Abstract: A double-walled tube is produced by providing an inner core tube and an outer casing tube, wherein the outer diameter of the inner core tube is smaller than the inner diameter of the outer casing tube, with consideration of manufacturing tolerances and/or noncircularity, by such a value that insertion of the core tube into the casing tube can easily be performed because of a sufficient amount of play, and the core tube is brought into abutment under the action of repeated, successive hardening steps with a stepped radial expansion, against the inner surface of the casing tube.

Abstract: Method and apparatus for flame hardening the inner wall surface of tube bends of a circular configuration wherein either a flame hardening burner is passed longitudinally through the internal cavity of a tube bend which is mounted in a fixed position, or alternatively the tube bend is moved over a stationary flame hardening burner. This relative movement between tube bend and burner is effected along a circular path corresponding to the longitudinal center line of the tube bend. Subsequently to flame hardening the heated inner wall surface of the tube bend is quenched with cooling water discharged from water sprinklers disposed downstream of the flame hardening zone. During flame hardening and quenching the tube bend is continuously maintained in positions such that the quenching water may leave the tube bend by gravity.