to nitrogen. By mineralogical content, oxygen inclusions divide into the following main groups: Free oxides — FeO, MnO, Cr2O3, SiO2 (quartz), Al2O3 (corundum) and others Spinels — compound oxides formed by bi and trivalent elements Ferrites, chromites and aluminates are in this group. silicates, which are present in steel like a glass formed with pure SiO2 or SiO2 with admixture of iron, manganese, chromium, aluminium and tungsten oxides and also crystalline silicates. Silicates are the biggest group among non-metallic inclusions. In liquid steel non-metallic inclusions are in solid or liquid condition. It depends on the melting temperature. By stability, non-metallic inclusions are either stable or unstable. Unstable inclusions are those that dissolve in dilute acids (less than 10%concentration). Unstable inclusions are iron and manganese sulfides and also some free oxides. Present-day levels of steel production allow to move off from the metal different inclusions. However in general the content of inclusions in different steels varies within wide limits and has a big influence on the metal properties. Influence of non-metallic inclusions to the properties of steel and alloys Present-day methods of steel and alloy production are unable to attain completely pure metal without any non-metallic inclusions. Inclusions are present in any steel to a greater or lesser extent according to the mixture and conditions of production. Usually the amount of non-metallic inclusions in steel is not higher than 0.1%. However, the number of inclusions in metal is very high because of their extremely small size. Non-metallic inclusions in steel are foreign substances. They disrupt the homogeneity of structure, so their influence on the mechanical and other properties can be considerable. During deformation, which occurs from flatting, forging, and stamping, non-metallic inclusions can cause cracks and fatigue failure in steel. When investigating the influence of non-metallic inclusions on the quality of steel, of great importance are the properties of these inclusions: size, shape, chemical and physical characteristics. All these properties depend on the chemical composition of steel, method of smelting and for certain steel grade. These properties can change within wide limits even within the same mode of production. Different methods for analysis of non-metallic inclusions have been developed and are now in use. These make it possible to determine content, structure and amount of non-metallic inclusions in steel and alloys with high accuracy. See also Aluminium alloy inclusions Entrainment defect Longitudinal facial crack References Casting (manufacturing) Metallurgy Steelmaking