Abstract
A set of different acicular ferrite type microstructures have been developed, with both isothermal treatments between 500°C and 400°C and continuous cooling after austenization at 1250°C, in a medium carbon forging steel. The inclusions responsible for the intragranular nucleation of the ferrite have been studied by means of transmission electron microscopy. The results obtained enabled the identification of a MnS core covered by hexagonal CuS, which seems to make nucleation at inclusions energetically more favourable than nucleation at grain boundaries. A good combination of strength and toughness has been obtained associated with a refined acicular ferrite microstructure, generated by continuous cooling, and with a mixture of acicular ferrite and pearlite, obtained at 500°C by isothermal treatment.
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