Creep Behavior, Deformation Mechanisms, and Creep Life of Mod.9Cr-1Mo Steel
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INTRODUCTION
TEMPERED martensitic 9 to 12 pct Cr steels have been favored as creep strength-enhanced ferritic (CSEF) steels for application to thick section boiler and turbine components of coal-fired ultra-supercritical (USC) power plants[1,2] with steam temperature at around 873 K (600 C) and also for application to future fast breeder nuclear reactor.[3] 9 to 12 pct Cr steels such as Mod.9Cr-1Mo steel specified as ASME Gr.91 (9Cr-1Mo-0.2V-0.05Nb) can offer the highest potential to meet the required flexibility for USC power plants, because of their smaller thermal expansion and larger thermal conductivity than austenitic steels and Ni base superalloys. At present, Gr.91 is one of the most popular CSEF steels widely used in USC power plants.[4] A deciding criterion for creep resistance of USC power plant steels is usually 100,000 hours creep rupture strength at operating temperature.[5] The 100,000 hours creep rupture strength is usually estimated by extrapolation using short-term creep rupture data. But it becomes evident that the stress rupture data of CSEF steels sometimes exhibit an inflection indicating the FUJIO ABE, Research Fellow, is with the Materials Reliability Unit, Environment and Energy Materials Division, National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan. Contact e-mail: [email protected] Manuscript submitted February 9, 2015. Article published online September 21, 2015 5610—VOLUME 46A, DECEMBER 2015
degradation in creep strength at long times and that the conventional time–temperature–parameter (TTP) methods such as Larson–Miller parameter method could not evaluate long-term creep life correctly but tended to overestimate it.[1,6,7] The stress extrapolation makes it difficult to evaluate long-term creep life of CSEF steels correctly. The new analysis methods, such as International Standard Organization Creep Rupture Data Assessment (ISO CRDA) method,[8] Wilshire and Sharning method,[9] region splitting analysis method,[10] and multi-region analysis method,[11] have been proposed as more reliable methods than the conventional TTP methods. For the improvement of reliability of long-term creep life estimation for Gr.91, efforts have also been paid to clarify the correlation between creep life and creep deformation behavior by the analysis of creep strain data.[4] There is an ever evolving microstructure in tempered martensitic 9 to 12Cr steels and the change in creep rate with time and strain reflects coupled elementary processes during creep, such as micro-grain growth, change in dislocation density, and change in precipitate volume fraction and size.[12] This implies that the creep strain behavior can be reasonably correlated with the creep life. The creep and creep rate curves provide us fundamental but very useful information on life assessment as well as on creep deformation mechanisms. The Omega method has successfully been applied to the remaining creep life estimation of Gr.91.[13,14] Prager proposed the Omega method for prediction of METALLURGICAL AND MATER
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