In C-Mn steel weld metals, it is reported that nucleation of various ferrite morphologies is aided by non-metallic inclusion; in particular oxygen-rich inclusions of a certain type and size are associated with the intragranular nucleation of acicular ferrite.
[2][3] Acicular ferrite is a fine Widmanstätten constituent, which is nucleated by an optimum intragranular dispersion of oxide/sulfide/silicate particles.
The interlocking nature of acicular ferrite, together with its fine grain size (0.5 to 5 μm with aspect ratio from 3:1 to 10:1), provides maximum resistance to crack propagation by cleavage.
Composition control of weld metal is often performed to maximise the volume fraction of acicular ferrite due to the toughness it imparts.
The efficacy of inclusions as nucleation sites in modern low alloy steel weld metals is such that fine-scale intragranular bainite can nucleate on them, both by continuous cooling and by isothermal transformation below the bainite start temperature.