Role of inclusions for formation of acicular ferrite in low-carbon steel weld deposits

Abstract

Acicular ferrite was investigated from the viewpoint of orientation relationships among acicular ferrite, prior austenite, and inclusions. The Baker-Nutting orientation relationship was observed between acicular ferrite and a TiO layer forming at inclusion surfaces. Meanwhile, the Kurdjumov-Sachs orientation relationship was observed between acicular ferrite and prior austenite. The results indicate that acicular ferrite is considered to nucleate due to a low misfit with TiO layers; however, there is still doubt. In case the TiO layer forms at molten steel, its orientation would be random with austenite. Thus, acicular ferrite 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 liquidtin quenching technique and the thermodynamic calculation. The results reveal that TiO forms not at high temperatures, such as molten steel temperature, but below the solid phase temperature, such as austenite or ferrite 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 the interface with acicular ferrite. Additionally, the Mn-depleted zone was also not observed. Therefore, acicular ferrite is considered to nucleate not due to the low misfit or the Mn-depleted zone but due to another mechanism.