A tube fitting assembly is provided in which the back sleeve 4 can be simply fabricated without any reduction in its corrosion resistance. The back sleeve 4 is fabricated by pressing the exterior edge of the front end of a short column with a roller 6 of greater hardness than the short column whereby a tapered face 42 is formed on the outer edge of the front end that is harder than the rest of the piece. A bore or cavity is then bored axially through the short column. Thereafter, the back sleeve 4 so formed can be assembled with the tube to be connected, the coupling body 2, the front sleeve 3, and the envelope nut 5 to form an effective tube fitting assembly.