σ-Phase in Lean Duplex Stainless Steel Sheets

doi:10.1007/s40195-014-0201-9

Abstract

Abstract:

The precipitation kinetics of secondary phases in two austeno-ferritic lean duplex stainless steels (lean DSS) were examined after aging the materials at 800 °C. Owing to the instability of ferrite, all DSS are known to be sensitive to solid-state phase transformations in the critical temperature range 600-1,000 °C and different secondary phases may form, depending on composition and microstructure. The performed thermodynamic simulations revealed the proneness to the precipitation of such phases also have been done in lean DSS, but only information on the equilibrium microstructures were achieved. Therefore, the materials were aged at various times, in order to verify the simulations and determine the precipitation kinetics. The occurred structural modifications were observed and quantified by scanning electron microscope and X-ray diffraction measurements, determining phase type, composition and volumetric fraction. At 800 °C, grade 2101 was found to be only affected by Cr₂N nitrides precipitation, whereas a significant amount of σ-phase was found to form in LDX 2404 for treatment longer than 1 h, almost totally replacing ferrite after 50 h. Up to now, the intermetallic σ-phase has been observed only in the high alloyed DSS, and the unexpected precipitation in grade 2404 highlighted that the increased content of molybdenum in this steel might be considered as determinant for the formation.

Key words: Lean duplex stainless steels, Isothermal heat treatments, σ-Phase, Chromium nitrides, Precipitation kinetics

Marco Breda, Massimo Pellizzari, Marco Frigo. σ-Phase in Lean Duplex Stainless Steel Sheets[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(3): 331-337.

Figures/Tables 10

Table 1 Chemical compositions of the investigated steels (in wt%)

Steel	C	Si	Mn	Cr	Ni	Mo	Cu	P	S	N	Fe
2101	0.025	0.65	5.13	21.57	1.56	0.28	0.30	0.019	0.0010	0.229	Bal.
2404	0.022	0.33	2.83	24.41	3.34	1.54	0.42	0.021	0.0004	0.253	Bal.

Fig. 1 Microstructures of the as-received samples. a LDX 2101, b LDX 2404 (dark—ferrite; white—austenite, rolling direction is horizontal)

Fig. 2 Equilibrium phase diagrams of the two steels. a LDX 2101, b LDX 2404

Fig. 3 Microstructures of the two steels after treated at 600 °C for 100 h (ferrite—dark gray, austenite—light gray, Cr2N—black, rolling direction is horizontal). a LDX 2101, b LDX 2404

Fig. 4 Microstructural evolution of LDX 2404 DSS after treated at 800 °C for different time (SEM-BSE) (ferrite—dark gray, austenite—light gray, σ-phase—bright, Cr2N—black, rolling direction is vertical). a 10 min, b 1 h, c 2 h, d 8 h, e 24 h, f 50 h

Table 2 Chemical composition of σ-phase in LDX 2404 after 1-h treatment at 800 °C (in wt%)

Cr	Mo	Ni	Mn	Fe
29.8 ± 0.5	5.9 ± 0.4	1.9 ± 0.1	3.0 ± 0.1	Bal.

Fig. 5 XRD patterns of LDX 2404 DSS. a As-received, b treated at 800 °C for 50 h

Fig. 6 SEM-BSE images (ferrite—dark gray, austenite—light gray, Cr2N—black, rolling direction is horizontal) of LDX 2101 DSS after treated at 800 °C for 2 h a, for 50 h b, the XRD pattern of LDX 2101 DSS after treated at 800 °C for 50 h c

Fig. 7 Precipitation kinetics at 800 °C. a σ-Phase in LDX 2404 steel, b Cr2N in LDX 2101 and LDX 2404 steels

Fig. 8 Cr2N precipitation pseudo-kinetics in LDX 2101 and LDX 2404 steels

References 16

[1]	J.O. Nilsson,Mater. Sci. Technol. 8, 685(1992)
[2]	R.N. Gunn (ed.), Duplex Stainless Steels: Microstructure, Properties and Applications (Abington, Cambridge, 1997)
[3]	I. Calliari, M. Pellizzari, E. Ramous,Mater. Sci. Technol. 27, 928(2011)
[4]	T. Liang, X.Q. Hu, X.H. Kang, D.Z. Li, Acta Metall. Sin. (Engl. Lett.) 26, 517(2013)
[5]	T.H. Chen, J.R. Yang, Mater. Sci. Eng. A 311, 28 (2001)
[6]	I. Calliari, P. Bassani, K. Brunelli, M. Breda, E. Ramous, J. Mater. Eng. Perform. 22, 3860(2013)
[7]	I. Calliari, M. Pellizzari, M. Zanellato, E. Ramous, J. Mater. Sci. 46, 6916(2011)
[8]	S. Wessman, R. Petterson, S. Hertzman,Steel Res. Int. 81, 337(2010)
[9]	M. Pohl, O. Storz, T. Glogowski,Mater. Charact. 58, 65(2007)
[10]	J.O. Nilsson, P. Kangas, T. Karlsson, A. Wilson, Metall. Trans. A 31, 35 (2000)
[11]	M. Knyazeva, M. Pohl,Metallogr. Microstruct. Anal. 2, 343(2013)
[12]	B. Sundman, B. Jansson, J.O. Anderson, CALPHAD 9, 153 (1985)
[13]	L. Kaufman, H. Bernstein, Computer Calculation of Phase Diagrams (Academic Press, New York, 1970)
[14]	I. Calliari, M. Breda, A.F. Miranda Pérez, E. Ramous, R. Bertelli, in Proceedings of the International Conference and Exhibition on Analysis and Testing of Metallurgical Processes and Materials (The Chinese Society for Metals, Beijing) (2012)
[15]	H.S. Cho, K. Lee,Mater. Charact. 75, 29(2013)
[16]	L. Duprez, B.C.De Cooman, N. Akdut, Steel. Res. 72, 311(2001)