Modeling of inclusion growth and dissolution in the weld pool
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I. INTRODUCTION
THE integrity of welded joints depends on the geometry, composition, macrostructure, and microstructure of the weldments. In low-alloy steel welds, the amounts of various ferrite morphologies, such as allotriomorphic ferrite, acicular ferrite, bainite, and martensite play a critical role in the final properties. Moreover, oxide inclusions in the weld also affect the weld properties by affecting microstructural development. Critical to the development of the weld-metal microstructure are the composition, morphology, and distribution of inclusions.[1] Oxide inclusions are most common in steel weldments. In general, the presence of inclusions is detrimental to weld properties. However, under a given set of conditions, in low-alloy steel welds, certain oxide inclusions promote the formation of an acicular ferrite phase, which improves toughness.[2–7] On the other hand, the presence of a very high volume fraction of inclusions may initiate premature ductile fracture. Although the importance of inclusions in affecting weldmetal properties has been recognized for a long time, it is only in recent years that systematic studies of inclusions in weld metals have begun. The oxide and other inclusions are affected by the concentrations of oxygen and alloying elements in the weld metal. Frost and Olson[2] showed that microsegregation during weld-pool solidification leads to significant enrichment of oxygen and deoxidants in the interdendritic liquid. These authors considered the effect of this enrichment on the sequence of oxide formations during the final stages of solidification. Barbaro et al.[3] concluded that oxide inclusions are likely to be effective in controlling acicular ferrite formation and grain coarsening, even in the T. HONG, Graduate Student, and T. DEBROY, Professor, are with the Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802. S.S. BABU, Development Staff, and S.A. DAVID, Corporate Fellow and Group Leader, are with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6095. Manuscript submitted October 22, 1998. METALLURGICAL AND MATERIALS TRANSACTIONS B
heat-affected zone (HAZ), as compared to carbide and nitride precipitates. The oxide inclusions containing g-Al2O3, titanium oxide (TixOy), and galaxite (MnO?Al2O3), with diameters larger than 0.4 mm, are often found to be effective nucleation sites for acicular ferrite.[4] Kluken and Grong studied mechanisms of inclusion formation in Al-Ti-Si-Mn steel weld metals[4] and precipitate stability in weld metals.[6] They calculated the volume fraction, size, and chemical composition of inclusions from weld-metal chemistry. They determined the volume fraction of inclusions from an empirical relation involving both oxygen and sulfur concentrations. They modeled the chemical composition of inclusions in low-alloy steel welds as a function of weld-metal chemistry, with stoichiometric relations and a fixed oxidation sequence based on the standard free energy of oxi
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