Effect of Mn Depletion on Intra-granular Ferrite Transformation in Heat Affected Zone of Welding in Low Alloy Steel

Role of Mn depleted zone formed around MnS precipitate in austenite (γ) in transformation of Intra-Granular Ferrite (IGF) in low alloy steel subjected to simulated thermal cycle of weld Heat Affected Zone (HAZ) has been studied. Mn distribution in γ adjacent to MnS were examined by Field-Emission type Transmission Electron Microscopy (FE-TEM) with Energy Dispersive X-ray Spectroscopy (EDS). Foil samples for TEM observation were prepared using Focused Ion Beam (FIB) in order to prevent ununiformity of thinning rate of MnS and steel, and to thin the nucleation site of IGF specified by optical or scanning electron microscopic observations. The authors demonstrated the direct measurement of Mn depleted zone at IGF nucleation site. Mn depletion varied from 0 to 0.4 mass% depending on annealing temperature and holding time at γ phase region. Calculated concentration profiles of Mn near the depleted zone for various thermal cycles showed good agreement with experimental results. Change in area fraction of IGF with thermal cycle conditions was well explained by the extent of Mn depletion. It was concluded that formation of IGF was controlled mainly by the Mn depletion effect in low alloy steels.