In-situ Observation of Nucleation and Growth of Acicular Ferrite in Weld Metal

Abstract

Acicular ferrite is regarded as being the most desirable microstructural feature, from the strength and toughness point of view, in mild and low alloy steel weld metals. Recent evolution and diversity of mechanical property for base metal demand the same property to the weld. Thus, the understanding of the formation mechanism of acicular ferrite microstructure in weld is one of the essential issues for low alloy steel welds.
We have suggested that high-temperature laser scanning confocal microscopy was suitable system to make clear the nucleation site of precipitate in rapid cooling process, such as welding. In the present work, the morphological development of acicular ferrite, was in-situ observed during weld cooling. The sample designed to form acicular ferrite microstructure was heated and cooled in infrared imaging furnace and the morphological developments were directly observed using laser scanning conforcal microscopy.
The nucleation and growth at inclusion, sympathetic nucleation and impingement events of acicular ferrite were directly shown in high-time resolution.