JCO Precision Bending Forming Technology for Pipeline Steel Pipes

Literature Overview

This paper by Li Jian, Zhao Jun, and Ma Rui, published in Optics and Precision Engineering in 2010, presents a precision bending forming technology for pipeline steel pipes using the JCO (Jog-Curl-Open) process. The research was supported by the National Natural Science Foundation of China (Youth Fund, No. 50805126) and conducted at Yanshan University and the First Heavy Industries Group Dalian Design and Research Institute. The core innovation is the integration of plastic bending engineering theory with machine vision measurement technology to achieve precise control of the bending process, reducing the ovality of the formed pipe blank to within 1.5 percent.

Technical Background of JCO Forming

The JCO forming process is a widely used method for manufacturing large-diameter line pipes. The process involves three main stages: the J stage, where the flat plate is pre-bent at one end; the C stage, where the plate is progressively bent into a C-shape through multiple rolling passes; and the O stage, where the C-shaped blank is closed into a full circle and welded. The ovality of the formed pipe blank is a critical quality parameter because it affects the subsequent welding quality, dimensional accuracy, and mechanical performance of the finished pipe. High ovality leads to uneven weld bead thickness, residual stress concentration, and potential defects at the weld toe.

Traditionally, the JCO forming process relies heavily on operator experience to adjust the roll positions and bending parameters. This empirical approach is inconsistent, time-consuming, and prone to errors, particularly when manufacturing pipes of varying diameters and wall thicknesses. The precision bending technology presented in this paper addresses this limitation by implementing a closed-loop control system based on machine vision measurement.

Core Technical Methodology

The precision bending technology proposed in this paper consists of three key components. First, the two-pass pre-bending method is employed in the initial forming pass to identify the springback behavior of the plate material. Springback is the elastic recovery that occurs after the plastic deformation of the plate during bending, and it is a primary source of forming error. By performing two pre-bending passes, the system can measure the springback amount and incorporate it into the prediction of the precise pressing stroke for the target forming angle.

Second, the error compensation technique is applied in subsequent forming passes. After each pass, the actual forming angle is measured using the machine vision system, and the deviation from the target angle is calculated. This deviation is then used to adjust the roll position for the next pass, creating a feedback loop that progressively reduces the forming error. This iterative compensation approach ensures that each forming pass achieves the target angle with high accuracy, regardless of material variability or process disturbances.

Third, the machine vision measurement system provides real-time monitoring of the forming angle. The system uses image processing algorithms to detect the straight edges of the pipe blank and convert the image angle to the actual forming angle. The camera external parameters are calibrated using a positive triangle as the calibration template, which allows rapid and high-precision calibration from a single image.

Technical Parameters and Performance

The following table summarizes the key technical parameters and performance metrics of the precision bending technology:

The reduction of ovality from the typical 2 to 3 percent range to within 1.5 percent is a significant improvement. This reduction directly translates to improved welding quality, as a more circular pipe blank results in more uniform gap control and weld bead geometry. The 0.2 degree forming angle monitoring error is well within the tolerance required for precision forming, ensuring that the cumulative error across multiple forming passes does not exceed acceptable limits.

Image Processing and Measurement System

The machine vision measurement system is a critical component of the precision bending technology. The image processing algorithm is specifically designed for the detection of straight edges on the pipe blank surface. The processing flow includes image acquisition, preprocessing, edge detection, line fitting, and angle calculation. The edge detection algorithm must be robust against surface variations, lighting conditions, and the presence of surface oxides or scale on the hot-rolled plate.

The calibration method using a positive triangle as the template is particularly elegant in its simplicity. A positive triangle provides three non-collinear points that define both the scale and orientation of the image plane relative to the object plane. By solving the perspective transformation equations using the known coordinates of the triangle vertices and their detected image coordinates, the camera external parameters (rotation and translation) can be determined from a single image. This approach eliminates the need for multi-view calibration, which is time-consuming and prone to errors.

Engineering Practice and Implementation Considerations

The implementation of precision bending technology in a JCO forming line requires careful integration of the machine vision system with the roll control system. The following table outlines the key implementation considerations:

The precision bending technology represents a paradigm shift from experience-based to measurement-based forming control. This shift requires changes in the production workflow, quality control procedures, and operator skill requirements. The system should be integrated into the overall process control system of the JCO forming line, with appropriate alarms and interlocks to prevent operation when measurement errors exceed acceptable limits.

Study Insights and Reflections

This paper presents a well-conceived and practically implemented approach to precision bending in JCO forming. The integration of machine vision measurement with iterative error compensation creates a robust closed-loop control system that significantly improves the dimensional accuracy of the formed pipe blank. The use of a positive triangle for camera calibration is a clever solution that balances accuracy with implementation simplicity. However, the study focuses on the bending process in isolation, and it does not address the interaction between the precision bending control and the subsequent welding process. In practice, the ovality of the pipe blank affects the welding gap, which in turn affects the welding parameters and weld quality. A comprehensive process control strategy should consider the entire forming-welding sequence as a coupled system. Additionally, the technology should be validated across a range of pipe diameters, wall thicknesses, and material grades to confirm its robustness under varying production conditions. The results presented here are encouraging and suggest that precision bending technology has the potential to become a standard feature in modern JCO forming lines.