de Siqueira, N.C. ; ALMEIDA, P. D. ; Miranda de Sá, AMFL ; ALMEIDA, L. H. ; PEREIRA, GABRIELA R. .
Resumo: The ultrasonic non-destructive evaluation is a valuable tool for materials characterization in the industry. It presents advantages such as high speed of testing, low operational cost and portability for field inspection. The great majority of engineering materials present microstructural heterogeneities such as pores, inclusions, second phase particles and precipitates, which may change their response to ultrasound. The Fast Fourier Transform is useful for converting signals from the time domain to the frequency domain in order to reveal information that is not evident in typical ultrasonic signals. The Fast Fourier Transform (FFT) is an important signal processing algorithms to obtain the Fourier Transform; its application in ultrasonic signals is already well established in the literature and has been subject of studies by several authors. Spectral analysis using the FFT of back wall echoes may allow the microstructural characterization of a material. The present study aims at the characterization of the aging condition of centrifugally cast heat-resistant HP stainless steels through the post-processing of ultrasonic signals. This material has been widely applied at the oil industry in reformer and pyrolysis furnaces tubes due to its high creep resistance and high mechanical performance while exposed to severe operational conditions. However, these conditions can cause microstructural changes in HP steels and may reduce their creep and mechanical resistance, leading the material to failure. Therefore, in order to ensure the reliability of these structures, the development of non-destructive techniques capable of monitoring the states of aging this steel is required. The FFT was applied on the ultrasonic signals obtained from three HP steel samples, each one presenting different states of aging. Spectral analysis was conducted in back wall echo of these specimens. The results indicate that it is possible to characterize different states of aging in HP steel samples through the proposed methodology.
