Double cone coupling nipple of a steel tubing and method of making same

Tube coupling assembly comprising:

• a steel tubing having a coupling nipple which is produced by cold forming an end area of the tubing and which has a first and second conical surface, the first conical surface being a sealing surface,
• a hollow counterpart provide with threads and having a cone shaped wall, the first conical surface being for face-side tight contact with the cone-shaped wall of the hollow counterpart which is adapted to contact said first conical surface, and
• a cap screw having threads thereon with a high tightening torque which is applied to the threads of the counterpart and supported on the second conical surface of the nipple,
• wherein the steel tubing is thin-walled and constructed of high-strength steel,
• wherein the conical area of the nipple defines a wall having a portion thereof with a thickness approximately double that of the remaining tubing area,
• wherein the material of the first and second conical surfaces of the nipple is harder than the material of the wall of the hollow counterpart,
• wherein a cone angle .alpha. of the first conical surface of the nipple measures over 100.degree., and
• wherein forming of the material in the conical area of the nipple produces a tensile strength of approximately 800 to 850 N/mm.sup.2 which is higher by approximately 300 N/mm.sup.2 than a tensile strength in the remaining tubing areas.