Life Prediction for Stress Corrosion Behavior of 316L Stainless Steel Elbow of Nuclear Power Plant
Shu GUO,En-Hou HAN(),Haitao WANG,Zhiming ZHANG,Jianqiu WANG
Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
Cite this article:
Shu GUO,En-Hou HAN,Haitao WANG,Zhiming ZHANG,Jianqiu WANG. Life Prediction for Stress Corrosion Behavior of 316L Stainless Steel Elbow of Nuclear Power Plant. Acta Metall Sin, 2017, 53(4): 455-464.
Stress corrosion cracking (SCC) is one of the main ageing mechanism in light water reactor (LWR). 316L austenitic stainless steel was adopted in nuclear industry for its relatively high corrosion resistance. The SCC of austenitic stainless steel may occur as it is subjected to both the tensile stress and the caustic medium, with regard to maintaining the structural integrity of components in nuclear power plant, an accurate prediction and efficient assessment of the component lifetime is significant and necessary. The stress corrosion crack propagation behavior of the 316L stainless steel elbow of nuclear power plant was investigated through a numerical simulation method. Firstly a finite element (FE) model was created for the stainless steel thick-walled elbow (the outer diameter is 355.6 mm, the inner diameter is 275.6 mm), with a semi-elliptical shaped 3D defect introduced at the internal surface of the elbow as the geometry of the crack, which was consistent with a practical crack, the crack opening displacement (δi) was determined by the calculations through the Dugdale model; subsequently, according to the FE calculation results, establish the fitting formula of the stress intensity factor (K) varying with the crack depth (a) and additional stress (P), and the fitting formula of the stress corrosion crack propagation rate (da/dt) for elbows under two types of cold work deformation was deduced through the combination with the experimental data, the crack propagation time was then calculated using a iterative method for cracks which evolved from different initial crack depth values to certain crack depth values. The calculation results provided effective reference criterion for the nuclear power plant safety assesment. This investigation demonstrated that, when the cold deformation extent of the elbow part is relatively small ( with the hardness of 230~245 HV) and it is under the ideal condition (no initial additional stress), it takes around 57 a for the stress corrosion crack to penetrate the elbow, when the initial additional stress was elevated to 200 MPa, the elbow failure time was shrinked to 1/5 (no stress release), 2/7 (half-stress release) and 3/7 (total stress release) of the former; keep the same initial additional stress (200 MPa) and increase the cold work deformation extent (the hardness was increased from 230~245 HV to 275~300 HV), the elbow failure time was shortened to 2/5 (no stress release), 3/8 (half-stress release) and 1/3 (total stress release) for the elbow part with higher cold deformation extent compared to the part with lower cold deformation extent, thus it was observed that both the decrease of the extent of stress relaxation and the increase of the extent of cold work deformation contributed to the reduction of the residual life of the nuclear power plant 316L stainless steel elbow.
Fig.1 Schematic geometry of a crack (a) and schematic for crack propagation at the inner surface of the elbow (b) (a—crack depth, δi —crack opening displacement at the middle of the crack)
Fig.2 Schematic for δi, δ and ry[11] (δi—crack opening displacement, δ—crack tip opening displacement, ry—plastic zone at the crack tip, l—crack length, α—the inclination angle of plastic zone radius ry, σ—stress distribution at the crack tip)
Fig.3 Schematic for mesh (a) elbow part with a crack embedded (b) enlarged area around the crack (c) nodes setting along the crack boundary line
Fig.5 Stress distributions of the elbow with the crack depth of 20 mm, under 100 MPa additional stress (a) whole elbow (b) enlarged area around the crack (c) sectional view at 1/2 crack length (d) enlarged area at the 1/2 crack length
Fig.6 Comparision between the assumed and the calculated value of crack opening displacement (δi) under different additional stresses (0~300 MPa) for cracks with different depth (20~30 mm) (The maximum inaccuracy between these two values is less than 3%)
Fig.7 Variation of stress intensity factor (K) with crack depth under different additional stresses
SCC test step
Small deformation (230~245 HV)
Large deformation (275~300 HV)
K MPam0.5
da/dt 10-7 mms-1
Duration h
Δa μm
K MPam0.5
da/dt 10-7 mms-1
Duration h
Δa μm
1
20
1.40
256.1
98.5
20
4.28
536.8
699.0
2
25
1.84
216.8
149.0
25
4.83
519.0
921.0
3
30
2.24
232.0
192.0
33
5.97
144.9
313.0
4
40
2.65
195.1
189.8
40
7.42
498.1
1338.5
Table 1 Parameters at different SCC test steps for both small deformation and large deformation parts of 316L stainless steel (Temperature is 310 ℃,concentration of dissolved oxygen is 0.1 mg/L, concentration of dissolved hydrogen<3×10-4 mg/L)
Fig.8 Variation of crack propagation time with crack depth for the elbow part with hardness of 230~245 HV (a) under 200 MPa constant additional stress, 0 MPa constant additional stress, and 200 MPa initial additional stress~0 MPa ultimate released stress, respectively (b) under 200 MPa constant additional stress, 100 MPa constant additional stress, and 200 MPa initial additional stress~100 MPa ultimate released stress, respectively
Fig.9 Variation of crack propagation time with crack depth for the elbow part with hardness of 275~300 HV (a) under 200 MPa constant additional stress, 0 MPa constant additional stress, and 200 MPa initial additional stress~0 MPa ultimate released stress, respectively (b) under 200 MPa constant additional stress, 100 MPa constant additional stress, and 200 MPa initial additional stress~100 MPa ultimate released stress, respectively
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