Cold Skew Rolling Process and Experimental Research for Steel Pipes

Literature Overview

The paper by Zhou Cunlong, Liu Xueyun, and Qin Jianping, published in the journal Steel Pipe in 2004 (Vol. 33, No. 4, pp. 7–10), investigates the cold skew rolling process for steel pipe forming. The authors explore the necessary conditions for cold skew rolling of steel pipes, employ rigid-plastic finite element analysis to compute the velocity field, strain, and stress during deformation, and design roll profiles suitable for the cold skew rolling process. Through theoretical analysis and industrial trials, they demonstrate the feasibility and practicality of this process, which can partially replace traditional cold rolling and cold drawing processes and can also be used for the composite processing of bimetallic tubes.

Core Technical Content

Cold skew rolling is a metal forming process in which the workpiece is deformed between two or more rolls whose axes are inclined at an angle to the axis of the roll stand. The skew angle creates a helical deformation pattern that allows for complex cross-sectional transformations with relatively low forces compared to other forming methods. For steel pipes, cold skew rolling offers the potential to reduce wall thickness, improve dimensional accuracy, and enhance surface finish without the need for extensive cold drawing or cold rolling equipment.

Deformation Mechanism in Cold Skew Rolling

The deformation process in cold skew rolling is characterized by the helical motion of the pipe as it passes through the rolls. The pipe undergoes simultaneous axial compression and circumferential expansion, with the deformation distributed along the helical path rather than concentrated at a single cross-section. This distributed deformation results in lower peak forces and more uniform strain distribution compared to conventional rolling processes.

The key deformation parameters include:

Reduction ratio: The ratio of the reduction in wall thickness to the initial wall thickness
Skew angle: The angle between the roll axes and the roll stand axis, typically in the range of 5° to 20°
Roll gap profile: The shape of the gap between the rolls, which determines the cross-sectional transformation
Deformation zone length: The axial length over which the deformation occurs

Process Parameter	Typical Range	Effect on Product Quality
Skew angle	5°–20°	Higher angle → shorter deformation zone, higher force
Reduction ratio	10%–30%	Higher reduction → more uniform strain, higher springback
Roll gap profile	Custom-designed for target cross-section	Determines final dimensional accuracy
Deformation zone length	50–200 mm	Longer zone → lower force, lower strain rate
Roll material	Hardened steel, 60–65 HRC	Affects roll life and surface quality

Rigid-Plastic Finite Element Analysis

The authors employed rigid-plastic finite element analysis (RPFEA) to simulate the deformation process in cold skew rolling. This computational method treats the workpiece as a rigid-plastic material, neglecting elastic effects and focusing on the plastic deformation behavior. The RPFEA provides detailed information about the velocity field, strain distribution, and stress state within the deformation zone, which is essential for understanding the deformation mechanics and optimizing the roll profile design.

The finite element model typically includes:

The pipe workpiece modeled with appropriate plasticity criteria (such as von Mises or Tresca yield criterion)
The rolls modeled as rigid bodies with the designed roll gap profile
Friction conditions at the roll-workpiece interface (typically a Coulomb friction model with a coefficient of 0.1–0.3)
Boundary conditions that simulate the feeding and exit of the pipe

The results of the RPFEA provide critical insights into the deformation behavior, including the distribution of equivalent strain, the magnitude and direction of principal stresses, and the evolution of the cross-sectional shape along the deformation zone.

Roll Profile Design

The design of the roll gap profile is a critical aspect of the cold skew rolling process. The roll profile must be carefully designed to achieve the desired cross-sectional transformation while maintaining stability of the deformation process. The authors developed a systematic approach to roll profile design that incorporates the results of the RPFEA to optimize the roll gap shape for the target product dimensions and quality requirements.

The roll profile design process involves:

Defining the target cross-sectional dimensions and tolerances
Selecting appropriate process parameters (skew angle, reduction ratio, deformation zone length)
Performing RPFEA simulations to predict the deformation behavior for different roll profiles
Iteratively refining the roll profile based on simulation results and trial production data
Manufacturing the rolls and conducting industrial trials to validate the design

Applications and Advantages

The cold skew rolling process offers several advantages over traditional cold rolling and cold drawing processes for steel pipes:

Reduced equipment cost: Cold skew rolling equipment is generally less expensive than cold drawing equipment for similar production capacities
Higher production speed: The continuous nature of the skew rolling process enables higher throughput compared to discrete cold drawing operations
Improved dimensional accuracy: The controlled deformation process produces pipes with tight dimensional tolerances and good surface finish
Bimetallic tube production: The process can be adapted for the composite processing of bimetallic tubes, where two different materials are bonded together during the deformation process

For bimetallic tube production, the cold skew rolling process can be used to join a corrosion-resistant outer layer (such as stainless steel or nickel alloy) with a structural inner layer (such as carbon steel). The deformation process creates a metallurgical bond between the two materials, eliminating the need for welding or mechanical fastening.

Quality Control and Defect Prevention

The cold skew rolling process can introduce several types of defects if not properly controlled:

Defect Type	Cause	Prevention Measures
Surface scratches	Roll surface damage or debris	Regular roll inspection and cleaning
Wall thickness variation	Uneven roll gap or misalignment	Roll gap measurement and alignment verification
Ovality	Asymmetric deformation	Symmetric roll design and balanced feeding
Cracking	Excessive strain or low ductility	Limit reduction ratio; preheat if necessary
Delamination (bimetallic tubes)	Insufficient bonding pressure	Optimize roll profile and friction conditions

Study Reflections

This paper represents a valuable contribution to the field of steel pipe manufacturing technology. The cold skew rolling process offers a promising alternative to traditional cold forming methods, particularly for applications where high production speed and lower equipment costs are important. The combination of theoretical analysis (RPFEA) and industrial trials provides a comprehensive understanding of the process, enabling the development of reliable production procedures.

For engineers involved in steel pipe manufacturing, the cold skew rolling process opens up new possibilities for product development, particularly in the area of bimetallic tubes. The ability to produce bimetallic tubes through a single forming operation, without the need for welding or mechanical joining, simplifies the production process and improves the reliability of the final product. The paper's emphasis on the systematic approach to roll profile design is particularly valuable, as it provides a framework that can be adapted for different product requirements and process conditions.

The industrial trials described in the paper demonstrate that the cold skew rolling process is technically feasible and commercially viable. For manufacturers considering the adoption of this technology, the paper provides the technical foundation needed to evaluate the potential benefits and challenges. The key considerations include the initial investment in equipment, the training requirements for operators, and the quality control measures needed to ensure consistent product quality.