Weld Examination in Pipes, Pressure Vessels & Structures — Austenitic & Dissimilar Welds

Acoustic Noise and Grain Scattering

The large columnar grain structure of austenitic weld metal scatters ultrasonic energy — generating structural noise that can mask defect signals. Frequency selection, focal law optimisation, and grain noise suppression techniques are required to distinguish genuine defect responses from material-related scatter. Low-frequency PAUT configurations (typically 1–2 MHz rather than the 5–10 MHz used in carbon steel inspection) are required to limit grain scattering while maintaining acceptable beam propagation.

Velocity Anisotropy and Beam Skewing

The preferred crystallographic orientation in columnar austenitic grains causes ultrasonic velocity to vary with propagation direction — an effect known as anisotropy. This causes refracted beams to deviate from their nominal angles, introducing positional error in defect location and through-wall sizing if the velocity anisotropy is not modelled and compensated. Our procedures include anisotropy characterisation and beam path correction for each weld configuration.

Dissimilar Metal Weld Complexity

Welds joining ferritic and austenitic materials — common in power generation boiler headers, refinery pressure vessels, and nuclear plant piping — present the combined inspection challenge of two different base materials and a weld metal that may itself be austenitic, nickel alloy, or a buttered transition composition. Access from both sides may be limited, and the weld geometry often includes butter layers and inconel overlay that further complicate beam propagation.

Low-Frequency Phased Array Configurations

PAUT probes optimised for austenitic inspection operate at 1–2 MHz with wide-aperture arrays to maintain beam coherence through the coarse-grained weld structure. Focal laws are developed using accurate material velocity models — not assumed from carbon steel tables — and validated on representative weld specimens before field deployment.

Creep Wave and Longitudinal Wave Techniques

For certain austenitic weld configurations, longitudinal wave PAUT and creep wave techniques provide detection sensitivity superior to shear wave inspection — particularly for near-surface and inter-run defects. Our procedure library includes both shear and longitudinal wave focal law sets, deployed based on the specific weld geometry and defect population of concern.

Procedure Validation on Representative Mockups

Every PAUT procedure for austenitic or dissimilar weld inspection is validated on a weld mockup that replicates the target weld's material specification, heat input, and welding procedure. Validation demonstrates detection of reference reflectors at the sensitivity level required by the applicable inspection code before the procedure is deployed on production welds.