Evaluation of carburization-induced degradation in service-exposed nickel–chromium alloys using time-of-flight diffraction (TOFD)

This research aims to investigate the detection of carburization layers in service-exposed 30Cr–45Ni radiant
heating coils and to analyze the behavior of sound waves using the Time-of-Flight Diffraction (TOFD) technique. The metallographic analysis revealed the precipitation of chromium carbides (Cr23C6) within chromium-depleted zones near the inner surface. Carbon from the environment diffused into the matrix and combined with chromium to form carbides, contributing to grain boundary thickening and indicating progressive microstructural degradation known as carburization. The carburization depth was assessed using two approaches. The first involved area fraction analysis to quantify variations in the carbide-to-matrix density ratio, while the second employed microhardness profiling to capture depth-dependent mechanical changes. Each method was used to define the carburization limit line or reference distance, which was then compared with wave signals obtained from the TOFD technique. Carburization deterioration significantly increased local hardness and introduced residual compressive stress due to phase expansion. These structural changes altered the material’s Young’s modulus, Poisson’s ratio and density, consequently affecting wave velocity and acoustic impedance. When the sound wave propagates into non-carburized regions, the TOFD signal shows no peak between the lateral wave and the back-wall echo. In contrast, when the sound wave propagates from the 30Cr–45Ni matrix into Cr23C6, the TOFD signal exhibits a small distinct peak between the lateral wave and the back-wall echo. However, a key point to consider is that the direction of the peak signal on the A-scan display is reversed relative to the lateral wave initial direction. To confirm that the signal originated from wave reflection at chromium carbides, the wave position was compared with the carburization limit line or reference distance. These findings highlight the promising potential of this technique for detecting degradation depth in materials without causing damage to the specimen.
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