Coking resistance to multilayer metal tube and its production method

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PROBLEM TO BE SOLVED : To improve the performances of a heat resistant metallic tube used under the conditions of high temperature, in which the precipitation and deposition of carbon and carburization are also easy to occur, and to realize a heat resistant multilayer metallic tube excellent in caulking resistance and also good in carburizing resistance by a simple process. SOLUTION : The face in contact with hydrocarbon in a tube base material 1 of a heat resistant metal is provided with a build-up welding layer 2 of a Cr-Ni-Mo series alloy by plasma transfer welding 2, in which build-up welding is executed, and powder is used as a filler material, to form the material 1 into a multilayer metallic tube. The alloy forming the build-up welding layer contains, by weight, 36 to 49% Cr, 35 to 63% Ni and 0.5 to 5% Mo. A part of Ni can be substituted for Co. B of 0.001 to 0.015% or further either or both of 0.001 to 0.015% Zr and 0.0001 to 0.002% rare earth metals are added, and the maximum amounts of impurities are controlled to C : 0.1%, N : 0.3%, Si : 1.5%, Mn : 1.5%, Fe : 10%, P+S : 0.02%, and O : 0.3%.