PURPOSE: To greatly decrease the voids from a joint boundary by cold drawing a blank in the state of coupling the mouth drawing part of an outside pipe and the mouth drawing part of an inside pipe, then sufficiently soaking and heating the blanks while allowing diffusion joining to take place at a specific temp., then hot rolling the blank.
CONSTITUTION: A tape-like insert material 2 is spirally wound around the outer peripheral surface exclusive of the mouth drawing part of the inside pipe 1 by having specific overlaps. The assembly of the inside and outside pipes is executed by inserting the inside pipe 1 into the outside pipe 3. Cold drawing is thereafter executed to bring the inner peripheral surface of the outside pipe 3 into pressurized contact via an insert material 2 onto the outer peripheral surface of the inside pipe 1 in order the eiliminate the gap between inside and outside tubes. The blank with which the cold drawing is competed is cut to a prescribed length and both end faces are subjected to welding. The pipe stock 6 welded at both end faces is sufficiently soaked and heated at the m.p. of the insert material +50°C or above and ≤1250°C for the purpose of diffusion joining of the insert material 2 and pipe making. The sufficiently soaked and heated pipe stock 6 is stretched and rolled by a plug mill 7 and is made into the double pipes.
OGAWA KAZUHIRO
YAMAKAWA TOMIO
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