Corrosion-resistant alloy (CRA) inspections apply ultrasonic inspection (UT) techniques to inspect austenitic welds. The UT inspection of austenitic welds differs greatly from inspection of low-alloy carbon steel welds.
The main difference associated with welds on corrosion-resistant alloy materials is a coarser grain structure than mild steel. The combination of a coarse grain structure and the anisotropic nature of CRA weld-filler materials affects the ultrasonic-wave propagation by way of sound attenuation. This can potentially result in CRA weld defects going undetected.
To minimize this issue, the probes make use of angled longitudinal waves/creep waves rather than the conventional shear-wave approach, generally used for mild-steel weld inspection. These probes are referred to as transmit and receive longitudinal (TRL) probes, and TRL inspection probes are widely used on coarse-grained materials because these provides a better signal-to-noise ratio than conventional probes.