Formation of intragranular ferrite in weld heat-affected zone and its influence on microstructure

The formation of intragranular ferrite during the welding process can improve the microstructure of the weld heat-affected zone(HAZ) and enhance its low-temperature impact toughness. This study employed hightemperature laser scanning confocal microscopy to conduct in-situ observations of the formation of intragranular fer‑rite and its behavior in partitioning austenite grains. Focused ion beam(FIB) was utilized for site-specific preparation of micro-interface samples between inclusions and between inclusions and ferrite. Transmission electron microscopy was employed for micro-interface analysis to elucidate the nucleation mechanism induced by TiN from the perspec‑tive of mismatch. The existence of a manganese-depleted zone was in-situ demonstrated using instruments such as nanoindentation. Finally, electron backscatter diffraction(EBSD) was used to analyze the crystallographic informa‑tion of intragranular ferrite. The research findings are as follows. Intragranular ferrite nucleates and grows near inclu‑sions to form acicular-like ferrite, and ceases growth upon encountering pre-existing ferrite or austenite grain bound‑aries. Intragranular ferrite can partition austenite grains and optimize the microstructure of the weld heat-affected zone. The composite inclusions in this steel induce intragranular ferrite through two mechanisms. The mismatch between the( 210) plane of TiN and the( 210) plane of ferrite is 4. 76%, which indicates a coherent relationship that effectively promotes nucleation. The hardness of the intragranular ferrite lath induced by MnS is significantly lower on the inclusion-adjacent side than on the side far from inclusions, indirectly confirming the existence of a manganese-depleted zone from the perspective of hardness variation. The dislocation density of primary intragranular ferrite is significantly higher than that of secondary intragranular ferrite, suggesting that the formation of secondary ferrite is stress-induced nucleation. Intragranular ferrite laths induced by the same inclusion and those induced by other inclusions are separated by high-angle grain boundaries.