Phased Array UT Weld Inspection

This tutorial was developed by Materials Evaluation using selected ASNT source material on phased array ultrasonic testing, weld inspection, scan planning, phased array parameter selection, and stainless-steel pipe weld applications. A list of source material appears at the end.

Introduction

Phased array ultrasonic testing (PAUT) gives inspectors powerful tools for weld examination: electronic beam steering, multiple angles in a single scan, real-time imaging, data storage, and improved visualization of the weld volume. But the reliability of a PAUT weld inspection does not begin when the technician starts scanning. It begins with a well-planned scan plan that connects the plan to reliable weld inspection.

What Is a Scan Plan?

A scan plan is a one- to two-page instruction sheet that outlines the steps for performing the specific inspection. The scan plan should include:

• the part thickness

• the weld profile

• a sketch showing probe placement with index positions

• Minimum channels in the phased array (PA) instrument

• PA probe

• sweep angle range

• probe active aperture

• focal depths for half vee and full vee

• calibration block

• calibration reflectors

The sketch in the scan plan must show the reflection of sound from expected discontinuities to confirm their detection. Simply illuminating the weld volume or flooding the weld with sound does not necessarily mean that discontinuities of interest will be detected. Figure 1 shows an example of probe placements. The probe placement shown in Figure 1a illuminates the weld but may not produce a reflection back to the PA probe from the sidewall lack of fusion (LOF). The scan shown in Figure 1b will produce a reflection from the LOF back to the probe.

What PAUT Changes, and What It Does Not

PAUT follows the same physical principles as conventional ultrasonic testing. Sound refracts, reflects, and mode converts according to geometry and discontinuity orientation. The difference is how the beam is produced and controlled.

In conventional angle beam UT, weld inspection commonly uses fixed refracted angles such as 45°, 60°, or 70°. In PAUT, a probe contains many small elements. By pulsing these elements at precisely timed intervals, the instrument can steer and focus the beam electronically (Figure 2).

Instead of changing probes or wedges to obtain different angles, PAUT can sweep through a selected range of angles. For shear wave weld inspection, that range is often ~40° to 70°, depending on the procedure, part geometry, and application. This sectorial scan, or S-scan, can display a cross-sectional image of the weld and surrounding material. PAUT can also use electronic scanning, or E-scan, to step a beam along the length of the probe. Some scan plans combine these approaches.

The advantage is flexibility. The risk is assuming that flexibility automatically equals coverage. It does not. Every PAUT setup must still be aimed at a useful part of the weld volume, and the scan plan must demonstrate that the relevant discontinuities in the volume are reflected back to the PA probe and detected, as shown in Figure 1b.

Start with the Weld, Not the Instrument

A good PAUT setup starts with the weld configuration. Before selecting a probe or entering sweep angle range, focal depth, or other parameters, the examiner must understand the weld thickness, bevel angle, cap condition, access, material, expected discontinuity types and orientation, and acceptance criteria.

The scan plan should answer several practical questions:

• What volume must be examined?

• Are beam angles favorably oriented to reflect sound back to the probe from expected discontinuities (index positions for probe placement)?

• Which surfaces can the probe access?

• Will the examination use half vee or full vee?

• Are multiple groups, wave modes, or focal depths needed?

In weld inspection, PAUT data are often interpreted from direct and reflected sound paths (Figure 3). In a half-vee path, the beam reaches the weld volume before reflecting from the opposite surface. In a full-vee path, the beam reflects from the inside or opposite surface and then interrogates another portion of the weld. Reflected indications may appear as mirror images in the S-scan display. The examiner must understand the sound path to avoid confusing the indication’s position, depth, or orientation.

Select Probe Parameters for Resolution, Not Convenience

Probe selection directly affects detection and sizing. A convenient probe is not always an adequate probe.

The most important probe parameters include active aperture, frequency, element size, number of elements, and wave mode. Active aperture is the number of active elements × element size. Instrument parameters include sweep range, sweep resolution, focal depth, gain correction, and scanning speed. These choices influence beam size, sensitivity, coverage, and image clarity.

Active aperture is especially important. A larger active aperture and higher frequency produce a sharper focus and smaller beam spot, within the limits of the near field and setup geometry. A smaller beam spot improves discontinuity definition and sizing. A poorly focused beam will oversize and blur separate reflectors together, as shown in Figures 4a–4b. For weld inspection, focusing improves both characterization and sizing. Characterization and sizing have a direct effect on discontinuity acceptance as per applicable codes and specifications.

Frequency also matters. Higher frequencies are more susceptible to attenuation. Lower frequencies may be necessary in coarse-grained or attenuative materials, but they can reduce resolution and oversize discontinuities. The goal is not to choose the highest frequency possible in all cases; the goal is to choose the highest practical frequency that provides adequate penetration and signal quality in the material being inspected. A frequency of 5 MHz works well for most carbon steel components.

PA probes can have anywhere from 16 to 128 elements. More elements do not guarantee better results. What matters is the number of active elements. Maximum active elements are limited by the PA instrument channels. For example, the maximum number of active elements on a 32:128 PA machine will be 32, regardless of the probe.

Choose the Wave Mode for the Material and Discontinuity

For many carbon steel welds, shear waves are commonly used because they can operate in both half-vee and full-vee modes. PAUT shear wave sectorial scans can be effective for weld discontinuities such as cracks, lack of fusion, lack of penetration, slag, and porosity when the material and geometry support the technique.

However, shear waves may not adequately penetrate welds in materials such as austenitic stainless steel, dissimilar metal welds, nickel alloy welds, and other anisotropic or coarse-grained materials. In austenitic stainless steel welds, large columnar grains and directional material properties can scatter, attenuate, or skew the ultrasonic beam. Shear waves are often more severely affected than longitudinal waves. Longitudinal waves may provide better penetration through the weld volume, but they are limited to half-vee mode, which may require the removal of the weld cap. L-waves do not support the same full-vee inspection approach as shear waves because they mode convert on reflection.

This does not mean one wave mode is always correct. For some root cracks near the weld in the parent material, both shear and longitudinal waves may be useful. Shear waves may provide a strong corner response when the reflecting surface is favorable. Longitudinal waves may provide additional information from the crack face. For internal discontinuities in coarse-grained weld metal, longitudinal waves may be more reliable.

The practical lesson is simple: choose the wave mode based on material behavior, expected discontinuity location, and sound path. PAUT is a platform; the inspection technique still has to fit the job.

Build the Scan Plan to Prove Coverage

A scan plan is more than an instrument setup file; it serves as the instruction sheet for the inspection.

At minimum, the scan plan should show weld geometry, probe position, PA instrument channels, PA probe, wedge angle, refracted angle range, first and last beam paths, active aperture, focal depth, and the sound paths needed to detect discontinuities in the required volume. It should also identify the essential variables—such as calibration block size and calibration reflector size—as required by the applicable code, standard, procedure, or customer specification.

A traditional S-scan uses a fanlike set of beams through a range of angles from the same active aperture. An E-scan steps a beam along the probe using different groups of elements. A compound S-scan combines aspects of S-scan and E-scan by using a fanlike series of beams through a defined range of angles and elements (Figure 5). This can improve coverage and workflow in some weld applications, especially where one setup must adapt to a range of weld bevels or thicknesses.

Beam resolution should be selected for the setup. Finer angular or beam-step resolution may improve image detail, but it increases the number of A-scans, file size, and data-processing demands. Excessive resolution can slow acquisition or complicate analysis without improving the inspection result. Too little resolution can reduce flaw definition or miss important information. For most inspections, a resolution of 0.5° to 1.0° works well. The correct setting balances coverage, resolution, scanning speed, and data quality.

Calibrate and Verify Before Production Scanning

Calibration confirms that the system response is meaningful. Verification confirms that the setup remains suitable for the inspection.

Calibration blocks must be made as required by the code or specification. Calibration reflectors may include side-drilled holes or notches. A successful calibration must show consistent sensitivity, such as 80% of full-screen height for all calibration reflectors over the entire range of angles. A correct depth should be measured for all calibration reflectors across the full range of angles.

For PAUT weld testing, calibration may include angle-corrected gain, time-corrected gain, adjustment of time delays (wedge delay), encoder calibration, and verification of position and depth accuracy. The examiner should confirm that all reflectors appear at the correct location across the active angle range.

Calibration reflectors should be appropriate for the inspection objective. Notches, side-drilled holes, and other reflectors do not always produce equivalent responses, especially when longitudinal waves or anisotropic materials are involved. For stainless steel and dissimilar welds, calibration blocks should match the material, diameter, thickness, and microstructure as closely as required by the procedure.

Interpret the Image Through the Sound Path

PAUT images are helpful, but they are not self-explanatory. A Level II examiner must interpret indications by connecting the displayed image to the scan plan and sound path.

Important questions include:

• What is the position of the discontinuity relative to the weld?

• Is it planar or volumetric?

• Does the response repeat from the opposite side?

Discontinuities found by PAUT must be evaluated in accordance with the code acceptance criteria.

Practical Checklist for Level II Examiners

Before scanning, confirm the following:

1. The weld geometry and examination volume are addressed in the scan plan.

2. Calibration block and reflectors meet code/specification requirements.

3. Probe, wedge, and wave mode are suitable for the material and discontinuity type.

4. Sweep range and focal depth cover the required weld volume.

5. Half-vee and full-vee sound paths are understood.

6. Sensitivity and gain correction are verified on calibration reflectors across the active angle range.

7. Essential variables and results are documented clearly.

Conclusion

PAUT can improve weld inspection by combining beam steering, real-time imaging, data storage, and flexible coverage. But its value depends on the decisions made before scanning begins.

A reliable PAUT weld inspection requires a scan plan that matches the weld and demonstrates detection of discontinuities of interest, a probe and wave mode matched to the material, calibration that meets the required code or specification, and sound interpretation practice.

For Level II examiners, the strongest habit is to question the setup, thoroughly testing on reflectors in the calibration block. When the scan plan is sound, the parameters are appropriate, and the data are interpreted in context, PAUT becomes more than an advanced display—it becomes a disciplined method for producing reliable weld inspection results.

ACKNOWLEDGMENTS

The editors acknowledge and thank Anmol Birring of Birring NDE for his technical review of this article.

SOURCE MATERIAL

1. Birring, A. S. 2008. “Ultrasonic Phased Arrays for Weld Testing.” Materials Evaluation 66 (3): 282–284.

2. Birring, A. S. 2008. “Selection of Phased Array Parameters for Weld Testing.” Materials Evaluation 66 (9): 931–934.

3. Lupien, V. 2007. “Principles of Phased Array Ultrasound for Nondestructive Testing.” Materials Evaluation 65 (1): 24–32.

4. Magruder, C. 2016. “Advances in Phased Array Weld Inspection Scan Plan Designs – Use of Compound S-scan for Improved Weld Flaw Detection and Sizing.” 2016 Annual Conference Paper Summaries (ASNT): 90–94.

5. Birring, A. S., and J. Williams. 2023. “Phased Array Ultrasonic Testing of Stainless Steel Pipe Welds.” Materials Evaluation 81 (6): 24–33.