / prefluxing of silver solder.

Hi all some help please.

In preparing for silver soldering has anyone fluxed joints before riveting and allowed the flux to dry out in the right place between the joint, say overnight; before silver soldering. My boiler sections are currently fixed together with screws and I would like to be sure the flux has fully coated the surfaces before actually removing these and finally replacing them with Rivets. I presume the water in the flux mix is only there to allow the flux to flow or seep into the joints which it would not do if dry, or does it serve some deeper chemical purpose? my concern would be that the air pressure of the flame might blow the dry flux out from the joints if applied directly Hope this makes sense.

Kind regards all

BK.

hi brian,

i partly do what you propose in that for miniature loco boiler work i apply the flux then let it dry out a bit for an hour or so.

as for flux between joints riveted together, there are perhaps 2 points worth considering. the joints should be cleaned beforehand and roughed up, then riveted. if you have less than 2 thou gap between the riveted joints you wont get silver solder penetration. there are tricks to overcome this - centre popping the joint all along leaving the raised bit created by the centre punch, or coarsely filing criss cross lines (something i do for foundation ring sections).

if the joint is clean before riveting and the riveting is done shortly before silver soldering ie not left on the bench for a few months, then it is only necessary to apply the flux either side of the joint and not between.

remember that all rivet heats must have silver solder applied, and ideally the rivets should not be tight.

just my own personal approach of course.

cheers,

julian

Thanks Julian.

My thinking was to pickle the individual boiler parts, and then flux and rivet up with a few bits of thin 2/3 thou shim adjacent to rivets between joints which is then pulled out. Leave overnight and solder up the following day, using a normal fluid flux mix to supplement the joints. I understand the need to coat the rivet heads. This isnt my first boiler but on the last one I did seem to get problems with what appeared to be lack of flux penetration.

Hi RRMBK Try mixing borax with meths into a paste ,when heated it does not bubble and also wets the joint right through.I always use this method when I silver solder my jewelry bits . I also use DIEWRSOL soldar liquid (spelled SOLDAR) ,this is expensive but works well on gold silver and all other non ferrous metals.

Maurice

Hi again misspelled DIEWERSOL Maurice.

I do it a little different, I make the joint then apply a weak (i.e. watery) load of flux to one edge so that it is drawn in by capiliary action, then apply a thick paste to either edge. The weak solution will find its way into the rivets and every where else. I often allow it to dry, seems to make no difference.

The job of the flux is to prevent oxidation of the joint when heat is applied. Oxidation will prevent the solder from flowing. Many propriety fluxes can do a limited amount of cleaning, but you shouldn't rely on that - its best to get the joint as clean as you can before applying the flux.. The water is simply to make it easy to apply the flux to the joint - it has no magic properties. When the joint gets close to the solder melting temperature, the flux will fuse into a clear liquid; as long as as it coats the joint then no oxygen can can intrude. There's a "gotcher" though, if you heat flux for to long, or at too higher temperature, you burn it and it loses its ability to exclude oxygen.

I also mix in a drop of washing up liquid to the flux paste mix. This makes the surface "Wet" and ensures that it doesnt run away from vertical surfaces and probably jelps with flux penetration of the joint.

I do this too; it helps with neutralising small amounts of grease or oil that might be present in the joint.

Thank you all for your advice and suport . greatly appreciated. I feel a lot more confident about the next steps now. Gary - one queery please re your " gotcha" any estimate of how long is too long ?

Kind regards All.

BK

Hi BK,

It depends on the flux: Easyflow is quite sensitive - if it goes brown then its burnt. This can occur as quickly as a minute or two. High temperature fluxes, such as Tenacity, can withstand higher temperature for longer, but there's a catch: when the joint has cooled down, the flux forms a hard, glass-like, covering. Very hot water removes the Easyflow, but has no effect on Tenacity which often has to be chipped off.

Never heat the solder directly, it will ball up and refuse to flow; the normal reaction is then to keep blasting the area which simply ends up burning the flux. Heat the joint; sneak up on it from various directions, but avoid directly heating the solder. When the joint is hot enough it will melt the solder which will then run into the joint.

hi brian,

re gary's 'gotcha' i use Thessco F or J&M Tenacity 4A for copper silver soldered loco boiler work where prolonged heating is involved. under no circumstances use ordinary easyflo flux. if the job doesnt get up to heat within say 10 minutes with propane and the joints being silver soldered not done within a few minutes then you will have flux exhaustion problems. (i have never found adding extra flux part way though satisfactory).

this is an idea of the quantity of flux i use - quite a lot. mixed to a stiff paste with as bob says a drop of washing up liquid, and then left to dry a bit before lighting up.

your 2 thou shim is a very good idea when riveting. i try and use as few rivets as possible.

good luck!

cheers,

julian

Edited By julian atkins on 13/01/2016 08:13:37

Julan,

Do you fully or partially thread your bushes before soldering in?

Gary and Julian thank you. What a great example of " a picture speaks a thousand words" I would never have considered the block in the firebox to conserve heat and protect other areas, and the view of the flux gives me a really good idea of the amount and consistency.

Kwil - I usually partly thread but being daft I often forget when making the bushes and go all the way through. Not had any failures with the fully threaded bushes yet.

Anyhow I can forget about boilers now for a fortnight - snow has fallen and the slopes are beckoning -- off skiing hooray !!

Kind regards all

BK.

Julian,

Thanks for your photo - very informative. Especially how you use Thermalite blocks. What size of burner would you use for this job when you do light up? I'm looking to have a go at a Minnie TE sized boiler. I suspect that the boiler in your photo is a little larger.

Regards

Phil

hi phil,

i have a sievert set and 13kg propane bottle. for boiler work i use the 2943 burner.

all bushes PB102/hard drawn phos bronze, and only partly tapped with a taper tap before silver soldering.

glad you like the pic -there are more of this boiler in 2 of my albums. doing the backhead is the final stage and always a bit nerve wracking. i also check beforehand that my propane bottle is still heavy!

cheers,

julian

Hi Julian,

Thanks for your reply - your album does tell a story.

The Sievert 2943 at 43.5 kw is, according to my research, <> 3.5 times the output of the Bullfinch 1250 that I used in my first failed efforts on a Minnie firebox, perhaps I am getting nearer to understanding what will be required to complete the boiler.

Regards

Phil

Hi all,

There's a distinct difference how the flux will perform when sliver brazing steel than when dealing with copper or brass; copper / brass / unleaded gunmetal/ drawn bronze etc, the flux will all work and react in much a similar manner and the Easyflo type flux is most suitable for the much smaller assemblies, but as Julian has said a more a suitable flux where prolonged heating is required such as boiler construction, mechanical cleaning is the norm, but not emery cloth etc, before the application of any flux. Also it is essential to reflect as much heat back onto the job whilst its being heated, the Themalite or similar block being very suitable. Steel should be treated in a very similar manner, mechanically cleaned but using a flux capable of more prolonged heating is required and plenty of it, remember flux is relatively cheap so don't skimp on it. and when silver brazing steel try to bring the job to the correct temperature as quick as possible, support the job off of the hearth on some odd pieces of fire brick rubble to allow the heat to circulate all around the job. Following any silver brazing job let it cool to black even down to a point where you can comfortably pick it up and let the job soak for a couple of hours in clean cold water, you will find the majority of the flux residue will then just rub off. One thing, avoid Borax etc like the plague, the flux when heated goes like glass and is nearly as hard, you'll spend hours trying to clean the job, use a flux compounded and matched to the sliver brazing process.

Regards,

Bob Youldon

The basic principle of silver soldering is capillary flow.

Everything that you do for strong sound joints and be successful is to promote that capillary flow.

For capillary flow to occur there must be a gap.

No gap = no flow = no joint

As described earlier, centre punching or roughing up the surface with a coarse file will help produce a 0.1mm gap. Placing foil in the joint will also work but consider using silver solder foil that you do not have to remove.

Pre-cleaning copper, chemically or otherwise, prior to silver soldering is often unnecessary

At the first instance that you apply heat, you create more oxide on the surface that you hve so painstakingly removed. Joint cleanliness, removing oxides and keeping the surface free of oxide, is the job of the flux. Simply make sure the joint is free of oil and grease which the flux cannot tackle. The temperatures involved will drive off any grease from the hands. Note. This comment does not apply when soft soldering

Use a flux that will operate for the heating period.

If necessary use a long life flux eg HT5. Its residues are more difficult to remove but clean the joint afterwards in 10% caustic soda. Flux which becomes exhausted will not keep the joint free of oxides.

The alloy will always flow to where it is hottest.

Heat the joint in the area to where you want the alloy to flow in relation to where you are applying the alloy. This ensures there are no cold spots in the joint that will cause the alloy to freeze, stop flowing and lead to a lack of penetration. As per Gary

HEAT THE JOINT NOT THE ALLOY

For more detailed information go to **LINK**

Stick to the principles af capillary flow and you will be successful!

Keith

Edited By CuP Alloys 1 on 13/01/2016 13:30:39

Hi Julian, I've sent you a PM.

Roger