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Stanovení vlivu geometrie vzorku na rozvoj creepové deformace pomocí metody akustické emise
Dvořák, Jiří ; Sklenička, Václav ; Král, Petr ; Kvapilová, Marie ; Svobodová, M. ; Šifner, J. ; Koula, V.
Tato práce se zabývá stanovením vlivu geometrie tělesa na vývoj porušení v průběhu creepové expozice. Creepové testy byly provedeny při teplotě 600 °C a napětí 180 MPa v režimu ochranné atmosféry argonu. Pro zkušební testy byl zvoleny vzorky z oceli P92 s konvekčním hladkým povrchem a vzorky opatřené V-vrubem. Výsledky creepové odolnosti a lomového chování byly doplněny podrobnou mikrostrukturní charakterizací k posouzení iniciace a vývoje creepového poškození. Na základě naměřených výsledků byla creepová data vyhodnocena pomocí neuronových sítí, které systematicky vyhledávají a zpracovávají signál z akustické emise (AE). Akustická emise byla použita jako účinná nedestruktivní metoda pro včasnou predikci počátku možné havárie komponent energetických a chemických zařízení.
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Creepové zkoušky zirkoniových povlakových tyčí jaderného paliva a jejich interpretace
Sklenička, Václav ; Kuchařová, Květa ; Kloc, Luboš ; Dvořák, Jiří ; Král, Petr ; Kvapilová, Marie ; Vrtílková, V. ; Krejčí, J.
Časově závislá plastická deformace-creep je jedním z nejdůležitějších degradačních faktorů, určujících životnost zirkoniových slitin, určených k pokrytí jaderného paliva vodou chlazených reaktorů v jaderné energetice. Příspěvek je věnován creepovým zkouškám povlakových tyčí slitiny Zr1%Nb (modifikace slitiny E110) používaných v reaktorech typu VVER. Vedle standardní creepové zkoušky při konstantním tahovém napětí bude demonstrována nestandardní speciální creepová zkušební metoda umožňující měření velmi malých creepových deformací při napětích blížících se reálným podmínkám namáhání v reálné praxi.
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Influence of severe plastic deformation and subsequent annealing on creep behaviour of martensitic 9% Cr steel
Král, Petr ; Dvořák, Jiří ; Sklenička, Václav ; Horita, Z. ; Tokizawa, Y. ; Tang, Y. ; Kunčická, Lenka ; Kuchařová, Květa ; Kvapilová, Marie ; Svobodová, M.
The objective of the study is to evaluate the effects of severe plastic deformation (SPD) and annealing on creep behaviour of advanced tungsten modified creep-resistant 9 % Cr martensitic P92 steel. The as-received P92 steel was deformed by high-pressure torsion (HTP), high-pressure sliding (HPS) and rotary swaging (RS) at room temperature prior creep testing. These SPD methods imposed significantly different equivalent plastic strain in the range from 1 up to 20. Constant load creep tests in tension were performed in an argon atmosphere at 873 K and applied stress ranging from 50 to 200 MPa. The microstructure and phase composition of P92 steel were studied using a scanning electron microscope Tescan Lyra 3 and a transmission electron microscope Jeol 2100F. The results show that under the same creep loading conditions the HPT and HPS-processed P92 steel exhibited significantly faster minimum creep rates, creep fracture strain and the decrease in the value of the stress exponent of the creep rate in comparison with as-received P92 steel. However, it was revealed that the RS-processed specimens exhibited one order of magnitude lower minimum creep rate and lower ductility compared to commercial P92 steel. The creep curves for the HPT and HPS-processed states exhibited a pronounced minimum of strain rate. The pronounced minimum of strain rate disappeared when these states were annealed at 923K/500h before application of creep loading. The microstructure changes occurring during creep and different creep behaviour between as-received and deformed states are discussed.
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DETEKCE PORUCH POMOCÍ AKUSTICKÉ EMISE (AE) V PODMÍNKÁCH VYSOKOTEPLOTNÍHO CREEPU
Dvořák, Jiří ; Sklenička, Václav ; Král, Petr ; Kvapilová, Marie ; Svobodová, M. ; Šifner, J. ; Koula, V.
Současné aplikace akustické emise (AE) jsou přednostně orientovány na průběžný monitoring provozu strojních komponent. AE je perspektivní nedestruktivní metodou pro včasnou predikci a identifikaci poruch materiálu dříve než dojde k havárii sledovaných zařízení. Příspěvek analyzuje naměřená data a rozbor signálů na úrovni jednotlivých emisních událostí v průběhu creepové zkoušky mědi na základě výsledků předchozí mikrostrukturní a fraktografické analýzy jejího creepového porušení a lomu. Cílem modelové studie je přispět k úspěšné aplikaci AE pro reálné provozní komponenty energetických zařízení a tím ke zvýšení jejich provozní bezpečnosti.
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HIGH - TEMPERATURE CREEP BEHAVIOUR OF CAST COBALT-BASE SUPERALLOYS
Dvořák, Jiří ; Král, Petr ; Kvapilová, Marie ; Hrbáček, Karel ; Sklenička, Václav
Two cast and heat-treated NbC and TaC – strengthened cobalt superalloys have been developed for a precision casting of spinner discs for glass wool industry. In this work constant load creep tests in tension were carried out in argon atmosphere at three testing temperature 900, 950 and 1000 °C and at the initial applied stresses ranged from 40 to 200 MPa. All the tests were continued until the final fracture. The results of creep testing were combined with microstructural and fractographic examinations by means of light and scanning electron microscopy. A mutual comparison of creep characteristics of the investigated superalloys under comparable creep loading conditions showed that NbC-strengthened superalloy exhibited longer creep life than TaC-strengthened one. Further, it was found that carbide precipitation is the primary strengthening mechanism in both cobalt-base superalloys under investigation and the amount, morphology\nand type of carbides have the decisive effect on the creep properties including creep damage and fracture processes. By contrast, NbC-superalloy exhibited a more brittle character of creep fracture mode than TaCstrengthened superalloy. This study was initiated to investigate in more details creep deformation processes and the effect of the creep microstructure and damage evolution on both investigated superalloys. The different behaviour and properties of studied superalloys were explained based on the received results of this study.
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Development of Creep Damage in Similar Weld Joints of P92 Steel Pipe
Král, Petr ; Sklenička, Václav ; Kuchařová, Květa ; Svobodová, M. ; Kvapilová, Marie ; Dvořák, Jiří
The microstructure and creep behaviour of the welded joints of P92 steel pipe were\ninvestigated in order to determine the influence of orbital heat welding technology on the creep\nresistance. Creep specimens were machined from the welded joints. Tensile creep tests of welded\njoints were performed at 873 K using different stresses. The microstructure of tested specimens was\ninvestigated by scanning electron microscope Tescan equipped with an electron-back scatter\ndiffraction. The creep results showed that the creep fracture strain of the welded joints decreases\nwith decreasing value of applied stress. Microstructure investigation showed that fracture behaviour\nof welded joints is influenced by an enhanced cavity formation at grain boundaries in the heataffected\nzone causing lower fracture ductility.
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Creep damage tolerance factor lambda of selected creep-resistant steels
Sklenička, Václav ; Kuchařová, Květa ; Dvořák, Jiří ; Kvapilová, Marie ; Král, Petr
The creep damage tolerance factor lambda as an important outcome of the continuum damage mechanics approach has been used to asses the creep fracture mode and the susceptibility of material to localized cracking at strain concentrations. In this work, using sets of our earlier published creep data of three advanced ferritic creep-resistant steels (T23 low alloy steel, P91 and P92 chromium steels) are analysed in terms of the creep damage tolerance factor lambda. It was found that the value of the creep damage factor lambda is not constant and depends on the creep loading conditions. The data analysis is followed by fractographic investigations, which is used to identify the creep fracture mode(s) experimentally.
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Microstructure Stability and Creep Behaviour of Alloys Processed by ECAP
Dvořák, Jiří ; Král, Petr ; Svoboda, Milan ; Kvapilová, Marie ; Sklenička, Václav
Coarse-grained binary Cu and Al alloys were processed by equal-channel angular pressing (ECAP) in order to evaluate the effect of severe plastic deformation on the thermal stability of resulting ultrafine-grained microstructure after ECAP followed by creep loading of the pressed alloys. ECAP was conducted at room temperature with a die that had an internal angle of 90° between the two parts of the channel. The subsequent extrusion passes were performed by route Bc. Microstructure of samples up to 12 ECAP passes was characterized by transmission electron microscope (TEM) and scanning electron microscope (SEM) equipped with the electron backscatter diffraction (EBSD) unit. Constant load creep tests in tension were performed at 0.3 - 0.5 Tm and at different applied stresses. It was found that the creep behaviour is influenced by high-angle grain boundaries created by ECAP and creep fracture occurred along the shear bands which are situated near shear plane of the last ECAP pass.
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Influence of Microstructure Instability on Creep Behaviour of UFG Pure Materials
Král, Petr ; Dvořák, Jiří ; Svoboda, Milan ; Kvapilová, Marie ; Sklenička, Václav
Experiments were conducted to determine an effect of creep temperature on creep behaviour of pure Cu. The ECAP pressing was performed at room temperature by route Bc. Constant load creep tests in tension were conducted at 373-573 K under different stresses. The values of the stress exponent n of the minimum creep rate for ultrafine-grained (UFG) and coarse-grained material were determined. Microstructure of samples was characterized by transmission electron microscope (TEM) and scanning electron microscope (SEM) equipped with the electron backscatter unit (EBSD). The microstructure analyses showed that microstructure of pure Cu processed by 8 ECAP passes and subsequent creep exposure contained large fraction of boundaries with coincidence sites lattice (CSL). The results showed that creep in UFG materials is influenced by additional creep mechanisms up to 0.5 Tm when the UFG microstructure in more or less stable.
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Influence of Heterogeneity of Microstructure on Creep Behaviour of SPD Metals
Král, Petr ; Dvořák, Jiří ; Kvapilová, Marie ; Svoboda, Milan ; Sklenička, Václav
The methods of severe plastic deformation enable a production of bulk metals with the submicrometer or even nanometer grain size. Experiments were conducted to determine an effect of equal-channel angular pressing (ECAP) on the creep behavior of pure metal. The ECAP deformation was performed at room temperature by route Bc. Constant load creep tests in tension were conducted at 373 K under different stresses. The values of the stress exponent n of the minimum creep rate for ultrafine-grained material were determined. Microstructure of samples was characterized by scanning electron microscope equipped with the electron backscatter diffraction unit. Heterogeneity of microstructure was evaluated by coefficient of variation CVa. It was found that the fracture behavior in UFG materials is influenced by GBS, formation of mesoscopic shear bands and cavitation.
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