EVALUATION OF THE NUCLEATION POTENTIAL OF INTRAGRANULAR ACICULAR FERRITE IN STEEL WELDMENTS

An empirical model has been developed to evaluate the relative nucleation potential (p) of intragranular acicular ferrite (IAF) in steel weldments. The p value was found to be a function of austenite grain size (g) and the volume fraction of IAF (upsilon(IAF)), or, alternatively, a function of the mean free path of inclusions and upsilon(IAF). The model indicates that the presence of inclusions promotes IAF by increasing the intragranular nucleation surface area, but excessive inclusions deteriorate the formation of IAF due to decreasing austenite grain size, increasing grain boundary associated inclusions, and impairing the intragranular nucleation potential. A transition from p < 1 to p > 1 is predicted to occur at the grain size of about 100 mum. Analyses of experimental data show that the minimum inclusion size to ensure a good nucleation potential is in the range of 0.25-0.8 mum.