Creep-Induced Microstructural Evolution and Acoustic Characterization in Tempered Martensitic Stainless Steel
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INTRODUCTION
AROUND the world, a great number of operating fossil, nuclear power, and petroleum plants were constructed during 1960s and 1970s and have exceeded the their original 30- to 40-year design life.[1,2,3] Structural metals used in plant components subjected to complex service environments—for example, exposure to elevated temperatures or loading—undergo degradation of mechanical properties such as strength, fracture toughness, etc., due to microstructural changes, corrosion, fatigue, creep, and combinations of these. Creep is one of the most critical factors that determine the structural integrity of the components. To save energy, meet recent regulatory requirements for CO2 emissions, and improve thermal efficiency, steam pressures and operating temperatures in the components have been increased, resulting in accelerated material degradation. Furthermore, economic and environmental circumstances prohibiting the construction of new plants increase the severity of this problem. Nondestructive evaluation (NDE), which probes the characterization of microstructures, deformation and damage initiation, and growth, may play an essential role because of the growing awareness of the benefit of using NDE techniques to assess the performance of components. In particular, NDE is assuming greater significance in the assessment of material degradation. To operate plants safely past their design life, an NDE technique that enables the evaluation of the current state of materials and predicts their remaining life, has long been sought.[4–7] TOSHIHIRO OHTANI, formerly Senior Researcher, with, Ebara Research Co., LTD, Fujisawa, Kanagawa, Japan, is Professor with the Shonan Institute of Technology, Department of Mechanical Systems Engineering, Fujisawa, Kanagawa 251-8511, Japan. Contact e-mail: [email protected] Manuscript submitted August 21, 2006. Article published online July 3, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
At present, surface replication, boat samples, and miniature test coupons are used to assess the creep damage after years of service.[1,8–11] While somewhat successful, such tests are limited by restrictions associated with the considerable pretest knowledge required to determine the proper location for testing. In addition, these tests can be destructive in nature and considerable surface preparation may be required. Therefore, it is essential that a selected technique provide a measurement simple and quick enough to deal with a large number of objects and give accurate information about microstructural changes. A number of alternate methods for creep-damage evaluation are available for detecting the damage. Ultrasonic methods offer a unique way of evaluating the internal damage state with comparatively simple and easy instrumentation, while X-ray, replication, and measurements of hardness and magnetic properties merely inspect the surface or near-surface region of the metals. The ultrasonic method has been extensively studied for assessing creep damage in a wide range of materials.[12–18]
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