Segregation behaviour and phase developments during solidification of Inconel 625; effect of iron and carbon

The key to improve the performance of Inconel 625 weld overlays applied for corrosion resistance in waste incinerator plants is to understand the solidification process: At present, enhanced resistance against fireside corrosion in waste incinerator boilers is commonly achieved by overlay welding with nickel based alloys i.e. Inconel grades. The final layer has a thickness of at least 2 mm. This layer has a dendritic morphology with a dendrite arm spacing of a few microns. In general, this combination of material and process technology provides an adequate solution. However, corrosive attacks of the protective layer is observed to occur predominately along either the dendrite cores, D.C. or in the inter-dendric regions, I.D. ¡V which occasionally results in unexpected large (and possibly detrimental) local corrosion rates. In this investigation microstructure characterization was performed on representative samples of the weld overlay using a combination of X-ray diffraction and scanning electron microscopy. It was found that the concentration of niobium and molybdenum was larger in the interdendritic matrix (denoted ƒ×ID) compared to the dendrite core (ƒ×DC). In addition, both elements were found to be present in the inter-dendritic material, either in an intermetallic phase ¡V commonly referred to as Laves phase or (when carbon is present) in niobium rich carbides. Accordingly, differences in composition and constitutions between ID and DC, which arise during solidification, are likely to cause the observed preferential corrosion behaviour.