Silver soldering contradiction

[Norman BillinghamParticipant @normanbillingham91454]

There's a fairly famous study from about 1939 which measured the strength of a torch-brazed butt joint in stainless steel as a function of the gap (it appears as a figure in Keith Hale's book on p. 19). It and several other studies show that the tensile strength falls off pretty linearly as the gap gets bigger, from a maximum of about 140,000 psi at 0.002" gap to 40,000 psi at 0.025" gap. In that study the strength also falls off below 0.002" gap but its been suggested that the reason for that is that a very small gap leads to weakening of the joint due to incomplete exclusion of the flux and the effect vanishes in vacuum or inert atmosphere brazing. In that test the strength was still 100,000 psi when the gap was nominally zero – i.e. the two pieces were in contact

As others have said there has to be some gap or there's nowhere for the alloy to go – but it doesn't have to be much – though even at 25 thou a strength of 40,000 psi is 18 tons psi or 3GPa which is still pretty strong (comparable to annealed copper). Bear in mind that as you scale to model sizes the stresses also scale.

As far as cleaning is concerned, organic contamination like surface grease, oil etc. will very easily burn off at brazing temperatures, so very careful degreasing is less important as long as the flux can spread. However it's so easy to give things a wipe over before putting on the flux that one may as well do it. Certainly Johnson Matthey in their video of the process, which we use in our training course at SMEE, advocate degreasing before fluxing.

The situation is very different for soldering. At the lower temperatures involved organic crud won't burn off so easily so careful degreasing is a good thing.

[Tim StevensParticipant @timstevens64731]

In my opinion (as an ex-apprentice silversmith, but a looong time ago) the need for a gap is a bit of a misunderstanding. There are joints where a press fit followed by soldering can be perfect, with flow right through the joint and a meniscus both sides. But, it all depends. The flux needs to be clean (no tiny emery grains, eg) and with a recipe which melts completely at the flow temperature of the solder. The solder needs to melt so that there are no tiny grains mixed in with the liquid metal. And there need to be no burrs and rough or loose places where tiny slivers can break loose.

The problems arise when these complex conditions are not met. If the flux or the metal are at all pasty, then flow into a tight joint will not be wonderful – there won't be room for the granules and they will get in the way.

So, it might seem unkind to suggest this, but if you always need a gap your technique, or materials, etc might not be perfect. Capilary attraction is a surface effect with liquids; get the surfaces and the liquids right and the closer the better.

Debate can now continue …

Tim

[JasonBModerator @jasonb]

Tim, what does this press fit actually fit like when at 650degrees?

usual advice on here when one part if stuck in another is to apply heat to get them apart, so this suggests that any gap increases when the parts are heated.

I wonder why Johnson Matthey give optimum gaps for their solders if a gap is not needed, maybe they are saying their materials are not perfect? 

 

Edited By JasonB on 12/04/2018 19:22:07

[Russell EberhardtParticipant @russelleberhardt48058]

Perhaps, when considering if the gap is big enough it might help to note that the diameter of a silver atom is about 300 picometres. So a gap of 0.1 mm is enough for about 300,000 layers of silver atoms!

Russell

[not done it yetParticipant @notdoneityet]

Posted by JasonB on 12/04/2018 18:59:58:

… usual advice on here when one part if stuck in another is to apply heat to get them apart, so this suggests that any gap increases when the parts are heated……

Doesn’t that rather depend on the relative coefficients of linear expansion? ‘Usual’ is probably right because many don’t think of the expansion coefficients.

[nigel jones 5Participant @nigeljones5]

Kwil – nothing to do with cavalier attitude, more to do with people who have no experience quoting inaccurate informational having read something in a book somewhere written by another person who probably did the same. If you know better then please enlighten us with your experience. I share mine in order to help others from the pitfalls of missguided information, whether it flies in the face of popular myth or not. Show me someone on here with more experience of silver soldering and I will bow to their word. Hundreds of boilers made, not a single return, complaint or even negative comment ever made. Jason – the gap grows condiderably when heated, almost to the point that it is too big a gap, hence a nice tight fit at the start yields perfect results once youve gone all over the piece. I will offer no further advice on any aspect of boilers, ask Kwill in future, no one wants to hear from a proffesional boiler maker! PM me if you have anything you want to ask.

[JasonBModerator @jasonb]

Fizzy you have just confirmed what I have been saying that when you make a boiler you get a gap between the flanged plate and the barrel when it is hot, this is why JM etc quote the gap of say 0.1mm as the gap at melting point not the gap when cold which could well be a push fit. Agree that too big a gap when cold will only get worse when heated and the solder simply won't bridge across and will just run to the lowest point.

This does not happen to anywhere near the same amount when you have two flat faces against each other that I why other means of creating a gap are needed.

I think Shaun who posted earlier and Keith both from CuP who are forum members here may know a thing or two from many years in the trade of selling silver solder. And I think Keith has written the book to which you may be reffering.

 

Edited By JasonB on 12/04/2018 20:21:55

[Bill PhinnParticipant @billphinn90025]

Posted by Jeff Dayman on 12/04/2018 16:12:49:

I've always tried for best cleanliness, enough good quality flux, and adequate heat when silver soldering, and have had consistently good results for many years. I typically leave a two or three thou gap between parts. I too read about Kozo Hiraoka's centre punch mark gap generation method many years ago and use it often.

Just my opinion and $0.02 worth.

Cleanliness is vital, i.e. surfaces to be joined should be free of grease, oxidation or any other contamination which might interfere with solder flow across the surfaces to be bonded.

 

 

When soldering silver and gold jewellery, or copper and brass for more utilitarian purposes, my aim has always been to get as close a fit as possible between the surfaces to be joined, not to consciously leave a gap. In practice there always is a gap, but ideally it will be the minimal kind of gap that can be achieved with accurate hand filing. A yardstick is that if I hold a ring shank before soldering up to the light and can see a chink of light passing through the joint anywhere, the gap is too large.

 

 

If the surfaces to be joined have been milled down to an accuracy of the kind found in gauge blocks, which "ring" together and stay together entirely of their own accord, and these surfaces are mechanically clamped together during soldering then quite probably the gap would be inadequate for good solder flow, but in practice I'd be surprised if the gaps left by most of us who pride ourselves on being competent solderers (and gauge this partly from how our work holds up over time) are ever too small for solder to properly flow into. Poor solder flow is nearly always a result of other factors than too tight a joint, and in my experience of people reporting problems with soldering, uneven, or slow or inadequate heating are the primary culprits, followed closely by insufficiently clean and insufficiently tight joints.

 

Edited By Bill Phinn on 12/04/2018 20:20:14

Edited By Bill Phinn on 12/04/2018 20:21:26

[Tim StevensParticipant @timstevens64731]

If the two parts are the same metal, and at the same temperature, they will be exactly the same amount bigger when hot as when cold. The problem is that sometimes the metals are different, but mainly that the temperatures are different. If this produces a tighter fit, then at red heat, something will give, almost inevitably – certainly so with copper (or silver). Such a joint, when cooled, or when both sides do reach the same temp, will be looser. In other cases, the temp difference itself produces looseness – a hot tube in a very hot hole, for example. So, in ideal cases (ie once in a blue moon*) the conditions stay the same, but almost always the gap can get bigger. Not because it has to, but because circumstances are not ideal.

* for those from the NW of England, the term 'once every Preston Guild' might be more familiar.

I do hope this is making sense to some of you …

Tim

[vintagengineerParticipant @vintagengineer]

I have silver soldered numerous vintage propshafts. I use silver solder strip and press the joint together and then heat up the joint. No gap.

[Pete RimmerParticipant @peterimmer30576]

In my work we have a guy that re-tips drill bits with silver solder. There's a spring clamp that presses the tips hard against the drill to solder them. These things see extreme duty and they are soldered with no gap.

[Keith HaleParticipant @keithhale68713]

Hi All,

Forgive me

NO GAP = NO METAL FLOW = NO JOINT

Keith Hale

BSc Hons (Metall) (for those who want info from a metallurgist!)

The chap from CuP Alloys that does the lectures!

49 years in the brazing/silver soldering business

ex JM Sales Technical Services

ex Product Manager Sheffield Smelting Co (Thessco) and Engelhard Industries

ex MD oc CuP Alloys Ltd and

ex MD CP Alloys Ltd (manufacturerer of brazing alloys)

NO GAP = NO METAL FLOW = NO JOINT

Keith

PS Yes alloy can take other capillary routes eg capillary paths around tube/plate joints around screw heads but if you want the best strongest joints, the filler matal needs to penetrate through the joint not simply around it. Recognize the problem of leaks around stays or boiler tubes? You don't get them if the alloy penetrates the joint.

Out!

KH

[duncan webster 1Participant @duncanwebster1]

I always used to believe that cleanliness was next to Godliness for silver soldering. Then I went to one of CuP Alloy's excellent lectures where he said that it's the flux that does the cleaning. Next day I dug up a bit of copper pipe which had been buried in the garden for at least a year as a support for launching fireworks, cut 2 pieces out of it and silver soldered them together. Perfect joint, and clean metal where the flux had been. I must admit to wiping the mud off, but nothing else. Would I do this with something that mattered? No, but it just shows you don't need to go mad

Edited By duncan webster on 13/04/2018 00:40:45

[Sam StonesParticipant @samstones42903]

Back in 2010 when I resumed work on my skeleton clock, there were about half a dozen small brass parts outstanding.

Cost precluded buying minimum quantities of plate and bar-stock, so I chose to fabricate from smaller sections rather than cut them from solid.

My M.E. photo album ‘John Stevens Skeleton Clock Part 1’

**LINK**

… shows parts which I silver soldered.

• Minute-wheel Cock (this is in 3 pieces)
• Lifting piece in place (minute-wheel cock in position)
• The Maintaining Detent (2 pictures … in 2 pieces before and after)
• The Bell `Standard' & Nut (the standard is in five pieces)

The faces to be joined were hand filed to shape and adjusted to as close as possible, i.e. no gap. Parts were held together and either weighted or, in the case of the maintaining detent, I built a jig. In every joint, the solder ran instantly. As required, the joints are hard to detect, although it was a bit careless of me to miss some of the fillets during cleanup.

I read somewhere that, during the process, brazing material can actually alloy with the parent material thus producing five layers. Any thoughts about this?

Sam

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Sam Stones on 13/04/2018 01:12:38:

 

… I read somewhere that, during the process, brazing material can actually alloy with the parent material thus producing five layers. Any thoughts about this?

Sam

.

Sam,

Did you read the piece that I quoted from, earlier ?

MichaelG.

.

[extract from the earlier quote] "the essential feature of a soldered joint is that a metallurgical bond is produced at the filler-metal/base-metal interface. The solder reacts with a small amount of the base metal and wets the metal by forming intermetallic compounds."[/extract from the earlier quote]

I think the problem with this topic might be that people are using common words like 'gap' and 'layer' without necessarily meaning the exact same thing as the next man.

Edit: apologies … I forgot to mention that the link describes Soldering rather than Brazing, and that 'Silver Soldering' is perhaps best seen as 'a little bit of both'.

Edited By Michael Gilligan on 13/04/2018 08:47:52

[Brian HParticipant @brianh50089]

KH mentions something that many people are unaware of, namely that the silver solder should be fed in one side and observed to appear at the other side. This shows that the solder has penetrated the joint.

Brian

[Michael GilliganParticipant @michaelgilligan61133]

Posted by BDH on 13/04/2018 08:24:32:

KH mentions something that many people are unaware of, namely that the silver solder should be fed in one side and observed to appear at the other side. This shows that the solder has penetrated the joint.

.

Or [alternatively, and perhaps more typically for small jobs] the solder is included within the joint area, either as 'paillons' or as rings of wire … and the flow comes out to the edge.

MichaelG.

[HopperParticipant @hopper]

Posted by Michael Gilligan on 13/04/2018 08:13:08:

I think the problem with this topic might be that people are using common words like 'gap' and 'layer' without necessarily meaning the exact same thing as the next man.

Therein lies the problem, quite likely.

If you beat or press a domed/flanged boiler end out of flat copper plate and make it a tight fit in the piece of copper pipe that forms the main boiler shell, does that mean it's a "no gap" fit? Or does it mean the end piece is a tight fit on the three points around the OD that are in contact with the ID of the copper pipe and there is possibly a one or two thou gap throughout the rest of the joint?

To mate two pieces of metal together with gaps of less than a thou or so often requires engineers' blue, scrapers, surface plates and the like. Would bits of copper sheeting and pipe as formed stand up as "no gap" fits if checked with engineers blue etc to a similar standard?

And if you prep the copper surfaces with emery paper, do the myriad surface scratches created constitute a "gap", being they could be a thou deep in each of the two pieces?

 

Edited By Hopper on 13/04/2018 08:48:20

Edited By Hopper on 13/04/2018 08:48:59

[Bob StevensonParticipant @bobstevenson13909]

The danger now with this topic is that people who are new to silver soldering will read this and assume that they are obliged to make provision for a 'gap' when preparing parts to allow for the solder ….In actual working practice one can forget about the 'gap' when making up parts and components by hand methods (and most machining too) as there is every chance that solder WILL flow into joints…and the solder WILL NOT fill any gaps…..

 

Again;..parts must be a good close fit……the parts MUST be clean of impurities………the joint MUST be hot enough (red, 'glowing&#39 before solder is applied.

 

BEWARE of;….impurities in fuel or torch that spoil cleanliness before temperature is reached……lack of heat due to poor torch choice, poor heating technique……touching the parts with bare hands after cleaning……..not dissolving flux, such as Borax, sufficiently before application so that the undisssolved 'grains' can't run into the joint.

Edited By Bob Stevenson on 13/04/2018 15:41:13

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Bob Stevenson on 13/04/2018 15:36:50:

The danger now with this topic is that people who are new to silver soldering will read this and assume that they are obliged to make provision for a 'gap' …

.

Sorry, Bob … I must beg to differ !!

If those people actually READ what has been written, I think they will be able to make a reasonably well-informed judgement.

MichaelG.

[Sam StonesParticipant @samstones42903]

Sam,

Did you read the piece that I quoted from, earlier ?

MichaelG.

Guilty as charged, Michael

I knew I read it somewhere.

Is this what happens when we get old?

Keep smiling,

Sam

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Sam Stones on 13/04/2018 20:08:27:

I knew I read it somewhere.

Is this what happens when we get old?

.

You're doing fine, Sam

MichaelG.

[vintagengineerParticipant @vintagengineer]

This is what I would use if using filler rod.

Tight fit

Negligible clearances which can be assembled or disassembled by hand – e.g. hubs, gears, pulleys, bushings, frequently removed bearings

H7/j6

[Sam StonesParticipant @samstones42903]

For a butt joint, I would interpret that to mean lightly clamped, VE !?

Does this help those who have never seen or heard of H7/j6 ?

https://en.wikipedia.org/wiki/Engineering_tolerance 

'Go to the back of the class, Sam!"

 

Edited By Sam Stones on 14/04/2018 20:37:26

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