| 摘要 |
A sheet metal is rolled double to form a cylinder having an inner plate 2 and an outer plate 3 , and form an overlapped portion 4 , with their circumferential end portions 2 a and 3 a crossed in the circumferential direction. Between the inner plate 2 and the outer plate 3 , a plurality of line grooves 5 are formed to extend outward of an outer tube 1 , with their tip end portions facing with each other to be apart from each other by a predetermined distance from a peripheral edge of one of the inner plate and outer plate, and these line grooves 5 are arranged in parallel with each other along a longitudinal direction of the outer plate 3 . And, at least one portion of the overlapped portion positioned between the neighboring line grooves is joined by welding W.
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