Changes in creep property and precipitates due to aging of T91 steel after long-term service
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DOI 10.1007/s12206-020-07 -y
Journal of Mechanical Science and Technology 34 (8) 2020 Original Article DOI 10.1007/s12206-020-0720-4 Keywords: · Boiler tube · Creep life · Image analysis · Precipitate · Service aging · Tempered martensite · T91
Correspondence to: Kee Bong Yoon [email protected]
Citation: Lok, V., Le, T. G., Yu, J. M., Ma, Y. W., Nguyen, V. P., Choi, S. T., Yoon, K. B. (2020). Changes in creep property and precipitates due to aging of T91 steel after long-term service. Journal of Mechanical Science and Technology 34 (8) (2020) ?~?. http://doi.org/10.1007/s12206-020-0720-4
Received April 20th, 2020 Revised
May 23rd, 2020
Accepted June 8th, 2020
Changes in creep property and precipitates due to aging of T91 steel after long-term service Vanno Lok1, Thi Giang Le1, Jong Min Yu1, Young Wha Ma2, Vinh Phu Nguyen1, Seung Tae Choi1 and Kee Bong Yoon1 1
Department of Mechanical Engineering, Chung-Ang University, 84 Heukseok-ro, Dongjak-gu, Seoul 2 06974, Korea, R&D Institute, Doosan Heavy Industries & Construction, Changwon, Korea
Abstract
The creep deformation behavior of virgin and long-term service-aged (77000 h) T91 steel of superheater tubes in a power plant was assessed via creep tests along with microstructure examination. A series of miniature tensile creep tests were conducted at 590 °C under applied stresses ranging from 120.0 to 224.1 MPa for both material types. The creep behavior was examined in terms of variations in the microstructural properties correlated with creep rupture life and ductility. X-ray diffraction, optical microscopy, field-emission scanning electron microscopy, hardness measurements, and image analyses were used to analyze the shape and morphology of the existing δ-ferrite, tempered martensite microstructure, and precipitates in both the virgin and long-term service-aged materials. The presence of larger residual δ-ferrite in the virgin microstructure was one of the causes of creep rupture strength reduction in the virgin material. On the contrary, the synergistic effect of the tempered martensite lath microstructure and additional precipitation strengthening of fine precipitates had beneficial effects translating to retention of the creep rupture strength of the longterm service-aged material.
† Recommended by Editor Chongdu Cho
1. Introduction
© The Korean Society of Mechanical Engineers and Springer-Verlag GmbH Germany, part of Springer Nature 2020
Grade 91 martensitic steels were developed owing to the demand for ferritic steels having higher creep strengths. However, to date, such materials have presented limitations with respect to long-term service at elevated temperatures, i.e., for different structural components such as headers, boiler tubes, and steam generators in thermal and nuclear power plants [1-3]. This steel grade has been widely used for steam pipes and tubes in the construction of hightemperature thermal boilers and steam generators for nuclear power plants since the 1980 s, and it was selected as the American Society of Mechanical Engineers (AS
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