In-situ observation of the fusion zone under different high heat input for hot-rolled ship plate steel with 0.012yttrium

For the application of steel for ocean engineering in the deep exploitation of marine resources, there are increased demands for dimension and performance standards. This has led to the rapid development of high-heat-input welding steel with a high welding efficiency. In this study, the fusion-zone microstructural evolution of hot-rolled ship plate steel with 0.012yttrium (Y) under high heat input welding was simulated by laser scanning confocal microscopy, and the dissolution and precipitation behaviors of the second phase in steel were discussed by thermodynamic analysis. Results revealed that in the solid–liquid transition process during simulated welding, three stages are observed: precipitation, migration, and aggregation growth. Insoluble yttrium-containing inclusions gathered through the cavity bridge, in addition to the simultaneous precipitation of the carbides and nitrides at the cavity bridge, finally forming larger flocculent-mixed inclusions. This process satisfies the two-step nucleation mechanism. The lower the interfacial energy, the more likely the occurrence of heterogeneous nucleation. A new grain boundary was observed at the sharp angle of aggregated yttrium oxide, which could refine grains. At a heat input welding of greater than 300 kJ cm⁻¹, rare-earth yttrium did not play a role in the grain refinement and maintenance of the structure stability. (paper)