Measurement of Thin Case Depth in Hardened Steel by Ultrasonic Pulse-Echo Angulation Techniques
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This study evaluated ultrasonic pulse-echo angulation as a nondestructive technique for measuring thin case depths (less than 2 mm) in hardened steel. In concept, this method has been used to measure case depths greater than 2 mm and has also been suggested as a technique for determining nominal grain size. It has not been applied to case depths less than 2 mm because impediments, such as the obscuring effect of the front interface echo, the extremely low signal-to-noise ratio of grain backscatter, and the small area localization usually required by industrial inspection procedures prevent accurate measurement.
Study results indicate that improved signal processing techniques (i.e., RF signal averaging, and smoothing) can overcome these obstacles. The final algorithm predicted effective case depth at 0.91, 1.40, and 1.98 mm at ultrasonically calculated values of 0.94, 1.40, and 2.11 mm, with respective standard deviations of 0.25, 0.23, and 0.81 mm. The extremely localized fluctuations in the case depth specimen contributed significantly to the measured standard deviation.
Key Wordsshear wave backscatter case depth ultrasonic pulse-echo RF signal averaging spatial averaging Hilbert transform moving average window martensite pearlite
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