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Eingeladener Vortrag
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Das hochaktuelle Thema der Integration und Wiederverwendung von Wissen und Daten aus Herstellung, Bearbeitung und Charakterisierung von Materialien ('Digitalisierung von Materialien') wird in den Projekten Innovationsplattform MaterialDigital (PMD, materialdigital.de) und Materials-open-Lab (Mat-o-Lab, matolab.org) adressiert. In diesem Beitrag werden die Weiterentwicklungen in diesen Projekten hinsichtlich der Speicherung von Zugversuchsdaten gemäß einer normenkonformen (DIN EN ISO 6892-1:2019-11) ontologischen Repräsentation vorgestellt. Das umfasst den Weg von der Entwicklung einer Ontologie nach Norm, der Konvertierung von Daten aus Standardtests in das interoperable RDF-Format bis hin zur Verknüpfung von Ontologie und Daten. Letztendlich können die entsprechenden Daten in einem Triple Store abgelegt und abgefragt werden.
Konventionelle Kraftwerke sind durch die Energiewende erheblichen Anforderungen ausgesetzt. Mangels ausreichender Stromspeicherkapazitäten müssen sie zur Netzstabilisierung flexibel auf die wetter- und tageszeitbedingte Fluktuation der Wind- und Sonnenergie reagieren, was gegenüber dem Grundlastbetrieb eine hohe Anzahl von Lasteingriffen oder Anfahrvorgängen bedeutet. Schnelle Lastwechsel und Anfahrvorgänge führen jedoch zu einer erheblichen Beanspruchung aller Bauteile des Dampfkreislaufs, die zu einer beschleunigten Bauteilschädigung und bis hin zu Anlagenausfällen führen können.
Vor diesem Hintergrund werden an der Bundesanstalt für Materialforschung und -prüfung (BAM) Untersuchungen zur Beständigkeit ferritisch-martensitischer Dampferzeuger-werkstoffe mit 9 12 Gew. % Cr (P91, P92, VM12 SHC) bei zyklischem Anlagenbetrieb durchgeführt. Ziel der Arbeiten ist, durch grundlegende Korrosions- und Mechanikversuche unter zyklischen Bedingungen und ergänzende Simulationen die Reaktion dieser Werkstoffgruppe auf lastflexible Betriebsprofile umfassend zu beschreiben. Aufbauend auf einer detaillierten Charakterisierung der auftretenden Schädigungsmechanismen werden in einem Folgeschritt Standardzyklen zur effizienten Prüfung einzelner Werkstoffe abgeleitet.
Der Beitrag gibt einen kurzen Überblick über die Konzeption und stellt nachfolgend die aktuellen Projektergebnisse vor. Im Bereich der Oxidations-/Korrosionsuntersuchungen steht dabei zunächst die Integrität der schützenden Oxidschichten, vor allem auf der Dampfseite der Komponenten, im Vordergrund. Hierzu werden zyklische Oxidationstests an Standardproben und bauteilnahen Probengeometrien durchgeführt und die Oxidationskinetik sowie die Entwicklung der Haftfestigkeit untersucht. Die mechanischen Untersuchungen bauen auf den bekannten Kriech- und Ermüdungseigenschaften auf. Der Schwerpunkt liegt deshalb auf der Untersuchung von Schädigung und Lebensdauer bei Kombination von stationären Zuständen (mit überwiegender Kriechschädigung) und transienten Zuständen inklusive Temperaturwechseln, die Kriechermüdungs- bzw. thermo-mechanische Ermüdungsprozesse auslösen und so eine frühzeitige Rissbildung bewirken können. Im Hinblick auf die Flexibilisierung von Bestandsanlagen werden betriebsbeanspruchte Chargen in die Untersuchungen einbezogen, um eventuelle zusätzliche Effekte durch Werkstoffalterung zu berücksichtigen.
Martensitic phase transformations were studied in ultrafine grained Ni-rich pseudoelastic NiTi wires during cyclic deformation under small imposed strain amplitudes. Small strain variation tests were complemented by a thermographic analysis of the emerging temperature distributions in the specimens. The characteristics of the observed thermal profiles result from the specific phase transitions which take place at different stress levels. Homogeneous temperature changes, corresponding to a non-localized transformation activity along the specimen length, were observed throughout the whole range of applied stresses, starting from values as low as 100 MPa. This behavior is in line with previous literature reports for the stress-induced transformation from B2 austenite to R-phase as long as the critical stress for B19' martensite formation is not reached. In the present study, similar type of transformation activity could also be demonstrated at higher strains/stresses, even after the stress induced transformation to B19' was apparently completed. These findings suggest that transformation activity involving the B2 phase is present throughout the whole pseudoelastic stressstrain cycle; i.e., it is not restricted to the initial loading portion. Finally, non-localized transformation to or from B19' was identified during small amplitude strain variations in the plateau-like coexistence ranges of the pseudoelastic cycle.
Results of an extended TMF test program on grade P92 steel in the temperature range of 620 °C - 300 °C, comprising in-phase (IP) and out-of-phase (OP) tests, partly performed with symmetric dwells at Tmax/Tmin, are presented. In contrast to previous studies, the low-strain regime is also illuminated, which approaches flexible operation in a power plant with start/stop cycles. At all strain amplitudes, the material performance is characterized by continuous cyclic softening, which is retarded in tests at lower strains but reaches similar magnitudes in the course of testing. In the investigated temperature range, the phase angle does not affect fatigue life in continuous experiments, whereas the IP condition is more detrimental in tests with dwells. Fractographic analyses indicate creep-dominated and fatigue-dominated damage for IP and OP, respectively. Analyses of the (micro)hardness distribution in the tested specimens suggest an enhanced microstructural softening in tests with dwell times for the low- but not for the high-strain regime. To rationalize the obtained fatigue data, the fracture-mechanics-based D_TMF concept, which was developed for TMF life assessment of ductile alloys, was applied. It is found that the D_TMF parameter correlates well with the measured fatigue lives, suggesting that subcritical growth of cracks (with sizes from a few microns to a few millimeters) governs failure in the investigated range of strain amplitudes.
Temperature-induced, stress-induced martensitic phase transitions and martensite reorientation process in Ni rich (50.9 at.%) NiTi pseudoelastic NiTi wires with ultra-fine grained (UFG) microstructure were studied by electrical resistance measurements. Measurements of the electrical resistance as a function of temperature at different constant mechanical loads accompanied by complementary experiments with variable loads at constant temperature were performed. Results show that the transformation mechanisms in UFG microstructures exhibit a higher level of complexity when compared with those characterizing the behavior of other microstructures (e.g., recrystallized or larger grains size). It was found that a threshold stress level below 150 MPa delimits the transition from a homogeneous (low stress) to localized but reversible Lüders type transformation (high stress) when the transformations are induced under constant applied stress and that reorientation processes require stresses of 100 MPa in the present UFG wires. Even though the strain evolutions do not always show two distinct yielding events during cooling or heating, electrical resistance measurements proved that a two-step transformation involving R-phase and B19' martensite was always present in the extended range of temperatures and stresses investigated here.
The use of miniaturized specimen geometries in mechanical testing allows extracting the sample material directly from critical components of power plants like e.g. headers and pipes. In this way, both the impacts of the manufacture of the component (including all shaping and tempering influences) and of the complex aging/loading/oxidation conditions during Service can be analyzed and representative mechanical properties are obtained. In the present study, example results of a comparative creep and creep rupture study on P91 tempered martensite ferritic Steel, involving Standard and miniature specimens that were all taken from one batch of material, are presented.
Cylindrical creep specimens with a minimum gauge length diameter of 3 mm were used, representing the smallest recommended test geometry of ISO 204. The test results of miniature specimens exhibit all characteristic creep features of tempered martensite ferritic steels, and analysis of the stress and temperature dependence of creep results in values that correspond well to literature data for P91. However, direct comparison with large scale specimen data reveals small but systematic variations in minimum creep rates, elongations at fracture and times to rupture. In our contribution, these differences are discussed in the light of literature findings on specimen size influences in other heat resistant alloys. Size effects need to be considered for a correct interpretation of results from miniature specimen creep tests.
The use of miniaturized specimen geometries in mechanical testing allows extracting the sample material directly from critical components of power plants like e.g. headers and pipes. In this way, both the impacts of the manufacture of the component (including all shaping and tempering influences) and of the complex aging/loading/oxidation conditions during Service can be analyzed and representative mechanical properties are obtained. In the present study, example results of a comparative creep and creep rupture study on P91 tempered martensite ferritic Steel, involving Standard and miniature specimens that were all taken from one batch of material, are presented. Cylindrical creep specimens with a minimum gauge length diameter of 3 mm were used, representing the smallest recommended test geometry of ISO 204. The test results of miniature specimens exhibit all characteristic creep features of tempered martensite ferritic steels, and analysis of the stress and temperature dependence of creep results in values that correspond well to literature data for P91. However, direct comparison with large scale specimen data reveals small but systematic variations in minimum creep rates, elongations at fracture and times to rupture. In our contribution, these differences are discussed in the light of literature findings on specimen size influences in other heat resistant alloys. Size effects need to be considered for a correct interpretation of results from miniature specimen creep tests.