Martensitic Transformation and Microstructure in Fusion Zone of Dissimilar Steel Welded Joint

The observation with elevated temperature microscope indicated that the martensite transformation really exists in the fusion zone of pearlitic base metal (ZG20SiMn) and austenitic welding consumables (A307、A457), and the transformation could occur from 400℃ to near room temperature. TEM analysis showed that martensite in the fusion zone is band type with low carbon, and its substructures are dislocation and a little twinning. The carbides in the fusion zone are mainly resulted from PWHT, typically M₂₃C₆ and some M₇C₃ precipitated on the martensite and bandaries between austenite and martensite. The results showed that the martensite and carbides region is mainly controlled by the Ni content of welding consumables. The increasing of Ni content makes the region narrow and gets higher charpy toughness in fusion zone.