Laves Phase Formation and Its Effect on Mechanical Properties in P91 Steel

doi:10.1007/s40195-015-0318-5

Abstract

Abstract:

Effect of Laves phase formation on mechanical properties in a pressurized T-junction of P91 steel pipe at 849 K for 58,000 h with 25.65 MPa vapor pressure was studied. Thermodynamic calculations had been performed by using the software Thermo-Calc to study the phase at equilibrium state. Counter plot of von Mises stress in the pipe during service life was calculated by finite element analysis to study the effect of the operated stress distribution on the evolution of Laves phase. The change in the microstructure and mechanical properties in the sites with different stress was also studied. The results indicated that the formation of Laves phase in P91 steel was a thermodynamically possible process due to enrichment of Mo and depletion of C adjacent to M₂₃C₆ particles or along martensite lath and packet boundaries. The formation of Laves phase had a detrimental influence on the mechanical properties in P91 steel. The mean size of Laves phase would be significantly increased with increasing operated stress, leading to a reduction in tensile properties and impact energy. In particular, crack initiation energy and crack growth energy during impact test rapidly decreased with increasing the mean size and volume fraction of Laves phase.

Key words: Heat-resistant steels, Laves phase, Creep, Precipitation, Phase stability

Zhi-Xin Xia, Chuan-Yang Wang, Yan-Fen Zhao, Guo-Dong Zhang, Lu Zhang, Xin-Ming Meng. Laves Phase Formation and Its Effect on Mechanical Properties in P91 Steel[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(10): 1238-1246.

Figures/Tables 11

Fig. 1 Finite element analysis of operated stress in the T-junction: a size of the T-junction, b finite element mesh model, c load and boundary conditions, d counter plot of von Mises stress

Table 1 Chemical composition of different specimens in the tee (in wt%)

Specimen	C	Si	Mn	V	Cr	Mo	Nb	S	P	N	Fe
A	0.12	0.30	0.44	0.20	8.62	0.93	0.06	0.003	0.017	0.044	Bal.
B	0.11	0.27	0.43	0.21	8.41	0.92	0.06	0.003	0.017	0.047	Bal.

Fig. 2 Equilibrium phase diagrams of P91 steel calculated by Thermo-Calc software with database SSOL5: a 9Cr-1Mo, b Mo concentration-temperature, c 9Cr-2Mo

Table 2 Mechanical properties of the tee and original pipe

Specimen	Yield strength (MPa)	Tensile strength (MPa)	Yield strength at 849 K (MPa)	Tensile strength at 849 K (MPa)	Impact energy at longitude (J)	Impact energy at transverse (J)
A	380	610	240	290	35	23
B	450	650	290	320	91	73
Original pipe	505	660	335	370	212	165

Fig. 3 Change in impact toughness and stress in the T-junction

Table 3 Impact absorbing energy during the process of the fracture in both specimens

	Crack initiation energy (J)	Crack growth energy (J)	Ratio of the crack initiation energy to impacting absorption energy (%)
Transverse of specimen A	13.5	8.5	61.4
Longitude of specimen A	28.7	5.5	83.2
Transverse of specimen B	45.7	49.6	47.9
Longitude of specimen B	59.9	51.8	53.6

Fig. 4 Waveform graph of instrumented impact testing of both specimens

Fig. 5 Microstructure and distribution of the precipitations in both steels: a, c specimen A; b, d specimen B

Fig. 6 Secondary electron images and backscattered electron images of both specimens: a, c specimen A; b, d specimen B

Table 4 Chemical composition of different specimens in different phases (wt%)

Specimen	Phases	C	Si	Cr	Mo	Fe
A	Laves	-	3.17	10.45	41.28	45.1
A	M₂₃C₆	8.86	-	59.57	6.92	24.65
B	Laves	-	3.22	9.91	41.75	45.12
B	M₂₃C₆	6.26	-	52.32	8.01	33.41
Thermo-Calc	Laves	-	-	1.19	45.45	53.36
Thermo-Calc	M₂₃C₆	5.14	-	70.82	20.40	3.64

Fig. 7 SEM images of impact fracture surfaces in the both specimens: a crack initiation zone and c crack growth zone in the specimen A; b crack initiation zone and d crack growth zone in the specimen B

References 25

[1]	F. Abe, Mater. Sci. Eng., A 319, 770 (2001)
[2]	P.J. Ennis, A. Zielinska-Lipec, O. Wachter, A. Czyrska-Filemonowicz, Acta Mater. 45, 4901(1997)
[3]	F. Abe, H. Araki, T. Noda, Metall. Trans. A 22, 2225 (1991)
[4]	Z.X. Xia, C. Zhang, H. Lan, Z.G. Yang, P.H. Wang, J.M. Chen, Mater. Sci. Eng., A 528, 657 (2010)
[5]	O. Prat, J. Garcia, D. Rojas, G. Sauthoff, G. Inden, Intermetallics 32, 362 (2013)
[6]	A. Mahmoudi, M.R. Mater. Charact. 62, 976(2011)
[7]	A. Kipelova, A. Belyakov, R. Kaibyshev, Mater. Sci. Eng., A 532, 71 (2012)
[8]	J.S. Lee, H.G. Armaki, K. Maruyama, H. Asahi, Mater. Sci. Eng., A 428, 270 (2006)
[9]	V. Knezevic, G. Sauthoff, J. Vilk, G. Inden, A. Schneider, R. Agamennone, ISIJ Int. 12, 1505(2002)
[10]	G. Dimmlera, P. Weinert, E. Kozeschnik, H. Cerjak, Mater. Charact. 51, 341(2003)
[11]	B.A. Senior, Mater. Sci. Eng., A 119, 5 (1989)
[12]	P.J. Ennis, C. Filemonowicz, Sadhana 28, 709 (2003)
[13]	Y. Tsuchida, K. Okamoto, Y. Tokunaga, ISIJ Int. 35, 317(1995)
[14]	H.J. Zhang, S.T. Liu, C.X. Fan, Electr. Power 40, 12 (2007)
[15]	J.H. Baek, S.H. Kim, C.B. Lee, D.H. Hahn, Met. Mater. Int. 15, 565(2009)
[16]	W.S. Ryu, S.H. Kim, Trans. Indian Inst. Met. 63, 111(2010)
[17]	Z.X. Xia, C. Zhang, Z.G. Yang, Mater. Sci. Eng., A 528, 6764 (2011)
[18]	Y. Hosoi, N. Wade, S. Kunimitsu, T. Urita, J. Nucl. Mater. 141, 461(1986)
[19]	M.I. Isik, A. Kostka, V.A. Yardley, K.G. Pradeep, M.J. Duarte, P.P. Choi, D. Raabe, G. Eggeler, Acta Mater. 90, 94(2015)
[20]	Z.X. Xia, C. Zhang, Z.Y. Yang, Mater. Sci. Technol. 27, 282(2011)
[21]	J. Cui, I.S. Kim, C.Y. Kang, K. Miyahara, ISIJ Int. 41, 368(2001)
[22]	F.R. Larson, J. Miller, Trans. ASME 74, 756 (1952)
[23]	R.L. Orr, O.D. Sherby, J.E. Dorn, Trans. ASM 46, 113 (1954)
[24]	Z.X. Xia, C. Zhang, N.Q. Fan, Y.F. Zhao, F. Xue, S.J. Liu, Mater. Sci. Eng., A 545, 91 (2012)
[25]	Y. Tomita, J. Mater. Sci. 26, 35(1991)