Wall Thickness Eccentricity Analysis of Seamless Steel Tube Oblique Rolling Piercing

Literature Overview

This paper by Lv Qinggong, Mou Renling, and Xu Wenjing from the School of Advanced Engineers at University of Science and Technology Beijing was published in Iron and Steel (Vol. 52, No. 8, 2017, pp. 65-69). The study addresses the critical issue of wall thickness eccentricity in seamless steel tubes produced by oblique rolling piercing. Wall thickness eccentricity is one of the most significant quality defects in seamless tube production, directly affecting the dimensional accuracy, mechanical properties, and service performance of the final product. This research is particularly relevant to engineers involved in seamless tube manufacturing, quality control, and downstream applications where dimensional tolerance is critical.

Core Technical Findings

The study characterizes the wall thickness eccentricity of oblique rolling pierced tubes through production trials and establishes an analytical calculation model. The fundamental characteristic of wall thickness eccentricity is described as an "eccentric helical type" pattern, which accounts for more than 70% of the total wall thickness non-uniformity. This helical pattern is directly related to the rotational motion of the tube during the piercing process and the interaction between the piercing plug and the tube坯 (billet).

A critical finding is that billet temperature eccentricity is a very important factor influencing wall thickness eccentricity, and it is recommended to control it within 10°C. This is a practically significant recommendation, as temperature uniformity during billet heating is a well-known challenge in industrial practice. The study also identifies several process parameters that can be adjusted to improve wall thickness eccentricity: increasing the oblique rolling piercing deformation amount, increasing the number of tube rotations, reducing the feed angle, increasing the plug diameter, reducing the billet diameter, increasing the roll transition zone length, and increasing the piercing speed.

Process Parameters and Their Effects on Wall Thickness Eccentricity

Interpretation of Technical Points

The "eccentric helical type" characterization of wall thickness eccentricity is a valuable observation. This pattern arises because during oblique rolling piercing, the tube rotates around its longitudinal axis while being simultaneously elongated and pierced. Any asymmetry in the deformation process, such as non-uniform temperature distribution, uneven contact pressure from the rolls, or slight misalignment of the plug, manifests as a helical variation in wall thickness. The helical wavelength is related to the rotation speed of the tube and the axial deformation rate.

The analytical calculation model developed in this study provides a useful tool for predicting wall thickness eccentricity under different process conditions. This is particularly valuable for process optimization, as it allows engineers to evaluate the effects of parameter changes without conducting expensive production trials. The model should be validated against experimental data for specific mill configurations and steel grades, as the deformation behavior may vary with material properties and processing conditions.

The recommendation to control billet temperature eccentricity within 10°C is highly practical. In industrial practice, achieving uniform temperature distribution in a large-diameter billet during heating is challenging due to the thermal inertia of the steel. The temperature gradient within the billet creates differential deformation during piercing, as the hotter regions deform more easily than the cooler regions. This differential deformation directly contributes to wall thickness eccentricity. Engineering measures to improve temperature uniformity include optimizing the heating furnace design, controlling the heating rate, and implementing temperature monitoring systems with feedback control.

Process Optimization and Quality Control

The process parameters identified in this study provide a comprehensive framework for optimizing wall thickness eccentricity in oblique rolling piercing. Each parameter can be analyzed using the FMEA (Failure Mode and Effects Analysis) methodology to determine its criticality and the required control measures.

From a metallurgical perspective, the chemical composition of the steel affects the deformation behavior during piercing. High-carbon and alloy steels tend to have higher flow stress and lower ductility compared to carbon steels, which can exacerbate wall thickness eccentricity. The deformation temperature range, which is determined by the steel grade, must be carefully controlled to ensure sufficient plasticity for uniform deformation. For example, low-carbon steels such as 20 steel can be pierced at relatively lower temperatures, while high-strength steels such as 42CrMo require higher piercing temperatures to achieve adequate ductility.

Quality control measures for wall thickness eccentricity should include both preventive and detective approaches. Preventive measures include process parameter optimization, equipment maintenance, and operator training. Detective measures include ultrasonic thickness measurement at multiple points around the tube circumference, optical measurement of outer diameter and inner diameter, and statistical process control (SPC) monitoring of eccentricity trends. According to standards such as GB/T 14976 and ASTM A513, the maximum allowable eccentricity is typically specified as a percentage of the nominal wall thickness, and compliance must be verified for each production lot.

The relationship between wall thickness eccentricity and downstream processing is also important. Tubes with significant eccentricity may experience uneven deformation during subsequent cold drawing or cold rolling, leading to additional dimensional inaccuracies. In applications where the tube is subjected to internal pressure, such as in pressure vessels or pipelines, wall thickness eccentricity creates stress concentrations at the thinnest wall location, which can reduce the fatigue life and increase the risk of failure. Therefore, controlling wall thickness eccentricity is not merely a dimensional accuracy issue but a structural integrity concern.

Key Questions and Reflections

Several aspects of this study warrant further consideration. First, the analytical model assumes idealized deformation conditions, and its accuracy may be limited for complex industrial scenarios with multiple interacting parameters. Second, the study focuses on the piercing stage, but wall thickness eccentricity can also be introduced or modified during subsequent processing stages such as rolling, sizing, and cutting. A comprehensive quality control strategy should consider the entire production chain.

The finding that increasing piercing speed improves eccentricity is interesting from a process engineering perspective. Higher piercing speeds reduce the residence time of the tube in the deformation zone, which may limit the development of temperature gradients and non-uniform deformation. However, increasing piercing speed also increases the deformation rate, which may lead to higher flow stress and potential defects such as surface cracks or internal microcracks. Therefore, the optimization of piercing speed must balance eccentricity improvement against other quality considerations.

The study does not extensively discuss the role of plug design and condition in wall thickness eccentricity. In practice, the geometry of the plug nose, the surface condition of the plug, and the lubrication between the plug and the tube坯 all influence the deformation behavior and the resulting eccentricity. Regular inspection and maintenance of the plug, including measurement of wear and replacement when necessary, is essential for maintaining consistent eccentricity levels.

Study Insights and Implications

This study provides a systematic understanding of wall thickness eccentricity in oblique rolling piercing, combining experimental characterization with analytical modeling. The identification of the "eccentric helical type" pattern and the quantification of its contribution to total eccentricity (over 70%) are valuable contributions to the field. The practical recommendations for process parameter optimization, particularly the temperature eccentricity control within 10°C, offer actionable guidance for industrial practice.

For engineers involved in seamless tube production, this paper reinforces the importance of process control and quality assurance at the piercing stage. The analytical model provides a useful tool for process development and optimization, while the experimental findings validate the model and provide confidence in its application. Future work should focus on extending the model to account for additional factors such as plug geometry, lubrication, and steel grade effects, and on developing real-time monitoring and control systems for eccentricity reduction. Overall, this study represents a meaningful contribution to the advancement of seamless tube manufacturing technology and quality control practices.