Double-walled tube with interface gap and production method therefor

A method for producing a double-walled tube with an interface gap, wherein the method comprises:

• providing an outer wall tube made of ferritic steel containing not less than 2% by mass of Cr,
• forming on said outer wall tube an oxide scale layer containing Cr,
• wherein said oxide scale layer is formed on an inner surface of said outer wall tube and is formed having a thickness of 10 to 30 μm,
• providing an inner wall tube made of steel or a steel alloy containing not less than 2% of Cr,
• forming on said inner wall tube an oxide scale layer containing Cr,
• wherein said oxide scale layer is formed on an outer surface of said inner wall tube,
• grinding or machining said outer surface of said inner wall tube such that said oxide scale layer on said outer surface of said inner wall tube is of a thickness not less than 0.1 mm, and
• inserting said inner wall tube into said outer wall tube and after said inner wall tube has been inserted into said outer wall tube, cold working an outside diameter of said outer wall tube with said inner wall tube inserted therein at a reduction rate of 5 to 30% in order to form said double-walled tube having said interface gap between said inner wall tube and said outer wall tube.