The cross-rolling technique is extensively used in the manufacturing of alloy steel pipes. It plays a key role in several critical processes such as rolling, equalizing, sizing, stretching, expanding, and spinning, in addition to the primary operation of piercing. Currently, two types of cross mills are commonly used in production: those with two rolls and those with three rolls. While these systems are applied in similar processes, their operational methods differ. However, the kinematic behavior of the rolling process remains largely consistent. What sets them apart is the stress state within the deformation zone, which varies significantly between the two systems.
The main distinction between cross-rolling and longitudinal rolling lies in the direction of metal flow. During longitudinal rolling, the metal flows in the same direction as the roll surface movement. In contrast, during cross-rolling, the metal moves perpendicular to the tool’s motion, creating a more complex flow pattern. Cross-rolling combines elements of both longitudinal and transverse rolling, with the metal moving at an angle relative to the roll’s direction. In addition to forward motion, the metal also rotates around its own axis, resulting in a spiral-like progression. This makes the kinematics and force distribution in cross-rolling far more intricate than in longitudinal rolling. Moreover, during cross-rolling, a mandrel or plug is typically used, subjecting the metal to axial forces that further complicate the stress conditions during deformation.
The cross-rolling method was first introduced for piercing. In the 19th century, Mannesmann pioneered the use of two rolls with a centrally supported head to achieve this. Later, Steffel simplified the roll design into a "punch" shape, leading to the development of widely used roll-piercing machines for alloy steel pipe production. Other variations of this method include disc-shaped rolls and conical rolls.
Regardless of the roll type, a central cavity may form during the piercing process. These cavities can grow and lead to internal defects like cracks and folds. Such flaws are challenging to eliminate in subsequent operations like reducing, expanding, or rolling, and they may even worsen, causing structural failure. For low-plasticity metals, large deformations are not feasible, limiting their workability and affecting the efficiency of the two-roll system.
To address the limitations of the two-roll system, various measures are taken in practice, such as optimizing the rolling reduction, modifying roll and head shapes, and increasing the feed angle to prevent center tearing. At the same time, researchers have explored alternative approaches, leading to the development of the three-roll system. In this setup, three rolls are arranged at 120-degree angles, generating compressive stress at the tube blank's center. As a result, no cavities appear, regardless of the deformation level. However, this system introduces new issues, such as the "triangular effect" and "tail-triangle" phenomenon, where metal is extruded between the rolls. These effects create alternating bending stresses in the pipe wall, potentially causing inner wall cracks. Additionally, the tail triangle may hinder smooth passage through the rolls, disrupting the rolling process. Thus, while the two-roll system faces challenges with cavities and the three-roll system deals with triangular issues, both have their unique drawbacks.
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