Effect of Intergranular Precipitation on the Internal Oxidation Behavior of Cr-Mn-N Austenitic Stainless Steels

doi:10.1007/s40195-015-0288-7

Abstract

Abstract:

The effect of intergranular precipitation on the internal oxidation behavior of Cr-Mn-N austenitic steels at 1000°C in dry air atmosphere was investigated using scanning electron microscope, transmission electron microscope, and X-ray diffraction analysis. The results show that intergranular M ₂₃C₆ carbide morphologies play an important role on the internal oxidation behavior of Cr-Mn-N steels. During the period of the oxidation, both discontinuous chain-shaped and continuous film-shaped intergranular M ₂₃C₆ carbides precipitated along the grain boundaries. Internal oxides of silica preferentially intruded into the matrix along grain boundaries with discontinuous M ₂₃C₆ carbide particles, while silica was obviously restricted at the interfaces between the external scale and matrix on the occasion of continuous film-shaped M ₂₃C₆ carbides. It is seemed that reasonable microstructure could improve the oxidation resistance of Cr-Mn-N steels.

Key words: M 23C6 precipitation, Internal oxidation, Cr-Mn-N austenitic steel

Lei-Gang Zheng, Xiao-Qiang Hu, Xiu-Hong Kang, Dian-Zhong Li. Effect of Intergranular Precipitation on the Internal Oxidation Behavior of Cr-Mn-N Austenitic Stainless Steels[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(8): 1008-1014.

Figures/Tables 9

Table 1 Chemical compositions of the studied steels (wt%)

Steel	C	Si	Mn	Cr	Ni	N	Fe
Cr-Ni	0.31	0.67	1.49	25.60	10.37	0.12	Bal.
Cr-Mn-N-1	0.24	1.96	9.49	20.71	1.88	0.36	Bal.
Cr-Mn-N-2	0.29	1.80	11.09	20.01	2.00	0.35	Bal.
Cr-Mn-N-3	0.32	2.12	10.10	19.97	2.10	0.31	Bal.

Fig. 1 SEM micrographs of the Cr-Ni a, Cr-Mn-N-1 b, Cr-Mn-N-2 c, Cr-Mn-N-3 d steels after solution-annealing treatment, respectively

Fig. 2 SEM micrographs of the Cr-Ni a, Cr-Mn-N-1 b, Cr-Mn-N-2 c, Cr-Mn-N-3 d steels after oxidation at 1000°C for 500 h in dry air atmosphere, respectively

Fig. 3 TEM bright field images and corresponding SAED patterns of M 23C6 on grain boundaries in Cr-Mn-N-2 a and Cr-Mn-N-3 b steels. (The corresponding SAED patterns in a and b showed a cube-on-cube orientation relationship of [001]γ//[001]M23C6 , ((100)γ//(100) M23C6) and [211]γ//[211]M23C6,((111)γ//(111)M23C6) between M 23C6 carbides and the matrix, respectively)

Fig. 4 Cross-sectional SEM secondary electron images of the Cr-Ni a, Cr-Mn-N-1 b, Cr-Mn-N-2 c, Cr-Mn-N-3 d steels isothermally oxidized at 1000°C for 1 h in dry air atmosphere, respectively (The EDS spectrum in b showed that the internal oxides are silica)

Table 2 Composition of the oxide scales for the studied steels oxidized at 1000°C for 1 h analyzed by EDS (wt%)

Steel	O	Mn	Cr	Fe
Cr-Ni	31.47	10.91	54.66	1.11
Cr-Mn-N-1	25.96	42.43	26.17	5.44
Cr-Mn-N-2	20.69	50.56	23.14	5.61
Cr-Mn-N-3	20.84	53.96	19.21	6.00

Fig. 5 XRD patterns for the oxide scales of the studied steels oxidized at 1000°C for 1 h

Fig. 6 Cross-sectional SEM backscattered electron images of Cr-Ni a, Cr-Mn-N-1 b, Cr-Mn-N-2 c Cr-Mn-N-3 d steels isothermally oxidized at 1000°C for 500 h in dry air atmosphere, respectively (The small images inserted in a and b displayed the details that silica intruded along austenite-ferrite interfaces and grain boundaries, respectively)

Fig. 7 Schematic map for the oxidation process of the studied Cr-Mn-N steels

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