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Acicular ferrite is regarded as the most desirable microstructural feature, in view of strength and toughness, in mild and low alloy 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 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 conforcal microscopy. The nucleation and growing at inclusion, sympathetic nucleation and impingement event of acicular ferrite were directly shown in high time resolution.

In situ characterization of directional solidification process during welding was carried out using the time resolved X-ray diffraction technique utilizing intense synchrotron radiation. Then, behaviour of dendrites in steels under welding conditions of a practical manufacturing process were investigated using the TRXRD method for in-situ weld observation with the uniquely-sensitive two-dimensional pixel detector. Consequently, the crystal growth during the rapid cooling was caught in detail and employed a systematic peak profile analysis in order to acquire the essential information for controlling the weld microstructure. Our results would suggest the microstructure formation process of low alloy in directional solidification during rapid cooling. Simultaneously, we discuss the possibility of detecting the nucleation.