Abstract
Nanosize TiO2 particles were added directly into welding molten pool through electrode for the difficulty of accurate control of oxygen potential and production processing parameters. The characteristics of phase transformation and thermal behavior of inclusions for Fe‐C‐Mn‐Si‐Ti‐O system and Fe‐C‐Mn‐Si‐TiO2 system were analyzed. Results show that the added TiO2 particles are more helpful for the formation of Mn‐Ti‐O complex inclusion and can induce the decrease of phase transformation temperature of austenite to ferrite. Intragranular ferrite can be obtained under the condition of continuous cooling transformation with cooling rate of 293 K/s–373 K/s. The inclusions in steel welds are spherical in shape and mainly composed of TiO2, Ti3O5, Ti2O3, MnO, and SiO2. The mean size of inclusions is 0.67 μm. These complex inclusions can supply a large number of nucleating cores for their precipitation at higher temperature, which will disturb the growth of columnar crystal during solidification. Moreover, Mn‐containing titanium oxides will promote the transformation of austenite to intragranular ferrite for the formation of manganese depleted zones in steel welds around oxides. So it can be concluded that nanosize titanium oxide added directly in welding molten pool can be effectively used to control phase transformation and achieve fine and favorable microstructure.
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