Phase Characterization and Formation Behavior in 6 wt% Si High-silicon Austenitic Stainless Steel during Isothermal Aging

doi:10.1007/s40195-020-01152-7

Abstract

Abstract:

The precipitation behavior of different phases in a high-silicon stainless steel (6 wt% Si) during aging at 600-1050 °C for 24 h was investigated. The morphology, crystal structure and composition of various precipitates were detailly characterized using optical microscopy, scanning electron microscopy and transmission electron microscopy. Four phases were mainly identified: χ-phase, M₆C carbides, σ phase and a new type of fcc-phase. During aging at 600-900 °C, the main precipitate was (Cr, Mo and Si)-rich χ-phase which was directly precipitated from γ matrix. The χ-phase was calibrated as bcc structure with a lattice parameter of 8.90 Å. The peak temperature for the precipitation of χ-phase was 800 °C, and it was dissolved when aging at temperatures above 1000 °C. The σ-phase was observed only at 700 °C and grew next to χ-phase. Above 700 °C, a new fcc-phase was found to be precipitated along with χ-phase, with a space group of Fd3c and a lattice parameter of 12.56 Å. The M₆C carbides started to be precipitated at 700 °C in the vicinity of χ-phase. And its amount basically increased with the increasing of temperature. An orientation relationship between M₆C/γ was found: [100]c//[100]γ, (001)c//(001)γ, i.e., the cube-on-cube relationship.

Key words: High-silicon austenitic stainless steel, Precipitates, χ-Phase, Precipitation behavior

Sihan Chen, Tian Liang, Yangtao Zhou, Weiwei Xing, Chengwu Zheng, Yingche Ma, JinMing Wu, Guobin Li, Kui Liu. Phase Characterization and Formation Behavior in 6 wt% Si High-silicon Austenitic Stainless Steel during Isothermal Aging[J]. Acta Metallurgica Sinica (English Letters), 2021, 34(5): 649-656.

Figures/Tables 11

Table 1 Chemical composition of SS-6Si (wt%)

C	Mn	Si	Cr	Mo	Ni	Cu	P	S	N	Fe
0.035	1.19	5.85	18.3	0.96	22.2	1.31	0.007	0.003	0.02	Bal.

Fig. 1 a OM, b SEM images of SS-6Si after solution treatment at 1150 °C for 2 h

Fig. 2 a OM, b SEM images of SS-6Si after aging at 600 °C for 24 h

Fig. 3 Detailed microstructure characterizations of SS-6Si aged at 600 °C for 24 h: a TEM image of precipitates, b SAED patterns obtained from χ-phase by large-angle tilting experiments. A bcc structure can be derived, c CBED patterns of the phase taken along [u.v.0] zone axis slight away from [100] zone axis, [11w] zone axis, respectively. There is no mirror symmetry perpendicular to [001] direction, as the arrow indicated

Table 2 Chemical compositions of χ-phase and γ-matrix aged at 600 °C for 24 h obtained by TEM/EDS (wt%)

	Si	Cr	Mn	Ni	Cu	Mo	Fe
γ-matrix	6.08	18.19	1.08	21.88	3.64	1.00	Bal.
χ-phase	6.38	23.09	1.45	20.21	0.64	2.88	Bal.

Fig. 4 OM image showing various precipitates in SS-6Si after aging at 700 °C for 24 h

Fig. 5 Chemical composition of phases observed in SS-6Si aged at 700 °C, obtained by SEM/EDS, refer to Fig. 4 for Phases 1-4

Fig. 6 Detailed characterizations of precipitates in SS-6Si after aging at 700 °C for 24 h. a1-a3 SEM images; b1-b3 TEM images; c1-c3 SAED patterns obtained from Phases 2-4, respectively. The inset in b1 shows the SAED pattern from Phase 1

Fig. 7 Microstructures of SS-6Si aged at 800 °C, 900 °C. a1, a2: OM images; b1, c1, b2, c2: SEM images showing details of precipitates along GBs, within grains

Fig. 8 Precipitation of M6C carbide in SS-6Si after aging at a1-c1 1000 °C, a2-c2 1050 °C: a1, a2 OM images, b1, b2 SEM images, c1, c2 TEM images. Inset in c1 shows the corresponding SAED pattern obtained from M6C carbide

Fig. 9 Volume fraction of a χ-phase, b M6C carbides in SS-6Si after aging at 600-1050 °C for 24 h

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