Acoustic damping characterization and microstructure evolution during high-temperature creep of an austenitic stainless
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I. INTRODUCTION
STRUCTURAL metals are subject to aging from fatigue, creep, corrosion, and their combination. Exposure to elevated temperatures promotes creeping and stress-corrosion cracking. Aged metals lose toughness, or the ability to absorb energy for stresses above the yield point. They cannot endure the occasional high load without fracturing. In-service degradation with creep is one of the most critical factors determining the structural integrity of the elevated-temperature components in power plants, chemical plants, and oil refineries the world over. For example, fossil plants have been in operation for such long durations that the critical components have exceeded the design life of 30 to 40 years. They have undergone progressive damage over time. To save energy and meet current regulations concerning CO2 emissions, as well as to improve thermal efficiency, the steam pressures and operating temperatures in the components have been increased. As a consequence, material degradation is accelerating. Economic and environmental matters prohibiting the construction of new plants increase the severity of this problem. Therefore, an in-service assessment of the state of the damage is important for ensuring safe operations, predicting remaining life, and promoting life-extending programs.[1,2] For this assessment, a nondestructive technology enabling the evaluation of the current state of the materials and the prediction of their remaining life has long been sought after.[3–11] Some candidates include magnetic, replication, small angle neutron scattering (SANS), and ultrasonic methods.[6] The magnetic and replication methods have been applied in the past.[6,7,12–18] The magnetic method measures the change in the magnetic properties from the magnetic hysteresis loop, the magnetic Barkhausen noise, and the acoustic Barkhausen emission associated with the evolution of the properties of a metal as creep progresses.[7,12–16] This method is sensitive to the damage, but it is limited to ferromagnetic materials. The TOSHIHIRO OHTANI, Senior Researcher, is with the Materials Lab., Ebara Research Co. Ltd., Fujisawa, Kanagawa 251-8502, Japan. Contact e-mail: [email protected] Manuscript submitted February 28, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
replication method is capable of detecting the microstructural state in highly localized surface regions.[17,18] It provides accurate information about microstructural changes such as grain coarsening, void formation, and microcrack and macrocrack formation. However, it gives no information about precipitation coarsening, the evolution of dislocation structure, or volumetric damage such as creep damage proceeding, not from the surface, but from the inside of a material.[19] In addition, this method requires empirical judgment and careful analysis, and is time-consuming and labor-intensive. The SANS method has firmly established that when a beam of neutrons enters a materials, it undergoes scattering by microstructural features. This method is very sensitiv
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