The cross-rolling technique is extensively used in the manufacturing of alloy steel pipes. It plays a key role in several essential processes such as rolling, equalizing, sizing, stretching, expanding, and spinning, in addition to the primary process of piercing. Currently, two types of cross mills are commonly used in production: those with two rolls and those with three rolls. Although they are applied in different ways across various stages, their basic kinematic principles remain similar. However, the stress conditions within the deformation zone differ significantly between the two systems.
The main distinction between cross-rolling and longitudinal or combined rolling lies in the direction of metal flow. In longitudinal rolling, the metal flows in the same direction as the roll surface movement. In cross-rolling, the metal moves perpendicular to the direction of the deforming tool. In a hybrid approach, the metal flow direction forms an angle with the roll's movement. Besides moving forward, the metal also rotates around its own axis, resulting in a spiral motion. This complexity makes the kinematics and force analysis of cross-rolling far more intricate than that of longitudinal rolling. Additionally, during cross-rolling, a plug or mandrel is typically present inside the steel pipe, subjecting the material to axial forces, which further complicates the stress state of the deformed metal.
Cross-rolling was first introduced for piercing. In the 19th century, Mannesmann pioneered the use of two rolls with a central support to achieve this. Later, Steffel simplified the design by introducing a "punch" shaped roll. This type of rolling machine became widely adopted in the production of alloy steel pipes. Other variations include disc-shaped rolls and conical rolls.
Regardless of the roll design, a cavity may form at the center of the product during the piercing stage. These cavities can lead to internal defects like cracks and folds, which are hard to eliminate in subsequent operations such as reducing, expanding, or rolling. They can even worsen, causing structural failure. For materials with low plasticity and poor deformability, large deformations are not feasible, limiting the efficiency of the two-roll system.
To address these limitations, manufacturers have implemented various strategies, such as optimizing the rolling reduction, modifying the roll and head shapes, and increasing the feed angle to prevent tearing at the front end. Meanwhile, research into the three-roll system has gained momentum. In this system, the three rolls are arranged at 120-degree angles, generating compressive stress at the center of the tube blank. As a result, no cavities appear, regardless of the deformation level. However, the three-roll system introduces new challenges, such as the triangular effect and the "tail-triangle" phenomenon, where metal is extruded between the rolls. This leads to alternating bending stresses in the pipe wall, potentially causing inner wall cracking. Worse still, the tail-triangle may get stuck, disrupting the rolling process. Thus, while the two-roll system faces issues with cavities, the three-roll system deals with problems related to triangles, each presenting unique challenges in the cross-rolling process.
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