copper -> steel conversion factors? Also copper to steel joints?
I am looking to build a boiler, but I want to build the thing in steel rather than copper. For our cousins in the US, europe and down in Oz, this will seem perfectly normal, but in the UK, raising the idea of a steel boiler in a group of model engineers gets a worse reaction that if you went into the car park and jumped up and down on their cars.
I'm hoping to stay away from the anger and indignation in this post, and get some basic facts.
Many of the designs I am looking at call for the use of 3mm copper for the main vessel. I also need to take into account a wastage factor of around 10%, so lets say it's 3.5mm. That would suggest a 3.5mm steel construction might be enough, if we assume that steel is as strong as copper, which is should be ( i think )
In my head, the copper will be in an annealed state after heating, so will go soft, where as the worst the steel will do is get harder.
However, a number of sources suggest that if building a boiler out of steel, significantly thicker sheet will be needed to get a like for like level of strength, with some suggesting up to 6mm sheet would be needed. This seems very thick to me, but I am absolutely prepared to accept that my world view is wrong on this, I just can't understand why.
Can anyone help me with this? What sort of conversion factors should I use when converting a copper based boiler to a steel boiler? Do I really need to double the thickness for what is a significantly stronger material?
Secondly, I will need to make some joins between copper and steel. My thoughts are that Silver Solder will be fine, but I wanted to check this to make sure - is there anything I need to be aware of? Special fluxes? Specific solders etc?