Common Weld Defects Detectable Using PAUT
Phased Array Ultrasonic Testing is an advanced nondestructive testing technique used to detect, locate and evaluate discontinuities inside welded components.
PAUT uses multiple ultrasonic elements to electronically steer and focus sound beams through the weld. This allows the inspection system to examine the weld from a range of angles and generate cross-sectional images of the examination volume.
Subject to the material, geometry and qualified procedure, PAUT can detect planar and volumetric weld discontinuities such as cracks, lack of fusion, incomplete penetration, slag inclusions and certain forms of porosity.
Discontinuity, Indication and Defect
These terms should not be used interchangeably.
➤ Discontinuity: An interruption in the normal physical structure of a material.
➤ Indication: A signal or response observed during NDT.
➤ Relevant indication: An indication believed to originate from a material or weld discontinuity.
➤ Defect: A discontinuity that does not satisfy the applicable acceptance criteria.
PAUT does not automatically classify every signal as a defect. The indication must be analysed, characterized and evaluated according to the applicable code or project specification.
1. Lack of Sidewall Fusion
Lack of sidewall fusion occurs when the weld metal does not fuse adequately with the prepared fusion face of the parent material.
Possible causes include:
➤ Insufficient heat input
➤ Incorrect electrode angle
➤ Poor welding technique
➤ Excessive travel speed
➤ Oxide or scale on the fusion face
➤ Improper weld preparation
Lack of sidewall fusion is normally planar and can be difficult to detect if the ultrasonic beam is not directed toward its surface.
PAUT can use multiple angles to improve the probability of producing a strong response from the expected fusion-face orientation.
2. Lack of Inter-Run Fusion
Lack of inter-run or interpass fusion occurs between adjacent weld beads or layers.
It may result from:
➤ Inadequate cleaning between passes
➤ Improper heat input
➤ Incorrect bead placement
➤ Poor electrode manipulation
➤ Slag remaining between weld passes
The discontinuity may have an irregular orientation. Multiple PAUT angles and scan directions can help detect and characterize the response.
3. Lack of Root Fusion
Lack of root fusion occurs when the weld metal fails to fuse properly at the root faces of the joint.
Possible causes include:
➤ Insufficient root gap
➤ Incorrect root-face dimensions
➤ Low heat input
➤ Poor electrode positioning
➤ Misalignment
➤ Improper welding parameters
PAUT inspection should include beam paths designed specifically to cover the weld root and lower fusion faces.
4. Incomplete Root Penetration
Incomplete penetration occurs when the weld metal does not extend completely through the root of a joint designed for full penetration.
It is commonly associated with:
➤ Insufficient root gap
➤ Excessive root face
➤ Incorrect electrode size
➤ Low welding current
➤ Poor joint alignment
➤ Inadequate access to the root
PAUT may produce a consistent linear response at the weld centreline. Evaluation must distinguish incomplete penetration from acceptable root geometry and other geometric responses.
5. Cracks
Cracks are planar discontinuities that may occur during welding, cooling or service.
Types of cracks include:
➤ Hot cracks
➤ Cold cracks
➤ Hydrogen-induced cracks
➤ Toe cracks
➤ Root cracks
➤ Centreline cracks
➤ Transverse cracks
➤ Longitudinal cracks
➤ Crater cracks
➤ Fatigue cracks
➤ Stress-corrosion cracks
Crack detection depends strongly on orientation. A crack perpendicular or favourably angled to the ultrasonic beam may produce a strong response, while an unfavourably oriented crack may require alternative beam angles or scanning directions.
PAUT may also help locate crack tips for through-wall sizing when the signals are sufficiently clear.
6. Slag Inclusions
Slag inclusions are non-metallic materials trapped within the weld.
They may be caused by:
➤ Inadequate cleaning between passes
➤ Incorrect electrode angle
➤ Poor welding technique
➤ Low heat input
➤ Improper weld-bead shape
Slag indications may appear as irregular, elongated or grouped responses. Their ultrasonic characteristics depend on size, shape and orientation.
PAUT can assist with determining their position and length, but classification must consider all available scan views and probe movements.
7. Porosity
Porosity consists of gas cavities trapped in the solidifying weld metal.
Common forms include:
➤ Individual pores
➤ Uniformly distributed porosity
➤ Cluster porosity
➤ Linear porosity
➤ Wormhole or elongated cavities
Porosity is generally volumetric and may produce multiple small, scattered signals.
Radiography can be particularly effective for displaying porosity patterns. PAUT may detect significant porosity, but sensitivity varies according to pore size, distribution, frequency and inspection setup.
8. Tungsten Inclusions
Tungsten inclusions may occur during gas tungsten arc welding when particles from the tungsten electrode become trapped in the weld.
The ultrasonic response depends on the inclusion’s size, location and surrounding weld structure.
PAUT may detect significant inclusions, although radiography can provide useful complementary information for this type of volumetric discontinuity.
9. Root Concavity
Root concavity occurs when the root surface of the weld is below the internal surface of the parent material.
Because it is a geometric condition rather than a fully enclosed discontinuity, its PAUT response may vary with:
➤ Concavity depth
➤ Surface profile
➤ Beam angle
➤ Pipe curvature
➤ Weld alignment
Accurate weld geometry and supplementary inspection may be needed to distinguish root concavity from lack of penetration or root cracking.
10. Excessive Root Penetration
Excessive penetration occurs when too much weld metal projects beyond the internal surface.
It can create geometric ultrasonic responses that may be confused with discontinuities.
PAUT evaluation should consider:
➤ Signal movement
➤ Position relative to the weld centreline
➤ Response from opposing scan directions
➤ Root-profile information
➤ Comparison with calibration or demonstration specimens
11. Undercut
Undercut is a groove melted into the parent material near the weld toe or root and left unfilled.
External undercut is usually detected more effectively through visual or surface examination. Internal or inaccessible undercut may produce an ultrasonic response.
PAUT can supplement visual, dimensional or conventional UT examination where volumetric information is required.
12. Lamination
Laminations are planar discontinuities within rolled products. They are generally parallel to the material surface and may exist near a weld before fabrication.
Laminations can:
➤ Interfere with angle-beam inspection
➤ Produce unexpected reflections
➤ Affect weld integrity
➤ Extend during welding or service
➤ Complicate indication positioning
A straight-beam lamination check should be completed where required before performing weld PAUT.
13. Burn-Through
Burn-through is a localized area where excessive penetration creates an opening or severe root depression.
It is often identifiable through visual examination or radiography. PAUT may detect the associated root-profile change, but interpretation depends on access and geometry.
14. Misalignment and Hi-Lo
Internal misalignment occurs when the joined components are not aligned correctly.
Misalignment can create strong geometric responses and change the expected weld profile. Although PAUT may identify the effect, dimensional or visual measurement is normally required to determine the actual amount of misalignment.
15. Service-Induced Cracking
PAUT may be used to investigate cracking that develops after a component enters service, including:
➤ Fatigue cracking
➤ Stress-corrosion cracking
➤ Creep-related cracking
➤ Hydrogen-related cracking
➤ Thermal-fatigue cracking
➤ Cracking near weld repairs
The inspection technique must be developed for the expected damage orientation and location. Specialized probes, lower frequencies or complementary NDT methods may be required.
Defect Detectability Summary
|
Discontinuity |
Typical form | PAUT suitability |
| Lack of sidewall fusion | Planar |
Generally strong when beam orientation is suitable |
|
Lack of root fusion |
Planar | Detectable with dedicated root coverage |
| Incomplete penetration | Linear/planar |
Generally detectable, but root geometry must be considered |
|
Cracks |
Planar | Strongly dependent on crack orientation |
| Slag inclusion | Irregular/volumetric |
Generally detectable depending on size and orientation |
|
Porosity |
Volumetric | Detectability varies; fine porosity can be difficult |
| Tungsten inclusion | Volumetric |
Detectability depends on size and position |
|
Root concavity |
Geometric | May be indicated but requires careful characterization |
| Undercut | Surface-connected |
Surface methods may be more appropriate |
|
Lamination |
Planar | Generally detectable using straight-beam examination |
| Misalignment | Geometric |
PAUT may show the response; dimensional confirmation required |
|
Service cracking |
Planar/branched |
Requires damage-specific technique development |
How PAUT Characterizes Indications
PAUT analysis may consider:
➤ Signal amplitude
➤ Indication position
➤ Sound-path distance
➤ Beam angle
➤ Indication length
➤ Through-wall location
➤ Signal behaviour during probe movement
➤ Response from multiple angles
➤ Response from opposite scanning directions
➤ Relationship to the weld geometry
➤ Tip-diffracted signals
➤ Associated mode-converted signals
No single coloured scan image should be used alone to classify a defect.
Defect Length Measurement
Indication length may be measured using:
➤ Encoded positional data
➤ Amplitude-drop techniques
➤ Defined reporting thresholds
➤ Signal-envelope analysis
➤ Code-specified methods
Length-sizing accuracy depends on scanner position, encoder calibration, beam spread, signal response and the selected sizing technique.
Through-Wall Height Sizing
Possible sizing approaches include:
➤ Tip-diffraction techniques
➤ Time of Flight Diffraction
➤ Amplitude-based methods
➤ Maximum-amplitude techniques
➤ Beam-boundary methods
➤ Code-approved sizing methods
Through-wall sizing should only be reported when the procedure, equipment and personnel are qualified for the selected method.
Geometric Indications
Not every PAUT signal represents a weld discontinuity. Responses may originate from:
➤ Weld root
➤ Weld cap
➤ Counterbore
➤ Internal radius
➤ Back wall
➤ Pipe curvature
➤ Flange face
➤ Nozzle geometry
➤ Thickness transition
➤ Cladding interface
➤ Mode conversion
➤ Surface roughness
Accurate drawings and knowledge of the actual component geometry are essential for reliable interpretation.
Factors Affecting Defect Detection
PAUT performance depends on:
➤ Defect size
➤ Defect orientation
➤ Material type
➤ Grain structure
➤ Component thickness
➤ Pipe curvature
➤ Probe frequency
➤ Element configuration
➤ Wedge design
➤ Beam angle
➤ Focal depth
➤ Surface condition
➤ Coupling stability
➤ Calibration-block suitability
➤ Scanner position
➤ Inspector competency
PAUT Limitations
PAUT may not provide reliable detection when:
➤ The expected defect is inaccessible to the beam
➤ Probe movement is severely restricted
➤ Material attenuation is excessive
➤ Surface condition prevents stable coupling
➤ Component geometry creates strong interfering signals
➤ The calibration block is not representative
➤ Actual weld dimensions are unknown
➤ Defects are extremely small or unfavourably oriented
These limitations should be evaluated during scan planning and documented in the final report.
Complementary NDT Methods
PAUT may be combined with:
➤ TOFD for through-wall sizing
➤ Conventional UT for confirmation
➤ Magnetic Particle Testing for surface cracks in ferromagnetic materials
➤ Liquid Penetrant Testing for surface-breaking discontinuities
➤ Radiographic Testing for volumetric weld discontinuities
➤ Visual Testing for weld-profile and surface conditions
➤ Eddy Current Testing for suitable surface and near-surface applications
Applicable Standards
PAUT weld examination may be performed with reference to:
➤ ASME Boiler and Pressure Vessel Code, Section V
➤ ISO 13588
➤ ISO 17640
➤ ISO 4761
➤ ISO 20601
➤ ISO 22825
➤ API 1104
➤ AWS requirements, where applicable
➤ Client specifications
➤ Approved PAUT procedures
The applicable acceptance standard determines whether a detected discontinuity is acceptable or classified as a defect.
Our PAUT Defect-Detection Services
Integrity & Advanced Inspection Solutions India Private Limited provides:
➤ PAUT weld inspection
➤ PAUT and TOFD combined inspection
➤ Crack detection and sizing
➤ Pipeline girth-weld inspection
➤ Pressure-vessel weld inspection
➤ Small-bore piping inspection
➤ Repair-weld inspection
➤ Complex-geometry assessment
➤ Procedure and scan-plan development
➤ Calibration and validation blocks
➤ Flawed demonstration specimens
➤ Encoded data analysis and reporting
Information Required for Inspection Planning
Please provide:
➤ Component and weld drawings
➤ Material specification
➤ Diameter and wall thickness
➤ Weld preparation
➤ Expected defect type
➤ Manufacturing or service history
➤ Applicable inspection code
➤ Acceptance criteria
➤ Surface access
➤ Inspection quantity
➤ Project location
➤ Required schedule
Request a PAUT Technical Assessment
Our technical team can review the component geometry and expected damage mechanism before recommending PAUT, TOFD or a complementary NDT approach.