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Nondestructive Testing of High-Temperature Alloys
Release Date:
2022/02/25
As a critical component of high-end equipment, the application of high-temperature alloys requires nondestructive testing at every stage—from raw materials and billets to finished products. Due to its convenience, safety, speed, and reliability, ultrasonic testing has been widely adopted in the field of high-temperature alloys. Nondestructive testing of high-temperature alloys plays a vital role in the inspection of high-end equipment, spanning the entire lifecycle of structural component fabrication—from raw material procurement and manufacturing processes to in-service inspection. The two most commonly used nondestructive testing methods are ultrasonic testing and fluorescent penetrant testing.
We will use the turbine disk as an example to introduce nondestructive testing methods for powder metallurgy superalloys:
Powder metallurgy superalloys are characterized by fine grain size, uniform microstructure, absence of macrosegregation, high degree of alloying, high yield strength, and excellent fatigue performance, making them the optimal material for manufacturing high-thrust-to-weight ratio turbine disks and shroud disks for next-generation engines. A certain engine model in China has already adopted powder metallurgy materials. The turbine disk is a critical component of aircraft engines, operating under severe high-temperature conditions and enduring extremely high loads; its service environment is highly demanding. Moreover, powder metallurgy disks are prone to initiation of fatigue cracks from very small critical defect sizes, meaning that even minute defects can significantly degrade component performance and potentially lead to catastrophic failure. Therefore, the adoption of advanced nondestructive testing technologies is of paramount importance.
There are three types of defects in powder metallurgy superalloys: primary grain boundaries, thermally induced porosity, and non-metallic inclusions. Among these, primary grain boundaries and thermally induced porosity can be mitigated through process optimization; however, inclusions cannot be completely eliminated by existing powder production and processing techniques. Therefore, nondestructive testing methods must be employed for control, requiring such techniques to reliably detect defects in accordance with the acceptance criteria. These acceptance criteria are based on damage-tolerance theory, which posits that all structures contain inherent defects; as long as these defects do not exceed a specified allowable size, the structure remains safe.
1. Ultrasonic testing:
Ultrasonic testing is one of the conventional nondestructive testing methods, characterized by strong penetration capability, high sensitivity and resolution, and the ability to detect changes in materials as ultrasonic waves propagate through them, enabling both localization and quantification.
Ultrasonic testing methods can be classified according to their principles into pulse-echo, time-of-flight diffraction, through-transmission, and resonance methods, with the pulse-echo method being the most widely used. In the pulse-echo method, a transducer emits ultrasonic pulses into the test piece, and defects are detected by analyzing the characteristics of the reflected waves returning from the workpiece.
The ultrasonic flaw detector is the primary instrument for ultrasonic testing. It generates high-voltage electrical pulses via a transmitting circuit to excite the piezoelectric transducer, causing it to vibrate and couple into the workpiece to produce ultrasonic waves. The reflected waves from the workpiece are then received, amplified, detected, and processed before being displayed. Display modes include A-scan, B-scan, C-scan, and D-scan. An A-scan is a waveform display in which the horizontal axis typically represents the travel time (or distance) of the ultrasonic wave, and the vertical axis represents the amplitude of the reflected wave. By analyzing the shape of the waveform, one can determine whether defects are present in the material, as well as their size and geometry. B-, C-, and D-scans, on the other hand, are grayscale images representing a specific cross-sectional view.
When scanning a test workpiece, if there is an interface or a defect within the workpiece, an interface wave or a defect wave will appear between the initial pulse and the bottom echo. The figure shows the A-scan waveform for a babbitt alloy, with the composite-layer interface wave located between the initial pulse and the bottom echo.
Powdered-part defects are characterized by their small size. To detect such minute defects in powdered parts, the key is to enhance both the amplitude of the ultrasonic reflection signals from small defects and the signal-to-noise ratio. How can we simultaneously increase the reflection amplitude of small defects and improve the signal-to-noise ratio? One approach is to use water-immersion focused transducers at high inspection frequencies. This is because the focused sound beam concentrates energy intensely in the focal zone, leading to a significant increase in acoustic pressure and thus higher reflection amplitudes from small defects; moreover, the volume of the base material traversed by the beam is relatively small, resulting in reduced scattering noise and, consequently, a better signal-to-noise ratio. However, different focused transducers have varying parameters; therefore, to ensure consistent detection sensitivity across the entire thickness of the part, it is crucial to carefully optimize the ultrasonic testing process parameters.
Traditional ultrasonic testing of high-temperature alloys employs a single water-immersion focused transducer, with the focal point positioned either at the component surface or at a specific depth. In contrast, ultrasonic inspection of powder metallurgy disks utilizes multiple transducers for segmented scanning; even if each transducer’s focal zone is located at a different depth within the component, this approach ensures high detection sensitivity across the entire thickness of the part.
2. Fluorescence detection:
Low-cycle fatigue test data indicate that approximately 25% of cracks in powder metallurgy parts initiate at surface defects, about 30% originate from near-surface defects, and the remainder arise internally; however, near-surface defects pose a greater hazard. Fluorescent penetrant inspection is used to detect surface-opening defects in parts. Given the small size of defects typically found in powder metallurgy components, the key to effective fluorescent penetrant inspection lies in optimizing the process parameters to reliably detect even minute defects.
Fluorescent penetrant:
The ultra-high-sensitivity post-emulsifiable fluorescent penetrant method offers the highest detection sensitivity for small defects; therefore, the fluorescent inspection procedure was finalized as the ultra-high-sensitivity post-emulsifiable fluorescent penetrant process.
To ensure that 0.2-mm defects in the powder tray can be reliably detected using fluorescent methods, the selection of post-emulsifiable fluorescent penetrant and the optimization of process parameters are critical. The specific fluorescent penetrant to be used must be determined based on the material properties and the nature of potential damage in the structural component.
Based on the type of fluorescent penetrant selected, choose the appropriate developer method; conduct the penetrant testing in accordance with the relevant standards for penetrant inspection; and then, according to the indications observed during fluorescent inspection, examine the results using a 20× binocular magnifying lens.
Conclusion
Although nondestructive testing of high-temperature alloys is of great importance, research reports on this topic are remarkably scarce. Most studies on high-temperature alloys focus on manufacturing processes and raw materials, with nondestructive testing lagging behind. Although ultrasonic and fluorescent penetrant testing have yielded some promising results, their procedures remain cumbersome and their accuracy relatively low due to various factors, underscoring the need for further research in this area. More time and effort must be devoted to advancing nondestructive testing methods for high-temperature alloys.
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