Acicular ferrite nucleation as a diffusion controlled process in high strength low alloyed (HSLA) steel weld metal

Acicular ferrite is a desirable microstructure in high strength low alloy steel weld metal. This is due to its improved toughness and the enhanced mechanical properties of the weld metal. Although the nucleation of acicular ferrite has been studied by many researchers, the exact mechanisms of its nucleation and growth are still under discussion and remained unclear. In this research work, the mechanism of acicular ferrite formation in the weld metal as cast structure has been clarified as diffusion controlled solid state phase transformation. This is based on the classic theory of nucleation and growth which can contribute to possible increase of nucleation sites and growth of intergranular ferrite in HSLA steel weld metal. Therefore, it could be considered that inclusions are not acting as a nucleation site for the intergranular acicular ferrite. Consequently, our results revealed that, in austenite transformation to pro-eutectoid and acicular ferrite, manganese as an austenite stabilizer alloying element is playing an important role in the nucleation and growth of the ferrite grains. It should be added that cooling rate accompanied with the presence of other alloying elements has influenced the type and morphology of the final ferrite microstructure and constituent products.