Abstract
This paper deals with the improvement of weld metal toughness by formation of fully acicular ferrite microstructure in laser welding. The addition of 5-20% oxygen into the He shielding gas for laser welding of Ti added base plate formed Ti containing oxide inclusions in weld metal, which acted as the efficient nucleation sites of acicular ferrite. Although coarse grain boundary ferrite was formed along prior austenite grain boundaries, B addition effectively suppressed the formation of grain boundary ferrite. It was shown that the microstructure of laser weld metal was consisted of fully acicular ferrite when Al/O ratio was less than 1.13 and B content was more than 1/2.22(O-0.89Al-0.33Ti). The laser weld metal of fully acicular ferrite exhibited excellent toughness (vTrs<-100°C).
