Detection of hard intermetallics in β-quenched and thermally aged Zircaloy-2 using ultrasonic measurements

Abstract

Zircaloy-2 is widely used for critical core components in the pressurised heavy water reactors. Components of Zircaloy-2 are usually fabricated from the β-quenched billets. Proper quenching treatment must be ensured to avoid the rejection of finished products made from Zircaloy-2. Metallographic techniques are used for this purpose. However, it is advantageous to use nondestructive testing (NDT) techniques such as ultrasonics. In this work, laboratory investigations are carried out on the feasibility for the assessment of the acceptability of the β-quenched microstructure using ultrasonic attenuation and velocity measurements in the frequency range 2-100 MHz. Precise velocity measurements with an accuracy of 0.2 ns by cross-correlation technique have been made in this study. Results indicate that low frequency (2-10 MHz) ultrasonic velocity measurements using both longitudinal and transverse waves are useful for revealing the presence of hard intermetallics. High-frequency (75 and 100 MHz) ultrasonic velocity measurements are useful for revealing the early-stage dissociation of β-quenched martensite to α-phase. Ultrasonic measurements are also correlated with hardness and density measurements in addition to microstructural features. As compared to the attenuation measurements, velocity measurements are more reliable and repeatable for microstructural characterisation of Zircaloy-2.