Hairline cracks in Silver solder

[IanmacParticipant @ianmac]

We have built a few boilers in our club over the past three years all have been satisfactory until the last two using cadmium free silver solder.

Two boilers built in the last two months have both thrown up leaks on first water test which appears to be coming through hairline cracks in the Solder. Both were in areas where there were butt joints. ie no rivets.

Given the problem showing up on the first of these two boilers particular care was taken to ensure the second was fully heated before soldering and cooled as gradually as we could achieve to avoid any differential expansion and contraction. Propane was used as the heat source.

Is there an issue with cadmium free Silver Solder that could be causing this ? ie less tolerant than the older solder to expansion and contraction across a joint

[IanmacParticipant @ianmac]

[KWILParticipant @kwil]

You say butt joints, where precisely?

[blowlampParticipant @blowlamp]

Butt joint on a soldered boiler – as in edge to edge?

Martin.

[Phil PParticipant @philp]

I suppose it would be inappropriate to make jokes about butt cracks You have my sympathies on using the so called equivalent silver solder, I have been dreading having to go down that route, as I have a couple of boilers to make soon.

Amazingly I have just been given (yes given) almost 2kg of 5mm x 1mm flat strips of easy-flo N° 2.

So I have no intention of buying any of the new silver solder (probably ever), but will need some more flux. Does anyone know if the easy flo fluxes available now are OK to use with good old fashioned Cadmium bearing silver solder ?

I assume it will be OK, but can anyone confirm this ?

Thanks

Phil

[John Ockleshaw 1Participant @johnockleshaw1]

Hello Ianmac,

I think you will find the cracking is caused by the joint design.

I was never able to get a, Austen-Walton designed, boiler for Twin Sisters completed because of the butt joint opening up as the boiler cooled.

Regards, Graham

[SpringbokParticipant @springbok]

Phil
If you PM me will give you enough flux to last you a lifetime, my old techie who was oirsh as they come had a great love of his job and still a dear friend, when he retired after about 30 years it came down to me to clear out the storeroom as metalwork was deemed to dangerous. even found a brand new workbench stored in the back, now in my workshop before the porters could skip it.. Also managed to rescue all sorts of sheet metal brass,copper, you name it. eventually before I retired even saw someone useing pkts of tesco spagetti and glue in an attempt to get kids to construct something sad…..

Bob.

[Phil PParticipant @philp]

Bob

I will send a PM with my address, that is a very generous offer thankyou.

If you reply with your address I will return the favour with some 5mm x 1mm flat strips of silvery looking metal.

Phil

[KWILParticipant @kwil]

Dont bother with EasyFlow fluxes, CuP Alloys HT5 is much better and lasts longer under the heat.

Butt joints are a NO NO in boilers, a butt strap overlapping the joint must be used or else TIG welding

is an alternative if you can get it done.

[MICHAEL WILLIAMSParticipant @michaelwilliams41215]

Worth mentioning that the advantages in putting a few screws or rivets in a joint to be silver soldered are the obvious one that they hold the joint together while silver soldering and the less obvoius one that they hold the joint together while silver solder is setting . Avoidance of movement while silver solder is setting is paramount .

As for Cadmium free silver solder this has been the norm in industry for many years and has been used without problems .

Most common problem in home workshops is that heating equipment that was just barely adequate for Easy Flo is probably inadequate for Cadmium free silver solder due to the slightly higher melting point . Also the higher temperature tends to exhaust standard flux a bit quicker .

Michael Williams .

[CuP AlloysParticipant @cupalloys]

The strength of easiflo 2 and the 55% silver cadmium free alternative is identical.

The ductility is very similar – 27% v 24%

Butt joints are the weakest joint design for brazing.As the alloy cools in the joint, it contracts leading to localized "necking". This acts as a "stress raiser" leading to the alloy itself bearing all the stress instead of the alloy carrying the stress from one part of the joint to the other. This is the basis of the phenomenon that in the vast majority of cases the weakest part of a brazed joint is the parent materials. All brazed joints should be designed to operate in shear or torsion – not tension. eg use a strap as per KWIL.

Ensure that the joint is allowed to cool naturally and so reduce any thermal stresses.

Another thought – are you building the crack into the back of the joint by not achieving full penetration of the alloy into the joint.

Make sure the whole of the joint is above the melting point of the alloy. Let the silver solder get its heat from the joint. Do you need a bigger burner fopr a little more heat as per Michael.

Get the part of the joint you want the alloy to flow to hotter than the point at which you feed the alloy.

Ensure the joint gap is maintained between 0.1 and 0.2mm. Too large a gap creates more "necking"

The cadmium free alloy is no more prone to cracking than easi flo2.

Ensure your flux has sufficient life for the heating time. Geneally speaking you can use the same flux for cadmium free and cadmium bearing silver solder. Dependent on the circumstances (temperature and heating time) you may be better off using a longer life flux.

A simple message to all model engineers;

if you stick to the basic principle of the brazing process you will be successful. Everything you do is to promote capillary flow. Get the joint design and materials right, flux them appropiately, heat it correctly. Most problems associated with silver soldering lie behind the torch not in front of it!

Embrace the cadmium free alloys. There are slight differences but there is no need to fear them.

keith

[michael howarth 1Participant @michaelhowarth1]

Keith……I have seen small boiler designs where the hollow end of a drawn copper tube is placed on a sheet of copper and silver soldered on, thus forming the backhead. Would you describe this as a butt joint ?

Mick

[jason moore 1Participant @jasonmoore1]

I would be interested in the answer to this as well, also smokebox tubeplates without a flange.

Seems to be the norm in gauge 1 designs which are new to me having only built 3 1/2" gauge boilers till now.

[CuP AlloysParticipant @cupalloys]

Hi Mick/Jason

It is a butt joint of a fashion but in these cases there are redeeming features.

1) as long as the tube is free to expand the silver solder will flow around and between the tube/sheet. In doing so, the alloy will create a fillet on the inside and outside of the tube.

2) there is no local "necking" in the joint material but a smooth fillet.

There is no stress raising element in the joint when transfering the stress from the sheet to the tube as in an edge to edge joint in sheet. This is where we came in!

From solely a brazing perspective with a view to ensuring complete penetration and a sound leaktight joint:-

Mick, I would prefer that the sheet in your case is made into a cup or cut to match the id of the tube.

and

Jason, I would prefer that the tubes passed through the sheet

But in either case, the pressure in the boiler is way below the strength of the silver solder so you should be OK – as long as the joint is sound.

Keith

PS You will probably find that you will use less alloy and spend less money by producing joints that do not rely on fillets.

[michael howarth 1Participant @michaelhowarth1]

Keith………Thanks for that advice, much appreciated.

Mick

[SpringbokParticipant @springbok]

Ian

Flux winging its way to you today, via snailmail. will be in a sealed bag so hopefully get to you in one bit.
Bob.

[IanmacParticipant @ianmac]

Thanks for the feedback and the detailed info from Keith

Ianmac

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