In the seamless pipe
mandrel mill process, a solid round steel billet is used. The billet is charged into a rotary hearth furnace. After the billet is discharged from the rotary hearth furnace, a small hole is punched into its end. This indentation acts as a starting point to aid in rotary piercing.
Rotary piercing is a very fast and dynamic rolling process that cross rolls the preheated billet between two barrel-shaped rolls at a high speed. The cross rolling causes high tensile stresses at the center of the billet. The design of the piercer rolls causes the metal to flow along the roll and over a piercer point as it exits the process. The piercer point is a high-temperature, water-cooled alloy tool designed to allow the metal to flow over it as a pipe shell forms from the rotary process. Once the pierced pipe shell is produced, it is immediately transferred to the floating mandrel mill.
The floating mandrel mill comprises eight rolling stands using 16 rolls and a set of mandrel bars. These mandrel bars are inserted into the pierced pipe shell and then conveyed into the mandrel mill and rolled into an intermediate size pipe shell. The mechanism of rolling the pipe shell through the mandrel mill with the mandrel bar inside acts as a control to create an intermediate standard outside diameter (OD) and a controlled wall thickness, based on the resultant dimension of the mandrel bar set.
The mandrel mill shell is then reheated in a reheat furnace so that it can complete the final rolling process and gain its final dimensions and micro-structural qualities. Upon exit from the reheat furnace, the tube receives a high-pressure water descale, removing the iron oxide scale that formed in the reheat furnace. A clean, scale-free surface is critical for good surface quality.
The pipe shell is further reduced to specified dimensions by the stretch mill. The pipes then are conveyed to the cutoff mechanism, where they are cut to the prescribed length. then these pipes are bundled and directed to subsequent finishing and inspection operations before shipment.