Effect of Nitrogen and Boron on the Development of Acicular Ferrite iN Reheated C-Mn-Ti Steel Weld Metals

The transformation behaviour of C-Mn-Ti alloyed steel weld metals with different levels of nitrogen and boron contents has been investigated using a dilatometric technique in combination with transmission electron microscopy (TEM) associated with microanalysis techniques, i.e. energy dispersive X-ray analysis (EDXA) and electron energy loss spectroscopy (EELS) in order to clarify the role of nitrogen and boron on the nucleation and growth processes of weld metal microstructure in particular acicular ferrite which gives high strength and improved impact toughness of steel weld metals. Results of this investigation showed that the addition of a small amount of boron, typically 40 ppm to a C-Mn-Ti weld metal was sufficient to significantly reduce the transformation start temperature with a decrease in the amount of grain boundary ferrite and a concommittant rise in the amount of acicular ferrite. Further decrease in the transformation start temperature was observed as the level of boron content was increased up to approximately 160 ppm resulting in the formation of bainitic microstructures. However, a subsequent addition of nitrogen around 240 ppm to this type of weld metal increased the transformation temperature and modified the weld microstructure marked by the presence of intragranular acicular ferrite and polygonal ferrite which nucleated on multiphase inclusions principally of the ‘TiO’ type but on which has formed BN. This finding seems to suggest that BN is a potent substrate for nucleating ferrite and the amount of acicular ferrite in Ti-B-N system is controlled by the balance between BN as an energetically favourable site for acicular ferrite nucleation and soluble boron which acts as a hardenability element suppressing grain boundary ferrite formation.