Inclusions nucleating intragranular polygonal ferrite and acicular ferrite in low alloyed carbon manganese steel welds

Abstract

The inclusion-assisted formation of two types of intragranular ferrite in low alloyed C-Mn steel welds, intragranular polygonal ferrite (IPF) and acicular ferrite (IAF), was investigated in relation to the inclusion characteristics (mainly size and chemistry) and the welding heat input. For this analysis, inclusions engulfed by one ferrite grain and those shared by two ferrite grains were considered as the IPF nucleants, and the rest of them (in contact with more than three ferrite grains) were considered as the IAF nucleants. All inclusions were multi-component oxides having Ti oxides as the nucleoid. The inclusion size exhibited a log-normal distribution and the average size increased as the heat input increased. Inclusions larger than ∼0.4 μm were found to be effective on IAF nucleation, and the effectiveness increased as the inclusion size increased. Accordingly, the overall IAF nucleant fraction increased as the heat input increased due to the corresponding inclusion size increase. However, the probability of the IAF nucleant in the same inclusion size range was insensitive to the heat input. There was no difference in the elemental distribution between the IAF and IPF nucleants except for Si. The Si distribution was uniform in the IPF nucleants while its content was relatively high at the inclusion periphery in the case of the IAF nucleants regardless of the heat input and the inclusion size.