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Abstract
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Austenite films with almost identical thicknesses of nanoscale bainitic ferrite plates in a bainitic sheaf and also
austenite micro blocks separating bainitic sheaves spreading in different crystallographic directions, are the
main factors controlling the toughness and ductility properties in nanostructured bainitic steels. This work
aims to investigate how different heat treatment procedures and production methods would affect the
impact toughness and tensile ductility in nanostructured bainitic steels. Primarily, bainitic transformation has
been applied at different heat treatment temperatures for optimum heat treatment times at which bainite
transformation completed and toughness and ductility properties have been assessed according to the
volume fraction, morphology and mechanical stability of retained austenite. Consequently, austempering
heat treatment has been conducted for times longer than the optimum heat treatment times and toughness
and ductility have been evaluated based on the microstructural characteristics and changes. Achieved results
have been compared with those of obtained at the end of optimum heat treatment conditions. Additionally,
toughness and ductility properties in nanostructured bainite achieved from primarily deformed parent
austenite have been investigated based on the microstructural evolutions and different size distributions of
microstructural constituents in ausformed samples and results have been compared with those of achieved
in simple heat treatment procedure. Finally step-austempering has been applied to investigate the
microstructural evolutions and their effect on comprehensive mechanical properties emphasizing on
toughness and ductility.
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