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Dissertation Saal Fakultät für Maschinenwesen Lehrstuhl für Umformtechnik und Gießereiwesen Quantitative Phasenanalyse von ausferritischem Gusseisen mithilfe der Neutronendiffraktometrie Patrick Gabriel Saal Vollständiger Abdruck der von der Fakultät für Maschinenwesen der Technischen Universität München zur Erlangung des akademischen Grades eines Doktor-Ingenieurs (Dr.-Ing.) genehmigten Dissertation. Vorsitzender: Univ.-Prof. Dr. mont. habil. Dr. rer. nat. h.c. Ewald Werner Prüfer der Dissertation: 1. Univ.-Prof. Dr.-Ing. Wolfram Volk 2. Univ.-Prof. Dr.-Ing. Babette Tonn, TU Clausthal Die Dissertation wurde am 17.05.2016 bei der Technischen Universität München eingereicht und durch die Fakultät für Maschinenwesen am 21.12.2016 angenommen. Vorwort Die Freiheit eigene Ideen zu erforschen und Dinge auszuprobieren, die auch fehlschlagen können, ist nicht selbstverständlich. Schließlich müssen die Mittel hierfür an anderer Stelle erwirtschaftet werden. Umso mehr schätze ich den Vertrauensvorschuss, den ich vor Beginn und während der Ausarbeitung dieser Dissertation erhielt. Ich danke der Deutschen Forschungsgemeinschaft (DFG) für die finanzielle Förderung des dieser Arbeit zugrundeliegenden Forschungsprojekts (VO 1487/5-1). Ebenfalls gilt mein Dank Prof. Hartmut Hoffmann, der mir das Vertrauen schenkte als wissenschaftlicher Mitarbeiter am Lehr- stuhl für Umformtechnik und Gießereiwesen (utg) zu arbeiten. Sein Nachfolger als Ordinarius, Prof. Wolfram Volk, begleitete mich bei der Anfertigung dieser Arbeit. Für die zahlreichen Dis- kussionen und Anregungen auch aus anderen fachlichen Richtungen möchte ich mich bei ihm bedanken. Gleichermaßen gilt mein Dank Prof. Babette Tonn für die Übernahme des Koreferats sowie Prof. Ewald Werner für die Bereitschaft, den Prüfungsvorsitz zu übernehmen. Die finanzielle Förderung sowie die wissenschaftliche Begleitung machten diese Arbeit erst mög- lich. Viele weitere Unterstützer haben jedoch einen ebenso bedeutenden Anteil am Gelingen. So- wohl durch Hilfestellungen fachlicher Art, insbesondere aber dadurch, dass die Zusammenarbeit mit ihnen stets sehr freundschaftlich war. Ich danke allen Mitarbeitern des Lehrstuhls utg, die mich in meiner Zeit als wissenschaftlicher Mitarbeiter unterstützten. Sei es tatkräftig bei Versuchsvorbereitungen oder einfach durch ein Ge- spräch. Mein besonderer Dank gilt Dr. Leopold Meier, der mich in jeder Phase dieser Arbeit mit bemerkenswerter Hingabe unterstützte. Ebenfalls möchte ich mich bei den zahlreichen Beteiligten der Forschungs-Neutronenquelle Heinz Maier-Leibnitz (FRM II) für ihre Mitarbeit bedanken. Ganz besonders gilt dies für Dr. Michael Hofmann, der enorm viel Zeit für die Durchführung von Experimenten, Diskussionen und mehr investierte. Stellvertretend für alle, die mich durch ihre studentischen Arbeiten während meiner gesamten Zeit am Lehrstuhl unterstützten, danke ich Martin Landesberger. Mein größter Dank gilt meiner Familie, die mir den nötigen Rückhalt gab. Meinen Eltern Anka und Wolfgang dafür, dass sie mir in jeder Phase meines Lebens eine Unbeschwertheit schenkten, ohne welche diese Arbeit nicht möglich gewesen wäre. Und meiner lieben Frau Olga für ihre Ge- duld, die Entbehrungen während der Erstellung dieser Arbeit und dass sie mir dabei half zu ver- stehen, was im Leben wichtig ist und was nicht. Für meine lieben Eltern. Kurzfassung Ausferritisches Gusseisen (ADI) ist ein Gusseisen mit Kugelgrafit, welches einer Wärmebehand- lung unterzogen wird. Diese besteht aus den Prozessschritten Austenitisieren, Abschrecken auf Ausferritisierungstemperatur sowie isothermem Ausferritisieren. Das ADI-Gefüge besteht aus plattenförmigem Ferrit, mit Kohlenstoff angereichertem Austenit sowie Grafit. Der Austenit ist bei Raumtemperatur in einem metastabilen Zustand und kann durch Unterkühlung sowie durch mechanische Belastung zu Martensit umwandeln. Im Rahmen dieser Arbeit wurden die Umwandlungsvorgänge während der Wärmebehandlung von ADI sowie die Umwandlung von metastabilem Austenit zu Martensit durch Unterkühlung und mechanische Belastung untersucht. Der Ausgangswerkstoff war Gusseisen mit Kugelgrafit mit 3,74 Gew.% C, 2,34 Gew. % Si und 0,15 Gew.% Mn, welcher gezielt mit Nickel legiert wurde. Dadurch standen drei Materialien mit ca. 0 Gew.%, 0,5 Gew.% und 1,5 Gew.% Ni zur Verfügung. Die Untersuchung der Phasenumwandlungskinetik erfolgte mittels Neutronendiffraktometrie wäh- rend der Wärmebehandlung in einem Laborspiegelofen, welcher in der Forschungs-Neutronen- quelle Heinz Maier-Leibnitz am Instrument STRESS-SPEC eingesetzt wurde. Durch die Analyse der gestreuten Intensitäten der Streuebenen Austenit (111) und Ferrit (110) wurden die Phasen- gehalte und die Kohlenstoffkonzentration des Austenits bestimmt. Die daraus resultierenden zeit- lichen Verläufe zeigten in den ersten Minuten der Ausferritisierung schnelle Ferritbildungen sowie Kohlenstoffanreicherung im Austenit. Diese verlangsamten sich mit fortschreitender Behand- lungszeit und kamen schließlich zum Erliegen. Aus diesen Ergebnissen wurden Zeit-Temperatur- Umwandlungsdiagramme abgeleitet. Erste Analysen einer ADI-Probe mittels einer tomografi- schen Atomsonde (APT) zeigten zudem, dass ein Gradient der Kohlenstoffkonzentration innerhalb des Austenits vorliegt, welcher mit der Neutronendiffraktometrie nicht festgestellt werden kann. Die Umwandlung von metastabilem Austenit zu Martensit durch Unterkühlung wurde mittels Di- latometrie und Neutronendiffraktometrie untersucht. Hieraus wurden Martensitstarttemperaturen in Abhängigkeit der Wärmebehandlungsparameter ermittelt. Für die Untersuchung der mecha- nisch induzierten Martensitbildung wurden deformierte Zug- und Druckproben im hochauflösen- den Pulverdiffraktometer SPODI analysiert. Die Ableitung der umgewandelten Martensitanteile erfolgte mittels Rietveld-Verfeinerung unter Berücksichtigung der Probentexturen. Der Anstieg des Martensitanteils mit zunehmender Deformation konnte als sigmoidal approximiert werden. Bei Zugbelastung zeigte sich zudem eine deutlich stärkere Umwandlungsrate als unter Druck. Die sigmoidale Approximation konnte durch In-situ-Messungen während Zug- und Druckversuchen am Instrument STRESS-SPEC bestätigt werden. Executive summary Austempered ductile iron (ADI) is a nodular cast iron, which has undergone a special heat treat- ment consisting of austenitisation, quenching and isothermal austempering. The ADI microstruc- ture consists of ferrite platelets, carbon enriched austenite and graphite. At room temperature the austenite is metastable and can transform to martensite when the material is supercooled or me- chanical load is applied. Within this work the transformation processes during the heat treatment of ADI and the trans- formation of metastable austenite to martensite induced by supercooling and mechanical loads were investigated. The base material was nodular cast iron with 3.74 wt.% C, 2.34 wt.% Si and 0.15 wt.% Mn alloyed with nickel in three steps. Thus three materials consisting approximately 0 wt.%, 0.5 wt. % and 1.5 wt.% Ni were used. The heat treatment was performed in a mirror furnace, which was used in the neutron source Heinz- Maier-Leibnitz at the instrument STRESS-SPEC. The phase transformation kinetics during the heat treatment was investigated using neutron diffraction. Analysing the scattered intensities of the austenite (111) and ferrite (110) reflections, phase fractions and austenite carbon contents were derived. Within the first minutes of austempering the results show a fast ferrite formation and austenite carbon enrichment. With ongoing heat treatment time the reaction ceases. On the basis of these results time-temperature-transformation diagrams were derived. First analyses of an ADI specimen using atom probe tomography (ATP) additionally showed that the carbon is distributed inhomogeneously within an austenite grain. This effect cannot be determined using neutron dif- fraction. The transformation of metastable austenite to martensite by supercooling was analysed using dil- atometry and neutron diffraction. Herefrom martensite start temperatures were derived depending on the heat treatment parameters. Strained tensile and compression specimens were used for the investigation of mechanically induced formation of martensite. The specimens were analysed at the high resolution powder diffractometer SPODI. The determination of the transformed marten- site fractions was performed using Rietveld refinement considering the specimen’s textures. The increase of martensite fraction with increasing deformation could be approximated with a sigmoid function. The transformation rate was considerably stronger for tension than for compression. The sigmoid approximation could be confirmed by in-situ measurements during tensile and compres- sion tests at the instrument STRESS-SPEC. I Inhaltsverzeichnis Verzeichnis der Kurzzeichen ...................................................................................................... IV Verzeichnis der Abkürzungen .................................................................................................... IX 1 Einleitung ................................................................................................................................ 1 2 Grundlagen und Stand der Technik ..................................................................................... 3 2.1 Herstellung von ausferritischem Gusseisen .................................................................. 3 2.1.1 Ausgangsmaterial ...........................................................................................
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