So what is the big deal?
Well for a start, this is a big deal financially. The materials to be joined are expensive, and some are difficult to obtain.
And the silver solder itself is expensive. We ended up using 4 sticks of 45% silver solder, and an unknown quantity of oxygen, acetylene and propane.
And the end result will be inspected by an expert boiler inspector, and if it is substandard, it will be rejected. No argument.
So yeah, it was a big deal. Look at the pics. A few friends called in to watch and help. It was a cold wet winter day, and I had the workshop wood heater working to keep the troops happy.
I used steel wool to expose the elemental copper.
Me applying flux. Note the silver solder rings which were made on the lathe. Don’t ask where the brush originated.
The firebox wrapper was placed on the aerated concreted form to keep the tubeplate level, and allow the tubes to penetrate exactly 2mm. The tube ends were fluxed, and silver solder rings placed into the join.
The 44 fluxed firetubes with their rings of silver solder are in place. The top tube plate will not be brazed at this time. It is there to keep the firetubes in the correct position. The firebox tubeplate is also fluxed and sitting in position on its Hebel block.
Me on the left with the propane weed flamer applying gross heat, Stuart with oxyacetylene applying local heat and silver solder, and Swen cheering. Brendan is the photographer, trying not to get burnt.
We probably should have reshaped the forge to reflect the heat more efficiently.
When the brazing seemed to be finished, the work was allowed to cool to 200c, then quenched in water then a 15″ soak in 20% sulphuric acid to remove any remaining flux, then a further water soak to remove any acid. Yes, the top plate is upside down. Yes, it will be reversed eventually.
So, not a perfect result, but not bad for a beginner. Stuart opined that the job would have been better if my silver solder was the older, (more dangerous) but more runny type which contained cadmium.