Simulation Analysis of Internal Hexagonal Defects in Seamless Steel Pipe Tension Reduction Process

Literature Overview

The paper authored by Yu Hui, Zang Xinliang, Du Fengshan, Wang Feixue, Pan Feng, and Zhou Xiaolan, published in Iron and Steel (Vol. 43, No. 3, 2008, pp. 53-56), presents a three-dimensional thermo-mechanical coupled finite element analysis of the internal hexagonal defect formation during the tension reduction (tension sizing) process of seamless steel pipes. The research was supported by the National Natural Science Foundation of China (Grant No. 50435010) and conducted jointly by Yanshan University and Shanghai Baosteel Steel Pipe Branch. The study focuses on an 18-stand tension reduction mill trial and uses MSC.Marc software to build a numerical model that reproduces the deformation characteristics observed in actual production.

Core Technical Points

Deformation Mechanism of Tension Reduction

The tension reduction process is a critical finishing operation in seamless steel pipe manufacturing, typically following the hot rolling or pilger mill stage. In this process, the pipe is pulled through a series of reducing rolls under applied tensile force, simultaneously reducing both the outer diameter and wall thickness while improving dimensional accuracy and surface quality. The key deformation feature is that the pipe is subjected to three-axial compression in the radial direction and tension in the axial direction, with the deformation occurring progressively through each stand.

The internal hexagonal defect is a geometric anomaly where the pipe's inner bore develops a six-sided cross-sectional profile instead of a true circle. This defect is particularly problematic for pipes used in high-pressure applications such as oil and gas pipelines, hydraulic cylinders, and structural tubing, where uniform wall thickness and a true circular bore are essential for pressure integrity and fatigue resistance.

Finite Element Model Development

The authors employed MSC.Marc to construct a three-dimensional thermo-mechanical coupled model that captures the following essential aspects:

• Geometric modeling: The pipe blank, reducing rolls, and tension device are modeled with appropriate mesh density, particularly at the contact interface between the pipe and rolls where large plastic deformation occurs.
• Material model: A temperature-dependent constitutive model is used to represent the plastic behavior of the steel pipe material during hot working, incorporating the flow stress curve at elevated temperatures.
• Thermal coupling: The temperature drop during deformation due to strain hardening, air cooling, and contact with rolls is coupled with the mechanical deformation to account for the changing material properties throughout the process.
• Contact formulation: A penalty-based contact algorithm handles the pipe-roll interaction, with friction coefficients calibrated from trial production data.

Wall Thickness Distribution and Defect Analysis

The simulation results were validated against measured data from the 18-stand trial, demonstrating high accuracy. The analysis of wall thickness at different points around the pipe cross-section at each stand exit revealed a systematic pattern:

The root cause analysis indicates that the internal hexagonal defect arises from the interaction between the number of reducing rolls (typically six rolls arranged in a hexagonal pattern), the tension force distribution, and the non-uniform wall thickness of the incoming pipe blank. When the incoming pipe has slight wall thickness variation or the rolls are not perfectly aligned, the deformation concentrates at specific angular positions, amplifying the geometric irregularity into a pronounced hexagonal shape.

Process and Standards Analysis

The tension reduction process must comply with dimensional tolerances specified in standards such as GB/T 8162 (general requirements for seamless steel tubes) and GB/T 8163 (seamless steel tubes for fluid transport). The wall thickness tolerance is typically ±10% for general applications but can be as tight as ±5% for high-pressure piping. The internal hexagonal defect, if not controlled, can cause local wall thickness reduction that violates these tolerances and creates stress concentration points that reduce fatigue life.

Key Process Parameters

Integration with Engineering Practice

In practical production at steel pipe mills, the internal hexagonal defect has been a persistent quality challenge. Based on the findings of this study, the following countermeasures can be implemented:

1. Incoming pipe quality control: Implement ultrasonic thickness measurement at multiple clock positions before the pipe enters the tension reduction mill to detect and reject blanks with excessive wall thickness variation.
2. Roll alignment verification: Perform regular laser-based alignment checks of the roll stands to ensure that the roll axes are properly aligned and the roll gap is uniform.
3. Tension control optimization: Use closed-loop tension control systems that adjust the pulling force in real time based on deformation resistance feedback, preventing excessive tension that can amplify geometric irregularities.
4. Post-reduction inspection: Deploy automated ultrasonic or optical bore inspection systems at the mill exit to detect hexagonal defects and trigger corrective actions.
5. Process simulation for new products: Use the validated FEA model as a design tool to predict defect formation before trial rolling, reducing the number of trial runs and associated material waste.

Key Questions and Reflections

This study raises several important questions for practitioners in the seamless pipe industry. First, the model assumes ideal roll geometry and perfect alignment, but in reality, roll wear, thermal expansion, and mechanical deflection all contribute to the deformation process. How sensitive are the simulation results to these real-world deviations? Second, the study focuses on a single trial with 18 stands, but production mills may have different configurations with varying numbers of stands, roll diameters, and reduction schedules. The generalizability of the findings needs further investigation. Third, the study does not address the effect of material grade on defect formation; different steel grades with different flow stress characteristics may exhibit different susceptibility to hexagonal defect development.

From a quality assurance perspective, this research highlights the value of numerical simulation as a complementary tool to traditional trial-and-error process optimization. By understanding the fundamental deformation mechanisms, engineers can make informed decisions about process parameters rather than relying solely on empirical adjustments. The FEA model also serves as a powerful training tool for new operators, helping them visualize the complex deformation patterns that occur inside the reducing rolls.

Study Insights and Implications

The most significant insight from this study is the direct correlation between the number of reducing rolls, the tension force distribution, and the formation of hexagonal internal defects. This understanding enables a systematic approach to defect prevention rather than reactive quality control. The validated FEA model provides a quantitative basis for process optimization, allowing engineers to predict the effect of parameter changes on product quality before implementing them in production. For steel pipe manufacturers targeting high-quality applications such as hydraulic cylinders, automotive tubing, and pressure vessels, this research provides actionable guidance for achieving tighter dimensional tolerances and superior internal surface quality.