Development of macro- and microstructures of carbon–manganese low alloy steel welds: Inclusion formation

Ladle steel deoxidation reactions are reviewed and the principles are extended to inclusion formation in steel weld metal. The dissolution of oxygen, the stability of various oxides, and the nucleation and growth of inclusions are discussed. Theoretical time–temperature transformation (TTT) diagrams are calculated for various oxide inclusions based on an overall kinetics approach using nucleation and growth rate expressions. These concepts are then extended to understand the development of weld metal inclusion characteristics. A strong correlation between the published inclusion composition and the stability of the oxides was found. An analysis of the TTT diagrams indicates that, during weld cooling, sequential oxidation of dissolved deoxidizing elements takes place, which agrees with the reported layered morphology of inclusions. The analysis indicates that the inclusion characteristics are quite sensitive to the oxygen content, the deoxidising element concentrations, the presence of preformed inclusions, and the reaction temperature. Inclusion coarsening and elimination of inclusions from welds are discussed in relation to the final inclusion characteristics.