Precipitate Phases in an 11% Cr Ferritic/Martensitic Steel with Tempering and Creep Conditions

doi:10.1007/s40195-014-0166-8

Abstract

Abstract:

Precipitates in an 11% Cr ferritic/martensitic steel containing Nd with tempering and creep conditions were investigated using transmission electron microscope with energy-dispersive X-ray spectroscopy. The precipitates in the steel with a tempering condition were identified to be Cr-rich M ₂₃C₆ carbide, Nb-rich/V-rich/Ta-Nb-rich MX carbides, Nb-rich MX carbonitride, and Fe-rich M ₅C₂ carbide. Nd-rich carbonitride, which is not known to have been reported previously in steels, was also detected in the steel after tempering. Most of the Nb-rich MX precipitates were dissolved, whereas the amount of Ta-rich MX precipitates was increased significantly in the steel after a creep test at 600°C at an applied stress of 180 MPa for 1,100 h. No Fe₂W Laves phase has been detected in the steel after tempering. (Fe, Cr)₂W Laves phase with a relatively large size was observed in the steel after the creep test.

Key words: Precipitate phase, High-Cr ferritic/martensitic steel, High temperature creep, Transmission electron microscope

Xiao-Ling Zhou, Yin-Zhong Shen, Zhi-Qiang Xu. Precipitate Phases in an 11% Cr Ferritic/Martensitic Steel with Tempering and Creep Conditions[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(1): 48-57.

Figures/Tables 12

Table 1 Chemical composition of the experimental steel (in wt%)

C	Si	Mn	Cr	W	Co	V	Nb	Ta	Nd	B	N	Fe
0.10	0.24	0.24	11.2	3.16	2.94	0.22	0.068	0.07	0.03	0.006	0.032	Bal.

Fig. 1 Optical micrographs of the 11% Cr FM steel with the normalized-and-tempered condition

Fig. 2 TEM micrographs of carbon replica prepared from the 11% Cr FM steel: a after tempering; b after creep test

Fig. 3 a TEM micrograph of carbon replica prepared from the 11% Cr FM steel after the tempering, showing M 23C6 precipitate A; b diffraction pattern in the zone axis of [001] taken from precipitate A; c EDX spectrum from precipitate A

Fig. 4 Comparison of size distribution for the precipitates with a similar morphology like M23C6 phase in the 11% Cr steel with tempering and creep conditions

Fig. 5 a TEM micrograph of carbon replica prepared from the 11% Cr FM steel after the tempering, showing spherical Nb-rich MX precipitates; b diffraction pattern in the zone axis of [011] taken from precipitate B; c EDX spectrum from precipitate B

Fig. 6 Contents of metallic elements in MX precipitates in 11% Cr FM steel with tempering and creep conditions

Fig. 7 Comparison of size distribution for the spherical MX precipitates in the 11% Cr steel with tempering and creep conditions

Fig. 8 a TEM micrograph of carbon replica prepared from the 11% Cr FM steel after the tempering, showing Nd-rich precipitate C; b diffraction pattern taken from precipitate C; c EDX spectrum from precipitate C

Fig. 9 a, b, c TEM micrographs of carbon replica prepared from the 11% Cr FM steel after the tempering, showing needle-like precipitates; d diffraction pattern taken from precipitate D. e EDX spectrum from precipitate D

Fig. 10 a TEM micrograph of carbon replica prepared from the 11% Cr FM steel after the creep test, showing elliptical Laves precipitate E; b diffraction pattern in the zone axis of [100] taken from precipitate E; c EDX spectrum from precipitate E

Table 2 Characteristics of the precipitates in 11% Cr FM steel with tempering and creep conditions

Condition	M ₂₃C₆	MX	Nd-rich Phase	M ₅C₂	Laves
After tempering	Cr-rich carbide, block-/plate-/rod-like, irregular shape; Relatively large size	Nb- rich/V- rich/Ta-Nb-rich MX carbide, sphere-/strip-like; Nb-rich MX carbonitride, sphere-like; Relatively small size	Nd-rich carbonitride, short rod-like	Fe-rich, needle-like	No detected
After creep test	Cr-rich carbide, Ellipse-/rod-like	Most of Nb-rich MX phases are dissolved; Ta-rich MX carbides is increased significantly; Composition of Ta-Nb-rich MX carbides is changed; Size of MX phases decreased significantly	No detected	No detected	(Fe, Cr)₂W, large-sized

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