The development of acicular ferrite in reheated Ti-B-Al-N-type steel weld metals containing various levels of aluminium and nitrogen

Effects of Al and N on the transformation of austenite to acicular ferrite during continuous cooling in Ti-B steel weld metals have been investigated using a dilatometric technique combined with (scanning) transmission electron microscopy (STEM/TEM) equipped with energy-dispersive Xray (EDX) facilities. Results showed that addition of 160 ppm Al to a Ti-B-N weld metal with a low or 'normal' N content (designated as N) accelerated the transformation kinetics resulting in acicular ferrite as the dominant microstructure. As the amount of Al was increased to 560 ppm, the transformation was retarded as indicated by its lower transformation start temperature, hence favouring upper bainite. It is interesting to note that N could give a beneficial effect in Ti-B-N weld metals with 560 ppm Al when the amount of N was increased to an intermediate level (N1), i.e. 120 ppm, marked by an increase in the transformation start temperature with upper bainite being replaced by intragranular acicular ferrite. A reversal effect was observed as the N level was increased up to 240 ppm (N2) where the growth of acicular ferrite had to compete with that of intragranular polygonal ferrite, and the mechanism in which acicular ferrite develops in Ti-B-Al-N welds is discussed.