PURPOSE: To manufacture a uniquely brazed double-wall tube by keeping a non-ferritic steel held in contact with a gaseous atmosphere at a brazing temp. for sufficient time for enabling fusing between a metal alloy and a non-ferritic steel surface.

CONSTITUTION: The temp. of the non-ferritic steel is rapidly elevated from the first temp. substantially equal to the vaporization temp. of a lubricating material sticking thereto the brazing temp. elevated sufficiently for initiating the fusion between the selected metal alloy and the non-ferritic steel surface. This temp. elevation is executed in the wet gaseous atmosphere consisting of a non-reactive carrier gas and a reactive gas which is suitable for attaining the fusion with this carrier gas and has a sufficient concn. for this purpose. The non-ferritic steel is kept held in contact with the wet gaseous atmosphere at the brazing temp. for the time sufficient to enable the fusion between the metal alloy and the non-ferritic steel surface. As a result, the extremely highly corrosion resistant tube may be provided.