How are black spots formed on precision cast stainless steel products?

After processing or polishing stainless steel castings, scattered regular spherical black spot defects may appear on the surface position, which contain elements such as 0, Si, Mn, etc.

The reason for the occurrence of black spot defects may be due to the presence of non-metallic inclusions (mainly metal oxides, sulfides, and silicates) in the molten steel.

The sources of non-metallic inclusions include external factors, such as sediment and impurities brought in by metal furnace materials, broken particles peeled off from the refractory materials of the furnace lining, and broken sand washed off from the steel liquid during the pouring process; There are also oxides generated inside the melt, such as the oxidation of elements in molten steel.

The melting point of these inclusions is lower than the temperature of the alloy melt and they exist in a liquid state. Due to their high surface tension and lack of mutual wetting with the molten steel, they aggregate into spherical shapes in the molten steel. After the steel solidifies, spherical inclusions are formed, resulting in regular spherical black spot defects on the surface of the casting.