Effect of Boron on Intra-granular Ferrite Formation in Ti-Oxide Bearing Steels

The effect of B on microstructure and toughness at the heat affected zone after welding thermal cycle simulations has been investigated in steels containing titanium oxide particles. The titanium oxide is identified as Ti2O3 with cation vacancies, which contribute to preferential nucleation of MnS and TiN precipitates on Ti2O3. The Mn-depleted zone is formed around Ti2O3 after the precipitation of MnS. The Ti2O3, particles with TiN and Mn-depleted zone act as preferential nulceation sites for intra-granular ferrite. The segregation of B at austenite grain boundaries suppresses effectively the nucleation of grain boundary ferrite. Ferrite nucleation at the interface between Ti2O3 and austenite matrix is not affected by B addition because of B-depleted zone arisen from the diffusion of segregated B into Ti2O3 via cation vacancies. Consequently the B addition to steels with Ti2O3 promotes the formation of fine intra-granular ferrite grains so that the toughness in heat affected zone is improved even after large heat input welding.