Ultrasonic Testing of Steel Welds

Ultrasonic testing of steel welds mainly conducts scientific testing and analysis on the internal defect detection of the welds, defect location and sizing, weld quality grade assessment, base material lamination defect screening, repair area verification testing and defect nature auxiliary discrimination, objectively assessing the product quality grade and issuing a detailed test report, providing a basis for manufacturers' quality control and downstream users' product acceptance.

Testing Items

Internal defect detection of the welds: Mainly detects the various discontinuous defects inside the fusion zone and heat-affected zone of the weld, including the blowholes, slag inclusions, lack of fusion, incomplete penetration and cracks. These defects will seriously weaken the load-bearing capacity and structural integrity of the welded joints, and are the primary concern of the ultrasonic testing.

Defect location and sizing: Precisely determines the position of the defect in the three-dimensional space of the weld, including the depth, horizontal distance and the position along the length direction of the weld. At the same time, the through-height (dimension in the plate thickness direction) and length of the defect are quantitatively measured, providing the key data for the subsequent defect assessment and repair.

Weld quality grade assessment: In accordance with the relevant standards (such as GB/T 11345 and ISO 17640), the weld quality is graded (such as the grades I, II, III and IV) according to the types, sizes, quantities and density of the detected defects. The assessment result is the important basis for judging whether the weld meets the design and use requirements.

Base material lamination defect screening: While testing the weld, the base material areas adjacent to the weld are scanned to screen whether there are the rolling defects such as the laminations and interlayers. These base material defects may propagate under the welding stress, affecting the structural safety.

Repair area verification testing: The weld areas that have undergone the repair treatments such as the excavation and grinding are re-inspected, verifying whether the original defects have been completely removed, and whether there are the new welding defects inside the repaired welds, ensuring the qualified repair quality.

Defect nature auxiliary discrimination: Through the analysis of the dynamic waveform characteristics of the defect reflected echoes (such as the waveform envelopes, number of the wave peaks and moving characteristics), combined with the process knowledge, the macroscopic nature of the defect (such as the planar defects and point defects) is preliminarily discriminated, providing the reference for the engineering decisions.

Testing Scope

Full penetration butt welds: Applicable to the full penetration butt welds of the steels with the plate thickness usually not less than 6 mm, including the welds in the various positions such as the flat welding, horizontal welding, vertical welding and overhead welding. This is the most widely applied and technically most mature field of the ultrasonic testing, used to ensure the complete fusion of the weld in the whole thickness direction.

T-joints, fillet joints and cruciform joint welds: Applicable to the joints with the complex geometric shapes, such as the T-joints, lap fillet welds and cruciform joints. During the testing, the specific scanning methods and probe angles need to be designed according to the joint forms, to effectively cover all the areas where the defects may occur.

Intersecting welds of the tubular joints: Applicable to the welds of the steel tube intersecting connections in the structures such as the offshore platforms and spatial grids. The spatial curves of these welds are complex and the testing is difficult, usually requiring the dedicated probe scanners or phased array technology, and the testing is carried out according to the dedicated testing procedure specifications.

Electroslag welding and electrogas welding welds: Applicable to the longitudinal or circumferential seams of the large-thickness steel plates (usually greater than 50 mm) welded by the electroslag welding or electrogas welding processes. Due to the coarse grains of the welds, the grass-like echo interference may be caused, and the probes of the lower frequencies and special signal analysis techniques need to be adopted.

Defect detection under the surfacing layers and corrosion-resistant layers: Applicable to the detection of the bonding state (lack of bonding defects) between the surfacing layer and the base material and the internal defects of the surfacing layer after surfacing the corrosion-resistant layers such as the stainless steel and nickel-based alloys on the base material surface. The twin-crystal straight-beam probes or small-angle longitudinal wave angle probes are usually adopted for the detection.

Periodic inspection of the welds of the in-service structures: Applicable to the welds of the in-service equipment such as the bridges, pressure vessels, pipelines and steel structure buildings, carrying out the periodic or irregular safety inspections, to find the active defects such as the fatigue cracks and stress corrosion cracks that may be generated during the use.

Testing Methods

Pulse-echo method: This is the most commonly used method. The probe transmits the ultrasonic pulses into the workpiece and receives the echoes reflected from the defects or the bottom of the workpiece. The defect is judged by analysing the position of the echo on the time base line (corresponding to the defect depth) and the amplitude (related to the defect size and orientation). According to the wave types, it can be divided into the longitudinal wave angle incidence method and transverse wave angle incidence method.

Time-of-flight diffraction method (TOFD): A pair of separated transmitting and receiving probes is adopted, and the detection and sizing are carried out based on the diffraction wave signals generated at the defect tips. This method has the high measurement precision for the defect height, can provide the B-scan or D-scan images of the weld cross-section, and has the high detection rate for the planar defects. It is commonly used in combination with the pulse-echo method.

Phased array ultrasonic testing (PAUT): A probe array composed of multiple elements is used, and the time delays of the transmitting/receiving ultrasonic waves of the various elements are controlled through the electronic system, realizing the deflection, focusing and scanning of the sound beam. This method has the high flexibility, can cover a large area in one scanning, and can generate the intuitive sector scans (S-scans) or end views. It is particularly suitable for the detection of the welds with the complex geometric shapes.

Multi-probe combined scanning technique: For the specific testing objects, multiple probes of the different angles or types (such as the 45°, 60° and 70° angle probes and straight-beam probes) are combined on one scanning device, and the detection of the multiple areas of the weld can be completed in one scanning, improving the detection efficiency and coverage. It is commonly used in the automated testing systems.

Tandem scanning technique: Two angle probes with the same characteristics are used, one transmitting and one receiving, symmetrically arranged on the two sides of the weld. It is mainly used to detect the planar defects approximately perpendicular to the testing surface (such as the transverse cracks and lack of fusion), and is insensitive to the defect orientations with the good detection stability.

Dynamic waveform analysis method: During the scanning process, the dynamic envelope characteristics of the defect reflected echo amplitudes changing with the probe movement are observed and recorded. The defects of the different natures (such as the blowholes, slag inclusions and cracks) have the different dynamic waveform characteristics. This method is helpful for the empirical auxiliary judgement of the defect natures.

Testing Standards

Domestic Standards:

1. GB/T 11345, Non-destructive testing of welds — Ultrasonic testing — Techniques, testing levels, and assessment

International Standards:

1. ISO 17640, Non-destructive testing of welds — Ultrasonic testing — Techniques, testing levels, and assessment

Note: The applicable standard can be selected according to the product use and customer trade requirements; standards agreed in the contract take precedence over general standards.

Testing Instruments and Equipment

A-scan analogue/digital ultrasonic flaw detector: The basic equipment, displaying the testing signals in the form of the time base line (the abscissa represents the sound path or depth) and echo amplitudes (the ordinate). The digital instruments have the functions such as the data storage, playback, automatic generation and application of the distance-amplitude curves (DAC/TCG) and report printing, and are the current mainstream equipment.

Ultrasonic probes (transducers): The core component, realizing the electro-acoustic signal conversion. According to the angles, they are divided into the straight-beam probes (0°) and angle probes (such as 45°, 60° and 70°); according to the elements, they are divided into the single-element and twin-crystal probes; according to the contact methods, they are divided into the contact and immersion probes. The frequency (commonly 2-5 MHz), element size and K value (tangent of the refraction angle) of the probes need to be selected according to the thickness of the tested workpiece and the standard requirements.

Phased array ultrasonic testing instrument and probe: The system includes the phased array instrument, multi-element array probes (linear arrays, matrix arrays, etc.), encoders and analysis software. The instrument channel numbers range from 16 to 256 or even more, and the focusing laws can be flexibly configured. The software can generate the various images such as the S-scans, B-scans, C-scans and end views in real time, which is convenient for the result interpretation and analysis.

TOFD testing system: A dedicated system, including the broadband pulse generator, acquisition unit with the high sampling rate A/D converter, a pair of broadband probes (usually the longitudinal wave straight-beam probes or small-angle longitudinal wave angle probes), encoders and analysis software. The system displays the results in the form of the A-scan signals and grey-scale images (B/D-scans), with the precise defect height measurement.

Scanning devices and couplants: The scanning devices are used to ensure that the probe moves on the workpiece surface in a stable manner (contact pressure, angle and speed), including the simple manual scanning frames and the complex mechanical or automated scanners (with the encoder positioning). The couplants (such as the machine oil, glycerol and dedicated coupling pastes) are used to eliminate the air between the probe and the workpiece surface, ensuring that the sound energy is effectively transmitted into the workpiece.

Standard test blocks and reference blocks: Used to calibrate the time base linearity, sensitivity of the instrument and make the distance-amplitude curves. The standard test blocks (such as the IIW blocks and CSK-IA blocks) have the specified materials and geometric shapes. The reference blocks (such as the RB series and SD series) usually have the same materials, thicknesses and surface conditions as the tested workpiece, and contain the specific artificial reflectors (such as the transverse holes, flat-bottomed holes and slots), used to set the testing sensitivity.

Data analysis and management system: The professional software equipped with the advanced testing systems, used to manage the testing process parameters, store the massive A-scan data, carry out the image synthesis and processing, automatically or semi-automatically identify and mark the defects, generate the standard-compliant testing reports, and realize the long-term archiving and traceability of the data.