Thermodynamic study of inclusion formation in low alloy steel weld metals

The formation and stability of inclusions in low alloy steel welds were investigated using equilibrium calculations. Based on the results, the origin of inclusions effective in acicular ferrite production is discussed. Particular emphasis is placed on the effect of the Al/O ratio and titanium addition on inclusion formation since these two factors are experimentally critical to acicular ferrite production. Mullite (2SiO(2).3Al(2)O(3)) is found to be formed in addition to Ti3O5 in the steel melt at 1800 K where the melt has a wt-%Al/wt-%O ratio of similar to 0.6, optimum for acicular ferrite production. However, the mullite is unstable in the austenite and galaxite (MnO.Al2O3), having the spinel structure, becomes stable at the lower temperatures. Therefore, it is proposed that galaxite is responsible for the nucleation of acicular ferrite. The formation of the galaxite should be in the solid state after weld solidification, being associated with preexisting mullite. Titanium additions are found to be beneficial to acicular ferrite production by decreasing the formation of ineffective glassy oxide. (C) 1997 The Institute of Materials.