Phased array ultrasonic testing has become the dominant method for volumetric weld inspection on pressure vessels and piping across oil and gas, petrochemical, and heavy fabrication sectors. It offers faster coverage, better imaging, and a permanent digital record compared to conventional contact UT, without the radiation management requirements of radiography.
This guide covers how PAUT works in the field for weld inspection, what the setup and procedure process involves, how defects are detected and sized, and what inspectors and clients should understand about the data that comes out of a PAUT examination.
The Basics of Phased Array Beam Steering
A phased array probe contains multiple small piezoelectric elements, often 16, 32, or 64, arranged in a line. Unlike a conventional single-element transducer that generates a fixed beam, the phased array instrument fires these elements in controlled sequences with precise time delays between elements.
By adjusting the delay pattern, the instrument can steer the beam to different angles without moving the probe. A sector scan, the most common weld inspection scan type, sweeps the beam through a programmed angular range (for example 40 to 70 degrees refracted shear wave) and records the response at each angle in real time.
The result is an S-scan image: a colour-coded cross-section of the weld showing signal amplitude and position at all inspected angles simultaneously. Where a conventional UT operator sees one A-scan trace at a time, a PAUT operator sees a full-volume image updated continuously during scanning.
Setup and Procedure Development
PAUT examination is only as good as its setup and procedure. Every application requires a written procedure and a technique sheet specific to the weld configuration, material, and thickness range being inspected. This is not a generic 'point and scan' exercise.
The procedure defines the focal law configuration (which element groups fire at which delay patterns to achieve each beam angle), the scanning parameters (index point position, scan increment, scan speed limit), the calibration requirements, the reference sensitivity setting, and the acceptance criteria.
A technique sheet, sometimes called a scan plan, shows geometrically how the beam paths cover the weld volume for the specific weld preparation and material thickness. The technique sheet confirms that the programmed focal laws achieve the required angular coverage and that there are no coverage gaps in the weld cross-section.
Under ASME Section V Article 4, the procedure must be demonstrated to detect calibration reflectors at the reference sensitivity level, and the Level III must approve the procedure before inspection begins.
Calibration in the Field
PAUT calibration is performed on a reference block made from the same material specification (or approved substitute) as the component being inspected, with the same surface condition and within the applicable thickness range.
The calibration typically involves sweeping the probe over side-drilled holes at multiple depths to verify sensitivity and beam positioning, and over notches or flat-bottomed holes (depending on the procedure and code requirements) to verify detection capability.
Calibration must be performed at the start of each inspection session, repeated after equipment changes, and verified at regular intervals during the inspection. If calibration verification fails, work performed since the last successful calibration must be re-examined.
Calibration records form part of the inspection package and must be submitted with the final report. A PAUT report without the corresponding calibration record is incomplete.
How Defects Are Detected and Sized
During scanning, the PAUT instrument records a full dataset for every scan position: amplitude and TOF (time of flight) at every focal law angle. This data is stored digitally and can be reviewed after the inspection, re-analyzed at different sensitivities, and measured with sizing tools in post-processing software.
A reportable indication is one that exceeds the reference amplitude at any angle in the sector scan. When an indication is detected, the PAUT operator documents position along the weld, depth, and extent. Sizing uses the 6dB amplitude drop method for amplitude-dependent sizing, or time-of-flight diffraction (TOFD) data if TOFD is included in the examination.
PAUT with TOFD is a highly capable combination. PAUT sector scanning provides good detection sensitivity across the weld volume. TOFD provides precise depth and through-wall sizing of embedded defects using the diffraction signals from defect tips rather than amplitude reflection. For fitness-for-service work where accurate flaw sizing drives remaining life calculations, TOFD sizing accuracy is significantly better than amplitude-based sizing from conventional UT.
A PAUT dataset stored digitally can be re-analyzed later without re-inspection. If sizing criteria change, or if a fitness-for-service analysis requires re-evaluation, the stored data can be re-processed against new parameters.
Common Weld Defects Detected by PAUT
PAUT weld inspection is designed to detect the full range of weld discontinuities that affect structural integrity:
- -Lack of fusion: Failure of the weld metal to fuse with the base metal or adjacent weld passes. Typically a planar, reflective defect that PAUT sector scanning detects well when beams are oriented perpendicular to the fusion face.
- -Lack of penetration: Incomplete root penetration in a weld joint. PAUT detects this as a reflector at the root line depth. Root pass defects are critical in pressure-retaining welds.
- -Porosity: Gas voids in the weld metal. Individual pores produce small, rounded indications. Cluster porosity creates multiple overlapping signals. PAUT detects porosity but RT is more sensitive to fine distributed porosity.
- -Slag inclusions: Non-metallic inclusions trapped in the weld metal. Typically elongated reflectors that PAUT images clearly in the S-scan.
- -Transverse and longitudinal cracking: Cracks oriented along or perpendicular to the weld axis. Transverse cracks require specific scan directions for detection. Longitudinal cracks within the heat-affected zone are well-detected by angle-beam PAUT.
- -Undercut: Groove at the weld toe. Primarily a VT finding but detectable by PAUT when scanning along the weld and can be sized for depth.
The PAUT Report Package
A complete PAUT weld inspection package includes:
- -Inspection procedure and technique sheet: The approved procedure and scan plan showing beam coverage geometry.
- -Calibration records: Pre- and post-inspection calibration verification with equipment serial numbers.
- -Personnel qualification records: PAUT inspector's current certification, including BINDT PCN or equivalent PAUT qualification.
- -Weld map: Identification of each weld examined, weld number, specification, dimensions.
- -Examination records: For each weld: scan direction(s), coverage achieved, any reportable indications with position, depth, and extent.
- -Indication disposition: Accept or reject decision per the applicable acceptance criteria, with Level III review for complex indications.
- -Digital data files: The stored PAUT dataset for each weld, available for client review and future re-analysis.
Norman QC delivers complete PAUT inspection packages with all of the above elements included. All procedures are reviewed and approved by an ASNT Level III UT with BINDT PCN PAUT certification.
FAQs
Does the PAUT procedure need to be qualified before use?
Under ASME Section V Article 4, written PAUT procedures must be qualified by demonstrating detection of calibration reflectors at the reference sensitivity. Some applications and client specifications require additional procedure demonstration and qualification on mock-up samples representative of the production weld geometry. The Level III approves the procedure after qualification.
Can PAUT data be re-analyzed after the inspection is complete?
Yes. The digital dataset stored during PAUT inspection can be re-analyzed using the same or updated sizing software. This is a significant advantage over conventional UT or RT: if a fitness-for-service evaluation requires sizing data not collected during the original inspection, the stored PAUT data can often provide it without a return visit.
How is PAUT affected by surface condition?
Like all contact UT, PAUT requires adequate coupling between the probe and the component surface. Rough or irregular surfaces, weld spatter, or heavy scale can affect coupling and introduce noise. Surface condition requirements are specified in the inspection procedure. In some cases, spot grinding is required at scan positions on rough surfaces to achieve the required coupling.