Analysis of Internal Fold Defects in Hot-Rolled Seamless Steel Pipes

Literature Overview

This paper, published in 2009 in the journal "Steel Pipe" (钢管) by Zhou Xiaofeng of Tianjin Steel Pipe Group, addresses one of the most persistent quality challenges in seamless steel pipe manufacturing: internal fold defects arising during the piercing operation. The study is particularly significant for engineers involved in large-diameter seamless pipe production for oil and gas applications, where internal surface integrity directly impacts pressure resistance and service life. The author systematically examines the relationship between continuous casting billet quality and piercing process parameters, proposing concrete preventive measures.

Core Technical Points

Internal folds are surface-breaking or subsurface defects located on the inner wall of the seamless pipe, typically manifesting as longitudinal creases or wrinkles that originate during the mandrel piercing stage. These defects compromise the structural integrity of the pipe by acting as stress concentrators, potentially initiating fatigue cracks or corrosion under stress conditions in service.

The paper identifies two categories of root causes:

• Intrinsic factors (internal): Related to the quality of the continuous casting billet, including segregation, internal porosity, cracks in the center of the billet, and non-uniform microstructure due to improper cooling rates.
• Extrinsic factors (external): Related to the piercing process parameters, including mandrel geometry, piercing temperature, piercing speed, and the ratio of piercing reduction.

Piercing Process Parameters and Fold Formation

The mechanism of fold formation can be understood through the following sequence: when the heated billet is pierced by the mandrel, the material flows around the mandrel surface. If the material temperature is insufficient or the deformation is too severe, the inner surface of the forming tube experiences excessive compressive strain. This compressive strain exceeds the material's stability limit, causing buckling or folding of the inner surface. The fold is then carried through subsequent rolling passes, becoming embedded in the final pipe wall.

Standards and Quality Requirements

Internal fold defects are classified under surface defect categories in standards such as API 5CT and GB/T 8163. According to these standards:

• API 5CT Class 1 and Class 2 pipes require freedom from longitudinal seams, cracks, and folds that could affect service performance.
• The maximum acceptable depth of surface imperfections is typically limited to 12.5% of the nominal wall thickness for non-destructive testing acceptance.
• Internal surface defects are particularly critical for pipes used in sour service (H2S environments), where even minor surface irregularities can initiate sulfide stress cracking.

Preventive Measures and Engineering Practice

The paper proposes a multi-layered prevention strategy:

1. Billet quality control: Implement strict incoming inspection of continuous casting billets, including ultrasonic testing for internal defects, chemical analysis for segregation indices, and visual examination of billet surface condition. Reject billets with center cracks, excessive segregation, or surface defects exceeding specification limits.
2. Optimized heating regime: Ensure uniform heating of the billet to the target piercing temperature with minimal temperature gradient across the cross-section. This requires well-calibrated heating furnaces with proper loading patterns and adequate heating time.
3. Piercing parameter optimization: Use finite element analysis to simulate the piercing process and determine the optimal combination of mandrel geometry, piercing temperature, and piercing speed that minimizes inner surface compressive strain.
4. Online monitoring: Implement temperature measurement systems and mandrel wear monitoring to detect deviations from optimal process conditions in real time.
5. Post-piercing inspection: Employ eddy current or ultrasonic testing immediately after piercing to detect folds before they propagate through subsequent rolling passes.

Study Insights and Reflections

This paper reflects a practical engineering approach to quality improvement, emphasizing the systematic analysis of defect formation mechanisms rather than relying solely on end-product inspection. The distinction between intrinsic and extrinsic factors is particularly useful for root cause analysis in production environments. From a quality management perspective, this approach aligns with the PDCA cycle: the analysis phase identifies causes, the corrective action phase implements parameter optimization, and the verification phase confirms effectiveness through reduced defect rates.

A key insight from this literature is that internal folds are not random events but are predictable outcomes of specific process conditions. This predictability means that prevention is achievable through disciplined process control, provided that the critical parameters are properly monitored and maintained within their optimal windows.

Reference Value and Outlook

The findings remain highly relevant for modern seamless pipe production, particularly for large-diameter pipes used in oil and gas pipelines where wall thickness tolerance and internal surface quality are critical. With the increasing demand for high-strength low-alloy (HSLA) seamless pipes, which are more susceptible to deformation-induced defects due to their higher yield strength, the preventive measures outlined in this paper deserve renewed attention. The integration of numerical simulation with process optimization represents a promising direction for further reducing internal fold occurrence rates in future production lines.