The 3/8” thick lower flue sheet is welded into the boiler shell first.  This is the sheet that is exposed to the most heat and is welded both on the outside as well as on the inside to insure strength.  At the same time as the inside of the lower flue sheet is welded to the shell, the seam in the shell is also welded on the inside.  Once all welding was complete, the welds were bead-blasted clean and inspected.