Evolution of Carbide Precipitates in 2.25Cr-1Mo Steel during Long-Term Service in a Power Plant
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TRODUCTION
DUE to their excellent mechanical properties, good formability, weldability, and satisfactory corrosion resistance, low-alloy ferritic steels are used extensively in power plants and petrochemical industries. Specifically, 2.25Cr-1Mo steel (T22) is commonly used for steam pipes and pressure vessels of power plants. In the nuclear industry, this steel has been considered for use in sodium-heated steam generators. Although the steel is heat treated to provide an optimum microstructure with a high density of finely dispersed acicular carbides, the instability of the initial carbides during long-term hightemperature exposure can result in a degradation of mechanical properties, especially the high-temperature creep-rupture strength and fracture toughness. Extensive research has been conducted to examine the precipitation processes in ferritic steels, as well as on the associated mechanical performance. Baker and Nutting studied the precipitation reaction sequence in 2.25Cr-1Mo steel after accelerated thermal aging.[1] The precipitation of coarse M6C carbides has been studied, and the kinetics of precipitation modeled in niobiumalloyed ferritic stainless steel during high-temperature aging up to 500 hours.[2] Carbide transformations in Cr-Mo steels with varying contents of chromium and molybdenum have been studied in order to determine the carbide reaction sequence.[3] Softening due to aging was clearly observed in annealed 2.25Cr-1Mo steel in YONG YANG, Assistant Research Scientist, KUMAR SRIDHARAN, Distinguished Research Professor, and TODD R. ALLEN, Associate Professor, are with the Engineering Physics Department, University of Wisconsin–Madison, Madison, WI 53706. Contact e-mail: [email protected] YIREN CHEN, Staff Scientist, is with Argonne National Laboratory, Argonne, IL 60439. Manuscript submitted June 27, 2007. Article published online March 17, 2010 METALLURGICAL AND MATERIALS TRANSACTIONS A
the temperature range 697 to 977 K (424 to 704 C).[4] However, most of these previous studies are based on simulated aging processes to limit the aging times to those deemed reasonable for laboratory experiments.[1,5–9] The goal of the present research is to study the carbide precipitate evolution in boiler pipe samples fabricated from 2.25Cr-1Mo steel (T22), which were removed from a supercritical water-cooled coal-fired power plant after service times of 17 and 28 years at 810 K (537 C).
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EXPERIMENTAL PROCEDURE
The T22 ferritic steels were procured from the Genoa Station 3 supercritical water coal-fired power plant operated by the Dairyland Power Cooperative, Genoa, WI. The samples were in the form of pipe components that were removed after 17 and 28 years service at 810 K (537 C) during the plant’s nearly 35 years of operation. The unexposed pipe has the same origin with the aged ones, and the bulk chemical compositions of the unexposed control sample and those of the two aging conditions are listed in Table I. The carbon was measured using LECO* CNS-2000; other alloying *LECO is a trademark of LECO Corp
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