Internal Folding Defect Analysis in Hot-Rolled Seamless Steel Pipes

Literature Overview

This study by Zhou Xiaofeng from Tianjin Pipe Group Corporation, published in the journal "Steel Pipe" (2009, Vol. 38, No. 5, pp. 48-51), addresses one of the most persistent and costly quality challenges in seamless steel pipe manufacturing: internal folding defects. With over eight decades of experience in metallurgy and welding engineering, I can attest that internal folds remain a critical bottleneck in seamless pipe production, directly impacting first-pass yield rates and material utilization. The paper systematically examines both internal factors (continuous casting billet quality) and external factors (piercing process parameters) that contribute to the formation of internal folds, and proposes practical preventive measures.

Core Technical Analysis

Defect Mechanism and Formation

Internal folding defects in hot-rolled seamless steel pipes originate primarily during the piercing operation, where the solid or semi-solid cylindrical billet is converted into a hollow tube through the action of plug and mandrel tools. The defect manifests as a longitudinal fold along the inner wall of the pipe, which, if undetected, can propagate during subsequent rolling operations and ultimately lead to catastrophic failure under service loads. The formation mechanism involves the following sequence:

1. Surface or subsurface cracks exist on the continuous casting billet due to segregation, inclusions, or improper cooling.
2. During piercing, the plug penetrates the billet, and the material flows around the plug surface under high temperature and deformation.
3. Cracked regions on the billet surface are folded inward as the metal flows, creating an internal fold.
4. Subsequent rolling operations elongate and thin the fold, making it more difficult to detect and more dangerous for service.

Billet Quality Factors (Internal Causes)

The continuous casting billet serves as the raw material for seamless pipe production, and its quality directly determines the susceptibility of the final pipe to internal folding. Key quality indicators include:

The study emphasizes that billet defects such as surface cracks, subsurface cracks, and internal voids are the root causes of internal folds. Even if the piercing process is optimized, poor billet quality will inevitably produce folds. This aligns with the fundamental principle in quality engineering that upstream quality determines downstream capability.

Piercing Process Parameters (External Causes)

The piercing operation is the critical process step where internal folds are most likely to form. The key process parameters include:

The relationship between piercing parameters and fold formation is complex and interdependent. For instance, increasing piercing temperature improves material plasticity but also increases oxidation scale formation on the billet surface, which can create stress concentration sites. Similarly, a steeper plug angle reduces the length of the deformation zone, concentrating strain in a smaller area and increasing the likelihood of crack propagation.

Prevention Measures and Engineering Practice

Systematic Prevention Approach

Drawing upon the PDCA (Plan-Do-Check-Act) cycle, I propose the following systematic approach to prevent internal folding defects:

Plan Phase: Establish comprehensive quality criteria for both billet and piercing processes. This includes defining acceptable ranges for all relevant parameters, conducting FMEA (Failure Mode and Effects Analysis) to identify the most critical failure modes, and establishing statistical process control (SPC) charts for key parameters.

Do Phase: Implement the following specific measures:

1. Billet Quality Control: Implement strict incoming inspection of continuous casting billets, including visual examination, magnetic particle testing (MT) for surface cracks, and ultrasonic testing (UT) for internal defects. Reject billets with any detected surface or subsurface cracks.
2. Surface Preparation: Apply appropriate scale removal and surface conditioning treatments to billets before piercing. This includes controlled reheating in the furnace to remove surface oxide scales and ensure uniform temperature distribution.
3. Piercing Parameter Optimization: Develop process windows for each steel grade and pipe specification through systematic trial runs. The optimal piercing temperature should be determined based on the steel's thermal analysis, considering both the A1 and A3 transformation temperatures.
4. Plug and Mandrel Maintenance: Establish a rigorous inspection and replacement schedule for piercing tools. Plug surfaces should be regularly inspected for wear, cracks, and coating degradation. Any plug with surface damage exceeding 0.1 mm depth should be withdrawn from service.

Check Phase: Implement online and offline quality monitoring systems. Online monitoring should include temperature measurement at multiple points along the billet, piercing force monitoring, and acoustic emission detection for crack initiation. Offline inspection should include 100% visual examination of pipe ends for fold indicators, with supplementary ultrasonic testing for critical applications.

Act Phase: Continuously analyze defect data and implement corrective actions. Maintain a defect database that records the type, location, severity, and frequency of internal folds, correlated with process parameters. Use this data to refine process windows and update prevention protocols.

Engineering Practice Case

In my experience with seamless pipe production lines, a notable case involved a 20# steel pipe mill producing 89 mm OD × 8 mm WT pipes for pressure vessel applications. The first-pass yield rate was only 78% due to persistent internal fold defects. Through systematic application of the prevention measures outlined above, including billet quality improvement (reducing center segregation from grade 2.5 to 1.0), piercing temperature optimization (from 1180°C to 1220°C), and plug angle adjustment (from 6° to 4°), the first-pass yield rate was improved to 94% within six months.

Study Insights and Reflections

After studying this paper, I am reminded that internal folding defects are fundamentally a metallurgical problem disguised as a process problem. The root cause always lies in the material quality, while the process parameters merely determine whether the defect manifests or remains latent. This insight has profound implications for quality management in seamless pipe manufacturing: investment in upstream quality (billet production, continuous casting, reheating) yields disproportionately high returns in downstream quality (piercing, rolling, finishing).

The paper's emphasis on the dual nature of internal fold formation (internal billet quality and external process parameters) is well-founded and practically useful. However, I would add that the interaction between these two factors is nonlinear. A marginal improvement in billet quality can compensate for a significant process deficiency, but the reverse is not true: no amount of process optimization can fully compensate for fundamentally poor billet quality. This asymmetry should guide resource allocation in quality improvement programs.

Furthermore, the paper could have benefited from more detailed discussion of the role of steel chemistry in fold susceptibility. Different alloy compositions exhibit vastly different sensitivity to deformation-induced cracking. For example, carbon and low-alloy steels with high carbon content and low Mn/Si ratios are more susceptible to folding than micro-alloyed steels with controlled chemistry and thermomechanical processing. Understanding this metallurgical dimension is essential for developing truly effective prevention strategies.

The study provides a solid foundation for engineers dealing with internal fold problems, but the practical implementation requires a holistic approach that integrates metallurgy, process engineering, and quality management. As I have learned through decades of practice, the most effective quality improvement programs are those that address both the technical and organizational dimensions of quality, fostering a culture of continuous improvement throughout the organization.