Seamless pipes after being extruded are produced by cold processing methods like Cold Pilgering and Cold Drawing. Cold Drawing is a comparatively slower process and gives lower reduction ratio of about 15-35% depending on alloys, but give a very fine and commendable grain structure. Both manufacturing processes have their own advantages depending upon the need. Cold working also has an advantage of quicker tool changes and low operational costs. Cold drawing is basically a metal working process where the metal is stretched by using tensile forces. Since it is done at room temperature, it is known as cold working. The principle used here is quite similar to cold pilgering where, the desired size of the pipe is achieved by forcing it between dies, reducing the lateral dimension of the pipe, i.e. diameter and increasing the longitudinal dimension, i.e. length. The machine used for the purpose is known as Draw Bench.
Draw bench uses only pressure and no heat to change the shape of the metal by cold working on steel. The drawing begins with procuring of raw materials with care to be taken about the material’s chemistry and dimensions including tolerances—size, wall thickness, concentricity, and straightness. The next process is pointing, which includes reduction of the material’s diameter at the tube end so that it can enter die. The common methods are push pointing, rotary swaging, and squeeze pointing. The draw bench is usually mechanical with a back bench, die head and front section. The cold drawing reduction process involves two types – one with mandrel inside the tube known as Rod drawing and one without mandrel known as sink drawing.
In rod drawing, a hardened steel mandrel is inserted inside the pipe followed by a gripper. The outer diameter of the pipe is determined by the size of the die and the inner diameter is determined by the size of the mandrel. After the tube has been introduced into the die, lubricating oil is pumped on the surface. Advantages of rod drawing are that drawing speeds are good enough and appreciable amount of reduction ratio can be obtained. Here, both the outer diameter and the wall thickness are reduced. This is not the case in Sink drawing, where the pipe is drawn from the die without any mandrel or internal support.
This causes reduction in outer diameter without affecting wall thickness to a great extent. This method can be used to draw the tube to a finished size and is also performed as a sizing pass after rod draw. The proper die angle will depend upon diameter to wall thickness ratio. The drawing of the tube in the die would involve considerable amount of friction and might result in unwanted wear, for this reason appropriate amount of lubrication needs to be provided, chlorinated oils are generally used for this purpose. The drawn product or the Cold Drawn pipe, exhibits a bright and polished finish, increased mechanical properties, improved machining characteristics and precise as well as uniform dimensional tolerances. Furthermore, annealing can be used for heat treatment in removing internal stresses and softening the product. After all the processes are completed, the final product is packed, shipped and delivered.