Solidification microstructures and phase transformations in Al-Ti-Si-Mn deoxidized steel weld metals

Abstract

The present investigation is concerned with basic studies of solidification mechanisms in Al-Ti-Si-Mn deoxidized steel weld metals. Assessment of the weld metal solidification micro-structures was done on the basis of optical microscopy in combination with secondary ion mass spectrometry (SIMS), while both scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for characterization of indigenous oxide inclusions. It is shown that nonmetallic inclusions play a critical role in the development of the weld metal columnar grain structure by acting as inert substrates for nucleation of delta ferrite ahead of the advancing interface. The nucleation potency of the oxides increases in the order SiO₂-MnO, Al₂O₃-Ti₂O₃-SiO₂-MnO, A1₂O₃, reflecting a corresponding increase in the inclusion/liquid interfacial energy. Moreover, a shift in the peritectic reaction (which facilitates growth of the austenite grains across the phosphorus-rich boundaries of the primary delta ferrite phase) has been observed in the presence of A1₂O₃ inclusions. Indications are that the resulting decrease in the local phosphorus concentrations at the austenite grain boundaries will strongly alter the kinetics of the subsequent solid-state transformation reactions by promoting growth of grain boundary ferrite sideplates at the expense of intragranularly nucleated acicular ferrite.