I agree, but you have raised an interesting thought and that is how small a weld can be made and what would you use to keep the weld from blowing out on small materials. Time for a little research on micro welding. Using steel poses a corrosion issue that copper does not. However I have seen an industrial boiler made of stainless that was made for a brewery. And I have considered a boiler design made of steel for small hobby stuff but it would be difficult to make it look like the copper models you often see. And yes I think there is a provision in the code for certifying a pressure system with a hydraulic test. But its been awhile since I have delved into such issues. What I remember from the code on small boilers if they run above a certain pressure 100 psig they are considered the same as the big boys and technically under all the same rules.Well, that's certainly interesting. I understand why it would need to be certified especially for commercial boilers, however, I would thimpfk that welding would be the very best way to make a steel boiler. Also, for small boilers, I would thimpfk pressure tests would be adequate.