Flange profile for air-tight jointing of two air duct sections of an air duct configured in its cross-section mainly as a an L-shaped hollow profile made of a folded sheet metal, wherein - the L-shaped legs (2, 3) of the said flange profile are configured with double walls each having an inner wall (4, 5) and an outer wall (6, 7), - the walls (4, 6) of the first L-shaped leg forming an end-side open insertion pocket for insertion of the wall of one of the air duct sections, - the walls (5, 7) of the second L-shaped leg forming a take-up compartment to take-up one leg of one angular corner piece, - the second L-shaped leg at its inner wall facing the parting line between the air duct sections having a beaded fold extending along the longitudinal extension of the flange profile into the interior of the hollow profile and reaching up to the outer wall of the second L-shaped leg, said beaded fold guiding the wall of the air duct section on its insertion into the insertion pocket and having a crest, wherein the outer wall of the second L-shaped leg in the transitional area towards the outer wall of the first L-shaped leg has an intermediate section being of an outwardly bulged and round shape complementary to the beaded fold, wherein after welding with a wall of the air duct section inserted into the insertion pocket the flange profile is so deformed that the crest of the beaded fold is pressed planar against the outer wall of the second L-shaped leg in the area of the intermediate section characterized in that a sealing agent strand extending along the longitudinal extension of the flange profile and arranged in the interior of the insertion pocket in the area of the crest of the beaded fold is provided in the hollow space of the flange profile and that a beading protruding into the interior of the air duct is configured at the inner wall of the first L-shaped leg at the level of the crest of the beaded fold which at least partly takes-up the sealing agent strand. - the L-shaped legs (2, 3) of the said flange profile are configured with double walls each having an inner wall (4, 5) and an outer wall (6, 7), - the walls (4, 6) of the first L-shaped leg forming an end-side open insertion pocket for insertion of the wall of one of the air duct sections, - the walls (5, 7) of the second L-shaped leg forming a take-up compartment to take-up one leg of one angular corner piece, - the second L-shaped leg at its inner wall facing the parting line between the air duct sections having a beaded fold extending along the longitudinal extension of the flange profile into the interior of the hollow profile and reaching up to the outer wall of the second L-shaped leg, said beaded fold guiding the wall of the air duct section on its insertion into the insertion pocket and having a crest, wherein the outer wall of the second L-shaped leg in the transitional area towards the outer wall of the first L-shaped leg has an intermediate section being of an outwardly bulged and round shape complementary to the beaded fold, wherein after welding with a wall of the air duct section inserted into the insertion pocket the flange profile is so deformed that the crest of the beaded fold is pressed planar against the outer wall of the second L-shaped leg in the area of the intermediate section