Air gap insulated exhaust pipe with branch pipe stub and method of manufacturing same

A method for manufacturing an air-gap-insulated exhaust pipe having a branch stub, as well as an exhaust pipe produced according to such a method and a tool for forming such an exhaust pipe. Two tubes are inserted into one another to form a double tube. The double tube is placed in a first internal high-pressure shaping tool, and exposed to internal high pressure fluid such that it expands to match the contours of the engraving of the first shaping tool, and a double-walled branch stub is blown out of the double tube. The partially formed double tube is then placed in a second internal high-pressure shaping tool, with the double tube being surrounded, between the two ends including the branch stub, circumferentially and throughout by a corresponding design of the engraving, by an expansion chamber. The outer tube is expanded between the outer tube and inner tube under high pressure up to a precise fit with the engraving of the second shaping tool to form an insulating air gap, with the end of the branch stub being supported externally without yielding via a second counterpunch. The finished double tube is removed from the second shaping tool and a cap area of the branch stub, containing the end, is cut off to form a through opening between the interior of the inner tube and the ambient environment outside the air-gap-insulated exhaust pipe.