Control of Microstructure in Heat Affected Zone of Welded Metal using Oxide, Sulfide and Nitride Fine Particles

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It is well known that the toughness at the heat affected zone of welding metal (HAZ) becomes wrong due to austenite grain growth after large-heat capacity welding process. In order to prevent this deterioration, control of microstructure in welded metal by using oxide, sulfide and nitride fine particles has been investigated.Fine oxide and sulfide particles, which were dispersed uniformly in liquid steel, became nucleation sites of austenite grains during solidification of steel. As a result, morphology of microstructure could be controlled. This sample was heated under the thermal condition similar to a large-heat capacity welding process by using welding thermal cycle simulator. The influence of composition, size distribution and dispersion of primary inclusion and secondary inclusions (sulfide and carbonitride precipitated during solidification) on grain growth in HAZ was clarified. The Influences of pinning effect of inclusion, solute drag effect of segregated elements, and dissolution and growth of precipitated sulfide and carbonitride on the control of grain growth at HAZ were also investigated. The influences of composition and size distribution of inclusion particles on the microstructure and nucleation of inter granular ferrite (IGF) were discussed.Fe-C alloy was melted in Ar atmosphere, and the initial oxygen, sulfur and nitrogen contents were controlled. The primary inclusion particles were formed by the addition of Fe-X alloy (X=Al,Mg,Zr,Ti,Si). The effect of these primary inclusion particles on the nucleation of microstructure during solidification and modification was examined. Relationships between the morphology of microstructure (columnar, cubic) and the composition and size distribution of primary inclusion particles was investigated. Since it was found that secondary inclusion (sulfide, carbonitride) was precipitated on the primary inclusion particle, the formation of multi-composed inclusion and its effect on IGF formation was discussed.