Hi Fizzy ,
You ask me a question with a hundred answers but here are some of the main points :
(1) As Jason makes clear it will be difficult to get heat into the right places in the firebox . Its not a question of getting overwhelming heat in there – it has to be an even heat and in quite a narrow band of allowable temperature . Too low a temperature and solder won't melt – to high and it will degrade or even vaporise .
(2) All heating will be done from one side of plate ie inside the firebox . With such thick plate and so much heat soak away to adjacent metal the other side of the plate will always be at a lower temperature . To get to soldering temperature right through the heated side will have to be very hot indeed and this may by itself take you outside the acceptance temperature for the solder .
(3) The big mismatch in thicknesses of tube and plate mean that tube will get hot almost instantly whereas plate will take (relatively) a long time . This means that there is the near certainty of burning through some tubes before the plate gets hot enough .
(4) The steel is heavily oxidised from welding . To silver solder properly it has to be absolutely scoured clean and bright – outside , inside and in the hole . It may not be impossible to get clean metal everywhere but it will be very difficult and a few missed bits will spoil the whole job .
(5) Separate from (3) above copper tube in contact with steel at high temperature will be chemically eroded . This is a slow process and normally doesn't matter but with a long heat and uncertain top temperature it will cause problems .
(6) The big variations in temperature in the boiler whilst silver soldering and the different expansion of copper tube to steel boiler shell means that the tubes will try to pull out of the joint as job cools down . This can lead to very weak joints . This effect has been a problem to a some people making all copper boilers but is likely to be worse with a steel / copper one .
Regards ,
Michael Williams .