Nucleation on irrational interphase boundaries in diffusional phase transformations of steels

Heterogeneous nucleation of ferrite on inclusions/precipitates dispersed in austenite is important in refining microstructures of low and medium carbon steels. In the present study, characteristics of intragranular ferrite nucleated on the B1-type complex inclusions/precipitates in austenite are discussed. The intragranular ferrite transformation is promoted remarkably with the presence of V(C,N) formed on the incoherent MnS inclusion. Those V(C,N) precipitates exhibit irrational and non-specific orientation relationships with respect to the austenite. Intragranular ferrite is idiomorphic for small undercooling but becomes acicular as the transformation temperature is lowered. The idiomorphic ferrites tend to hold low-energy orientation relationships with respect to V(C,N), resulting in irrational and non-specific orientation relationships between the ferrite and austenite. For the acicular ferrite, near Kurdjumov-Sachs orientation relationships are achieved with respect to the austenite. V(C,N) precipitation at austenite grain boundaries leads to an increase in the irrationality of the orientation relationship between grain boundary ferrite and the austenite matrix.