Study on grain growth behavior of acicular ferrite in weld deposited metal

In this study, having acicular ferrite (AF) as the matrix the high heat input weld metal was taken as the research object and acicular ferrite (AF) grain orientation relationships (ORs) in the deposited metal was identified from Metallographic microscope (OM), electron bacK-Scatter diffraction device (EBSD), automatic static phase change instrument, etc, the influence of different heat input on the growth behavior of AF grain in the weld metal was analyzed. The results showed that, When the cooling rate of the weld metal is below the critical rate, AF in the weld metal nucleated from the inclusions with polygonal ferrite morphology, and deviated from K-S OR with respect to parent phase γ. When the temperature of the deposited metal decreased to the transformation start temperature, AF grew up with slender needle morphology, and exhibited K-S OR with respect to parent phase γ. However, with the decrease of the welding heat input, AF nuclear size reduced, and the approach to K-S OR with respect to parent phase γ was accelerated, when the cooling rate of the weld metal exceeds the critical rate, AF grains can grow rapidly without nucleation.