Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
5454815 | Materials Characterization | 2016 | 11 Pages |
Abstract
At the present time 9-12% Cr ferritic/martensitic (F/M) steels with target operating temperatures up to 650 °C and higher are being developed in order to further increase thermal efficiency so as to reduce coal consumption and air pollution. An 11% Cr F/M steel was prepared by reference to the nominal chemical composition of SAVE12 steel with an expected maximum use temperature of 650 °C. The precipitate phases of the 11% Cr F/M steel normalized at 1050 °C for 0.5 h and tempered at 780 °C for 1.5 h were investigated by transmission electron microscopy. Except for Cr-/Cr-Fe-Co-rich M23C6, Nb-/V-/Ta-Nb-/Nd-rich MX, Fe-rich M5C2, Co-rich M3C and Fe-Co-rich M6C phases previously identified in the steel, two types of sigma phases consisting of Ï-FeCr and Ï-FeCrW were found to be also present in the normalized and tempered steel. Identified Ï-FeCr and Ï-FeCrW phases have a simple tetragonal crystal structure with estimated lattice parameters a/c = 0.8713/0.4986 and 0.9119/0.5053 nm, respectively. The compositions in atomic pct of the observed sigma phases were determined to be approximately 50Fe-50Cr for the Ï-FeCr, and 30Fe-55Cr-10W in addition to a small amount of Ta, Co and Mn for the Ï-FeCrW. The sigma phases in the steel exhibit various blocky morphologies, and appear to have a smaller amount compared with the dominant phases Cr-rich M23C6 and Nb-/V-/Ta-Nb-rich MX of the steel. The Ï-FeCr phase in the steel was found to precipitate at δ-ferrite/martensite boundaries, suggesting that δ-ferrite may rapidly induce the formation of sigma phase at δ-ferrite/martensite boundaries in high Cr F/M steels containing δ-ferrite. The formation mechanism of sigma phases in the steel is also discussed in terms of the presence of δ-ferrite, M23C6 precipitation, precipitation/dissolution of M2X, and steel composition.
Related Topics
Physical Sciences and Engineering
Materials Science
Materials Science (General)
Authors
Yinzhong Shen, Xiaoling Zhou, Tiantian Shi, Xi Huang, Zhongxia Shang, Wenwen Liu, Bo Ji, Zhiqiang Xu,