Formation of G-phase in 20Cr32Ni1Nb Stainless Steel and its Effect on Mechanical Properties

doi:10.1007/s40195-017-0589-0

Abstract

Abstract:

A series of tensile tests, Charpy impact tests, optical microscopy observations, and field emission-scanning electron microscopy examinations, were carried out to investigate the mechanical properties and microstructural evolution of 20Cr32Ni1Nb steel. Experimental results indicate that the as-cast microstructure of the steel typically consists of a supersaturated solid solution of austenite matrix with a network of interdendritic primary carbides (NbC and M₂₃C₆). In the ex-service samples, large amounts of secondary carbides precipitate within austenite matrix. Besides the growth and coarsening of NbC and M₂₃C₆ carbides during service condition, the Ni-Nbsilicides known as G-phase (Ni₁₆Nb₆Si₇) are formed at the interdendritic boundaries. The microstructural evolution results in the degradation of the mechanical properties of the ex-service steel. In addition, the precipitate rate of G-phase, depending in part on Si content, varies greatly for the 20Cr32Ni1Nb steel, which plays a key role in the long-term microstructural stability of the steel. Based on the X-ray diffraction data, time-temperature-transformation curve for the steel is obtained from the aged specimens.

Key words: 20Cr32Ni1Nb, Embrittlement, G-phase, TTT diagram

Xiao-Feng Guo, Ying-Ying Ni, Jian-Ming Gong, Lu-Yang Geng, Jian-Qun Tang, Yong Jiang, Xian-Kai Jia, Xin-Yu Yang. Formation of G-phase in 20Cr32Ni1Nb Stainless Steel and its Effect on Mechanical Properties[J]. Acta Metallurgica Sinica (English Letters), 2017, 30(9): 829-839.

Figures/Tables 16

Table 1 Chemical compositions (mass%) of the as-cast and ex-service 20Cr32Ni1Nb steels

Material	C	Cr	Ni	Nb	Si	Mn	Fe
As-cast	0.062	20.88	31.34	0.93	1.13	0.94	Bal.
Ex01	0.097	20.36	31.72	1.14	0.79	0.80	Bal.
Ex02	0.85	20.37	32.29	0.94	0.59	0.76	Bal.
CT15C	0.05-0.15	19.0-21.0	31.0-41.0	0.5-1.5	0.5-1.5	0.15-1.5	Bal.

Fig. 1 XRD patterns of extracted precipitates in the as-cast 20Cr32Ni1Nb steel and the sample Ex01

Fig. 2 Optical microstructures of the as-cast 20Cr32Ni1Nb steel at different magnifications

Fig. 3 Typical back-scattered electron (BSE)-SEM images of 20Cr32Ni1Nb steel in the as-cast condition: adark contrast corresponds to M23C6 and bright contrast to NbC carbides; b, c are the typical EDS analyses of M23C6 and NbC

Fig. 4 Optical microstructures of the specimen Ex01 at different magnifications

Fig. 5 Optical microstructures of the specimen Ex02 at different magnifications

Fig. 6 Back-scattered electron (BSE)-SEM images of the sample Ex01: aa typical casting microstructure; b EDS analysis of Cr-rich M23C6; cNb-rich NbC; d G-phase

Fig. 7 Precipitates in the specimen Ex01 detected using SEM elemental mapping technique

Fig. 8 Back-scattered electron (BSE)-SEM images of the sample Ex02 at different magnifications

Fig. 9 Time-temperature-transformation diagram of 20Cr32Ni1Nb steel determined from X-ray diffraction data

Fig. 10 Representative XRD patterns of extracted residues in the 20Cr32Ni1Nb steel after aging at: a 900 °C; b 1025 °C

Fig. 11 Charpy impact toughness values of the as-cast and ex-service 20Cr32Ni1Nb steels at different temperatures

Table 2 Tensile properties of the as-cast and ex-service 20Cr32Ni1Nb steels

Condition	Temperature (°C)	0.2% proof stress, σ_ys (MPa)	Ultimate tensile strength, σ_ult (MPa)	Elongation, δ (%)	Reduction in area, φ_f (%)
As-cast	20	208	506	41	25
Ex01	20	172	424	9.2	3.3
Ex02	20	203	463	31	17
CT15C	20	170	435	20	-

Fig. 12 SEM micrographs of the fracture surface of: a the as-cast sample tested at ambient temperature; b the as-cast sample tested at 890 °C; c the sample Ex01 tested at ambient temperature; d the sample Ex01 tested at 890 °C

Fig. 13 Typical SEM fractograph of the Charpy impact sample Ex01 tested at room temperature

Fig. 14 SEM micrographs of the fracture surface of: a the as-cast sample tested at ambient temperature; b the as-cast sample tested at 890 °C; c the sample Ex01 tested at ambient temperature; d the sample Ex01 tested at 890 °C

References

[1]	F.Z. Shen, G.X. Ma, X. Ling, X. Yang, X.Z. Zheng, J. Ruan, Y. Lu, Acta Metall. Sin. (Engl. Lett.) 12, 105 (1999)Google Scholar
[2]	M. Santos, M. Guedes, R. Baptista, V. Infante, R.A. Cláudio, Eng. Fail. Anal. 56, 194 (2015)CrossRefGoogle Scholar
[3]	L.M. Shen, J.M. Gong, Y. Jiang, L.Y. Geng, Acta Metall. Sin. (Engl. Lett.) 24, 235 (2011)Google Scholar
[4]	C.J. Liu, Y. Chen, Mater. Des. 32, 2507 (2011)CrossRefGoogle Scholar
[5]	L. Bonaccorsi, E. Guglielmino, R. Pino, C. Servetto, A. Sili, Eng. Fail. Anal. 36, 65 (2014)CrossRefGoogle Scholar
[6]	L.M. Shen, J.M. Gong, Y. Jiang, L.Y. Geng, Acta Metall. Sin. (Engl. Lett.) 25, 279 (2012)Google Scholar
[7]	J.B. Yan, Y.F. Gu, Y.Y. Dang, X.B. Zhao, J.T. Lu, Y. Yuan, Z. Yang, H.F. Yin, Mater. Chem. Phys. 175, 107 (2016)CrossRefGoogle Scholar
[8]	L.A. Spyrou, P.I. Sarafoglou, N. Aravas, G.N. Haidemenopoulos, Eng. Fail. Anal. 45, 456 (2014)CrossRefGoogle Scholar
[9]	X.W. Wang, J.Q. Tang, J.M. Gong, L.Y. Geng, Y. Jiang, H. Liu, Eng. Fail. Anal. 57, 350 (2015)CrossRefGoogle Scholar
[10]	S. Shi, J.C. Lippold, Mater. Charact. 59, 1029 (2008)CrossRefGoogle Scholar
[11]	M.P. Dewar, A.P. Gerlich, Metall. Mater. Trans. A 44, 627 (2013)CrossRefGoogle Scholar
[12]	J.J. Hoffman, G.E. Gapinski, Proceedings of the 46th International Conference Annual Safety in Ammonia Plant and Related Facilities Symposium, Montreal, p. 17-20, 2000Google Scholar
[13]	Q.Z. Chen, C.W. Thomas, D.M. Knowles, Mater. Sci. Eng. A 374, 398 (2004)CrossRefGoogle Scholar
[14]	D.M. Knowles, C.W. Thomas, D.J. Keen, Q.Z. Chen, Int. J. Pressure Vessels Pip. 81, 499 (2004)CrossRefGoogle Scholar
[15]	R.C. Ecob, R.C. Lobb, V.L. Kohler, J. Mater. Sci. 22, 2867 (1987)CrossRefGoogle Scholar
[16]	R.A.P. Iba?ez, G.D.A. Soares, L.M. Almeida, I.L. May, Mater. Charact. 30, 243 (1993)CrossRefGoogle Scholar
[17]	B. Piekarski, Mater. Charact. 47, 181 (2001)CrossRefGoogle Scholar
[18]	J.J. Hoffman, J. Magnan, Corrosion (NACE International, Houston, p. 1-11, 2003)Google Scholar
[19]	M. Mostafaei, M. Shamanian, H. Purmohamad, M. Amini, A. Saatchi, Eng. Fail. Anal. 18, 164 (2011)CrossRefGoogle Scholar
[20]	A. Alvino, D. Ramires, A. Tonti, D. Lega, Mater. High Temp. 31, 2 (2014)CrossRefGoogle Scholar
[21]	K.G. Buchanan, M.V. Kral, C.M. Bishop, Metall. Mater. Trans. A 45, 3373 (2014)