Role of non-metallic inclusions in the formation of acicular ferrite in low alloy weld metals

A critical investigation into the role of non-metallic inclusions in the nucleation of acicular ferrite in low alloy C-Mn-Ni weld metals was carried out using rapid in situ quenching, transmission electron microscopy, energy dispersive X-ray spectroscopy analysis, and dilatometric techniques. The relationship between the transformed acicular ferrite, the inclusions, and the chemical nature of the inclusions which had actually nucleated ferrite laths was examined in detail. By employing weld metals with either intermediate (200-350 ppm) or low (15-30 ppm) oxygen levels, the inclusion chemistry and nucleation behaviour ofacicular ferrite in welds with different oxygen contents were compared. In addition, the influence of the [Al]/[O] ratio in lower oxygen content weld metals was also examined. It was found that the first acicular ferrite laths nucleated from the inclusions at both oxygen levels, and the chemical composition of the active inclusions varied considerably. The inclusions nucleated acicularferrite by acting simply as inert substrates with the ability to reduce the free energy barrier to nucleation. The number and size distribution of the inclusions appeared to be the dominant factors; the thermal stress-strain fields and lattice disregistry were less important. The most favourable type of inclusions were within the size range 0.3-0.9 ?m for the higher oxygen content weld metals and 0.35-0.75 ?m for the lower oxygen level weld metals. A high [Al]/[O] ratio depresses the formation of acicular ferrite by increasing the soluble matrix aluminium content, which encourages extensive ferrite sideplate formation; the concurrent variation in inclusion chemical composition did not appear to influence the nucleation of acicular ferrite