The effect of Al/O ratio on the development of acicular ferrite microstructure development and the toughness in weld material containing low oxygen has been studied. Three base steel plates with different aluminum contents were prepared by vacuum melting method. The addition of 0∼10% CO2 into Ar shielding gas and the Ar gas double shield technique during arc welding of base plates formed various oxygen levels from 16 to 307 ppm. The samples with different Al/O ratio were thus obtained. The optical microstructures showed that coarse grain boundary ferrite was formed along prior austenite grain boundaries in all samples. When Al/O ratio was from 0.39 to 1.13, some of the inclusions appeared to be acted as nucleation site for acicular ferrite and weld metal with acicular ferrite exhibited good toughness (vTrs < 243 K). In this case, SEM-EDX chemical composition analysis of these inclusions indicated that inclusions were rich in oxygen, manganese and sulfer with a small amount of aluminum and silicon. It was assumed that the MnS precipitated on Al2O3-MnO-SiO2 inclusion and become effective nuclei of acicular ferrite.
Mizuguchi et al. (Tue,) studied this question.