coupling 1 consists of a tubular female half coupling 2 welded to the end of the tube 4 coaxially therewith and of a tubular male half coupling 3 welded to the end of the tube 5 coaxially therewith the male half coupling 3 comprises a first annular section 6 of cylindrical shape which has substantially the same diameter dimension as the outside diameter of the tube 5 a second annular section 7 of frusto conical shape and tapering outwardly to define together with said first tubular section 6 an annular shoulder 7a this annular shoulder 7a is of conical shape tapering in the opposite direction to the section 7 above a keyway 26 is formed in said shoulder 7a a third section 8 of said half coupling 3 is cylindrical and has an outside diameter which is substantially equal to the least diameter on the frusto conical section 7 the outer surface of this third section 8 is threaded with a trapezoidal thread and fluted with a plurality of axial grooves 9 defining a corresponding plurality of ribs 10 formed outwardly with said thread this tubular section 8 forms a first annular connector as explained hereinafter the female half coupling 2 comprises a cylindrical tubular body 11 adapted.