Detecting and Measuring Flaws in Cast Austenitic Stainless Steels

Cast Austenitic Stainless Steels (CASS) used across many critical components often feature irregular and large grain size, significantly impacting propagation of ultrasonic waves and resulting in difficult diagnostics.

Commonly used in nuclear power reactor systems because of its corrosion resistance and service durability, Cast Austenitic Stainless Steel is a coarse grained, elastically anisotropic material. Austenitic alloys including Inconel, 304 and 316 stainless steels are corrosion resistant because they contain chromium which forms a self-healing protective surface film. Yet, austenitic alloys are not impervious to cracking. The assets where they are used typically have a low tolerance to leaks for security (i.e. radiation) or efficiency reasons. Therefore, it is important to accurately detect the presence of cracks in austenitic alloy welds and characterize their severity. Manufacturing processes typically result in a variety of microstructures that are difficult to inspect through ultrasonic testing. The challenge in reliable Non-Destructive Evaluation (NDE) of CASS components is mostly due to detrimental effects of wave interactions with coarse-grain microstructures inherent to this material class. Shear waves in CASS travel under severe attenuation, reducing the ultrasound inspection only to longitudinal waves.

Specific Dual Matrix Array (DMA) ultrasonic probes with large apertures controlled by a PAUT instrument capable of firing 64-element probes simultaneously – such firing power being available, for example, with a 64:128PR configuration.

Beam profile simulation of a DMA probe

Visualization of DMA data using Enlight

To overcome this inspection challenge, industry has recommended the use of dual probes in Transmit-Receive Longitudinal (TRL) waves. These probes are referred to as dual linear and dual matrix arrays, or DLA and DMA.

In the TRL technique, also known as pitch-catch technique, the transmitter and receiver transducers are unique so that collected signals originate from the location where both beams cross each other. By using a separate pulser and receiver, the wedge size is reduced, and the probe can be used closer to a weld for higher sensitivity. Combining this approach with a large aperture and precise steering enables increased Signal-to-Noise Ratio (SNR) and Probability of Detection (PoD) in thick specimen. In terms of equipment solutions, it translates into 64+ parallel architecture systems driving probes with 64+ elements. Both the M2M Gekko® and M2M Panther™ systems offer 64:64 and 64:128 architectures with a standard DMA/DLA interface. Inspectors can configure the entire inspection procedure directly from the unit using either Capture™, Gekko software, or Acquire™, Panther software. These dual probes can be used with a set of optimized techniques for each inspection case. Phased array sector scanning can be set and adjusted by the user as well as Total Focusing Method (TFM and others) in real-time. Depending on the attenuation of the material under analysis, inspectors can optimize their inspection procedure for the best possible result, leveraging SNR and PoD.

The proposed solution includes:

• Gekko or Panther units to handle high channel count dual probes, live software configuration, high resolution sector scanning, TFM, and automatic TCG;
• Capture or Acquire software with the most up-to-date version;
• 1- or 2-axis scanner for data encoding and defect positioning;
• Linear, DLA and DMA probes and wedge kits;
• Enlight for data analysis.