Manufacturing Process
Welding
Steel pipes are produced by two methods which result in either welded or seamless pipe. However welded tubes have an edge over seamless as it is low cost. Raw steel is the primary material here which is used to produce the tubes. The tubes are produced by bending the cold rolled sheets or a strip produced in coils. It is then rolled in circular shapes with the help of plate bending machine and then passed under a welding head which melts the edges.
Contemporary manufacturers use filler material but here, no filler or flux material is used, maintaining the primary component throughout. A weld bend is formed on both I.D and O.D surfaces. The weld area is not seen with a naked eye and can be seen only through microscopic examination. The weld area crystallizes in less time, becoming the same like parent material. It has same:
Appearance and properties
Malleable and ductile
Same mechanical properties
Welded and drawn tubes have several advantages:
Surface defects are easily identified before welding
Strip can be rolled to a precise thickness
Uniform wall thickness is maintained after welding
Welded tubes are cost efficient compared to seamless tubes
Cold Drawing
It is a working process which uses tensile forces to stretch metal to a desired shape and thickness. It employs pulling the tube through a die, smaller in diameter than the O.D of the tube. The I.D is controlled on the inner surface by a tool, when passed through the die.
Cold drawing reduces the ductility of the alloy and it is repeated till the desired size is attained. The tubing is annealed between the drawing operations to prevent brittleness and increase ductility.
Bright Annealing
They facilitate better finish on the tube surface. Distortion is minimized and the tubes require no further finishing. In OST, the tubes are subjected to Bright Annealing for relieving the stress by putting in thermal energy. The material does not get scaling or oxidized in this process. Any loss of chromium is checked by the bright surface produced by the inert atmosphere in the furnace, unchanging the corrosion properties. The Bright Annealing furnaces are imported from Japan.
