| 1. |
- Bjurman, Martin, et al.
(författare)
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Fracture mechanical testing of in service thermally aged cast stainless steel
- 2016
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Ingår i: Fatigue and Fracture Test Planning, Test Data Acquisitions and Analysis. - : ASTM International. - 9780803176393 ; , s. 58-80
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Konferensbidrag (refereegranskat)abstract
- Embrittlement of Duplex Stainless Steels by thermal aging shortens the service life of structural components in Light Water Reactors (LWRs). This is an important issue when life extension programs are aiming at 60-80 years in service, as ductile failure is a design prerequisite. Cast and welded austenitic stainless steels, which contain some ferrite, are known to be affected by thermal aging. Historically, many LWR components of complex geometry have been cast in the Mo-containing quality CF8M. Aging is mainly attributed to two types of phase transformations occurring within the minor ferritic phase; Demixing of the ferrite by spinodal decomposition into Cr-rich a´ and Fe-rich a regions; and precipitation of G-phase, carbides and other secondary phases.The present program of two in-service aged pipe bend castings from the Pressurized Water Reactor (PWR) Ringhals 2 Steam Generator. These components are large castings of stainless steel quality CF8M. The manufacturing process produces a non-uniform microstructure with coarse ferrite and a high degree of directionality affecting properties as well as the methodology for testing.The materials were exposed to primary circuit PWR water for 72 kh at 291ºC and 325ºC, respectively, followed by 22 kh at a reduced service temperature.Fracture mechanical evaluation using the J-R technique at RT and 300ºC as well as instrumented Charpy-tests ranging from -196ºC to +400ºC are conducted. Effects of large microstructural heterogeneity and anisotropy from the casting and heat treating processes are tested and evaluated. The change of these parameters effect on aging embrittlement and fracture mechanisms within each phase as well as phase interaction are also studied.
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| 2. |
- Bjurman, Martin, et al.
(författare)
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Microstructural evolution of welded stainless steels on integrated effect of thermal aging and low flux irradiation
- 2019
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Ingår i: Minerals, Metals and Materials Series. - : Springer International Publishing. - 2367-1696 .- 2367-1181. ; Part F11, s. 703-710
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Konferensbidrag (refereegranskat)abstract
- The combined effect of thermal aging and irradiation on cast and welded stainless steel solidification structures is not sufficiently investigated. From theory and consecutive aging and irradiation experiments, the effect of simultaneous low rate irradiation and thermal aging is expected to accelerate and modify the aging processes of the ferrite phase. Here, a detailed analysis of long-term aged material at very low fast neutron flux at LWR operating temperatures using Atom Probe Tomography is presented. Samples of weld material from various positions in the core barrel of the Zorita PWR are examined. The welds have been exposed to 280–285 °C for 38 years at three different neutron fluxes between 1 × 10 −5 and 7 × 10 −7 dpa/h to a total dose of 0.15–2 dpa. The aging of the ferrite phase occurs by spinodal decomposition, clustering and precipitation of e.g. G-phase. These phenomena are characterized and quantitatively analyzed in order to understand the effect of flux in combination with thermal aging.
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| 3. |
- Bjurman, Martin, et al.
(författare)
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Microstructural evolution of welded stainless steels on integrated effect of thermal aging and low flux irradiation
- 2019
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Ingår i: Proceedings of the 18th International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors. - Cham : Springer International Publishing. ; , s. 1919-1926
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Konferensbidrag (refereegranskat)abstract
- The combined effect of thermal aging and irradiation on cast and welded stainless steel solidification structures is not sufficiently investigated. From theory and consecutive aging and irradiation experiments, the effect of simultaneous low rate irradiation and thermal aging is expected to accelerate and modify the aging processes of the ferrite phase. Here, a detailed analysis of long-term aged material at very low fast neutron flux at LWR operating temperatures using Atom Probe Tomography is presented. Samples of weld material from various positions in the core barrel of the Zorita PWR are examined. The welds have been exposed to 280–285 °C for 38 years at three different neutron fluxes between 1 × 10 −5 and 7 × 10 −7 dpa/h to a total dose of 0.15–2 dpa. The aging of the ferrite phase occurs by spinodal decomposition, clustering and precipitation of e.g. G-phase. These phenomena are characterized and quantitatively analyzed in order to understand the effect of flux in combination with thermal aging.
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| 4. |
- Bjurman, Martin, et al.
(författare)
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Phase separation study of in-service thermally aged cast stainless steel – atom probe tomography
- 2015
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Ingår i: International Conference on Environmental Degradation of Materials in Nuclear Power Systems – Water Reactors. - : Canadian Nuclear Society (CNS). - 9781510813953
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Konferensbidrag (refereegranskat)abstract
- Embrittlement of Duplex Stainless Steels by thermal aging shortens the service life of structural components in LWRs. This is an important issue when life extension programs are aiming at 60-80 years in service. Cast and welded austenitic stainless steels, which contain some ferrite, are known to be affected by thermal aging. Historically, many LWR components of complex geometry have been cast in the Mo-containing quality CF8M. Aging is attributed to two types ofphase transformations; Demixing of the ferrite by spinodal decomposition into Cr-rich ´ and Fe-rich regions; and precipitation of G-phase, carbides and other secondary phases.A study was conducted on two in-service aged large casting CF8M elbows exposed for 72 kh at 291ºC and 325ºC, respectively, followed by 22 kh at a reduced service temperature. Atom Probe Tomography was used to characterize the decomposition of the ferrite for both aging states. Spinodal decomposition and nucleation of precipitates, i.e. G-phase, have been identified. The extent of phase transformation increases with exposure temperature, and the mechanical properties follow the same trend.
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| 5. |
- Boåsen, Magnus, et al.
(författare)
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A generalized probabilistic model for cleavage fracture with a length scale - Influence of stress state and application to surface cracked experiments
- 2019
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Ingår i: Engineering Fracture Mechanics. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0013-7944 .- 1873-7315. ; 214, s. 590-608
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Rapport (övrigt vetenskapligt/konstnärligt)abstract
- A probabilistic model for the cumulative probability of failure by cleavage fracture with a material related length scale is further developed in this study. A new generalized effective stress measure is proposed, based on a normal stress decomposition of the stress tensor, capable of describing a state of normal stress in the range from the mean stress to the maximum principal stress. The effective stress measure associated with a material point is evaluated from the stress tensor averaged over the material related length scale. The model is shown to be well capable to predict both the fracture toughness at loss of both in-plane and out-of-plane constraint by model application to two different datasets from the open literature. The model is also shown to be well capable of predicting the probability of failure of discriminating experiments on specimens containing semi-elliptic surface cracks. A comparison where the master curve methodology is used to predict the probability of failure of the experiments is also included.
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| 6. |
- Boåsen, Magnus, 1991-
(författare)
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Modeling framework for ageing of low alloy steel
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Ageing of low alloy steel in nuclear applications commonly takes the form as a hardening and an embrittlement of the material. This is due to the evolution of the microstructure during irradiation and at purely thermal conditions, as a combination or separate. Irradiation introduces evenly distributed solute clusters, while thermal ageing has been shown to yield a more inhomogeneous distribution. These clusters affect the dislocation motion within the material and results in a hardening and in more severe cases of ageing, also a decreased work hardening slope due to plastic strain localization into bands/channels. Embrittlement corresponds to decreased fracture toughness due to microstructural changes resulting from ageing. The thesis presents a possible framework for modeling of ageing effects in low alloy steels.In Paper I, a strain gradient plasticity framework is applied in order to capture length scale effects. The constitutive length scale is assumed to be related to the dislocation mean free path and the changes this undergoes during plastic deformation. Several evolution laws for the length scale were developed and implemented in a FEM-code considering 2D plane strain. This was used to solve a test problem of pure bending in order to investigate the effects of the length scale evolution. As all length scale evolution laws considered in this study results in a decreasing length scale; this leads to a loss of non-locality which causes an overall softening at cases where the strain gradient is dominating the solution. The results are in tentative agreement with phenomena of strain localization that is occurring in highly irradiated materials.In Paper II, the scalar stress measure for cleavage fracture is developed and generalized, here called the effective normal stress measure. This is used in a non-local weakest link model which is applied to two datasets from the literature in order to study the effects of the effective normal stress measure, as well as new experiments considering four-point bending of specimens containing a semi-elliptical surface crack. The model is shown to reproduce the failure probability of all considered datasets, i.e. well capable of transferring toughness information between different geometries.
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| 7. |
- Efsing, Pål, 1965-, et al.
(författare)
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Swedish RPV Surveillance Programs
- 2018
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Ingår i: INTERNATIONAL REVIEW OF NUCLEAR REACTOR PRESSURE VESSEL SURVEILLANCE PROGRAMS. - : ASTM INTERNATIONAL. - 9780803176515 ; , s. 219-231
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Konferensbidrag (refereegranskat)abstract
- Because the reactors of the Swedish reactor program were erected over a limited period of time, there are significant similarities regarding used materials and manufacturing methods between the different units. Each individual plant is supplied with a plant-specific surveillance program that reflects the materials utilized in the belt-line area form the start of operation. The programs were originally based on U.S. Nuclear Regulatory Commission guidance and supported by ASTM codes and standards, and the reactors were originally constructed for an estimated operating time of 40 years. The programs have been updated to reflect the fact that current planning calls for up to 60 years of operation for several of the most recent plants. The surveillance programs are to be validated and accepted by the Swedish Radiation Safety Authority.
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| 8. |
- Huotilainen, C., et al.
(författare)
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Electrochemical investigation of in-service thermal aging in two CF8M cast stainless steels
- 2019
-
Ingår i: Journal of Nuclear Materials. - : Elsevier. - 0022-3115 .- 1873-4820. ; 520, s. 34-40
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Tidskriftsartikel (refereegranskat)abstract
- In-service thermal aging of CF8M cast austenitic stainless steel was investigated in materials removed from the steam generator inlet and crossover elbows of the Ringhals 2 pressurized water reactor nuclear power plant unit after approximately 92kh of full operating time. The thermal aging of these materials was investigated using the double loop electrochemical potentiokinetic reactivation method, coupled with indentation hardness measurements and microstructural characterizations, to identify correlations between the electrochemical behavior and traditional methods of investigating thermal aging embrittlement effects in cast stainless steels. While this electrochemical method can be easily employed to quantify thermal aging effects in materials aged at higher temperatures (e.g. greater than 350 degrees C), this study highlights the difficulties encountered when electrochemically evaluating the aging of materials exposed to nuclear power plant operating conditions.
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| 9. |
- Hyde, Jon, et al.
(författare)
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A sensitivity study using maximum entropy to interpret SANS data from the Ringhals Unit 3 NPP
- 2018
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Ingår i: Journal of Nuclear Materials. - : Elsevier. - 0022-3115 .- 1873-4820. ; 509, s. 417-424
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Tidskriftsartikel (refereegranskat)abstract
- SANS experiments were performed on a high Ni weld surveillance sample from the Ringhals NPP and the Maximum Entropy method was applied to determine the most probable size distribution of irradiation-induced scattering features. The results were shown to be consistent with atom probe observations. The sensitivity of the data analyses with respect to constraints such as the limited experimentally available Q range was explored. The calculated volume fraction and the mean volume-weighted diameter of the precipitates were found to be relatively insensitive to Qmax (the maximum scattering vector) greater than ∼0.40 Å−1. However, use of a lower Qmax results in a shift of the size distribution to larger diameters and a reduced particle number density. Simulations demonstrated that the experimentally observed decrease in the A-ratio at higher Q values is consistent with the presence of vacancies or higher Mn contents in smaller features. Importantly, features which are experimentally unresolvable do not add to the apparent volume fraction of the features which are resolved.
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| 10. |
- Konstantinović, Milan
(författare)
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Radiation induced solute clustering in high-Ni reactor pressure vessel steel
- 2019
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Ingår i: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 179, s. 183-189
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Tidskriftsartikel (refereegranskat)abstract
- The thermal stability and the structure of solute-vacancy clusters formed by neutron irradiation are studied by means of positron annihilation spectroscopy and hardness measurements of post-irradiation annealed reactor pressure vessel steels with high and low Ni contents. Two distinct recovery stages were observed and assigned to (a) the dissolution of vacancy clusters at about 650 K, and (b) the dissolution of solute-vacancy clusters at about 750 K. In steels with high Ni content, hardening mainly recovers during the second stage. Atomistic and coarse grain models suggest that during this stage, the removal of vacancies from vacancy-solute clusters leads to complete cluster dissolution, which indicates that solute clusters are radiation induced.
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| 11. |
- Lindgren, Kristina, 1989, et al.
(författare)
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Cluster formation in in-service thermally aged pressurizer welds
- 2018
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Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 504, s. 23-28
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Tidskriftsartikel (refereegranskat)abstract
- Thermal aging of reactor pressure vessel steel welds at elevated temperatures may affect the ductile-to-brittle transition temperature. In this study, unique weld material from a pressurizer, with a composition similar to that of the reactor pressure vessel, that has been in operation for 28 years at 345 °C is examined. Despite the relatively low temperature, the weld becomes hardened during operation. This is attributed to nanometre sized Cu-rich clusters, mainly located at Mo- and C-enriched dislocation lines and on boundaries. The welds have been characterized using atom probe tomography, and the characteristics of the precipitates/clusters is related to the hardness increase, giving the best agreement for the Russell-Brown model.
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| 12. |
- Lindgren, Kristina, 1989, et al.
(författare)
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Evolution of precipitation in reactor pressure vessel steel welds under neutron irradiation
- 2017
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Ingår i: Journal of Nuclear Materials. - : Elsevier. - 0022-3115 .- 1873-4820. ; 488, s. 222-230
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Tidskriftsartikel (refereegranskat)abstract
- Reactor pressure vessel steel welds are affected by irradiation during operation. The irradiation results in nanometre cluster formation, which in turn affects the mechanical properties of the material, e.g. the ductile-to-brittle transition temperature is shifted to higher levels. In this study, cluster formation is characterised in high Ni (1.58%) low Cu (0.04%) steel welds identical to Ringhals R4 welds, using atom probe tomography in both surveillance material and in material irradiated at accelerated dose rates. Clusters containing mainly Ni and Mn, but also some Si and Cu were observed in all of the irradiated materials. Their evolution did not change drastically during irradiation; the clusters grew and new clusters were nucleated. Hence, both the cluster number density and the average size increased with irradiation time. Some flux effects were observed when comparing the high flux material and the surveillance material. The surveillance material has a lower cluster number density, but larger clusters. The resulting impact on the mechanical properties of these two effects cancel out, resulting in a measured hardness that seems to be on the same trend as the high flux material. The dispersed barrier hardening model with an obstacle strength factor of 0.15 was found to reproduce the increase in hardness. In the investigated high flux materials, the clusters' Cu content was higher.
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| 13. |
- Lindgren, Kristina, 1989, et al.
(författare)
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On the Analysis of Clustering in an Irradiated Low Alloy Reactor Pressure Vessel Steel Weld
- 2017
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Ingår i: Microscopy and Microanalysis. - : Cambridge University Press. - 1435-8115 .- 1431-9276. ; 23:2, s. 376-384
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Tidskriftsartikel (refereegranskat)abstract
- Radiation induced clustering affects the mechanical properties, that is the ductile to brittle transition temperature (DBTT), of reactor pressure vessel (RPV) steel of nuclear power plants. The combination of low Cu and high Ni used in some RPV welds is known to further enhance the DBTT shift during long time operation. In this study, RPV weld samples containing 0.04 at% Cu and 1.6 at% Ni were irradiated to 2.0 and 6.4×10 23 n/m 2 in the Halden test reactor. Atom probe tomography (APT) was applied to study clustering of Ni, Mn, Si, and Cu. As the clusters are in the nanometer-range, APT is a very suitable technique for this type of study. From APT analyses information about size distribution, number density, and composition of the clusters can be obtained. However, the quantification of these attributes is not trivial. The maximum separation method (MSM) has been used to characterize the clusters and a detailed study about the influence of the choice of MSM cluster parameters, primarily on the cluster number density, has been undertaken.
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| 14. |
- Roudén, Jenny, et al.
(författare)
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Towards Safe Long-Term Operation of Reactor Pressure Vessels
- 2015
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Ingår i: ATW. Internationale Zeitschrift für Kernenergie. - : Nucmag.com. - 1431-5254. ; 60:5, s. 287-293
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Tidskriftsartikel (refereegranskat)abstract
- This publication summarizes the long term operation (LTO) conditions on European NPPs and provides recommendations on reactor pressure vessel (RPV) irradiation surveillance based on the work preformed in the work package 7 "Surveillance guidelines" of the LONGLIFE international project. The LONGLIFE project "treatment of long term irradiation embrittlement effects in RPV safety assessment" was 50% funded by the Euratom 7th framework programme of the European commision. The project coordinated by the Helmholtz-centrum Dresden Rossendorf successfully finalized in 2014.
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| 15. |
- Sedlak, Michal, et al.
(författare)
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A cohesive element with degradation controlled shape of the traction separation curve for simulating stress corrosion and irradiation cracking
- 2018
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Ingår i: Engineering Fracture Mechanics. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0013-7944 .- 1873-7315. ; 193, s. 172-196
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Tidskriftsartikel (refereegranskat)abstract
- A cohesive element with extended environmental degradation capability was developed and implemented into an Abaqus user element. The element uses a virgin and a fully degraded Traction Separation Law (TLS) as input. The use of the potential based PPR model enables flexibility in the softening shapes for both TSL. When the element is degraded, the TSL gradually goes from the shape of the virgin material to the fully degraded TSL shape. This transition was made with a new parameter. that can govern a more ductile or brittle crack growth behaviour at degradation. The effect on the plastic zone due to changing the softening shape is shown, where the convex shaped softening TSL gives higher plastic dissipation and larger plastic zones than the concave and more brittle TSL. The new degradation method was evaluated against a Hydrogen Embrittlement (HE) experiment showing improved agreement with the experiment compared to the literature. The effect of different susceptibility zones at the crack tip was also investigated, showing that a uniform degradation throughout the susceptible zone is more influenced by the. parameter than a triangular susceptible zone.
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| 16. |
- Sedlak, Michal, et al.
(författare)
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A coupled diffusion and cohesive zone model for intergranular stress corrosion cracking in 316L stainless steel exposed to cold work in primary water conditions
- 2019
-
Ingår i: Engineering Fracture Mechanics. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0013-7944 .- 1873-7315. ; 217
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Tidskriftsartikel (refereegranskat)abstract
- A multi-physics model was developed to simulate intergranular stress corrosion cracking (IGSCC) in austenitic stainless steel. The model is implicit, coupled with a segregated solution scheme including a diffusion equation based on Fick's second law and a cohesive zone description for the fracture mechanics part. The degradation is modelled with an anodic slip-dissolution equation that uses the effective cohesive traction and concentration as the main parameters. The diffusivity in Fick's second law creates a moving boundary. The cohesive zone is modelled using the PPR model with extended degradation properties using the degradation parameter chi. The model was evaluated against experiments on the effects of cold work on IGSCC. The model showed good agreements for both shifting amount of cold work, illustrated by only changing the yield stress in the bulk material and for shifting the stress intensity factor. The model versatility was also shown by simulating IGSCC in Alloy 600, also with good agreements. The change in the bulk material made crack propagation more disadvantageous for the lower yield stress where the crack blunts, creates more plastic strain and lowers the cohesive traction. The model predicts that cold work of the bulk material creates a faster crack growth velocity due to lower amount of plastic deformation in the bulk and higher cohesive traction. The higher crack growth rate is a coupled effect of both fracture and oxidation properties.
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| 17. |
- Sedlak, Michal, et al.
(författare)
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Modelling of IG-SCC mechanism at LWR conditions through coupling of a potential-based cohesive model and Fick’s second law
- 2015
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Ingår i: International Conference on Environmental Degradation of Materials in Nuclear Power Systems. - : Canadian Nuclear Society.
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Konferensbidrag (refereegranskat)abstract
- A fracture mechanic and diffusion model was coupled to simulate the behavior of Intergranular Stress Corrosion Cracking (IG-SCC). To ensure correct physical behavior some assumptions were made, the ion travel, the non-reversible adsorption, the oxide growth dependencies and the diffusion dependency on damage. The model was implemented in a user subroutine in ABAQUS using a cohesive element formulation and an extra adsorption term in Fick’s second law. The coupling was achieved by assuming proportionality between the total adsorption and fracture energy. The physical assumtion were verified on a DCB model.
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| 18. |
- Sedlak Mosesson, Michal, et al.
(författare)
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Modelling of IGSCC mechanism trough coupling of a potetial-based cohesive model and Fick's second law
- 2017
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Ingår i: ICF 2017 - 14th International Conference on Fracture. - : International Conference on Fracture. - 9780000000002 ; , s. 689-690
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Konferensbidrag (refereegranskat)abstract
- A model has been developed to predict crack growth velocities at IGSCC for varied stress and ion concentration. The model is a coupling between Fick's second law and a newly developed cohesive element with degradation of damage resisting properties, implemented into a user element in ABAQUS. High stresses at the crack tip are assumed to drive the corrosion process and change the diffusivity. The stress and ion concentrations are varied which shows that higher stresses or higher ion concentrations gives different oxide thicknesses and higher crack propagation velocities.
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| 19. |
- Shen, Rickard, et al.
(författare)
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INVESTIGATION OF THE RELATIONSHIP BETWEEN LOCAL PLASTIC STRAIN ESTIMATED BY EBSD AND LOCAL NANOINDENTATION HARDNESS IN ALLOY 690
- 2015
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Ingår i: International Conference on Environmental Degradation of Materials in Nuclear Power Systems. - : Canadian Nuclear Society.
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Konferensbidrag (refereegranskat)abstract
- Plastic strain distribution in Alloy 690 has been of interest since laboratory experiments showed that cold deformation may trigger susceptibility to stress corrosion cracking. In operating plants, the plastic strains in Alloy 690 generally originate from manufacturing processes, e.g. grinding, tube straightening or welding. In recent years, the plastic strains from such operations have typically been mapped using electron backscatter diffraction. This method quantifies curvature of the crystal lattice, which has been shown to correlate with plastic strain on both the macroscopic and the mesoscopic levels, and has a high enough spatial resolution to potentially show the plastic strain distribution within individual grains. In this work, the correlation between local estimated plastic strains and nanoindentation hardness has been investigated. Local estimated plastic strains were able to predict the spatial distribution of local increases and decreases in hardness, but vastly overestimated the magnitude of variation. It is believed that the calibration curve used to estimate macroscopic plastic strain from macroscopic average misorientations overestimates local plastic strains where local misorientations are high, and underestimates the strains where the local misorientations are low. A calibration curve based on local strain measurements and local misorientations could possibly be a suitable alternative.
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| 20. |
- Shen, Rickard, 1985-, et al.
(författare)
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Microstructural study of Alloy 690 base metal and HAZ from mockup components – Influence of Ti(C,N) banding
- 2015
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Ingår i: Fontevraud 8 - Contribution of Materials Investigations and Operating Experience to LWRs’ Safety, Performance and Reliability. - : International Atomic Energy Agency.
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Konferensbidrag (refereegranskat)abstract
- The Alloy 690 base metal and heat affected zones (HAZs) of three component mockups have been studied using light optical microscopy and scanning electron microscopy. All mockups were manufactured as part of the production of replacement components using commercial heats. Welding and post weld heat treatments were performed in the same way as for components in field in order to obtain microstructures representative to the ones in operating plants.All three Alloy 690 base metals exhibited Ti(C,N) banding, but to a different extent. In all cases, the bands extended through the material’s longitudinal direction. In two of these materials the Ti(C,N) bands were correlated with M23C6 carbide coarsening, fine grain banding and reduced grain boundary tortuosity in the longitudinal direction.Full carbide dissolution was observed near the fusion line. The coarser carbides in the banded regions were overall less affected, but were also fully dissolved when close enough to the fusion line. The region affected by carbide dissolution spanned roughly 300‑1000 μm wide depending on mockup. The Ti(C,N) precipitates however appeared unaffected. The lack of carbides near the fusion line, where the weld induced strains typically are highest, suggest that inducing cold work after a solution anneal may produce a material more representative of the HAZ than cold working the material in the thermally treated state.
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| 21. |
- Shen, Rickard Ruici, 1985-, et al.
(författare)
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Overcoming the drawbacks of plastic strain estimation based on KAM
- 2018
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Ingår i: Ultramicroscopy. - : Elsevier B.V.. - 0304-3991 .- 1879-2723. ; 184, s. 156-163
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Tidskriftsartikel (refereegranskat)abstract
- Plastic strain estimation using electron backscatter diffraction (EBSD) based on kernel average misorientation (KAM) is affected by random orientation measurement error, EBSD step length, choice of kernel and average grain size. These sensitivities complicate reproducibility of results between labs, but it is shown in this work how these drawbacks can be overcome. The modifications to KAM were verified against a similar misorientation metric based on grain orientation spread (GOS), which does not show sensitivity to these factors. Both metrics were used in parallel to estimate the plastic strain distribution in Alloy 690 heat affected zones from component mockups, and showed the same results where the grain size was correctly compensated for.
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| 22. |
- Shen, Rickard, 1985-, et al.
(författare)
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Spatial correlation between local misorientations and nanoindentation hardness in nickel-base alloy 690
- 2016
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Ingår i: Journal of Materials Science and Engineering. - : Elsevier. - 2161-6213 .- 0921-5093. ; 674, s. 171-177
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Tidskriftsartikel (refereegranskat)abstract
- Misorientation increases with plastic strain in metals, and this observation has been used as an empirical assessment of plastic strain in recent years. The method has been validated for a sample area corresponding to a 100 µm×100 µm square, but on the micrometer scale misorientations no longer seem to correlate with plastic strain. Misorientations are however not dependent on plastic strain but rather on dislocation density, which means it should also be related to hardness. Therefore, we have in this work compared maps of predicted hardness calculated from misorientation determination with maps of actual hardness measured by nanoindentation. It was shown that the predicted and measured hardness maps do indeed correlate spatially in nickel-base Alloy 690, although the measured values have a significantly smaller hardness variation. This is explained by a presumably high and uniform density of statistically stored dislocations, which contribute to hardness but do not affect the misorientation determination from electron backscatter diffraction. Thus local misorientation can be used to qualitatively map the local effective plastic strain distribution, for example to identify regions of increased hardness.
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| 23. |
- Styman, Paul, et al.
(författare)
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Post-irradiation annealing of Ni-Mn-Si-enriched clusters in a neutron-irradiated RPV steel weld using Atom Probe Tomography
- 2015
-
Ingår i: Journal of Nuclear Materials. - : Elsevier BV. - 0022-3115 .- 1873-4820. ; 459, s. 127-134
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Tidskriftsartikel (refereegranskat)abstract
- Atom Probe Tomography has been performed on as-irradiated and post-irradiation annealed surveillance weld samples from Ringhals Unit 3. The weld contains low Cu (0.07at.%) and high Ni (1.5at.%). A high number density (~4x1023 m-3) of Ni-Mn-Si-enriched clusters was observed in the as-irradiated material. The onset of recovery was observed during the annealing for 30 min at 450°C. Much more significant dissolution of clusters occurred during the 10 minute 500°C anneal, resulting in a reduction in mean cluster size and a halving of their volume fraction. Detailed analyses of the changes in microstructure demonstrate that the dissolution process is driven by migration of Mn atoms from the clusters. This may indicate a strong correlation between Mn and point defects. Dissolution of the clusters is shown to correlate with recovery of mechanical properties in this material.
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