Sigma phases in an 11%Cr ferritic/martensitic steel with the normalized and tempered condition

Journal Article · Wed Dec 14 23:00:00 EST 2016 · Materials Characterization

DOI:https://doi.org/10.1016/J.MATCHAR.2016.10.031· OSTI ID:22689672

Shen, Yinzhong ^[1]; Zhou, Xiaoling; Shi, Tiantian; Huang, Xi ^[1]; Shang, Zhongxia ^[2]; Liu, Wenwen; Ji, Bo; Xu, Zhiqiang ^[1]

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1. School of Mechanical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240 (China)
2. School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240 (China)

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 M{sub 23}C{sub 6}, Nb-/V-/Ta-Nb-/Nd-rich MX, Fe-rich M{sub 5}C{sub 2}, Co-rich M{sub 3}C and Fe-Co-rich M{sub 6}C phases previously identified in the steel, two types of sigma phases consisting of σ-FeCr and σ-FeCrW were found to be also present in the normalized and tempered steel. Identified σ-FeCr and σ-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 σ-FeCr, and 30Fe-55Cr-10W in addition to a small amount of Ta, Co and Mn for the σ-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 M{sub 23}C{sub 6} and Nb-/V-/Ta-Nb-rich MX of the steel. The σ-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, M{sub 23}C{sub 6} precipitation, precipitation/dissolution of M{sub 2}X, and steel composition. - Highlights: •Precipitate phases in normalized and tempered 11%Cr F/M steel are presented in detail. • σ-FeCr phase, 50Fe50Cr (in at.%), was identified in normalized and tempered 11%Cr F/M steel. • σ-FeCrW phase, 30Fe55Cr10W (in at.%), was identified in normalized and tempered 11%Cr F/M steel. • Sigma phase in the 11%Cr F/M steel was found to precipitate at δ-ferrite/martensite boundaries. • δ-ferrite may rapidly induce the sigma phase formation at δ-ferrite/martensite boundaries.

• OSTI ID: 22689672
• Journal Information: Materials Characterization, Journal Name: Materials Characterization Vol. 122; ISSN 1044-5803; ISSN MACHEX
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
CHEMICAL COMPOSITION
CHROMIUM
COMPARATIVE EVALUATIONS
CRYSTAL STRUCTURE
DISSOLUTION
FERRITE
FERRITIC STEELS
IRON-ALPHA
IRON-DELTA
LATTICE PARAMETERS
MARTENSITE
MARTENSITIC STEELS
MORPHOLOGY
PRECIPITATION
TEMPERATURE RANGE 0400-1000 K
TEMPERING
THERMAL EFFICIENCY
TRANSMISSION ELECTRON MICROSCOPY