Hot Piercing Mill

Seamless pipes are manufactured by a number of cold as well as hot rolling processes. The hot rolling process used to manufacture pipes at about 1250-1300 deg. Celsius is known as Hot Piercing process. The piercing is carried in a plant known as the piercing mill, which consists of a number of dies, of which, two primary contoured work rolls rotate in the same direction with their axes at 3 to 6 degree with respect to the horizontal stock plane. The process is also known as rotary piercing or seamless tubing.

A conditioned steel round of appropriate weight, grade and diameter is heated to a suitable forging temperature which is to be pierced in one or two of the mills. Rotary hearth furnace is usually used in the process to heat the steel into a round or a billet shape. These steel pieces are already cut in their required lengths and are indexed and arranged according to the customers’ requirements. The billets are then given a 1-inch indentation by billet centering machine at its one end to act as a starting point for rotary piercing. This indentation aids the piercing tool during actual tube formation. Rotary piercing is a dynamic and a rapid process that produces the tube shell in just a few seconds. As the billet passes in between the cross rollers, the mandrel or the piercing roll tool gives the billet its tube shape by piercing the billet through its centre. The process makes the material elongate between 1.5 to 2 times and reduce its cross section between 33 to 50%.

The cross rolling of the billet causes high tensile stress at the centre of the billet. The load imparted by the roller makes sure that the rotation and translation is happening at its core segments. The design of the piercer roll makes the metal flow over the piercer point and to the exit of the process.

The piercer point is a high-temperature, water-cooled alloy tool, with a design to allow the metal to flow over the piercer roll as a tube shell form. The piercing point is attached to a rod which stays in its place after rolling. The roll gap is usually closed by a non-driven support roll at the top and a support shoe at the bottom. After the piercing of the tube, if required, it can go through a pilgering process for the purpose of cold reduction. After the processing, the pipes are brought to a cut-to-length mechanism where they are cut to a required length. The material is then brought to its required yield strength with appropriate heat treatment method.

The features of this process involve two aspects – one is that the process results in a seamless tube that is homogenous in microstructure and physical properties, second is that the process involves high production of standard seamless pipes to be used specifically in oil and gas industries.

Inspection tests make sure there are no defects in the surface and sub-surface layers. The tubes are then gone through a pickling bath to remove scales. The approved and tested pipes are packed and delivered.