Corrosion resistant steam generator and method of making same

A method is described of fabricating a steam generator comprising the steps of: providing inlet and outlet chambers and a secondary coolant chamber and at least one primary coolant tube passing through the secondary coolant chamber and connecting the inlet chamber to the outlet chamber; positioning a tubesheet between one of the inlet or outlet chambers and the secondary chamber, the tubesheet having at least one passage therethrough; upsetting ends of the primary coolant tube to form a transition region in the ends, the transition region having smoothly varying wall thickness; reducing the outside diameter of a portion of the upset ends to accept a sleeve; installing the sleeve on the outside diameter portion; closing any space between the sleeve and the reduced outside diameter portion along an interface therebetween; welding the sleeve to the primary coolant tube to form a sleeve and tube assembly; inserting the sleeve and tube assembly into the passage in the tubesheet; and welding the sleeve and tube assembly to the tubesheet