Influence of N on precipitation behavior, associated corrosion and mechanical properties of super austenitic stainless steel S32654

doi:10.1016/j.jmst.2019.10.011

Abstract

Abstract:

The influence of N on the precipitation behavior, associated corrosion, and mechanical properties of S32654 were investigated by microstructural, electrochemical, and mechanical analyses. Increasing the N content results in several alterations: (1) grain refinement, which promotes intergranular precipitation; (2) a linear increase in the driving force for Cr₂N and Mo activity, which accelerates the precipitation of intergranular Cr₂N and π phase, respectively; (3) a linear decrease in the driving force for σ phase and Cr activity, which suppresses the formation of intragranular σ phase. The total amount of precipitates first decreased and then increased with the N content increasing. Furthermore, the intergranular corrosion susceptibility depended substantially on the total amount of precipitates and also first exhibited a decreasing and then an increasing trend as the N content increased. In addition, aging precipitation caused a considerable decrement in the ultimate tensile strength (UTS) and a remarkable increment in the yield strength (YS). Both the UTS and YS always increased with N content increasing throughout the solution and aging process. Whereas the elongation was considerably sensitive to the aging treatment, it exhibited marginal variation with the N content increasing.

Key words: Super austenitic stainless steel, Nitrogen, Precipitation behavior, Intergranular corrosion, Mechanical properties

Shucai Zhang, Huabing Li, Zhouhua Jiang, Zhixing Li, Jingxi Wu, Binbin Zhang, Fei Duan, Hao Feng, Hongchun Zhu. Influence of N on precipitation behavior, associated corrosion and mechanical properties of super austenitic stainless steel S32654[J]. J. Mater. Sci. Technol., 2020, 42: 143-155.

Figures/Tables 15

Table 1 Chemical compositions of experimental super austenitic stainless steels S32654 (wt%).

Steel	C	Si	Mn	P	S	Cr	Ni	Mo	Cu	N	Fe
L-N	0.013	0.40	2.95	0.005	0.002	24.49	22.51	7.35	0.47	0.45	Bal.
M-N	0.012	0.38	2.91	0.005	0.002	24.46	22.52	7.34	0.46	0.50	Bal.
H-N	0.012	0.39	2.92	0.005	0.002	24.45	22.58	7.38	0.46	0.54	Bal.

Fig. 1. Typical microstructures of S32654 after solution treatment at 1200 °C for 30 min: (a) L-N ; (b) M-N ; (c) H-N.

Fig. 2. Microstructures of S32654 samples aged at 1000 °C for various time: (a) L-N 30 min; (b) L-N 2 h; (c) L-N 6 h; (d) M-N 30 min,; (e) M-N 2 h; (f) M-N 6 h; (g) H-N 30 min; (h) H-N 2 h; (i) H-N 6 h.

Fig. 3. High magnification microstructures of S32654 samples aged at 1000 °C for various time: (a) L-N 30 min; (b) L-N 2 h; (c) L-N 6 h; (d) H-N 30 min; (e) H-N 2 h; (f) H-N 6 h.

Fig. 4. TEM bright-field images and corresponding SAED patterns of precipitates in S32654 samples: (a) intergranular σ phase; (b) intragranular σ phase; (c) Cr2N; (d) π phase.

Fig. 5. Area fraction of precipitates in S32654 steels aged at 1000 °C: (a) intergranular precipitates; (b) intragranular precipitates; (c) total precipitates.

Table 2 Elemental compositions of σ phase, π phase and Cr2N (wt%).

Phase	Cr	Mo	Ni	Mn	N	Fe
σ phase	30.15	21.14	11.80	2.37	-	34.54
π phase	42.19	20.31	14.32	1.19	3.26	18.73
Cr₂N	78.19	6.49	1.40	1.05	8.02	4.85

Fig. 6. Typical DL-EPR curves of S32654 steels: (a) L-N ; (b) M-N ; (c) H-N.

Fig. 7. IGC attack morphologies of S32654 samples aged at 1000 °C for various time: (a) L-N ST; (b) L-N 30 min; (c) L-N 2 h; (d) L-N 6 h,; (e) M-N ST; (f) M-N 30 min; (g) M-N 2 h; (h) M-N 6 h; (i) H-N ST; (j) H-N 30 min; (k) H-N 2 h; (l) H-N 6 h.

Fig. 8. IGC attack areas of S32654 steels after DL-EPR tests: (a) intergranular; (b) intragranular; (c) total.

Fig. 9. DL-EPR values of S32654 steels aged at 1000 °C for various time.

Fig. 10. Tensile properties versus aging time for S32654 steels aged at 1000 °C: (a) strength; (b) elongation.

Fig. 11. Cross-section microstructure near the fracture surface of S32654 steels aged at 1000 °C for various time: (a) L-N 30 min; (b) L-N 2 h; (c) L-N 6 h; (d) M-N 30 min; (e) M-N 2 h; (f) M-N 6 h; (g) H-N 30 min; (h) H-N 2 h; (i) H-N 6 h. The white arrows indicate the tensile direction.

Fig. 12. Magnified SEM images of the cross-section microstructure near the fracture surface of S32654 steels aged at 1000 °C for 6 h: (a) L-N ; (b) M-N ; (c) H-N. The white arrows indicate the tensile direction.

Fig. 13. Effects of N content on the driving force and element activity in S32654 steel aged at 1000 °C: (a) driving force; (b) element activity.

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