Manufacturing Process


Welded and drawn tubing begins as a cold rolled sheet or a strip which is produced in coils. The metal strips are shaped to a tube form and passed under a welding head which melts the edges of the open seam to form a fusion juncture. No filler material or flux is used and therefore the weld bead is of the same analysis as the parent metal. As per the method of welding employed, a weld bead is formed on either on both the I.D. and the O.D. surfaces. After the cold drawing process is complete, the weld area can be detected only by etching or careful microscopic examination. Actually the weld area is quite homogeneous with the parent metal since the cold working and intermediate annealing causes the weld area to recrystallize, thus it becomes equivalent to the parent metal in ductility, appearance and mechanical properties. Welded and drawn tubing from cold rolled strip has several advantages

Strip can be rolled to a precise thickness thus maintaining a uniform wall thickness after welding.

Surface defects can be spotted visually before welding.

Welded tubing often costs less than the seamless


Cold Drawing consists of pulling the tubing through a die which is smaller in diameter than the O.D. of the tube. While it passes through the die, it is usually supported on the inner surface by a tool which controls the I.D., wall thickness and the length. The cold drawing process is repeated till the desired size is attained. Cold drawing reduces ductility (Pliability) of the alloy. To prevent this, the tubing is normally annealed between the drawing operations which increases the ductility and prevents brittleness


The fine tubes being produced by OST are subjected to Bright Annealing for relieving the stresses and complete solutionizing of Austenitic matrix, by putting in thermal energy which allows the stresses to relax. In this unique process of bright annealing the material does not get oxidized and hence no scaling. The inert atmosphere present in the Furnace gives a bright surface and checks any loss of Chromium, thus the corrosion property remains unaltered. In OST this function is performed by the latest type of Bright Annealing Furnaces imported from Japan.