Zhuojin Pipe Fitting Co., LtdBurst Failure Analysis of Seamless Steel Tubes in High-Pressure Boilers
Literature Overview
This paper by Wang Peng, Li Fang-po, Lu Cai-hong, Liu Yong-gang, Li Lei, Wang Xin-hu, and Song Sheng-yin, published in Physicochemical Testing (Physical Part) in 2011, presents a detailed failure analysis of a seamless steel tube that burst during a hydrostatic pressure test in a high-pressure boiler system. The research was conducted at the Petroleum Tube Engineering Technology Research Institute of China National Petroleum Corporation. The study employed a comprehensive combination of macroscopic and microscopic examination, mechanical property testing, chemical composition analysis, and metallographic inspection to determine the root cause of the failure.
This case study is of paramount importance to pipe manufacturing and quality assurance professionals, as it illustrates the consequences of manufacturing defects in seamless steel tubes and demonstrates the systematic approach required for failure investigation. High-pressure boiler tubes are critical safety components, and any defect can lead to catastrophic failure with severe consequences.
Core Findings and Failure Mechanism
The investigation revealed that the burst occurred at the location of a maximum crack opening, where an inward fold defect was present along the axial direction of the steel tube. This inward fold defect reduced the effective load-bearing wall thickness and created a stress concentration, which ultimately led to axial bursting during the hydrostatic pressure test.
| Analysis Method | Key Finding | Significance |
|---|---|---|
| Macroscopic examination | Axial burst at maximum crack opening | Identified failure location and mode |
| Microscopic examination | Inward fold defect along axial direction | Identified root cause defect |
| Mechanical property testing | Properties met specification | Ruled out material non-conformance |
| Chemical composition analysis | Composition within acceptable range | Ruled out compositional defects |
| Metallographic inspection | Confirmed inward fold and stress concentration | Verified failure mechanism |
Defect Formation Analysis
The inward fold defect is a manufacturing defect that typically occurs during the hot rolling or piercing process of seamless steel tubes. During the piercing operation, the mandrel (or plug) pushes the heated billet through a roll gap, and if the process parameters are not properly controlled, material can fold inward, creating an internal defect. This defect is particularly insidious because it may not be detectable by external surface inspection and can only be identified through internal quality control methods such as ultrasonic testing (UT) or eddy current testing (ET).
The failure mechanism can be described as follows: the inward fold reduces the effective wall thickness at the defect location, creating a geometric stress concentration. Under the internal hydrostatic pressure, the stress at the defect root exceeds the local yield strength of the material, initiating plastic deformation. As the pressure continues to increase, the defect propagates, and eventually, the tube wall ruptures in an axial direction, following the path of the fold.
Standards and Quality Control Implications
This failure case directly relates to the quality control requirements specified in standards such as GB/T 5310 (Steel seamless tubes for high-pressure boiler), ASTM A213 (Seamless austenitic stainless steel boiler, superheater, and heat exchanger tubes), and EN 10216-2 (Thermo-mechanically controlled fine grain structural hollow sections). These standards mandate specific internal quality requirements, including the absence of internal defects such as folds, seams, and inclusions.
| Standard Requirement | Relevance to This Case |
|---|---|
| Internal surface quality (UT/ET inspection) | Would have detected the inward fold defect |
| Hydrostatic test pressure | The test pressure was appropriate but could not compensate for the defect |
| Wall thickness tolerance | The effective wall thickness at the defect was below the minimum specification |
| Heat treatment specification | Proper heat treatment would not remedy a manufacturing fold |
FMEA Perspective
Applying the Failure Mode and Effects Analysis (FMEA) methodology to this case, the inward fold defect represents a critical failure mode with the following characteristics:
- Failure Mode: Inward fold defect in seamless steel tube wall
- Effect: Reduced effective wall thickness, stress concentration, burst failure under pressure
- Severity: 10 (catastrophic failure with potential for injury and equipment damage)
- Occurrence: Low to moderate (depends on manufacturing process control)
- Detection: Moderate (requires internal NDT methods such as UT or ET)
- RPN (Risk Priority Number): High (due to extreme severity)
The FMEA analysis underscores the critical importance of implementing robust internal quality control measures, including 100% ultrasonic testing or eddy current testing for high-pressure boiler tubes, and the necessity of maintaining tight process control during the piercing and rolling operations.
Key Questions and Reflections
Several important questions arise from this failure analysis. First, what specific manufacturing process parameters led to the formation of the inward fold defect, and how could these have been controlled to prevent the defect? Second, why was the defect not detected during the incoming quality inspection, and what NDT methods could have been employed to identify it? Third, what are the implications for the design and specification of future high-pressure boiler tube procurement contracts?
From a metallurgical perspective, the inward fold defect represents a fundamental material discontinuity that compromises the integrity of the tube wall. Unlike surface defects that can be ground or repaired, internal folds cannot be remedied without destroying the tube. This makes prevention through process control the only viable strategy. The study reinforces the principle that in high-pressure applications, the quality of the seamless tube manufacturing process is paramount, and any compromise in process control can have catastrophic consequences.
Study Insights and Implications
This failure analysis provides a compelling case study for pipe manufacturing engineers and quality assurance professionals. The systematic approach employed in the investigation—combining macroscopic examination, microscopic analysis, mechanical testing, chemical analysis, and metallographic inspection—serves as a model for failure investigation in critical pressure components. The finding that the inward fold defect was the root cause, rather than material non-conformance or improper heat treatment, highlights the importance of manufacturing process control in seamless tube production. For engineers involved in the procurement and inspection of high-pressure boiler tubes, this case underscores the necessity of specifying and verifying internal quality through appropriate NDT methods, and of maintaining rigorous supplier qualification and process audit programs to ensure that manufacturing defects are prevented at the source.