Acicular ferrite is regarded as the most desirable microstructural feature, in view of strength and toughness, in high strength pipeline steel weld metals. Recent evolution and diversity of mechanical property for base metal demand the same property as the weld. Therefore, understanding of the formation mechanism for acicular ferrite microstructure in weld is one of the essential problems for low carbon low alloy steel weld. In the present work, the morphological development of acicular ferrite in situ, was observed during weld cooling. The sample designed to form acicular ferrite microstructure was schematically heated and cooled by infrared imaging furnace and the morphological developments were directly observed using laser scanning confocal microscopy. The nucleation and growing at inclusion, sympathetic nucleation and impingement event of acicular ferrite were directly shown in high time resolution. To clarify the evolution mechanism, the inclusions related to acicular ferrite formation were investigated from the crystallographic points of view.

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