Research ArticleThe Effect of Welding Residual Stress for Making ArtificialStress Corrosion Crack in the STS 304 Pipe
Jae-Seong Kim,1 Bo-Young Lee,2 Woong-Gi Hwang,2 and Sung-Sik Kang3
1 Institute for Advanced Engineering, Yongin 449-863, Republic of Korea2School of Aerospace and Mechanical Engineering, Korea Aerospace University, Goyang 412-791, Republic of Korea3Korea Institute of Nuclear Safety, Daejeon 305-338, Republic of Korea
Correspondence should be addressed to Sung-Sik Kang; [email protected]
Received 5 December 2014; Accepted 3 February 2015
The stress corrosion crack is one of the fracture phenomena for the major structure components in nuclear power plant. Duringthe operation of a power plant, stress corrosion cracks are initiated and grown especially in dissimilar weldment of primary loopcomponents. In particular, stress corrosion crack usually occurs when the following three factors exist at the same time: susceptiblematerial, corrosive environment, and tensile stress (residual stress included).Thus, residual stress becomes a critical factor for stresscorrosion crack when it is difficult to improve the material corrosivity of the components and their environment under operatingconditions. In this study, stress corrosion cracks were artificially produced on STS 304 pipe itself by control of welding residualstress. We used the instrumented indentation technique and 3D FEM analysis (using ANSYS 12) to evaluate the residual stressvalues in the GTAW area. We used the custom-made device for fabricating the stress corrosion crack in the inner STS 304 pipewall. As the result of both FEM analysis and experiment, the stress corrosion crack was quickly generated and could be reproduced,and it could be controlled by welding residual stress.
1. Introduction
Environmentally assisted crack, such as stress corrosion crack(SCC) of the NPP structural materials, has been one of thecauses for the shutdown of the power plant resulting in asignificant loss, incapacitating the production electric power.The resultant repair and replacement of components in lightwater reactors (LWR) remains as one of the limiting factorsfor the safe and economic operation of LWRs, especiallyin the plant life extension period. Stress corrosion crackingusually occurs when the following three factors exist atthe same time: susceptible material, corrosive environment,and tensile stress (including residual stress). Among thesefactors, the residual stress becomes critical problem for stresscorrosion cracking when it is difficult to improve thematerialcorrosivity of the components and their environment underoperating conditions [1, 2]. Generally, the residual stresses areinduced by welding process. The welding is mostly used formanufacturing of structure materials such as nuclear powerplant and vehicle. Horikawa et al. reported that the crack
propagation rate increased at the tungsten inert gas- (TIG-)welded joint. Tani et al. reported that chloride induced SCCof the austenitic stainless steel occurred independently of thetensile residual stress value. They have performed the SCCtest on the austenitic SS using loading device and showedthat the specimen of type 304L SS was fractured even whenthe applied stress was below 200MPa which was less than0.2% proof stress of type 304L SS [3]. Most of the studieshave used the slow-strain-rate test (SSRT), which is notuseful for studying the effect of stress level [4]. Since theresearch of Mazille and Rothea, [5, 6] it is well known thatNi-based alloy and stainless steel are susceptible to stresscorrosion cracking (SCC) in deaerated pure water at hightemperature, but the test was performed in autoclave whichis possible to contain a specimen. The conventional methodof manufacturing artificial stress corrosion cracks is difficultto imitate environmental conditions of the NPP because theresulting cracks are not obtained by using a pipe actuallyused for equipment of nuclear power plants but by using asimulation specimen.
Hindawi Publishing CorporationAdvances in Materials Science and EngineeringVolume 2015, Article ID 932512, 7 pageshttp://dx.doi.org/10.1155/2015/932512
2 Advances in Materials Science and Engineering
Figure 1: The stress corrosion crack forming equipment.
In this study, the stress corrosion crack was artificiallyproduced on the materials of the nuclear power plant byusing the custom-made device. The device provided realenvironment of nuclear power plant and the semiautomaticGTAW process was used for giving the residual stress in theinner of the pipe. The fracture time, which is defined bypressure drop in a SCC test, of the STS 304 pipe was used toconfirm the residual stress effect by welding heat input. Also,3D FEM (Finite ElementMethod) analysis model is designedabout STS 304 pipe and performed heat transfer analysis andresidual stress analysis.
2. Experimental Methods
2.1. Forming the Stress Corrosion Crack. The test materialwas austenitic STS 304, which is used as pipelines in theReactor Coolant System of nuclear power plants (O.D. =89mm, 𝑡 = 7.7mm). The stress corrosion crack formingequipment is shown in Figure 1. The equipment for formingstress corrosion cracks consisted of heater, pressure sensor,AE (acoustic emission) sensor, load cell, and DAQ (dataacquisition) system, and so forth. Generally, an autoclave wasused to make an environment condition of high pressureand high temperature for making a SCC. But, in this study,the autoclave was not used. The heating coils were directlyimposed on the surface of pipe to generate high steampressure and temperature of inner pipe, and the inside of pipespecimen was filled with corrosive solutions. The test wasperformed using the stainless steel 304 pipe in 2M Na
2SO4
and 1M NaOH solutions. The length of specimen is 150mm.Table 1 showsmechanical properties of the STS 304.The yieldstress of the STS 304 material is 139MPa and vapor pressureof the specimen according to ideal gas equation is 165 bar at350∘C. In this case, hoop stress was 124MPa caused by vaporpressure.
The GTAW process was used for the combined actionof the chemical environment and the residual stresses in thecustom-made system. Figure 2 shows the semiautomaticGTAwelding system for giving the residual stress. And Table 2 isthe welding conditions for GTAW.
2.2. For Inducing Residual Stress Inner Pipe. Table 3 showsexperimental conditions for welding. We welded an innerpipe using the same condition on the same line. Figure 3
Figure 2: The semiautomatic GTA welding system for giving theresidual stress.
Figure 3: BOP bead inner STS 304 pipe.
shows BOP (Bead on Plate) bead inner of the pipe using theGTAW process.
2.3. 3-D FEM Analysis. For the heat transfer analysis, wehad divided a step of heating and cooling analysis for eachpass by using the thermal physical properties of STS 304according to temperature and typically melting point ofsteel. Table 4 shows thermal physical properties of STS 304according to temperature. For the FEM stress analysis, wepredicted a variation of the welding residual stress using thethermal physical properties and boundary conditions, whichis a temperature variation by heat transfer analysis [7]. Thecommercial code ANSYS 12 was used for the nonlinear heattransfer analysis and stress analysis. Figure 4 shows 3D FEMmeshedmodel of piping. Element typewas selected for 70, 3Dthermal solid model and 45, 3D structural solid model. Thenumber of node was 14904, and the number of elements was12558.
2.4. For Evaluating Residual Stress Test. We used the instru-mented indentation technique to evaluate the residual stressvalues in the GTAW area (see Figure 5). The measuringpositions of residual stress, as seen in Figure 5, are 5, 10, and30mm spot from the welding center line.
3. Results and Discussion
3.1. Fabrication of the SCC. The stress corrosion crack wasfabricated using the custom-made manufacturing system.Figure 6 shows the temperature and pressure variation duringthe test. The maximum temperature and pressure, whichwere measured at 358∘C and 157 bar by thermocouple andpressure sensor, were similar environmental condition inthe nuclear power plant. It was confirmed that the newforming equipment for artificial stress corrosion cracks couldbe simulated by the environmental conditions in theNPP.Thevapor pressure was decreased after about 5 hours.
It was confirmed that the pipe was fractured by stresscorrosion crack. This means that the crack was alreadyinitiated before leaking. The vapor pressure induced hoopstress in the pipe. Circumferential stress in a cylindricalshaped part as a result of internal or external pressure is
called hoop stress. In a closed pipe such as our specimen,force applied to the cylindrical pipe wall by a rising innerpressure will ultimately induce hoop stress. During the test,maximum vapor pressure was 157 bar; that is, hoop stressof the inner surface was more than 87% of yield stress.It is estimated that stress corrosion crack was acceleratedby additional hoop stress as well as susceptible material,corrosive environment, and including residual stress. Many
Figure 4: Meshed model of piping.
Weldingcenter line
1 23
1
2
3
5mm10mm30mm
from welding center line
Figure 5: Evaluation of residual stress using the instrumentedindentation technique.
cracks in the inner surface (a top and bottom view) of thepipe were observed by the visual test. Figure 7 shows theouter shape of the specimen and cross section (×12.5) of theinner surface pipe at the cutting 1. Figure 8 is the opticalmicroscope fractography of points (a), (b), (c), and (d) atcutting 1. It is confirmed that the cracks were propagatedalong the grain boundary. The intergranular stress corrosioncracks were clearly revealed, which was a typical characterfor primary water stress corrosion crack in major pipes ofpressurized water reactor.
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0 10000 20000 30000 40000 50000
−100
−50
0
50
100
150
200
250
300
350
400
Time (s)
Vapo
r tem
pera
ture
(∘C)
and
pres
sure
(bar
)
Vapor temperature (∘C)Vapor pressure (bar)
Max pressure: 157bar
4hr 54min
Figure 6: Temperature and pressure in STS 304 pipe.
(A)
(B)400𝜇m
Figure 7: Outer shape of the specimen and cross section.
3.2. The Results of FEM Analysis. Because the melting pointof STS 304was about 1400∘C, the bead temperature of the firstwelding pass was decided as 1400∘C and it was assumed thatthe temperature would be increased by additional weldingpass. The heating time was the same as welding time. Thecooling time was decided as 10min in the FEM analysis,because the experimental cooling time was 10min. The heatinput of the pipe (volume energy per hour) was modifiedby a temperature of the HAZ (Heat Affected Zone). Thetemperature variation calculated by thermal transfer analysiswas used as boundary condition for analysis of the residual
stress in the pipe. Figure 9 shows a temperature gradient ofthe specimen after first welding pass.
Analysis was bilinear kinematic hardening method usingVon Mises or hill plasticity. Figure 10 shows thermal proper-ties of bilinear kinematic hardening for STS 304.
Figure 11 shows the results of circumferential (hoop)maximum tensile residual stress in the inner surface of thepipe. The residual stress is the highest at 90∼100 degreeposition which was well corresponded to position of crackappearance. The tensile residual stress of the FEM analysisconfirmed that the number W1 specimen is the lowest, and
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50𝜇m 50𝜇m
50𝜇m50𝜇m
(a) (b)
(c) (d)
Figure 8: Fractography of points (a), (b), (c), and (d) at cutting 1.
151.063
210.426
269.789
329.152
388.515
447.878
507.241
566.604
625.967
685.330
Ther
mal
stre
ss re
sidua
l
Figure 9: Temperature gradient of specimen after first welding pass.
the others are of similar level. So, we selected and evaluated aninstrumented indentation test for the specimens of numberW1 and number W3.
3.3.The Effect of the Residual Stress. Figure 12 shows the resultof residual stress for W1 and W3. The maximum position ofthe residual stress is about 15mm from welding center line.The residual stress value of W3 was about 20MPa higherthan that of W1. This means that increasing of welding passhas made a residual stress higher than the first welding pass.Table 5 shows relationship between number of welding passand the fracture time of pipe. All the experimental conditions
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5
0
400
800
1200
1600
2000
2400
2800
3200
3600
4000
×10∗∗−3EPS
×10∗∗5
SIG
T1 = 30.000
T2 = 200.00
T3 = 600.00
T4 = 1000.0
T5 = 1400.0
Figure 10: Bilinear kinematic hardening table for STS 304.
such as corrosion environment, heating temperature, andwelding condition are the same. So it can be considered thatdifference of through-wall fracture time is effects of residualstress.The fracture time of the specimenW5 that was weldedby 15 pass was relatively shorter than other conditions. Andit is well corresponded to the highest tensile residual stress atthe 90∼100 degree position in Figure 11.
Figure 13 shows a shape and position of artificial stresscorrosion crack using the SCC forming device. Even though
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0 40 80 120 160 200
−30
0
30
60
Resid
ual s
tress
es (M
Pa)
Degree from pipe bottom (0∘) to pipe top (180∘)Specimen number W1
Specimen number W2
Specimen number W3
Specimen number W4
Specimen number W5
Figure 11: Hoop residual stress distribution.
0 5 10 15 20 25 30 35
Distance (mm)
−20
0
20
40
60
80
Resid
ual s
tress
(MPa
)
1 pass residual stress10 pass residual stress
Figure 12: Residual stress graph on inner surface of STS304 pipe.
Table 5: The fracture time of STS 304 pipe.
Specimen number Pass number The fracturetime (min) Inner pressure
the SCC was initiated at a random position, it was confirmedthat SCC was affected by a residual stress in the sameenvironment.
W2
W3
W3, W4
W5 (60∘)W1 (70∘)
W2
(90∘)
180∘
270∘
90∘
0∘
Figure 13: Artificial SCC shape and position according to experi-mental conditions.
4. Conclusions
(1) The custom-made device was used, which simulatedan environment condition of NPP (nuclear powerplant), for fabricating the stress corrosion crack inthe inner pipe wall. And stress corrosion crack wasartificially produced on STS 304 pipe by a weldingresidual stress.
(2) The residual stress was increased by increasing weld-ing pass. The fracture time of the pipe by stress cor-rosion cracking was decreased by increasing residualstress according to welding heat input. Even thoughthe SCC was initiated at a random position, it wasconfirmed that SCC was affected by a residual stressunder the same environment.
(3) The new system was developed for manufacturingstress corrosion crack in the inner surface of the pipespecimen.And it is possible tomanufacturemass SCCproduction of real size pipes in our system.
Conflict of Interests
The authors declare that there is no conflict of interestsregarding the publication of this paper.
Acknowledgments
This work was supported by the Korea Radiation SafetyFoundation (KORSAFE) funded by National Safety andSecurity Commission (NSSC). Test equipmentwas supportedby the National Research Foundation (NRF).
References
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