Effects of rare earth on microstructure and impact toughness of low alloy Cr-Mo-V steels for hydrogenation reactor vessels

doi:10.1016/j.jmst.2019.03.012

Abstract

Abstract:

The effects of rare earth (RE) on the microstructure and impact toughness of low alloy Cr-Mo-V bainitic steels have been investigated where the steels have RE content of 0 to 0.048 wt.%. The results indicate that the normalized microstructures of the steels are typical granular bainite (GB) composed primarily of bainitic ferrite and martensite and/or austenite (M-A) constituents. The M-A constituents are transformed into ferrite and carbides and/or agglomerated carbides after tempering at 700 °C for 4 h. The addition of RE decreases the onset temperature of bainitic transformation and results in the formation of finer bainitic ferrite, and reduces the amount of carbon-rich M-A constituents. For the normalized and tempered samples, the ductile-to-brittle transition temperature (DBTT) decreases with increasing RE content to a critical value of 0.012 wt.%. Lower DBTT and higher upper shelf energy are attributed to the decreased effective grain size and lower amount of coarse agglomerated carbides from the decomposition of massive M-A constituents. However, the addition of RE in excess of 0.012 wt.% leads to a substantial increase in the volume fraction of large-sized inclusions, which are extremely detrimental to the impact toughness.

Key words: Low alloy Cr-Mo-V steel, Rare earth, Microstructure, Impact toughness, Granular bainite, Martensite-austenite constituents

Zhonghua Jiang, Pei Wang, Dianzhong Li, Yiyi Li. Effects of rare earth on microstructure and impact toughness of low alloy Cr-Mo-V steels for hydrogenation reactor vessels[J]. J. Mater. Sci. Technol., 2020, 45: 1-14.

Figures/Tables 17

Table 1 Chemical compositions of the investigated five 2.25Cr-1Mo-0.25 V steels (wt.%).

Steel	C	Si	Mn	Cr	Mo	V	P	S	RE	Fe
Ref.	0.15	0.08	0.57	2.24	0.88	0.24	0.005	0.002	--	Bal.
0.007RE	0.14	0.10	0.59	2.23	0.89	0.23	0.005	0.002	0.007	Bal.
0.012RE	0.14	0.08	0.57	2.24	0.88	0.24	0.005	0.002	0.012	Bal.
0.020RE	0.13	0.10	0.59	2.23	0.89	0.23	0.005	0.002	0.020	Bal.
0.048RE	0.14	0.08	0.57	2.24	0.88	0.24	0.005	0.002	0.048	Bal.

Table 2 Mechanical properties of the investigated five 2.25Cr-1Mo-0.25 V steels.

Samples	YS (MPa)	UTS (MPa)	A (%)	USE (J)	DBTT(℃)
Ref.	684	787	20.8	240	-56
0.007 RE	680	780	20.8	237	-61
0.012 RE	683	788	20.5	255	-70
0.020 RE	676	778	21	232	-49
0.048 RE	683	784	20.5	230	-24

Fig. 1. Charpy absorbed energy low alloy 2.25Cr-1Mo-0.25 V steels with different RE contents at test temperature ranging -120 °C to 20 °C. (a) Ref., (b) 0.007RE, (c) 0.012RE, (d) 0.028RE, and (e) 0.048RE steel.

Table 3 Quantitative statistics of inclusions and prior austenite grain size in the experimental steels.

Steel	f	d₀	R	g	D
Ref.	0.017	1.70	1.37	68.2	33.7
07RE	0.017	1.68	1.37	70.4	34.6
12RE	0.020	1.72	1.36	82.3	33.1
20RE	0.032	1.89	1.38	94.4	32.7
48RE	0.066	2.56	1.36	143	31.3

Fig. 2. SEM images of polished cross-sections of Ref. steel, the EDS results for the inclusions seen in (a) and (b) are shown in (c) and (d), respectively.

Fig. 3. (a) TEM image of the inclusion in the 0.012RE steel, (b) the corresponding SAED pattern, (c) the EDS results for the particle pointed by arrow in (a).

Fig. 4. (a) SEM micrographs of a polished cross-section of 0.048 RE steel. (b) from the region of enlarged rectangle in (a), (c) the EDS results for the inclusion seen in (b).

Fig. 5. ΔL-T curves of the investigated fives steels obtained at the cooling rate of 3 °C/s.

Fig. 6. SEM micrographs of the normalized samples, (a) Ref., (b) 0.007RE, (c) 0.012RE, (d) 0.020RE, and (e) 0.048RE steels. Etched by nital.

Fig. 7. CLSM micrographs of the normalized samples, (a) Ref., (b) 0.007 RE, (c) 0.012RE, (d) 0.020RE, and (e) 0.048 RE steels. Etched by Lepera reagent.

Table 4 Volume fractions of M-A constituents, GB1, GB2, and retained austenite (RA), and average size of M-A constituents and effective grain size in the investigated five 2.25Cr-1Mo-0.25 V steels.

Steels	Volume fraction of M-A (%)	Average size of M-A (μm)	Volume fraction of GB1 (%)	Volume fraction of GB₂ (%)	Volume fraction of RA (%)	Effective grain size (μm)
Ref.	9.6 ± 1.8	2.6 ± 0.1	81.2 ± 10.7	18.8 ± 10.7	5.4	6.74
0.007RE	8.7 ± 1.2	2.3 ± 0.1	68.1 ± 10.5	31.9 ± 10.5	4.8	6.07
0.012RE	6.6 ± 0.5	1.7 ± 0.1	43.2 ± 6.5	56.8 ± 6.5	3.1	4.92
0.020RE	5.4 ± 0.8	1.5 ± 0.1	36.7 ± 6.7	62.3 ± 6.7	2.6	4.39
0.048RE	5.6 ± 0.6	1.5 ± 0.1	34.6 ± 7.6	65.4 ± 7.6	2.5	4.42

Fig. 8. Carbon concentration measured by EPMA for two types of M-A constituents. (a) and (c) massive M-A constituents, and (b) and (d) elongated M-A constituents.

Fig. 9. Misorientation maps of Ref. (a) and (b) 0.012RE steels, showing grains with high-angle (>15 deg.) boundaries. Corresponding the morphology and distribution of retained austenite (red color) shown in (c) and (d), respectively.

Fig. 10. XRD patterns of the normalized samples.

Fig. 11. TEM micrographs showing (a) massive M-A constituents in GB1 region before tempering, (b) elongated M-A constituents in GB2 region before tempering, (c) the decomposition of massive M-A constituents into ferrite and carbides aggregates, and (d) carbides distributed at lath bainitic ferrite interfaces in GB2 region after tempering.

Fig. 12. Schematic illustration on GB transformation in a low alloy Cr-Mo-V steel during continuous cooling process.BF: bainitic ferrite.

Fig. 13. Typical fracture surfaces of Charpy impact specimens of 0.048RE steel tested at (a) -120℃ and (b) 20℃, (c) and (d) are EDS analysis of inclusions in (a) and (b) pointed to by a white arrow, respectively.

References 38

[1]	N.S. Cheruvu, Metall. Trans. A 20A (1999) 87-97.
[2]	R.L. Klueh, A.M. Nasreldin, Mater.Trans. A 18 (1987) 1279-1290.
[3]	Z. Jiang, P. Wang, D. Li, Y. Li, Mater. Sci. Eng. A 742 (2019) 540-552.
[4]	Z. Li, N. Xiao, D. Li, J. Zhang, Y. Luo, R. Zhang, Acta Metall. Sin. 50 (2014) 777-786.
[5]	G. Yan, L. Han, C. Li, X. Luo, J. Gu , J. Nucl. Mater. 483 (2017) 167-175.
[6]	L. Lan, C. Qiu, D. Zhao, X. Gao, L. Du, Mater. Sci. Eng. A 558 (2012) 592-601.
[7]	D. Tian, L.P. Karjalainen, B. Qian, X. Chen, JSME Int. J. Ser. A 40 (1997) 179-188.
[8]	H. Zhang, X. Cheng, B. Bai, H. Fang, Mater. Sci. Eng. A 528 (2011) 920-924.
[9]	Z. Qiao, Y. Liu, L. Yu, Z. Gao , J. Alloys. Compd. 475 (2009) 560-564.
[10]	H.K. Sung, S. Lee, S.Y. Shin, Metall. Mater. Trans. A 45A (2014) 2004-2013.
[11]	E. Bonnevie, G. Ferriere, A. Ikhlef, D. Kaplan, J.M. Orain, Mater. Sci. Eng. A 385 (2004) 7.
[12]	Y. Li, D.N. Crowther, M.J.W. Green, P.S. Mitchell, T.N. Baker, ISIJ Int. 41 (2001) 46-55.
[13]	K.A. Lanskaya, V.V. Yarovoi, O.V. Basargin, L.V. Kulikova, Met. Sci. Heat Treat. 1 (1988) 14-17.
[14]	X.W. Zeng, J. Rare Earth 23 (2005) 434-436.
[15]	W. Lu, H. Liu, T.Y. Hsu, Scr. Metall. Mater. 29 (1993) 273-274.
[16]	D. Li, P. Wang, X.Q. Chen, P. Fu, Y. Luan , et al., Submitted Nat. Mater. (2019).
[17]	Y.L. Liang, Y.L. Yi, S.L. Long, Q.B. Tan , J. Mater. Eng. Perform. 23 (2014) 4251-4258.
[18]	Y. Liang, Q. Tan, Heat Treat. Met. 34 (2009) 48-51.
[19]	H. Bai, H. Ren, Q. Fang, M. Qin, W. Sun, Heat Treat. Met. 39 (2014) 124-127.
[20]	Y. Hong, W.R. Tyson, B. Faucher, Acta Metall. Sin. 24 (1988) 282-287.
[21]	R. Knight, W.R. Tyson, G. Sproule, Metals Technol. 11 (1984) 273-279.
[22]	W. Oldfield, ASTM Stand. News (United States) 3 (1975) 11.
[23]	Z.J. Xie, G. Han, W.H. Zhou, C.Y. Zeng, C.J. Shang, Mater. Charact. 113 (2016) 60-66.
[24]	N. Huda, A.R.H. Midawi, J. Gianetto, R. Lazor, A.P. Gerlich, Mater. Sci. Eng. A662 (2016) 481-491.
[25]	T.Y. Hsu, ISIJ Int. 38 (1998) 1153-1164.
[26]	H.K. Sung, D.H. Lee, S.Y. Shin, S. Lee, J.Y. Yoo, B. Hwang, Mater. Sci. Eng. A 624 (2015) 14-22.
[27]	H. Okada, F. Matsuda, K. Ikeuchi, Z. Li, Weld. Int. 9 (1995) 211-216. DOI URL
[28]	Z. Jiang, P. Wang, D. Li, Y. Li, Acta Metall. Sin. 51 (2015) 925-934. DOI URL
[29]	Y.L. Zhou, T. Jia, X.J. Zhang, Z.Y. Liu, R.D.K. Misra, Mater. Sci. Eng. A 626 (2015) 352-361. DOI URL
[30]	Z. Jiang, P. Wang, D. Li, Y. Li, Mater. Sci. Eng. A 699 (2017) 165-175.
[31]	Q. Lin, W. Ye, Y. Du, Z. Yu, Chin. J. Met. Sci. Technol. (1990) 415-420.
[32]	H. Fang, B. Bai, X. Zheng, Y. Zeng, X. Chen, R. Zhao, Acta Metall. Sin. 22 (1986) a283.
[33]	D. Zhang, C. Wu, R. Yang, Mater. Sci. Eng. A 131 (1991) 93-97.
[34]	X. Jiang, S.H. Song, Mater. Sci. Eng. A 613 (2014) 171-177.
[35]	Y. Wu, L. Wang, T. Du, J. Less-Common Met. 110 (1985) 187-193.
[36]	C.J. Liu, H.L. Liu, M.F. Jiang, Adv. Mater. Res. 163-167 (2010) 61-65.
[37]	Z. Liu, Z. Yang, Acta Metall. Sin. 23 (1987) 531-533.
[38]	F. Wang, J. Li, Q. Han , J. Rare Earth (1995) 196-201.