Microstructural Analysis of Non-Metallic Inclusions in Atmosphere Melted and Poured Stainless Steel Investment Castings

Non-metallic inclusions are one of the common defects in stainless steel castings and have received widespread attention. In this paper, the morphology and composition of non-metallic inclusions in stainless steel investment casting were studied by metallography. The experimental results suggest that there are numerous non-metallic inclusions in this stainless steel casting. Due to the different formation mechanisms, these non-metallic inclusions vary greatly in morphology and chemical composition. The main inclusions are circular or spindle-shaped endogenous composite inclusions composed of a variety of oxides, whose substrate is manganese silicate (MnO-SiO₂), rich in Al₂O₃, CaO, Cr₂O₃ and TiO₂. A small number of inclusions are sickle-shaped exogenous inclusions produced by the furnace lining repair materials SiC-Al₂O₃ involved in the melt. The endogenous inclusions appeared as black granules with clear internal structure under optical microscopy (OM), with equivalent diameter distribution from 2 to 8 µm, and a few circular inclusions with sizes up to 80 µm. The endogenous inclusions are encapsulated in a niobium carbide shell, which was quite different from the exogenous inclusions. The exogenous inclusions are larger in size and aspect ratio and have multiple endogenous inclusions attached to the surface. The amount and size of coarse inclusions in stainless steel castings can be reduced by measures such as promptly keeping the furnace lining clean and sufficiently removing the steel slag from the melt. Graphical Abstract: (Figure presented.)