Patent ID: 8020272

Claim:
A method for joining tubes comprising: providing an outer tube; providing an inner tube to fit within the outer tube; forming a ridge on an outer surface of an end of the inner tube; after forming the ridge, fitting the end of the inner tube into an end of the outer tube to form an overlapped region of tube wall in which the outer tube overlies the ridge of the inner tube, and an additional region of at least one of the tubes that is not overlapped; after fitting the end of the inner tube into the end of the outer tube, roll forming the outer tube about the ridge of the inner tube to create a pressure tight joint between the overlapped region of the tubes; after roll forming to create a pressure tight joint, heating the overlapped region of the tubes and also heating the additional region of the at least one of the tubes; after heating the tubes, placing the heated overlapped region and the heated additional region of the at least one of the tubes in a pair of dies, the dies having a plurality of ring shaped cavities that encircle the overlapped region of the tubes and an additional die cavity adjacent the additional region of the at least one of the tubes; and, after placing the tubes in a pair of dies, introducing pressure into the tubes to expand the overlapped region of the tubes outwardly into the plurality of ring shaped cavities that encircle the overlapped region of the tubes and thereby form an interlocking joint by forming a plurality of annular corrugations between the outer tube and the inner tube at the overlapped region and also expanding the additional region of the at least one tube outwardly to form a bulged wall section whereby the interlocking joint is free of the formed ridge.