A section of the tube not less than 63 mm in length shall be flattened cold between two parallel flat surfaces to a distance between the plates (H) as calculated by the formula given below, without showing any sign of a crack of flaw.

            (1 + e)t

                H = -----------

             e + t/D

where    t = specified thickness of tube (mm)

            D = specified outside diameter of the tube (mm)

            e = a constant, as given below

            e = 0.08 for ferritic alloy steel

            e = 0.09 for austenitic steel