Dissimilar Metal (DM) welds typically join two or more different materials and mostly involve Inconel Alloys. They are often used in nuclear power plant design, to connect clad carbon steel vessels to stainless steel piping.
DM welds are very challenging configurations for Phased Array Ultrasonic Testing (PAUT) examinations. The anisotropic coarse-grain structure of the austenitic weld results in specific behavior of the ultrasonic beams when propagating through the material: high attenuation, high grain noise, redirection and distortion of the beam, and a low-pass filter effect blocking waves at higher frequency.
In addition, dissimilar metal welds typically join two or more different materials, so multiple acoustic interfaces are present together with a sometimes very complex geometry, including nozzles and tapers (see Figure 1).
Figure 1: Typical nuclear DM weld configuration
For challenging austenitic welds, standard phased array probe and wedge combinations will not provide the required inspection capability to achieve the desired results. Effective phased array inspection techniques for dissimilar metal welds are typically based on low-frequency (1.5 to 3.5 MHz) DMA probes. Such probes can be used to generate either shear waves for inspection of base material, or longitudinal waves for better propagation in through coarse-grain austenitic weld material and the successive acoustic interfaces. The dual transmit/receive configuration offers better sensitivity and Signal-to-Noise Ratio (SNR) due to the “convolution” of transmitter and receiver beams and avoids “ghost echoes” caused by internal wedge reflections. Eddyfi Technologies develops and manufactures DMA probes at its facility in State College (PA). View our complete probe catalog here.
Plane Wave Imaging (PWI) is an advanced firing technique that uses a multi-element aperture for pulsing instead of firing each element individually like Full Matrix Capture (FMC). Receiving is done individually with each element. The firing sequence typically consists of multiple focal laws, with varying angles and/or apertures. Just like for FMC firing, TFM imaging can be generated from the raw A-scan signals live during inspection.
PWI data acquisition has several benefits when compared to FMC data recording. The emitted pulse from the full aperture has more energy and is more directional than a single element excitation. It provides greater sensitivity and excellent SNR at higher scanning speed because of a significantly shorter firing sequence. When looking at simulated branched cracks in stainless steel base material, you can see in Figure 2 that the multiple facets of the cracks are well resolved in the TFM images from both PWI and FMC firing. But the maximum acquisition rate for the PWI firing sequence is 2x higher which allows a 2x faster inspection.
Figure 2: Austenitic steel reference block with simulated branched cracks. Middle: Plane Wave Imaging/TFM with envelope. Right: Full Matrix Capture/TFM with envelope.
Figure 3 shows a scan plan in the UltraVision® Classic software for inspection of a 25-millimeter (mm) or 1-inch (in) thick dissimilar metal weld with a multiple-line scanning sequence with PWI firing and TFM imaging. A 1.5 MHz DMA probe is used with wedges for longitudinal wave generation.
Figure 3: Scan plan in UltraVision for multiple-line scanning with PWI-TFM inspection technique looking for circumferential flaws in dissimilar metal weld
When using Eddyfi Technologies Emerald compact, industrial PAUT and TFM system controlled by UltraVision, a scanning speed of more than 100mm or 4in per second can be achieved for this inspection. The detection capability of the technique is clearly illustrated in the next figures. Figure 4 shows the image of a real thermal fatigue crack in the interface between Inconel buttering and carbon steel nozzle. High-energy PWI firing through the austenitic weld and buttering materials allows to clearly resolve the tip and corner signals of the crack in the TFM image. And Figure 5 shows the result of the examination for axial flaws on the same weld mock-up: a mechanical fatigue crack in the nozzle material and three notches in the middle of the weld are reliably detected and accurately positioned in the Polar View.
Figure 4: Image of circumferential thermal fatigue crack in dissimilar metal weld, detected with PWI-TFM technique using L-waves from 1.5 MHz DMA probe assembly; tip and corner echoes can be resolved for sizing
Figure 5: Image of axial mechanical fatigue crack and three axial notches in dissimilar metal weld, detected with PWI-TFM technique using L-waves from 1.5 MHz DMA probe assembly
Eddyfi Technologies Emerald (see Figure 6) is a compact phased array ultrasonic testing instrument that includes all the advanced methods discussed, delivers fast performance and intelligent results, and features exceptional signal quality meaning it can achieve high amplification without signal distortion. With real-time total focusing method processing onboard and offline, combined with its full matrix capture and plane wave imaging data acquisition features, the Emerald phased array system can produce faster and more detailed inspection results than most instruments on the market. Given its advanced capabilities, it is the ideal solution for addressing stainless steel and dissimilar metal weld inspections.
Figure 6: EMERALD compact phased array UT instrument
The procedure for encoded examination with the presented PWI-TFM inspection techniques was successfully qualified at PDI (Performance Demonstration Initiative) by Curtiss-Wright Nuclear Division/LMT, a US based NDT inspection vendor, in the summer of 2024, and was deployed on-site during a fall outage in a nuclear power plant immediately after. The procedure includes flaw detection, length and through-wall sizing, and covers a range from small diameter nozzles up to the heavy-wall safe-end to nozzle welds that connect steam generators to the main coolant piping.
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