σ-Phase Precipitation Mechanism of 15Cr-15Ni Titanium-Modified Austenitic Stainless Steel During Long-Term Thermal Exposure

doi:10.1007/s40195-017-0624-1

Abstract

Abstract:

The σ-phase precipitation in 20% cold-worked 15Cr-15Ni titanium-modified austenitic stainless steel was studied during long-term aging treatment. During the isothermal aging stage, the amount of σ-phase was significantly increased at beginning and gradually became constant. The aging time only slightly affected the size and morphology of the σ-phase. Conversely, during the isochronal aging stage, the amount of σ-phase grew rapidly with the increase in the aging temperature. The σ-phase with a large amount of stacking faults was prone to nucleate around the (Ti, Mo)C particles or at the grain boundaries. The (Ti, Mo)C particles can act as effective nucleation sites, where the σ-phase directly precipitates from the austenitic matrix. From this work, the growth of σ-phase is found to be influenced by the aging temperature, and a new mechanism of σ-phase precipitation from austenite has been proposed.

Key words: Austenitic stainless steel, Aging treatment, σ-phase precipitation, Carbides

Zhi-Nan Wang, Tian Liang, Wei-Wei Xing, Ai-Bing Du1, Ming Gao, Ying-Che Ma, Kui Liu. σ-Phase Precipitation Mechanism of 15Cr-15Ni Titanium-Modified Austenitic Stainless Steel During Long-Term Thermal Exposure[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(3): 281-289.

Figures/Tables 13

Fig. 1 a BSE micrographs of the cold-rolled sample BF STEM image of the cold-rolled sample; b BF STEM image; c the TEM-lining analysis of MC-type carbide

Fig. 2 BSE micrographs of the samples with different aging treatments: a-e samples aged at 700 °C for 72, 96, 120, 144 and 168 h; f SEM-EDS analysis of a new precipitate phase

Fig. 3 TEM micrograph of the sample aged at 700 °C for 168 h

Table 1 The chemical compositions (wt%) of the σ-phase from EDX measurements

Element	Ti	Cr	Mn	Ni	Mo	Si	V	Fe
σ-matrix GB	1.31	10.98	1.119	6.478	34.41	4.103	0.111	Bal.
σ-around MC	1.097	11.61	1.994	5.787	42.91	2.323	0.891	Bal.

Table 2 The chemical compositions (wt%) of the MC phase from EDX measurements

	C	Si	Ti	V	Cr	Mn	Ni	Mo	Fe
MC-in matrix	18.3	0.12	16.4	0.06	3.86	1.53	8.81	5.75	Bal.
MC-GB	29.1	-	57.5	1.05	0.29	-	-	11.46	Bal.

Fig. 4 Volume fraction of the σ-phase as a function of the isothermal aging time of 72-168 h at 700 °C

Table 3 Average length of σ-phase

Sample	700 °C-72 h	700 °C-96 h	700 °C-120 h	700 °C-144 h	700 °C-168 h
Length of σ-phase (μm)	0.45	0.44	0.47	0.45	0.47

Fig. 5 BSE micrographs of the samples with different aging temperatures: a 600 °C, b 700 °C, c 750 °C for 96 h

Fig. 6 Volume fraction of the σ-phase as a function of the isochronal aging temperature of 600, 700 and 750 °C for 96 h

Table 4 Average length of the σ-phase

Sample	600 °C-96 h	700 °C-96 h	750 °C-96 h
Average length of σ-phase (μm)	-	0.44	1.53

Fig. 7 TEM micrographs, SAED of samples with different aging treatments: TEM micrographs: a 600 °C for 96 h; b, c 700 °C for 96 h and 144 h; d 750 °C for 96 h; e SAED pattern of the (Ti, Mo)C and σ-phase in Fig. 7d; f HRTEM image of the σ-phase and the (Ti, Mo)C with the arrow in Fig. 7d

Fig. 8 15Cr-15Ni titanium-modified austenitic stainless equilibrium phase fractions of precipitates calculated by Thermo-Calc software

Fig. 9 TEM micrographs of the samples aged 750 °C for 96 h; high fault density in σ-phase precipitated on a grain boundary and b at the interface of (Ti, Mo)C and matrix

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