Ultrasonic testing of LSAW steel pipe

In the ultrasonic inspection of longitudinally submerged arc welded (LSAW) steel pipes, distinguishing between defect and non-defect signals is crucial. This process involves analyzing the characteristics of ultrasonic testing echoes, including those caused by segregation, inclusions, cracks, and base metal delamination. By understanding these signals, we can better locate defects and improve inspection accuracy.

This paper examines the causes and identification methods for four types of pseudo-defective waves: coupling agent reflection waves, weld surface reflection waves, reflection waves caused by weld misalignment, and non-defect waves from weld height or undercuts. These insights offer new approaches to enhance the accuracy of ultrasonic testing for LSAW pipes.

Pseudo-Defective Waves in Ultrasonic Inspection

Due to the welding process of steel pipes, the oscilloscope screen often displays pseudo-defect echoes in addition to the initial and defect waves. These pseudo-defective waves can interfere with defect identification. Below are the causes and identification methods for four common types of pseudo-defective waves:

- Coupling Agent Reflection Wave

Cause: The coupling layer’s thickness is theoretically required to be an integral multiple of the wavelength radius. If the coupling agent is too thick or lacks proper fluidity, it can accumulate in front of the probe. This accumulation converts part of the longitudinal wave into a surface wave, causing a reflection signal.

Identification Method: If the coupling agent wave is the cause, the wave will disappear when the coupling agent is evenly spread.

- Weld Surface Welding Trace Reflection Wave

Cause: This type of reflection wave often appears in butt welds of steel pipes. Deviations in pipe length and cumulative errors in multi-layer structures can result in excessive welding width, creating multi-channel weld marks on the weld surface.

Identification Method: To identify this wave, fix the probe and gently tap the weld with a hand-held coupler. The reflection wave will gently oscillate if caused by weld marks.

- Reflection Wave Caused by Weld Misalignment

Cause: This reflection wave primarily appears during the field inspection of butt welds in steel pipes. It is caused by axial misalignment when connecting the pipes.

Identification Method: The wave can be identified by analyzing its position and behavior during inspection.

- Non-Defect Waves from Excessive Weld Height or Undercuts

Cause: When the weld is too high or undercuts are present, corner reflections can occur, complicating defect discrimination.

Identification Method:

Calculate Sound Path: Accurately calculate the sound path distance and determine the depth or horizontal position of the defect based on the display data.

Hand Tap Test: Gently tap both sides of the weld with a hand-held coupler. If the reflected wave signal oscillates, it is likely due to corner reflections from excessive weld height or undercuts.

Manual Grinding Test: Manually grind the weld height or remove the scar, then retest. If the reflection wave disappears, it confirms the wave was a non-defective wave.

Conclusion

In the ultrasonic inspection of LSAW steel pipes, identifying and determining echoes is critical for accurate defect detection. By understanding the causes and characteristics of pseudo-defective waves, such as those from coupling agents, weld surface traces, weld misalignment, and excessive weld height or undercuts, inspectors can improve the accuracy and reliability of ultrasonic testing. This knowledge provides a new direction for enhancing the effectiveness of ultrasonic inspections in the industry.