The role of Mn depletion in intra-granular ferrite transformation in the heat affected zone of welded joints with large heat input in structural steels

Intra-Granular Ferrite (IGF) transformation in the Heat Affected Zone (HAZ) of welded joints has been practically applied in structural steels, but the mechanism of IGF nucleation has not been elucidated yet. Then, the role of Mn depletion was investigated to elucidate the mechanism of IGF nucleation. It was first demonstrated to prove the direct evidence of Mn depletion at the interface of MnS/ferrite matrix by the nano-probe analysis of Field-Emission Transmission Electron Microscope equipped with Energy Dispersive X-ray Spectroscopy (FE-TEM-EDS). The presence of Mn depletion is also believed to promote IGF nucleation at the interface of MnS newly precipitated on (Mn, Si) oxide, which is considered to have no coherency with ferrite matrix.