Austenitic Stainless Steel

Seamless pipes can be manufactured either by extrusion or piercing. Of which, extrusion is a widely known process because it provides a uniform outer diameter and concentric inner diameter and is the only process that is suitable for long-length seamless tubing. This process is different from ERW pipes since it does not have a welded joint along their length and can be extruded to any desired length. The raw material is collected in the form of hot extruded seamless pipe which is then supposed to be cold reduced. The process of drawing a tube to a particular size includes the shrinking of a large diameter tube into a smaller one, by drawing the tube through a die. The result of this process is high quality tubing with precise dimensions, excellent surface finish, and the added strength of cold working. The process used in tube reduction is known as Cold Pilgering. There are two important facts about the pilgering process – First, pilgering is a cold working process and therefore it greatly improves the material structure. Second, it can cut down a large amount of outside diameter, inside diameter and thickness of the tube with a reduction rate of 50-60%. The equipment includes a saddle assembly which holds the pair of dies- upper die and lower die. These dies are forced over the tube’s outer diameter. There is a mandrel inside the tube which maintains its inside diameter while the dies reduce its outer diameter. The mandrel is tapered and stationary but does rotate at specific time in a cyclic process. Due to the forcing of the dies in between the tube, its lateral dimension decreases whilst its longitudinal dimension increases. That is why, the tube obtained is considerably longer. The process is then followed by degreasing and drying of the pipe so that the heat treatment on the metal can be appropriately carried out. The heat treatment process used on the pipe is annealing, which includes heating of the pipe to a temperature, maintaining that temperature for a certain period of time and then cooling. Annealing becomes important since it gives the metal its required toughness and removes internal stress from the drawing process. The metal pipes then go through the straightening process in the presence of rollers, for the purpose of levelling and removing any deviations. The approved seamless metal pipes undergo the process of cutting and deburring. The cutting operation cuts the tube to required lengths and the deburring process removes any and all burs. Both of these operations are carried out for the purpose of improving the surface finish and removing all sort of surface impurities such as stains, contaminants, rust, etc. The processed product goes to the testing department where one or more non-destructive tests are carried out. These test includes hydrostatic testing and eddy current testing. Hydrostatic testing detects the strength of the metal as well finds out for any leaks in the pipe. Eddy current test detects any surface as well as subsurface flaws in the pipe. Mechanical tests may include tests for hardness and yield strength. Finally, there is a quality check done by the quality control department after which the product is packed and shipped. Being ISO 9001: 2008 certified company, Shalco Industries has always strived to provide their customers the highest quality of products.

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.