Comprehensive microstructural characterization in modified 9Cr-1Mo ferritic steel by ultrasonic measurements
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MODIFIED 9Cr-1Mo steel (T91/P91), with a high creep strength, high thermal conductivity, low thermal-expansion coefficient, and good resistance to corrosion and stress-corrosion cracking, is widely used as a structural material in power-generating and petrochemical industries. The alloy is recommended for use in the normalized and tempered condition. Because of the transformable nature of the ferritic alloy, it is susceptible to the formation of undesirable microstructures during fabrication and/or heat-treatment processes. It is important to nondestructively characterize the microstructure of this steel for quality control during fabrication and heat treatment to ensure the desired microstructure and mechanical properties. The ultrasonic technique offers promise in this regard. The ultrasonic technique is used for characterization of microstructures, assessment of defects, and evaluation of material properties. The use of ultrasonic measurements during fabrication and heat treatment ensures the absence of unacceptable discontinuities and the presence of a desired microstructure with acceptable properties.[1] Ultrasonic inservice inspection is carried out to detect any unacceptable degradation in microstructure and the formation and extension of defects during the operation of a component.[2] Ultrasonic parameters, such as velocity and attenuation, have been correlated with the microstructural features evolved during heat treatments in ferritic steels,[3,4] superalloys,[5] aluminum alloys,[6] and many other materials. Ultrasonic techniques have been used for the determination of yield ANISH KUMAR, Scientific Officer/D, K. LAHA, Scientific Officer/F, T. JAYAKUMAR, Head, NDT&E Section, K. BHANU SANKARO RAO, Head, Mechanical Metallurgy Division, and BALDEV RAJ, Director, Metallurgy, Chemical and Reprocessing Group, are with the Indira Gandhi Centre for Atomic Research, Kalpakkam - 603 102, India. Manuscript submitted September 21, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
strength,[7] fracture toughness,[8] grain size,[9] volume fraction of second phases,[10] etc. Ultrasonic spectroscopy has also been used for characterization of defects and microstructural features.[11] It has been reported that, in stainless steel, as the grain size increases, the peak frequency and the full width at half-maximum of the autopower spectrum of the first backwall echo decrease.[12] The shift in peak frequency has also been used for evaluation of structural variations induced by tensile deformation in SUS304 stainless steel.[13] The spectral peak frequency in SUS304 steel was found to increase with an increase in the tensile elongation, which was attributed to the formation and growth of martensite structures due to tensile deformation resulting in smaller crystalline grains, thus reducing the attenuation due to ultrasonic scattering.[13] Gericke has illustrated the use of ultrasonic spectroscopy for grain-size determination.[14] The frequency characteristic of the transducer was found to show two well-defined humps centere
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