My attempt at silver soldering the central heater box together tonight was not exactly a success. On the first try I used some flux that was left over from a couple of nights ago, and it completely failed to work. As a result the solder didn't stick at all. I also melted part of the cheap mole grips I was using to hold the pieces together. On the second try, I had a lot of trouble getting the inside of the joint up to temperature - everything around the joint was glowing bright red and the metal distorted quite a lot, but the top half of the overlapping portion of the joint shielded the bottom half from the flame and thus prevented it from heating up enough to melt the solder. I did eventually manage to get it to melt, but when it cooled down again the joint stayed in the distorted position that resulted from overheating the steel. So it is
stuck together, but the joint looks terrible. I suspect some combination of severe lack of skill and inappropriate equipment are to blame.
Now that MIG welding and silver-soldering have been abandoned I have to decide on Plan C, and I suspect the dreaded pop-rivets may feature prominently in it. It's likely though that I will put the heater project on the back burner for the time being and concentrate on the things I need to do to get Fenchurch through her MOT test instead - I want to get that over with before the end of this month if possible, so spending another week on the heater now would probably be unwise.
On a more positive note, here's a picture of me and Fenchurch. That weird facial expression which you probably recognise from my default userpic is me trying to fake a smile for the camera and failing badly at it.Update:
I've taken today off work, so I decided to have one last try at silver-soldering. I managed to get the ratio of water to flux powder better than I did previously and the flux paste went on quite nicely. I rearranged things so neither side of the joint was touching the (highly heat-absorbent) firebricks of my makeshift hearth. I quickly heated it up to brazing temperature, and the middle of the joint bowed way out due to insufficient clamping. I soldered the ends of the joint, then let it cool down hoping the bow would go away when it cooled down and the metal contracted, but it didn't. I rearranged the clamps to press the middle down and reheated it. The middle of the joint didn't stick (presumably the flux was exhausted by the first heating), and the outer ends came partially unstuck, leaving a joint that is poorly stuck together at the ends and has a huge gap in the middle. Doh :( I suspect the proper way to clamp a joint like this for brazing may involve fitting temporary screws or steel rivets along the length of it, then cutting them off and filling in the holes afterwards. I can't be bothered with that, so I officially give up on silver soldering. I have a tentative Plan C forming which involves using a thicker gauge of steel that doesn't instantly burn through and distort when I try to MIG weld it.