Mn Depletion Behavior at Oxide/matrix Interface in Low Oxygen Weld Metal of Low Carbon Steel

The structure of the oxide as a nucleus for intragranular ferrite formation in the low O weld metal and the formation mechanism of Mn depleted zone (MDZ) at the oxide/matrix interface were studied using liquid-tin quenched specimens at high-temperature conditions during laser welding of low carbon Ti added steel. At a high temperature of 1720 K, MDZ is formed around the complex oxides of (Ti,Mn)(3)O-5, (Ti,Mn)(2)O-3, and liquid phase oxides (containing Si, Mn, Ti, and S). The width of the MDZ increases with cooling, and at low temperatures (1275 K) MDZ is formed around the complex oxide consisting mainly of (Ti,Mn)(2)O-3 with MnS and Si-Mn oxides. These MDZs are formed all around the complex oxides, regardless of the kind of oxide. The formation of MDZs is considered to promote the ferrite transformation around the oxides. The equilibrium Mn concentration in each of the oxide phases increases during the cooling process and the thermodynamically stable phase changes from (Ti,Mn)(3)O-5 with a low equilibrium Mn concentration to (Ti,Mn)(2)O-3 with a high equilibrium Mn concentration, which drives the diffusion of Mn from the matrix phase to the oxide. In this process, MDZs are formed all around the complex oxides.