Effect of aluminium content on acicular ferrite formation in low carbon steel weld metals

Abstract

This paper describes the effect of Al content on the formation of acicular ferrite in low carbon steel welds. Inclusions and the microstructures of welds with three levels of Al content were analysed. Acicular ferrite was formed in the low and medium Al content samples, but not in the high Al content sample. In the low and medium Al samples, a TiO layer, which encourages acicular ferrite nucleation, was formed around the inclusions. Grain boundary ferrite was formed only in the low Al content sample. Inclusions in the low Al sample were predominantly composed of the amorphous phase, whereas those in the medium Al sample were composed of spinel structures. It was confirmed by electron energy loss spectroscopy analysis that boron existed in the amorphous phase. The formation of the amorphous phase enhances grain boundary transformation by absorbing boron, which stabilises austenite grain boundary, whereas the formation of spinel structures enhances intragranular transformation by suppressing the boron absorption.