Characterization of the aging state of modified HP steels by ultrasonic signal processing and the effect of creep voids in the interdendritic region

Abstract

HP austenitic stainless steels are extensively used in the oil and gas industry as radiant tubes in reformer and pyrolysis furnaces. These steels exhibit high resistance to oxidation, thermal stability, and creep resistance at elevated temperatures. Their microstructure changes as a function of temperature, leading to different aging states. In addition, creep voids can be formed due to furnace operating conditions such as time and stress. Therefore, the characterization of the microstructural conditions is necessary for service life assessment. This study aims at characterizing two modified HP steel samples with different aging states and volumetric fraction of creep voids via scanning electron microscopy and ultrasonic testing. Fast Fourier transforms were applied to the second backwall echo of the acquired ultrasonic signal. Spectral analysis established a correlation between the microstructural conditions and the signal response. According to the methodology developed, the ultrasonic parameters and spectral analysis results were sensitive to microstructural changes caused by aging and the presence of creep voids.