Seamless tube is made from solid round billets of stainless steel. Billets are semi-finished products used to produce stainless steel tubes, bars and rods. The billet is pulled or pushed over a mandrel that extrudes the billet into a hollow tube shape.
Following the extrusion process, the hollowed billet is rolled and stretched to the desired length, diameter and wall thickness. This can be achieved via one of two options: cold drawing and pilgering – also known as cold rolling.
During the cold drawing process, the tube is brought to room temperature and is given a nose or a point – a small tip at the end of the hollow tube. With the nose formed, the tube is fed through a die (just big enough for the nose to fit through) to reduce the material to the desired diameter.
The drawing of the tube through the dye is often done multiple times and can involve the use of different dyes throughout the process. Once drawn to the desired size, they are given a final thermal treatment.
After the treatment stage, the tube goes through a machine to straighten the tube – this resolves any warping that may have occurred during the earlier thermal treatment stage.
While the process is the same, there are three different methods to control the internal diameter of the tube: plug drawing, rod drawing and tube sinking.
Plug drawing consists of a mandrel situated at the end of the die. The die determines and shapes the inner diameter of the tube, as it passes through the die and around the plug.
This is one of the oldest tube drawing methods and provides one of the best inner surface finishes available through cold drawing.
Rod drawing replaces the plug at the end of the die, with an internal mandrel – or rod – that is drawn with the tube. By drawing the tube through the die with the mandrel inside it, the internal rod determines the internal surface finish and inner diameter throughout the process.
Rod drawing provides a level of precision and consistency that the other methods can’t guarantee.
Finally, Tube Sinking, also known as “free tube drawing”, is the most economical method and doesn’t involve a mandrel inside the tube. Without an internal mandrel, the inner diameter is determined by several factors including the diameters of the pre-drawn tube, the outer diameter of the final product and the amount of friction between the tube and die.
While it may be the most cost-effective method, it doesn’t provide the best surface finish, nor does it provide as consistent and accurate results as the other methods.
Pilgering (cold rolling)
An alternative method to cold drawing is pilgering – a longitudinal cold-rolling process.
During the pilgering process, a tapered mandrel is inserted inside the tube and is fixed in position by a mandrel thrust block. The tube and mandrel are then rocked back and forth through a pair of grooved ring dies. The dies rotate in unison, shaping the tube and reducing its diameter to the desired size, as the mandrel thrust block rotates the mandrel within the tube. The combination of the movements and rotations results in a superior surface finish and a potential reduction of up to 90% of the hollow tube’s original diameter.
Pilgering is a fast and efficient process, best suited for large production runs. It does have limitations though, especially when it comes to smaller OD (outer diameter) components or low volumes. The operation’s change-out is complex and time-consuming making, and the process doesn’t achieve as drastic changes on smaller OD tubing than it does with larger sizes.
While the production cost of seamless tube can make it an expensive choice for most applications, seamless still has its niches. Due to the difficulty to produce a small welded tube with a heavy wall thickness, seamless is still a popular choice within the production of microelectronics, semiconductors and similar applications.
Seamless’ lack of weld seam – commonly known as bead – avoids any potential defects or formability limitations that can be related to the weld on a tube. Being seamless as well provides it superior pressure. The combination of these properties makes Seamless the preferred choice for high pressure and critical applications, such as the Oil & Gas sector and the transference of fluids.