Cracking Austenitic Code: PAUT on Stainless and Dissimilar Metal Welds

If carbon steel welds are a puzzle, inspecting austenitic and dissimilar metal welds (DMWs) can feel like solving one in the dark. Coarse grains, anisotropy, and mode conversion create beam distortion and noise that can bury real indications. Yet, with the right approach, PAUT can still deliver reliable and interpretable data.

Material Challenges in Austenitic and Dissimilar Metal Welds

The challenge lies in the materials themselves. Coarse, directional grains scatter shear waves and distort sound paths, while anisotropy bends angles unpredictably. At dissimilar interfaces, such as stainless-to-low-alloy joints, acoustic mismatches further complicate amplitude and timing.

Field-Proven PAUT Strategies

Field-proven strategies begin with frequency. Lower frequencies, typically between 1 and 3.5 MHz, often outperform higher ones by offering better penetration and signal-to-noise ratio, though at the expense of some resolution. Refracted longitudinal (LL) or mixed-mode techniques can outperform shear waves in coarse-grained material, so consider separate groups for LL-root, LL-sidewall, and corroborating SW scans. Adding skew scans of ±10° to 20° can expose indications hidden by grain orientation, and scanning from multiple planes reduces the risk of missed defects.

Advanced Imaging Techniques: FMC and TFM

Use shorter apertures with depth-specific focusing to control noise, rather than over-focusing broad regions. For particularly complex bevels, full matrix capture (FMC) and total focusing method (TFM) imaging can provide clarity around fusion lines and roots when configured with tight regions of interest to manage file size and review time.

Calibration and Configuration Best Practices

Calibration should always reflect real-world conditions. Use representative blocks with similar microstructures, overlays, or butter layers where possible, and verify coverage using realistic reflectors, not just side-drilled holes. A practical starting point includes a 2–3.5 MHz probe, an LL-capable wedge, and angle groups covering LL 30°–60° for roots and sidewalls, SW 45°–60° for comparison, and skewed groups at ±15°. Keep index steps small, no greater than half the beam width at the fusion line, and use dynamic gates along the sound path with a near-surface validation pass.

Data Interpretation and Quality Assurance

When interpreting data, prioritize consistency across adjacent index positions and angles rather than isolated frames. Tip-diffraction cues and TFM-based sizing methods can stabilize your calls, and a quick verification pass using another angle or mode often resolves uncertainty. Combining PAUT with TOFD adds value, particularly for through-wall cracking and sizing accuracy, and a quick second-tech review is often the cheapest form of quality assurance. For help with your toughest PAUT applications, contact our team at contact@vermon-ndt.com