Effect of Oxides and Their Volume Fraction on Intragranular Ferrite Formation in Steel

Formation of intragranular ferrite is important for the refinement of microstructure in weld metal and coarse grained heat affected zone (CG-HAZ) in welding of steel. It is well known that titanium oxide (Ti(2)O(3)) works as a inclusion with high ferrite forming ability in Ti-B bearing steel welds. But in the welds, TiN and MnS also precipitate on Ti(2)O(3), and the beneficial effect of Ti(2)O(3) is often assisted by these precipitates and Mn depleted zone in the vicinity of Ti(2)O(3). Thus the role and the effectiveness of Ti(2)O(3) itself are still unclear. For the intragranular ferrite formation, not only the ferrite forming ability of inclusion particles but also the distribution of the nucleation sites is also important. In this research, ferrite forming ability of Ti(2)O(3) is evaluated in two methods: the measurement of undercooling in solidification of bcc-Fe on Ti(2)O(3) and the microstructure observation of steels containing high volume fraction of oxide particles prepared by powder metallurgy. Effect of oxide chemistry and the volume fraction of oxides were evaluated.