Burst Failure Analysis of High-Pressure Boiler Seamless Steel Pipe

Literature Overview

The paper by Wang Peng and colleagues from the PetroChina Pipe Technology Research Institute (2011) documents a forensic engineering investigation into the bursting failure of a high-pressure boiler seamless steel pipe during hydrostatic pressure testing. This case study is of considerable practical value to quality control engineers and materials scientists working in the oil, gas, and power generation industries, as it illustrates a systematic approach to failure analysis and highlights the critical role of manufacturing defects in pressure vessel failures.

Failure Investigation Methodology

The investigation employed a multi-disciplinary approach combining several analytical techniques:

Investigation Method	Purpose	Key Finding
Macroscopic examination	Visual assessment of fracture morphology	Axial burst with maximum crack opening at a specific location
Microscopic fractography	Identification of crack initiation site and propagation path	Crack initiated from an internal fold defect
Mechanical property testing	Verification of material performance	Properties met specification requirements
Chemical composition analysis	Confirmation of steel grade compliance	Composition within acceptable range
Metallographic examination	Detection of internal defects	Internal fold defect found at maximum crack location

Root Cause Analysis

The investigation identified a single internal fold defect oriented along the pipe axis as the direct cause of the burst failure. This defect effectively reduced the load-bearing wall thickness at the defect location, creating a severe stress concentration under hydrostatic pressure. The analysis followed a logical FMEA (Failure Mode and Effects Analysis) framework:

Failure Mode: Axial burst of the pipe during hydrostatic pressure test.
Failure Cause: Internal fold defect reducing effective wall thickness and creating stress concentration.
Failure Location: Maximum crack opening coincided with the internal fold defect location.
Contributing Factor: The fold defect was likely introduced during the hot rolling or piercing process of seamless pipe manufacturing, when material flow instability at the mandrel or plug creates folding of the pipe wall.

Defect Formation Mechanism

Internal folds in seamless steel pipes typically originate during the hot rolling process, particularly in the plug mill or floating plug mill stages. When the plug pressure is insufficient or the reduction schedule is aggressive, the pipe wall can fold inward rather than flowing smoothly around the plug. This creates a localized region of reduced wall thickness with a sharp internal corner that acts as a crack initiation site under cyclic or sustained pressure loading.

Stress Concentration Analysis

The presence of an internal fold creates a stress concentration factor (Kt) that can significantly exceed the nominal stress level at the defect location. Under hydrostatic pressure, the hoop stress is uniformly distributed across the pipe circumference, but the fold defect creates a local geometric discontinuity. The effective wall thickness at the fold location may be only 50-70% of the nominal thickness, leading to local stresses that can reach 1.5 to 2 times the nominal hoop stress. When the local stress exceeds the material's fracture strength, crack initiation occurs at the fold tip, followed by rapid unstable propagation leading to burst.

Quality Control Implications

This case underscores the importance of several quality control measures in seamless pipe manufacturing:

Process Control: Plug pressure, reduction schedule, and temperature control during hot rolling must be tightly managed to prevent fold formation. Process windows should be established and monitored using statistical process control methods.
Non-Destructive Testing: Internal fold defects are difficult to detect by conventional ultrasonic testing (UT) because they may not produce strong echoes if they are oriented parallel to the inspection beam. Eddy current testing or phased array UT with multiple scan angles may be more effective for detecting axial folds.
Acceptance Criteria: For high-pressure applications, zero tolerance for internal defects should be enforced, and any indication of wall thickness variation beyond specification limits should trigger detailed investigation.
Traceability: Full material traceability from the billet through all manufacturing stages is essential for effective root cause analysis when failures occur.

Study Insights and Reflections

This failure analysis case is a textbook example of how a single manufacturing defect can lead to catastrophic failure in a pressure-containing component. The seamless pipe met all chemical and mechanical property requirements, yet still failed catastrophically because of a geometric defect that was not detected during quality inspection. This reinforces the principle that material properties alone are insufficient to guarantee structural integrity; geometric integrity and defect control are equally critical. For engineers involved in pressure equipment design and procurement, this case argues for enhanced NDT coverage, particularly for internal defect detection, and for the implementation of rigorous process control in seamless pipe manufacturing. The systematic forensic approach used in this investigation—combining macroscopic, microscopic, mechanical, chemical, and metallographic analyses—serves as a model for effective failure analysis methodology.