Role of inclusions for formation of acicular ferit in low carbonsteel weld deposits

Acicular ferit was investigated from the viewpoint of orientation relationships among acicular ferit, prior austenite, and inclusions. The Baker-Nutting orientation relationship was observed between acicular ferit and a TiO layer forming at inclusion surfaces. Meanwhile, the Kurdjumov-Sachs orientation relationship was observed between acicular ferit and prior austenite. The results indicate that acicular ferit is considered to nucleate due to a low misfit with TiO layers; however, there is still doubt. In case the TiO layer form at molten steel, its orientation would be random with austenite. Thus, acicular ferit nucleating with the Baker-Nutting orientation relationship should have no orientation relationship with austenite. In order to address the issue, the inclusion formation process was investigated by using the liquid-tin quenching technique and the thermodynamic calculation. The results reveal that TiO form not at the high temperature such as molten steel temperature but at below the solid phase temperature such as austenite or ferit temperature. As the result of orientation analysis between prior austenite and TiO, TiO is not considered to form in austenite. It can be assumed that TiO layers form in order to match interface with acicular ferit. Additionally, the Mn depleted zone were also not observed. Therefore, acicular ferit is considered to nucleate not due to the low misfit or the Mn depleted zone but due to another mechanism.