Factors Affecting Steel Pipe Thickness Measurement by Ultrasonic Thickness Gauges

Literature Overview

This technical paper by Shen Haihong, Kang Lihong, Wan Ce, Jiang Mingming, Liu Yujie, and Yao Yifei, published in Physical Testing (2025, Vol. 43, No. 5, pp. 43–48), addresses a practical quality control challenge in steel pipe manufacturing: the accurate measurement of steel pipe wall thickness using ultrasonic thickness gauges. The authors discuss the principles of ultrasonic thickness measurement, identify causes of measurement anomalies, and propose a systematic approach using A-scan pulse-echo ultrasonic flaw detectors for verification and correction.

Core Technical Findings

The study identifies two primary causes of thickness measurement anomalies:

The key finding is that ultrasonic thickness gauges measure the time-of-flight of the ultrasonic pulse to the back wall. If an internal defect exists between the transducer and the back wall, the defect reflection can be mistaken for the back wall reflection, resulting in an erroneously low thickness reading. This is a critical distinction that has significant implications for quality control decisions.

Ultrasonic Thickness Measurement Principles

Ultrasonic thickness measurement operates on the pulse-echo principle:

1. An ultrasonic transducer emits a short-duration acoustic pulse into the material.
2. The pulse travels through the material at a known velocity (calibrated for the specific material).
3. The pulse reflects off the back wall and returns to the transducer.
4. The thickness is calculated as: t = v × Δt / 2, where v is the sound velocity and Δt is the round-trip time.

The accuracy of this method depends on:

• Correct sound velocity calibration for the specific material
• Clean and properly coupled transducer-material interface
• Absence of internal defects that could produce false back-wall echoes
• Adequate signal-to-noise ratio

Verification Methodology Using A-Scan Pulse-Echo Flaw Detectors

The authors propose a systematic verification procedure using A-scan pulse-echo ultrasonic flaw detectors:

Step 1: Equipment Setup

• Set the background noise level to 10% of full screen height
• Calibrate the sound velocity for the specific steel grade
• Select appropriate probe frequency (typically 2.5–5 MHz for steel pipe wall thickness measurement)

Step 2: Scanning Procedure

• Scan the area near the thickness measurement point
• Observe the A-scan display for reflections before the back-wall echo

Step 3: Interpretation

Engineering Practice Implications

For quality control engineers in steel pipe manufacturing:

• Standard compliance: The study recommends that national product standards should specify that when ultrasonic measurement results conflict with caliper measurements, caliper measurements should be considered authoritative. This recommendation addresses a practical challenge where ultrasonic measurements can be misleading due to internal defects.
• Equipment selection: A-scan pulse-echo flaw detectors should be available in quality control laboratories for verification of ultrasonic thickness measurements, particularly for critical applications.
• Training requirements: Quality control personnel should be trained to distinguish between true thickness reduction and apparent thickness reduction caused by internal defects. This requires understanding of ultrasonic principles and practical experience in A-scan interpretation.
• Documentation: When ultrasonic thickness measurements are performed, the A-scan verification results should be documented to provide traceability and support for quality decisions.

Key Questions and Reflections

Several aspects of this study merit further consideration:

• The study focuses on steel pipes; the same principles apply to other steel products (plates, forgings, castings), but the specific defect types and acceptance criteria may differ.
• The effect of pipe curvature on ultrasonic thickness measurement is not addressed. For thin-walled pipes with small diameter-to-thickness ratios, the curved surface can affect the accuracy of thickness measurements.
• The study does not discuss the effect of material anisotropy (common in rolled steel) on ultrasonic sound velocity and thickness measurement accuracy.
• The impact of surface roughness, paint, or other coatings on ultrasonic coupling and measurement accuracy is not addressed.

Study Insights and Practical Recommendations

The most valuable contribution of this study is the systematic approach to resolving thickness measurement anomalies. The proposed verification procedure using A-scan pulse-echo flaw detectors provides a practical solution to a common quality control challenge.

For steel pipe manufacturers, the following recommendations are proposed:

1. Implement dual measurement: Use both ultrasonic thickness gauges and calipers for critical thickness measurements. When results conflict, use the A-scan verification procedure to determine the cause.
2. Train quality control personnel: Ensure that quality control personnel understand the limitations of ultrasonic thickness gauges and can perform A-scan verification when needed.
3. Update quality procedures: Incorporate the A-scan verification procedure into quality control procedures, particularly for applications where internal defects are likely (e.g., recycled steel, low-quality steel, or pipes with known manufacturing defects).
4. Advocate for standard updates: Support the recommendation to update national product standards to specify the authority of caliper measurements when ultrasonic and caliper results conflict.

The study highlights an important aspect of quality control that is often overlooked: the need to understand the limitations of measurement tools and to implement verification procedures when measurement anomalies are encountered. This approach ensures that quality decisions are based on accurate and reliable data.