Effects of surface grinding for scratched alloy 690TT tube in PWR nuclear power plant: Microstructure and stress corrosion cracking

doi:10.1016/j.jmst.2021.11.009

Abstract

Abstract:

Alloy 690TT tube samples with different scratch depths were repaired by grinding treatments using abrasive papers of two different particle sizes. The microstructure and stress corrosion cracking (SCC) behavior were studied in detail. During grinding, the plastic accumulation zone vulnerable to SCC was removed. Meanwhile, some residual slip steps remained in the scratched area. Corrosion tests lasting 1000, 2000, 3000, and 4000 h show that the sensitivity and risk of SCC in the scratched area are decreased by grinding. Treatment using abrasive particles of a smaller size is more effective. Nevertheless, deep scratches remained hazardous even after the grinding.

Key words: Alloy 690TT, Scratch, Surface, Stress corrosion, High temperature corrosion, Intergranular corrosion

Bin Wu, Hongliang Ming, Fanjiang Meng, Yifeng Li, Guangqing He, Jianqiu Wang, En-Hou Han. Effects of surface grinding for scratched alloy 690TT tube in PWR nuclear power plant: Microstructure and stress corrosion cracking[J]. J. Mater. Sci. Technol., 2022, 113: 229-245.

Figures/Tables 21

Fig. 1. (a) Metallography images of as-received Alloy 690TT used in test, (b) schematic diagram of samples cutting.

Table 1. Chemical compositions of Alloy 690TT (wt.%).

Ni	Cr	Fe	Si	Mn	Al	Ti	C	S	P
Bal.	29.55	9.81	0.06	0.01	0.11	0.12	0.022	<0.01	<0.01

Table 2. Parameters of two grinding treatment.

Scratch depth (μm)	Treatment time (s)	Empty Cell
Empty Cell	Grinding treatment A (1000 r-600 #)	Grinding treatment B (1000 r-1000 #)
10	3-5	3-5
30	5-7	5-10
50	5-7	5-10
70	5-7	5-10
90	7-10	7-15
110	7-10	7-15

Fig. 2. The three-dimensional morphologies of samples with different scratch depths: (a1-c1) 30, 70, 110 μm, respectively, (a2-c2) 30, 70, 110, μm treated by GT-A, respectively, (d1-e1) 50 μm before treated by GT-A, GT-B, respectively, (d2-e2) 50 μm treated by GT-A, GT-B, respectively.

Fig. 3. Cross-sectional metallographic microstructure of samples with different scratch depths: (a-c) 30, 70, 110 μm, treated by GT-A, respectively, (d) 50 μm, treated by GT-A, (e) 50 μm, treated by GT-B.

Fig. 4. Cross-sectional metallographic microstructures of samples with different scratch depths by SEM: (a-c) 30, 70, 110 μm, treated by GT-A, respectively, (d) 50 μm,treated by GT-A, (e) 50 μm, treated by GT-B.

Fig. 5. EBSD results of sample with a scratch depth of 50 μm treated by GT-A: (a) IPF, (b) KAM maps, (c) GB maps.

Fig 6. Nanoindentation test results: (a) scratch depth of 50 μm sample treated by GT-A, (b-c) samples before and after treated by GT-A, respectively.

Table 3. Nanoindentation test results of two grinding treatments.

Average hardness (GPa)	GT-A	GT-B	Empty Cell	Empty Cell
Empty Cell	Original pipe region	Grinding region	Original pipe region	Grinding
region
Row 1	3.509	3.402	3.720	3.292
Row 2	3.410	3.201	3.304	3.340
Row 3	3.284	3.192	3.236	3.247
Row 4	3.183	3.090	3.232	3.164
Row 5	3.110	3.072	3.210	3.256

Fig. 7. TEM observation of the area ground by different two treatments: morphologies at low magnification, BFTEM, DFTEM and SAED images of (a1-d1) the area ground by GT-A, (a2-d2) the area ground by GT-B.

Fig. 8. The observation of microstructure in the area ground by GT-A: (a-b) BFTEM and DFTEM images of the ‘G1’ area in Fig. 7(a1), (c-d) HRTEM images of the ‘c’ and ‘d’ area in (a).

Fig 9. The observation of microstructure in the area ground by GT-B: (a-b) BFTEM and DFTEM images of the ‘G2’ area in Fig. 7(a2), (c) HRTEM images of the ‘c’ area in (a), (d) HRTEM images at the ‘d’ area in (c).

Fig. 10. Cross-sectional and local morphology of samples with scratch depth of 110 μm treated by different grinding treatment after 2000 h corrosion test: (a-b) GT-A, (c-d) GT-B.

Fig. 11. Cross-sectional and local morphology of the sample with a scratch depth of 50 μm treated by different grinding treatment after 1000, 2000 h, 3000 and 4000 h corrosion tests: (a1-a4) GT-A, (b1-b4) GT-B.

Fig. 12. Relationship between crack number, average length and scratch depth by different grinding treatment in corrosion tests with the durations of 1000, 2000, 3000 and 4000 h: (a1-a4) treated by GT-A, (b1-b4) treated by GT-B. (Note: the result of crack number is the sum of three parallel samples with the same depth, and the result of average length is calculated from that three parallel samples).

Fig. 13. SCC behavior in scratched area after corrosion test for 1000 h with depth of 30 μm, 50 μm and 110 μm, respectively: (a1-a3) no grinding, (b1-b3) treated by GT-B.

Fig. 14. Cross-sectional morphologies of samples with scratch depths of 30, 70 and 110 μm treated by GT-A after 2000 h corrosion test: (a1-a3) overview of scratched area, (b1-b3) the location of left APR, (c1-c3) the location of right APR, (d1-d3) the magnified images of (b1-b3), (e2-e3) the magnified images of the bottom of scratch groove, (f1-f3) the magnified images of (c1-c3).

Fig. 15. Cross-sectional morphologies of samples with scratch depths of 10, 30, 50, 70, 90 and 110 μm treated by GT-A after 4000 h corrosion test: (a1-a3) the location of left APR, (b1-b3) overview of scratched area, (c1-c3) the location of right APR.

Table 4. Information of cracks observed on the sidewall of scratch groove treated by GT-A.

Length of Cracks on the sidewall (μm)	Scratch depth (μm)	Test time (h)
33.6	70	2000
15.47, 7.19	110	2000
6.53, 4.83	70	4000
8.28, 6.62, 5.77, 8.14, 13.6	90	4000
6.03, 10.5	110	4000

Fig. 16. Cross-sectional morphologies of cracks observed on the sidewall of scratch groove treated by GT-A.

Fig. 17. Schematic diagram of effects of grinding treatment on SCC behavior in scratched area.

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