After Silver Soldering

[Runner]

Hi all,

I am dismayed by how soft copper based materials are after silver soldering and therefore easily damaged. My latest problem is the manufacture of an axle driven pump for a locomotive that required two gun metal cylinders silver soldered together to produce a T shaped form one for the ball valves, the other for the pump. The pump is fixed to the cross member by cutting a thread 9/16" x 26 TPI on the pump cylinder, one to enable a lock nut to lock it to the cross member and two to facilitate a sealing nut. In attempting to cut the thread I had to clamp the cylinder in a vise, which because of the softness of the material went slightly oval, therefore the plunger which was a good sliding fit wouldn't operate in the bore. I am now getting to the point of my enquiry. How can the assembly be hardened after silver soldering so that subsequent work doesn't damaged the assembly.

Does leaving the assembly alone work harden it after time?

Thanks in advance

Brian

 

[chipenter]

Cut all the threads first then silver solder them , gunmetal is pretty soft to start with and maybe you would have damaged the barrel any way .

 

[goldstar31]

My views- others may agree or not!

Frankly, I think that there is no way in which you can now harden copper with a heap of silver solder with lower melting points and which it has already 'mixed' with the parent metal. I think that you face correcting the damage- mechanically. You may have to make up a reamer or a lap or whatever variant. On that, I cannot comment other than to tell you what not to do next time- and this you know already.

Without hanging on bits, you can harden copper by heating it up exactly opposite to iron and steel. Copper/copper alloy tools were hardened by taking them up to red heat and letting them cool- slowly. Copper will work harden and most people have the opposite problem to you in that copper boilers and whatever harden during silver soldering- and have to be 'let down' to continue. I suppose that you know this already.

If I had the problem, I'd make up a soft reamer dead to size in mild steel rather like a D bit. I've hit worse than you in the past. I'd made up a cast iron head for a Westbury Milling machine that I was making. All was well -until I split it-- and it closed up on me!
I wasn't alone- the great George Thomas was making his pillar tool- and split the bores- and they closed. I got to the Quorn and realised that it would close up as well.

As Tom Jones would sing-- 'It's not unusual.'

Regards

Norman

 

[GWRdriver]

My views- others may agree or not! . . .

Unless I misunderstand your wording, I can't agree with this. It flies in the face of known conventional wisdom.

you can harden copper by heating it up exactly opposite to iron and steel. Copper/copper alloy tools were hardened by taking them up to red heat and letting them cool - slowly.

That is the perfect description of annealing (softening) copper, and it doesn't matter if it's quenched or allowed to air cool or blanketed to control cooling. The result is the same, soft copper. Bronzes are alloys so each one will vary in their response to annealing, and some won't be soften in the process. Whatever hardness was imparted to copper or bronze tools was done by working (beating and forming), thus the term "work hardening."

copper boilers and whatever harden during silver soldering- and have to be 'let down' to continue.

This simply isn't true. Silver soldered copper boilers, whether quenched or air-cooled, end up in an almost completely annealed state. They will however subsequently regain almost 100% hardness with the passage of time and repeated pressurization.

 

[goldstar31]

Well, there is no harm in having a friendly disagreement.

Can I go off for a bit of a tangent? A soldering iron- a proper one that goes into the fire or whatever. It will not pick up solder. What does one do? I'm interested. Of course, I know!

Regards

Norman

 

[GWRdriver]

Well, there is no harm in having a friendly disagreement - Norman

No, none at all. I do want to add something about threading and silver solder. You can of course pre-thread bushings and fittings which will be silver-soldered, but here is the problem (for me) which I try to avoid.

The amount of heat copper (or bronze or GM) will absorb and retain, in a give flame, in a given period of time, is directly related to its mass . . .that is, how much metal the is there to heat. The less metal, the less time and heat it takes to get it red. IF you pre-thread your bushings the thinnest part of your boiler will then be the thread crests. At the same relative heat, the threads will heat far more quickly and to a greater temperature than will the surrounding bushing, so much so that there is a real danger of the threads being burned (ie, material oxidized away), or even burned off. In some cases this is unavoidable, the average person with a torch simply cannot control the amount and placement of heat to the degree required to avoid overheating threads.

There is a way to avoid this problem, which isn't perfect, but then you just pick yer poison. When I machine bushings I drill the thread holes through and then (in the lathe) I run a tap in far enough to form maybe 1 fully formed thread, and several partial threads. This will give me a nice square and centered starter thread, which I can use after I'm done soldering and pickling, to run a full thread through. Even if the partial threads burn a little they will be reformed when the final tapping is done. I need to warn you that regardless of what is used for flux and pickle, or how well I've washed out, the acidic residue left in a boiler is highly corrosive to HHS tools so after tapping the taps (or dies) should be thoroughly cleaned, de-acidified, and well oiled. Otherwise the next time I come back to use them they will be covered in acidic corrosion. I learned this the hard way.

 

[Rivergypsy]

Nope, to anneal Cu, you get red heat and the quench in water, not air cool...

 

[GWRdriver]

Hi all, . . . an axle driven pump for a locomotive that required two gun metal cylinders silver soldered together to produce a T shaped form one for the ball valves, the other for the pump - Brian

Brian,
Many folks I know who build up pump bodies and such take the approach that what they are assembling is a "casting" and just as they would with a casting, all machining is done after the fact. This still leaves the problem, in this case, of excessive metal softness, so all things considered perhaps this one should be declared a scrapper and another attempt made, using a harder bronze alloy. I have used continuous cast bearing bronze stick for just such assemblies and it has worked out very well.

I do want to put another thought out for your consideration. Occasionally when I have built-up bits to make which are delicate, or which like most axle pumps will only see water pressure and won't be directly exposed to boiler or fire heat, I will use a good silver-bearing soft solder for assembly. If the mating connection is well made soft solder is plenty strong for the job and much if not all of the machining CAN be done before soldering.

 

[GWRdriver]

Nope, to anneal Cu, you get red heat and the quench in water, not air cool...

I have annealed hundreds of components for boilers and either way, quench or air cool, it ends up the same, annealed. Quenching is BETTER for the builder, because in addition to making the work go quicker, the thermal shock of quenching knocks off quite a bit of the copper oxide which appears during soldering, but it doesn't make the copper any softer.

 

[ausdier]

True Harry.
Air or water = same results.
Water is just faster.

 

[Runner]

Thanks to all who has provided input. After Norman's initial response I thought that I had the answer. Having quenched it to rid the assembly of the black stuff, I thought that was the wrong thing to do, instead let it air cool. Subsequent inputs show that either way the material is still in the annealed state.

Harry as always you have provided a detailed response. I was fortunate that I was able to rework the assembly and it works very well. I agree that soft solder and or loctite could be used in low pressure/temperature applications but silver soldering appears to be the 'sine qua non' for all applications of metal joining in the model engineering scene. The philosophy being "better safe than sorry".

Brian

 

[goldstar31]

I was having an strange thought. Of course, the Americans never had a Bronze Age!

Come to think of it, there is quite a thought about it elsewhere. Tin was in short supply and had to be imported in the really involved countries and supplies were erratic. So how did they get around making tools- which they clearly did- out of plain copper?

Don't get me wrong, they were quite 'with it' We are still trying to unravel their distant knowledge. But we would, wouldn't we?

 

[goldstar31]

Brian, As a serious reply to your problem, silver soldering is one of the joining techniques for relatively low temperature metal joining. Again, it is ideal for joining dissimilar metals- which we constantly have to address.

I was reading some of the quite interesting comments and realised that I have been retired for 27 years and 'did' my City And Guilds in Motor Vehicle Restoration at that time.
I always used silver solder paste- (and lead solder paste where appropriate) prior to the delight of making the joins. It is worth giving it a try.

 

[Rivergypsy]

Ah, sorry about that - I've been quenching to anneal for 25 years without realising an air cool would work. You learn something new every day!

 

[Runner]

Thanks goldstar31 about silver solder paste. But isn't it just a convenient and costs saving way of applying silver solder to small items like jewelery, you still have to heat the item to 750+ degs C, so you will have the same problem of leaving the item in an annealed state? Or am I missing something?

[email protected], I agree that we learn something new every day. My problem is remembering it when it's needed.

Brian

 

[GWRdriver]

Harry as always you have provided a detailed response. I was fortunate that I was able to rework the assembly and it works very well.

Brian,
Very happy to hear it.