Vedyn ja heliumin vaikutus matala-aktiivisiin Fe-Cr ferriittis-martensiittisiin ja ODS teräksiin

Significant amounts of hydrogen and helium are generated in the structural materials of the nuclear reactor systems by the interaction of the alloying elements with both fast and thermal neutrons. Hydrogen can also be effectively absorbed by other environmental processes. Helium and hydrogen stabilize the small vacancy clusters and facilitate the further formation of the voids that causes the swelling of the structural steels. At the same time, hydrogen plays an important role in degradation of the mechanical properties of the structural steels due to the hydrogen embrittlement (HE). In the thesis, the deleterious effects of hydrogen and helium are studied on the reduced activation ferrite-martensite (RAFM) and oxide dispersion strengthened (ODS) RAFM steels, which are promising materials for Gen IV nuclear reactor systems. Hydrogen and helium uptake in RAFM and ODS-RAFM steels are studied by thermal desorption spectroscopy (TDS) evidencing the higher hydrogen and helium concentration in ODS-RAFM steel after hydrogen charging and helium irradiation, respectively, than that observed in the base RAFM steel. Activation analyses for the hydrogen and helium desorption are performed using the obtained TDS curves. Possible role of the dispersoid phase of yttrium oxide nanoparticles in hydrogen and helium trapping is discussed. Mechanical properties of the RAFM and ODS-RAFM steels are studied in terms of their sensitivity to hydrogen embrittlement after electrochemical hydrogen charging at room temperature (RT). The obtained results evidence that there is a critical hydrogen concentration above which the ODS steel suffers from hydrogen embrittlement in form of intergranular fracture. The instrument for hydrogen charging from glow discharge plasma is developed and the suitable conditions of hydrogen charging were obtained experimentally. The mechanical properties of RAFM and ODS-RAFM steels are studied during continuous hydrogen charging using the developed hydrogen charging process instrument at room and elevated temperatures. Sensitivity to HE of the ODS steel is found to be less pronounced at elevated temperatures compared with that at RT, while the susceptibility to hydrogen of the matrix material remains approximately the same at all the testing temperatures. Hydrogen-induced cracking in the studied steels initiates preferably from non-metallic inclusions (NMI) such as chromium and tungsten carbide particles. Possible mechanism of hydrogen interaction with NMIs is discussed.Ydinreaktorin rakennemateriaaleihin seosaineiden ja nopeiden sekä termisten neutronien vuorovaikutuksessa muodostuu huomattavia määriä vetyä ja heliumia. Vety voi absorboitua tehokkaasti materiaaleihin myös muissa ympäristövaikutteisissa prosesseissa. Helium ja vety stabiloivat pieniä vakanssiryhmiä ja helpottavat tyhjän tilan muodostumista, joka aikaansaa teräksen turpoamista. Teräkseen absorboitunut vety johtaa mekaanisten ominaisuuksien heikkenemiseen vetyhaurauden seurauksena. Väitöskirjassa on tutkittu vedyn ja heliumin haitallisia vaikutuksia matala-aktiivisissa Fe-Cr ferriittis-martensiittisissa (RAFM) ja ODS RAFM teräksissä, jotka ovat lupaavia materiaaleja Gen IV ydinreaktorin rakennemateriaaleiksi. Vedyn ja heliumin loukkuuntumista RAFM ja ODS-RAFM teräksissä tutkittiin termisen desorptio spektroskopian (TDS) avulla. ODS-RAFM terästen vety- ja helium pitoisuudet vedyn sähkökemiallisen latauksen ja helium säteilytyksen jälkeen ovat korkeammat RAFM teräksiin verrattuna. Vedyn ja heliumin desorption aktivaatioanalyysi on tehty mitatuille TDS käyrille. Yttriumoksidi nanopartikkelien dispersion vaikutusta vedyn ja heliumin loukkuuntumiseen arvioidaan tulosten perusteella. Vetyhaurauden vaikutus RAFM ja ODS-RAFM terästen mekaanisiin ominaisuuksiin määritettiin sähkökemiallisen vetylatauksen jälkeen huoneenlämpötilassa. Tulokset osoittavat, että on olemassa kriittinen vetypitoisuus, jonka ylittäminen johtaa vedyn aiheuttamaan raerajamurtumaan. Tutkimuksessa suunniteltiin ja rakennettiin vedyn lataukseen plasmasta soveltuva erikoislaitteisto, jonka avulla voidaan aikaansaada kontrolloitu vedyn absorptio teräksiin myös korkeissa lämpötiloissa. Sopivat vetylatauksen parametrit määritettiin kokeellisesti. RAFM ja ODS-RAFM terästen mekaaniset ominaisuudet testattiin vetylatauksen aikana huoneenlämpötilassa ja korotetuissa lämpötiloissa in-situ kokeissa tällä laitteistolla. ODS teräkset ovat alttiimpia vedyn vaikutukselle huoneenlämpötilassa, mutta perusmateriaalin alttius vedyn vaikutukseen pysyy samana kaikissa tutkituissa lämpötiloissa. Vedyn aiheuttama murtuma ydintyy yleensä epämetallisista kromi- ja volframikarbidi partikkeleista. Tutkimuksessa arvioidaan vedyn ja epämetallisten partikkelien vuorovaikutusmekanismeja

Role of retained austenite in hydrogen trapping and hydrogen-assisted fatigue fracture of high-strength steels

The Interaction of hydrogen with retained austenite under fatigue loading of dual-phase and complex-phase high-strength steels with strength of about 1200 MPa was studied. A load-controlled fatigue test was performed in the air with the maximum applied tensile strength of 900 MPa, which is just below the offset yield point of the studied steels. The trapping of hydrogen accumulated into the studied steels under the fatigue loading was studied by thermal desorption spectroscopy. Measurements of hydrogen trapping evolution and microstructure changes during fatigue testing reveal a complicated hydrogen trapping behavior driven by hydrogen interaction with deformation defects and retained austenite. Hydrogen concentration increases in the studied steels during the fatigue testing in the air without preceding hydrogen charging. The fracture surfaces were studied by scanning electron microscopy evidencing the relationship between the hydrogen concentration increase related to retained austenite and initiation of the intergranular fatigue fracture. The role of retained austenite in hydrogen-assisted fatigue cracking is discussed and a possible mechanism of the hydrogen-assisted fatigue crack initiation in the high-strength steels is proposed.Peer reviewe

Evaluation of steels susceptibility to hydrogen embrittlement

A novel approach has been developed for quantitative evaluation of the susceptibility of steels and alloys to hydrogen embrittlement. The approach uses a combination of hydrogen thermal desorption spectroscopy (TDS) analysis with recent advances in machine learning technology to develop a regression artificial neural network (ANN) model predicting hydrogen-induced degradation of mechanical properties of steels. We describe the thermal desorption data processing, artificial neural network architecture development, and the learning process beneficial for the accuracy of the developed artificial neural network model. A data augmentation procedure was proposed to increase the diversity of the input data and improve the generalization of the model. The study of the relationship between thermal desorption spectroscopy data and the mechanical properties of steel evidences a strong correlation of their corresponding parameters. A prototype software application based on the developed model is introduced and is openly available. The developed prototype based on TDS analysis coupled with ANN is shown to be a valuable engineering tool for steel characterization and quantitative prediction of the degradation of steel properties caused by hydrogen.Peer reviewe

Study of correlation between the steels susceptibility to hydrogen embrittlement and hydrogen thermal desorption spectroscopy using artificial neural network

Steels are the most used structural material in the world, and hydrogen content and localization within the microstructure play an important role in its properties, namely inducing some level of embrittlement. The characterization of the steels susceptibility to hydrogen embrittlement (HE) is a complex task requiring always a broad and multidisciplinary approach. The target of the present work is to introduce the artificial neural network (ANN) computing system to predict the hydrogen-induced mechanical properties degradation using the hydrogen thermal desorption spectroscopy (TDS) data of the studied steel. Hydrogen sensitivity parameter (HSP) calculated from the reduction of elongation to fracture caused by hydrogen was linked to the corresponding hydrogen thermal desorption spectra measured for austenitic, ferritic, and ferritic-martensitic steel grades. Correlation between the TDS input data and HSP output data was studied using two ANN models. A correlation of 98% was obtained between the experimentally measured HSP values and HSP values predicted using the developed densely connected layers ANN model. The performance of the developed ANN models is good even for never-before-seen steels. The ANN-coupled system based on the TDS is a powerful tool in steels characterization especially in the analysis of the steels susceptibility to HE.Peer reviewe

Influence of microstructural deformation mechanisms and shear strain localisations on small fatigue crack growth in ferritic stainless steel

Microstructurally small fatigue crack growth (FCG) rate in body-centred cubic (BCC) ferritic stainless steel is investigated by using a novel domain misorientation approach for EBSD microstructural deformation analyses, in conjunction with in situ digital imaging correlation (DIC). The DIC analyses revealed that shear strain local- isations occur ahead of the crack tip during propagation and correlate well with the FCG rate retardations. Grain boundaries can be found at both peaks and valleys of the FCG rate curve and alter the interaction between crack growth and shear strain localisations. At the microstructural level, the deformation is associated with the dislocation-mediated plastic deformation process, showing increased formation of grain sub-structures in the regions of the strain localisation. Consequently, material experiences local hardening causing the FCG retarda- tion events. If the crack avoids the hardened material region through a macroscopic cross-slip mechanism, retardation is minor. On the contrary, if the crack penetrates the hardened region, retardation is significant.Peer reviewe

Prediction of hydrogen concentration responsible for hydrogen-induced mechanical failure in martensitic high-strength steels

Funding Information: This researcher was supported by the Public Research Networked with Companies (Co-Innovation) program of Business Finland via the projects 7743/31/2018 (ISA Aalto-HydroSafeSteels) and 7537/31/2018 (ISA-Intelligent Steel Applications). Publisher Copyright: © 2022 The Author(s)Hydrogen, at critical concentrations, responsible for hydrogen-induced mechanical property degradation cannot yet be estimated beforehand and can only be measured experimentally upon fracture with specific specimen sizes. In this work, we develop two deep learning artificial neural network (ANN) models with the ability to predict hydrogen concentration responsible for early mechanical failure in martensitic ultra-high-strength steels. This family of steels is represented by four different steels encompassing different chemical compositions and heat treatments. The mechanical properties of these steels with varying size and morphology of prior austenitic grains in as-supplied state and after hydrogen-induced failure together with their corresponding hydrogen charging conditions were used as inputs. The feed forward back propagation models with network topologies of 12-7-5-3-2-1 (I) and 14-7-5-3-2-1 (II) were validated and tested with unfamiliar data inputs. The models I and II show good hydrogen concentration prediction capabilities with mean absolute errors of 0.28, and 0.33 wt.ppm at test datasets, respectively. A linear correlation of 80% and 77%, between the experimentally measured and ANN predicted hydrogen concentrations, was obtained for Model I and II respectively. This shows that for this family of steels, the estimation of hydrogen concentration versus property degradation is a feasible approach for material safety analysis.Peer reviewe

Full-field Strain Measurements for Microstructurally Small Fatigue Crack Propagation Using Digital Image Correlation Method

A novel measurement approach is used to reveal the cumulative deformation field at a sub-grain level and to study the influence of microstructure on the growth of microstructurally small fatigue cracks. The proposed strain field analysis methodology is based on the use of a unique pattering technique with a characteristic speckle size of approximately 10 µm. The developed methodology is applied to study the small fatigue crack behavior in body centered cubic (bcc) Fe-Cr ferritic stainless steel with a relatively large grain size allowing a high spatial measurement accuracy at the sub-grain level. This methodology allows the measurement of small fatigue crack growth retardation events and associated intermittent shear strain localization zones ahead of the crack tip. In addition, this can be correlated with the grain orientation and size. Thus, the developed methodology can provide a deeper fundamental understanding of the small fatigue crack growth behavior, required for the development of robust theoretical models for the small fatigue crack propagation in polycrystalline materials.Peer reviewe

Improved accuracy of thermal desorption spectroscopy by specimen cooling during measurement of hydrogen concentration in a high-strength steel

Thermal desorption spectroscopy (TDS) is a powerful method for the measurement of hydrogen concentration in metallic materials. However, hydrogen loss from metallic samples during the preparation of the measurement poses a challenge to the accuracy of the results, especially in materials with high diffusivity of hydrogen, like ferritic and ferritic-martensitic steels. In the present paper, the effect of specimen cooling during the experimental procedure, as a tentative to reduce the loss of hydrogen during air-lock vacuum pumping for one high-strength steel of 1400 MPa, is evaluated. The results show, at room temperature, the presence of a continuous outward hydrogen flux accompanied with the redistribution of hydrogen within the measured steel during its exposure to the air-lock vacuum chamber under continuous pumping. Cooling of the steel samples to 213 K during pumping in the air-lock vacuum chamber before TDS measurement results in an increase in the measured total hydrogen concentration at about 14%. A significant reduction in hydrogen loss and redistribution within the steel sample improves the accuracy of hydrogen concentration measurement and trapping analysis in ferritic and martensitic steels.Peer reviewe