TOFD Inspection of Butt Welds Between Clad Shell and Nozzle

Literature Overview

This 2012 paper by Duan Wei, Wang Zhenzhong, and Chen Jianchun from Xi'an Nuclear Equipment Co., Ltd. addresses the application of Time of Flight Diffraction (TOFD) technique to the inspection of butt welds joining a clad shell to a nozzle in pressure vessels. The connection between a clad vessel shell and a nozzle is a critical weld in the fabrication of bimetal pressure vessels, as it must provide both structural integrity and metallurgical compatibility between the base metal and the corrosion-resistant overlay layer. The TOFD technique, which relies on the diffraction of ultrasonic waves from defect tips, offers superior defect sizing capability compared to conventional pulse-echo UT and is increasingly being adopted for the inspection of critical welds in pressure vessel fabrication.

Technical Background

In bimetal pressure vessels, the shell is typically fabricated from clad plate (such as carbon steel base with stainless steel or nickel-based alloy cladding), and the nozzles may be either clad pipe or lined pipe. The butt weld joining the clad shell to the nozzle must be inspected to ensure freedom from defects that could compromise the structural integrity or the corrosion resistance of the vessel. The presence of the clad layer introduces several challenges for UT inspection:

• The clad layer has different acoustic impedance from the base metal, causing reflections and mode conversions at the clad-base interface.
• The clad layer thickness may be insufficient to provide a clear acoustic path for conventional UT techniques.
• The weld geometry is complex, with the clad layer potentially being interrupted or thinned at the weld.

TOFD Technique Principles

TOFD is a non-contact ultrasonic inspection technique that uses two transducers (a transmitter and a receiver) placed on opposite sides of the weld. The technique relies on the diffraction of ultrasonic waves from the tips of planar defects such as cracks and lack of fusion. Key advantages of TOFD include:

• Superior defect sizing capability, particularly for crack-like defects.
• Insensitivity to defect orientation (unlike conventional UT, which is highly orientation-dependent).
• Ability to detect both planar and volumetric defects.
• Reduced need for complex scanning patterns and signal interpretation.

Inspection Methodology

The authors describe the TOFD inspection methodology for the clad shell-to-nozzle butt weld as follows:

1. Surface preparation: The weld surface is ground flush with the surrounding material to ensure good acoustic coupling and to provide a flat scanning surface. The clad layer surface is carefully ground to avoid excessive removal of the corrosion-resistant overlay.
2. Probe selection and arrangement: Dual-element probes with a center-to-center spacing of 10–15 mm are used. The probe frequency is typically 5 MHz, which provides a good balance between resolution and penetration. The probes are placed on opposite sides of the weld, with the transmitter and receiver aligned along the weld centerline.
3. Calibration: The system is calibrated using a reference block containing side-drilled holes (SDH) or notches of known dimensions. The calibration establishes the relationship between time of flight and defect depth, and the amplitude threshold for defect detection.
4. Scanning: The probes are scanned along the length of the weld in both longitudinal and circumferential directions. The scanning speed is typically 100–200 mm/s, with the probe spacing adjusted to ensure complete coverage of the weld volume.
5. Signal analysis: The TOFD signals are analyzed using a time-of-flight-diffraction (TOFD) A-scan or B-scan display. Defect indications are identified as diffracted signals arriving between the direct wave and the back-wall echo. The defect depth is determined from the time of flight, and the defect height is estimated from the time difference between the top and bottom diffraction signals.

TOFD Inspection Parameters

Signal Interpretation in the Presence of Clad Layer

The presence of the clad layer introduces several signal interpretation challenges:

• Clad-base interface reflection: The acoustic impedance mismatch between the clad layer and the base metal produces a strong reflection at the clad-base interface. This reflection may mask or be confused with defect diffraction signals.
• Mode conversion: At the clad-base interface, longitudinal waves may convert to shear waves and vice versa, producing additional echoes that complicate signal analysis.
• Reduced acoustic path: The thin clad layer provides a short acoustic path, which may limit the resolution of defect depth measurement.
• Weld geometry complexity: The clad layer may be interrupted at the weld, creating a complex geometry that produces multiple reflections and diffractions.

To address these challenges, the authors recommend the following approaches:

• Time-gating: Setting time gates to focus on the expected defect region and reject interface reflections.
• Signal subtraction: Using a reference scan from a defect-free area to subtract background signals and enhance defect indications.
• Multi-frequency scanning: Using probes of different frequencies to cross-verify indications and improve defect characterization.
• Complementary NDE: Combining TOFD with other NDE techniques such as PAUT, MT, or RT for comprehensive defect detection.

Comparison with Conventional UT

The following table compares the TOFD technique with conventional pulse-echo UT for the inspection of clad shell-to-nozzle butt welds:

Engineering Practice Insights

From practical experience in pressure vessel fabrication, the following observations are relevant:

• TOFD inspection of clad welds is most effective when the clad layer is relatively thin (3–5 mm) and the weld is well-prepared with a flush ground surface. Thicker clad layers may require additional signal processing to distinguish defect signals from interface reflections.
• The TOFD technique is particularly valuable for detecting lack of fusion and cracks at the clad-base interface, which are the most critical defects in clad welds. These defects are often difficult to detect with conventional UT due to their orientation.
• The combination of TOFD with PAUT provides complementary coverage: TOFD excels at detecting planar defects, while PAUT provides better imaging of volumetric defects and complex geometries.
• Operator training is critical for successful TOFD inspection, as the signal interpretation requires a good understanding of diffraction principles and the ability to distinguish true defect signals from noise and geometric reflections.

Key Questions and Reflections

• How does the TOFD technique perform when the clad layer is thicker than 6 mm, such as in heavy-wall nickel-based alloy clad vessels?
• Can the TOFD technique be adapted for the inspection of clad welds in the presence of a significant thickness mismatch between the shell and the nozzle?
• What is the minimum detectable defect size for TOFD inspection of clad welds, and how does it compare with the acceptance criteria specified in applicable codes?
• Is there a standardized procedure for TOFD inspection of clad welds in current codes and standards, or is the technique still in the research and development phase?

Reference Value and Outlook

This paper is a valuable contribution to the NDE community, as it demonstrates the feasibility and effectiveness of TOFD inspection for clad shell-to-nozzle butt welds. The work provides practical guidance on probe selection, calibration, scanning technique, and signal interpretation, which can be directly applied in production environments. The authors' experience at Xi'an Nuclear Equipment Co., Ltd., a leading manufacturer of nuclear pressure vessels, ensures that the findings are grounded in real-world engineering practice. Future work should focus on standardizing TOFD procedures for clad welds in applicable codes and standards, developing automated TOFD scanning systems for production efficiency, and conducting comparative studies with other advanced NDE techniques such as PAUT and TFM.