How Seamless Tubing is Made

Seamless tubular products are made by one of two hot forging process, rotary piercing and rolling, or extruding.

Rotary Piercing and Rolling

Rotary Piercing and rolling produce the bulk of seamless tubular products. A conditioned steel round of proper grade, diameter and weight is heated to a suitable forging temperature and pierced in on of several types of mills, which work the steel, causing it to flow around a piercer point, thus yielding a hollow billet. This billet is then roller elongated in a succession of mills in which internal support is provided by a mandrel or plug. Finally, the elongated steel tube is sized by further rolling without internal support. Three possible sequences of the piercing and rolling operations are shown in the below illustrations:

Extrusion

The Extrusion process also starts with a conditioned steel round of desired grade, diameter and weight. The billet is either cold drilled, cold drilled and hot expanded, or punch-pierced either separately or in the extrusion process. The drilled or punched billets are then hot extruded by axially forcing the material through a die and over a mandrel as illustrated below:

Finishing

Finishing may include thermal treatment, cold reduction and other operations to condition the surface. These operations are similar to those used for bar except provisions are made for maintaining a hollow product. While seamless tubing is different than steel bar - because of the hole - there are many similarities to the extent that seamless tubing is frequently referred to as "hollow bar".