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Ductile iron is nowadays widely used in key industrial sectors like off-shore, transport and energy production, accounting for as much as 25 % of the total casting production in the world. It is well known that ductile iron parts, depending on their size, may contain residual stresses developing over distances of a few millimeters or more, which arise due to the presence of constraints that hinder the free thermal contraction of the material during cooling. Fortunately, dedicated studies performed in the last few decades have provided a detailed understanding of the phenomenon, and today reliable tools exist that allow predicting and coping with the problem in almost all practical cases.
On the other hand, the intrinsic composite nature of ductile iron suggests the possible formation of another type of residual stresses, at much shorter length scales, associated with the thermal contraction mismatch between the two main metallurgical phases forming the material microstructure: the graphite nodules and the metallic matrix. Surprisingly, the subject has not received much consideration in the past, probably due the common belief that the graphite particles are very soft and unable to withstand any kind of loading. As a matter of fact, however, experimental evidence exists for their mechanical importance, especially at relatively high temperature and under compressive loadings, indicating that ductile iron might not be considered as a merely “voided material” in all situations.
Taking this as point of departure, the present work initially focuses on finding a satisfactory description of the nodules’ thermo-elastic behavior, which is shown to be missing in the published literature, by means of micro-mechanical homogenization analyses based on a representative unit cell. These, combined with the application of elastic bound theory for polycrystalline materials, lead to the conclusion that the nodules cannot be considered as homogeneous and isotropic at the microstructural scale. Consequently, a novel strategy to simulate their elastic response is proposed, which consists in modeling their characteristic internal structure, composed of graphite platelets arranged into conical sectors, in an explicit manner. The resulting anisotropic model turns out to provide homogenized values for the ductile iron thermo-elastic properties at the macro-scale in excellent agreement with the experiments. In addition, it also indicates that the average thermal contraction of the nodules is likely 3 to 4 times smaller compared to that of the surrounding matrix, hence confirming the existence of a driving force for the formation of stresses at the local scale. In order to investigate this last aspect, the final stages of the manufacturing process are simulated numerically, accounting for the different thermal expansion of the nodules and of the matrix during both the eutectoid transformation and the subsequent cooling to room temperature. The results show the formation of significant residual stresses in the matrix region close to the nodules, which are mainly deviatoric and strongly affected by the number of conical sectors forming the graphite particles.
To support the numerical findings, whose relevance calls for an adequate experimental validation, two techniques are employed. The Oliver-Pharr nano-indentation method is considered first, with the aim of obtaining some direct information concerning the constitutive behavior of the individual graphite particles. Unfortunately, the technique turns out to feature a number of assumptions that pose strong limitations to its applicability to brittle, inhomogeneous and anisotropic structures like the nodules. Interestingly, one of them is related to a concealed way of accounting for the particular contact condition arising between the indenter and the sample during the test, which is revealed in detail in this work for the first time in literature.
The second technique considered is a novel 3D X-ray diffraction method based on synchrotron radiation. This time, the experiments are successful and lead to the determination of the residual stress state around a single nodule lying beneath the material surface. The results are the first ever produced, and confirm the theoretical predictions that local stresses up to approximately half the macroscopic yield strength may remain in the ductile iron microstructure after manufacturing. Needless to say, this new type of residual stresses is expected to play an important role in determining the properties of ductile iron. Knowledge of the factors controlling it will pave the way for further optimization of the material performance under in-service loading.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark (DTU)
Number of pages206
ISBN (electronic)978-87-7475-476-3
StatePublished - 2017
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