Crystal Orientation Relationships between Acicular Ferrite, Oxide, and the Austenite Matrix

Crystal orientation relationships between acicular ferrite (AF), oxide and the austenite matrix have been investigated in low carbon steel weld by the submerged arc welding process. In particular, this study focused on the formation mechanism of the crystal orientation relationships. The AF microstructure was observed in weld metal containing titanium. Oxide particles were composed of MnTi2O4, amorphous phase and TiO2. The AF nucleated on MnTi2O4 having the Baker-Nutting (B-N) orientation relationship with the MnTi2O4 and Kurdjumov-Sachs (K-S) orientation relationship with the austenite matrix. This result implies that the MnTi2O4 had a rational orientation relationship with the austenite matrix. The orientation relationship is considered to be (001),(MnTi2O4)//[(1) over bar 11)gamma, [100](MnTi2O4)//[211]gamma from the viewpoint of the lattice coherency. It is supposed that the MnTi2O4 can be formed within oxide particles having this orientation relationship with the austenite matrix at high temperature during welding process. This mechanism allows the coexistence of both B-N and K-S orientation relationships, which lowers the AF/MnTi2O4 and AF/austenite interfacial energies. This results in the decrease of the activation energy for AF nucleation.