Soft Solder/Silver Solder mix

[runner42]

I did an experiment to determine the consequences of silver soldering copper after soft soldering had been undertaken. The soft solder was high melting point with 5% silver content and LEAD FREE. Firstly the soft solder was applied to a copper test piece, although it was a resin cored type that had flux included, I added a little Bakers Fluid. Next, I put a piece of silver solder on the same place that had previously been tinned with the soft solder and applied heat to the test piece. As expected the soft solder melted first and formed a meniscus around the silver solder and joined the un-melted silver solder to the copper. Further heat saw the silver solder melt and appeared to mix with the soft solder. My conclusion was that the copper is unaffected by this soldering process.

I understand that the consequence of applying silver soldering temperatures to tin/lead soft solder is that the lead attacks the copper at this elevated temperature and causes severe degradation. So using LEAD FREE soft solder is not a killer.

Brian

Soft_Silver Mix by Brian Leach, on Flickr

 

[jasonb]

My understanding was that the lead in the soft solder boils and gives off gasses at silver soldering temperatures which will causing a weaker silver soldered joint not that the copper was unduly affected.

As your two allows have flowed together I would think there will be some weakening of the resulting alloy

[runner42]

Oct 13, 2021 6:30:46 GMT jasonb said:

My understanding was that the lead in the soft solder boils and gives off gasses at silver soldering temperatures which will causing a weaker silver soldered joint not that the copper was unduly affected.

As your two allows have flowed together I would think there will be some weakening of the resulting alloy

I stressed that it is a LEAD FREE soft solder so your point although informative is not applicable in this case. I accept that the resulting alloy is going to weaker than the silver solder alone, however I am not interested in the resulting alloy per se, only to verify that it is not incompatible and causes unspecified issues. The silver soldering is the process used for establishing strength jointing between each part and the high melting point soft solder may be used sparingly as a caulking medium to correct small leaks. If however, there is a need to recommence silver soldering on adjacent parts then this maybe undertaken without producing other problems.

Brian 

[oldnorton]

You make an interesting point Brian.

Silver solder 55 is listed by Johnson Matthey as being Ag55%, Cu21%, Zn 22% and Sn2%; so not a lot of Sn in the mix. If that is alloyed with more Sn from the 'new soft solder' in a copper joint I don't think we can say if the resultant alloy will be weaker or stronger.

I am using the same 95% Sn + 5% Ag modern soft solder to join my brass tender together. Although it melts at a slightly higher temperature than the 'old' lead mix solders the job is going well.

Norm

[SteveW]

Guys,
The bottom line bas to be what the boiler inspectors think of your new joint of unknown qualities. In essence you've got a joint made with a metal possibly new to science.

I recall the original issue of mixing soft solder with hard was the resulting tin/lead/copper alloy melted below the temperature of the old silver solders which back then likely included cadmium. Maybe there was a catalyst action between all the metals alloying together that caused the problem.

Another issue could be determining the lead-free-ness of the soft solder.

Soft solder is easily scraped off so why actually do this. That said, yes a useful experiment.

[jem]

I welded a piece of mild steel thick walled tube to a piece of cast iron with my mig welder, this seems to have stuck the two together very well! only trouble is that I found when I turned down the weld to the desired dimension, the weld was extremely hard, and so would not turn down to the same dimension as the steel and cast iron, do you think that the cast iron hardened to weld metal?

sorry this is not soft solder v silver solder, but perhaps similar vane!

best wishes

Jem

[SteveW]

Oct 16, 2021 17:54:07 GMT jem said:

I welded a piece of mild steel thick walled tube to a piece of cast iron with my mig welder, this seems to have stuck the two together very well! only trouble is that I found when I turned down the weld to the desired dimension, the weld was extremely hard, and so would not turn down to the same dimension as the steel and cast iron, do you think that the cast iron hardened to weld metal?

sorry this is not soft solder v silver solder, but perhaps similar vane!

best wishes

Jem

Jem, you might have discovered that adding a bit of the free carbon floating around in the cast iron to your mild steel turns it into high carbon steel.