Zhuojin Pipe Fitting Co., LtdAnalysis of Reduced Ultrasonic Thickness Readings in Steel Pipes Due to Internal Inclusions
Literature Overview
This technical paper by Tian Dang, Xiu Xuefeng, Hou Wei, Li Xiuqin, Xu Qiulong, and Zhang Qin (2011), published in the journal Steel Pipe, addresses a practical quality control problem encountered during ultrasonic thickness measurement of steel pipes. The authors from Yangtze Steel Pipe Co., Ltd. systematically investigated cases where ultrasonic thickness gauges recorded values lower than the actual wall thickness, accompanied by surface defects such as bulges and lifts on the inner surface of the pipes. This is a significant industrial problem because it can lead to false rejection of acceptable pipes or, more dangerously, acceptance of pipes with hidden internal defects.
Root Cause Analysis
The study classified and statistically analyzed the occurrences of reduced ultrasonic readings and conducted detailed sampling and metallographic examination of affected specimens. The investigation revealed that the reduced readings are caused by abnormal inclusions distributed within the wall thickness, particularly those located in the mid-wall region and near the inner surface. The inner surface bulges and lifts are specifically associated with abnormal inclusions located near the inner surface edge of the wall thickness.
Defect Classification and Mechanism
| Defect Type | Inclusion Location | Ultrasonic Effect | Surface Manifestation |
|---|---|---|---|
| Reduced reading only | Mid-wall thickness | Partial wave reflection/scattering | No visible surface defect |
| Reduced reading with bulge | Near inner surface | Strong reflection from inclusion boundary | Inner surface bulge |
| Reduced reading with lift | Near inner surface edge | Interface between inclusion and matrix | Inner surface lift/scale |
The authors make an important distinction between inner surface lifts caused by inclusions and inner folds resulting from the two-high inclined rolling piercing process. These are fundamentally different defects with different origins and implications for pipe quality. The inclusion-related lifts arise from the localized weakness at the inclusion-matrix interface, where the bonding is poor and the material can separate under subsequent forming operations. In contrast, piercing folds are geometric defects caused by improper deformation during the piercing step of the hot rolling process.
Ultrasonic Interaction with Inclusions
When an ultrasonic pulse encounters an inclusion within the pipe wall, the acoustic impedance mismatch between the inclusion and the surrounding matrix causes partial reflection and scattering of the wave energy. If the inclusion is located in the mid-wall region, the reflected energy reduces the amplitude of the back-wall echo, which can be misinterpreted by the gauge as a shorter path length, resulting in a lower thickness reading. When the inclusion is near the inner surface, the interaction becomes more complex, and the inclusion boundary can act as a partial reflector that creates false echoes. The presence of bulges and lifts on the inner surface further complicates the measurement by introducing additional acoustic interfaces.
Engineering Practice and Quality Control Implications
This study has direct implications for ultrasonic testing procedures in steel pipe manufacturing and inspection. The findings suggest that a reduced ultrasonic reading should not be automatically interpreted as wall thinning. Instead, it should trigger a secondary inspection using complementary methods such as magnetic particle testing (MT), eddy current testing (ET), or direct visual examination of the inner surface after cutting. The distinction between inclusion-related defects and piercing folds is critical for proper classification and disposition of affected pipes.
Recommended Inspection Protocol
- Perform initial ultrasonic thickness measurement across the full circumference of the pipe.
- Flag any reading that deviates significantly from the nominal wall thickness or from adjacent measurement points.
- Conduct MT or ET inspection on the flagged regions to detect surface and near-surface discontinuities.
- For pipes showing bulges or lifts, perform a cross-sectional metallographic examination to determine the nature and extent of the inclusion.
- Classify the defect according to the relevant standard (e.g., ASTM A530, GB/T 196) and make a disposition decision.
Study Insights and Practical Recommendations
From my experience in pipe manufacturing quality control, this paper addresses a problem that is frequently encountered but rarely thoroughly investigated. The systematic approach taken by the authors, combining statistical analysis, metallographic examination, and ultrasonic characterization, provides a model for similar investigations in other pipe grades and production methods. The key takeaway for engineers is that ultrasonic thickness measurement, while fast and non-destructive, has inherent limitations when internal inclusions are present. A robust quality control system must integrate multiple inspection methods and maintain a clear understanding of the physical mechanisms behind measurement anomalies. The distinction between inclusion-related defects and process-induced folds is essential for correct root cause analysis and effective corrective action.