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Title: Modelling of MicrostructuralEvolution in AustemperedDuctile Iron
Author: Putman, Duncan Colin
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2004
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Austempered ductile iron (ADI) has a microstructure consisting mainly of high carbon austenite, bainitic ferrite and graphite nodules, produced by a two stage austenitisation and austempering heat treatment. The resulting microstructure gives these materials a combination of high strength and toughness, making them attractive for a wide range of applications. To increase surface hardness, ductile iron alloys can also be cast into chilled moulds to induce carbide formation in the required areas of components. These chilled ductile iron alloys can also be subjected to austenitisation and austempering heat treatments, therefore further improving the mechanical properties of the components core, whilst retaining the hard carbides present in the surface layers. This work encompasses three main areas; two are concerned with the production of generic microstructure models, which work in conjunction with thermodynamic modelling software MTDATA, and one relates to high temperature X-ray diffraction experiments. The first modelling section details how a computer program was developed that can be used to investigate how chemical composition influences the chill tendency of ductile iron alloys. The model predictions were shown to be in good agreement with a wide range of experimental measurements. The second modelling section considers ADI alloys. A computer program was developed which, given the chemical composition and austenitisation and austempering temperatures, produces a prediction of the microstructure of the alloy at the end of stage 1 of the austempering heat treatment, taking into account segregation of alloying elements. Experimental segregation profiles produced during this work showed good agreement with the model predictions. Furthermore, predictions of the stage 1 transformation kinetics as a function of alloying element segregation, are also made by the model. Therefore, the local microstructural transformation times during austempering can be predicted. Good agreement has been observed between phase volume fractions, transformation times and mechanical property predictions made using the model and those found in literature, therefore a useful tool for new alloy development has been produced. High temperature X-ray diffraction experiments were also performed as part of this work. Microstructures typical of ADI alloys were produced during these experiments, although small quantities of pearlite were observed in the samples, and care was taken to minimise any effects of decarburisation and/or oxidation. The austenite carbon content was monitored during austenitisation and austempering, enabling comparisons to be made between high temperature and low temperature X-ray diffraction measurements in ADI alloys.
Supervisor: Not available Sponsor: Not available
Qualification Name: Not available Qualification Level: Doctoral
EThOS ID:  DOI: Not available