Silver soldering question

[DanP]

I was just told by a fellow that once you silver solder two pieces of metal together you cannot unsolder them. The conversation came up when I said I wanted to remove a bushing from a boiler and replace it with a new one. Is this true?

 

[old-and-broken]

I believe a new alloy is produced at the interface of the metals, and the new melting point for it becomes much closer to that of the parent material. It can be done I believe, but may present some danger of over heating the nearby surfaces.

 

[jasonh]

Pieces of metal that are silver brazed/soldered can be un-soldered. I've had to do it in the course of correcting mistakes. I can't say I noticed any detriments when I re-brazed the joint - but it wasn't a scientific study.

 

[skyline1]

Hi Dan

Yes it can be done under certain circumstances but it is tricky and best avoided if at all possible I had to unsolder a boiler bush myself once and there are two problems, one is the alloying effect that old-and-broken mentioned which means that, depending on the grade of silver solder used, the temperature needed to re-melt the joint is quite high. A particular problem with brass bushes as it is quite close to the melting point of brass.

If the bush doesn't actually melt it will degrade and crumble due to burning off of the Zinc. If you heat a piece of brass to near its melting point you will see what I mean, don't breathe any of the zinc fumes though as zinc oxide fumes are quite dangerous.

The other problem is a mechanical one. It is good practice to ensure the joint is mechanically sound before soldering, the silver solder should merely seal the joint. This means that not only do you have to melt the joint but mechanically break it whilst it's red hot, not easy!

with a stepped brass bush what I did was to drill it out to the diameter of the boiler hole thus removing the stepped portion and then unsolder the "washer" that is left on the outside. If you are a little over zealous with the drilling, as long as you don't go too mad it won't make any difference just make a slightly bigger bush.

Regards Mark

 

[gus]

Hi Dan

Yes it can be done under certain circumstances but it is tricky and best avoided if at all possible I had to unsolder a boiler bush myself once and there are two problems, one is the alloying effect that old-and-broken mentioned which means that, depending on the grade of silver solder used, the temperature needed to re-melt the joint is quite high. A particular problem with brass bushes as it is quite close to the melting point of brass.

If the bush doesn't actually melt it will degrade and crumble due to burning off of the Zinc. If you heat a piece of brass to near its melting point you will see what I mean, don't breathe any of the zinc fumes though as zinc oxide fumes are quite dangerous.

The other problem is a mechanical one. It is good practice to ensure the joint is mechanically sound before soldering, the silver solder should merely seal the joint. This means that not only do you have to melt the joint but mechanically break it whilst it's red hot, not easy!

with a stepped brass bush what I did was to drill it out to the diameter of the boiler hole thus removing the stepped portion and then unsolder the "washer" that is left on the outside. If you are a little over zealous with the drilling, as long as you don't go too mad it won't make any difference just make a slightly bigger bush.

Regards Mark

Hi Mark,

You are right. Drill bigger hole and put a bigger socket.

 

[DanP]

Thanks for the answers guys.

Drill bigger hole and put in a bigger socket

Yes this is what we did.

a new alloy is produced at the interface of the metals

this is what I was told and that is the problem.

 

[lensman57]

I was just told by a fellow that once you silver solder two pieces of metal together you cannot unsolder them. The conversation came up when I said I wanted to remove a bushing from a boiler and replace it with a new one. Is this true?

Hi,

Once two pieces of metal have been soldered together, the solder forms a metalic bind between the two metals, this is not a joint in the conventional way but a metallurgical one. It is therefore very difficult if not impossible to unsolder the joint if the soldering was done correctly in the first place.

Regards,

A.G

 

[Entropy455]

Where I work it’s a common practice to un-sweat valves, flanges, piping components, etc. and re-braze the joints – either with installing, or not installing new piping components. Sometimes piping sections are un-sweated simply so that mechanics may gain physical access to other machinery.

The brazing material is silver based, and melts anywhere between 1300 and 1600 degrees F (depending on the silver alloy used).

I would agree that the silver cannot be completely removed from the surface of the parts (brass, copper, copper-nickel, etc.) This is because of the wetting action of the brazing material - where the silver physically bonds to the surface of the base materials. Nonetheless, if you melt the silver brazing material, the two parts will gain a degree of freedom, and they will come apart.