Proeutectoid ferrite (bcc α) nucleation on the inclusions and precipitates dispersed in austenite (fee γ), denoted as intragranular ferrite transformation, has been used to refine the microstructure in the heat affected zone to improve the toughness of low and medium carbon steels. It is currently considered that Bl(NaCl)-type complex inclusions/precipitates (e.g. oxide, sulfide, carbide, nitride) are promising ones as ferrite nucleation sites. The present study aims to study the ability of various complex inclusions/precipitates in austenite as nucleation sites in proeutectoid ferrite transformations by standard and analytical electron microscopy.
Table 1 shows the chemical composition of Fe-2Mn-0.2C hypoeutectoid steels with various alloying element additions to change the inclusions/precipitate in austenite. Steel A contains a large density of MnS with the sulfur addition of 470ppm. Steels B and C also contain similar dispersion of MnS as Steel A.