Nonmetallic Inclusions

(in steel and alloys), mainly chemical compounds of metals with oxygen, sulfur, nitrogen, and other unavoidable nonmetallic impurities, which are present as a separate phase. Nonmetallic inclusions impair the quality of metal, because they may be failure centers during use. A distinction is made among oxide, sulfide, nitride, and phosphate inclusions, according to chemical composition.

The most common oxide inclusions are encountered in the form of simple oxides (FeO, Al₂O₃, and SiO₂), complex oxides of the spinel type (FeO. Cr₂O₃, and MgO.A1₂O₃) and the aluminate type (nCaO.mAl₂O₃), and silicates and lime silicate glasses (2FeO. Si0₂ and 3Al₂O₃.2Si0₂). Sulfide inclusions are most often present in steel as manganese and iron sulfides, which form a continuous series of FeS-MnS solid solutions. The sulfides CaS, TiS, and ZrS are also found. Nitride inclusions are contained in considerable quantities in steels and alloys made with nitride-forming elements; the most common are TiN, ZrN, A1N, NbN, and VN.

A distinction is made among endogenous, exogenous, and exoendogenous nonmetallic inclusions, according to origin. Endogenous inclusions are formed in the reaction of steel components with dissolved oxygen, sulfur, and nitrogen. Exogenous inclusions are products of erosion of refractory materials, slag particles, and inclusions derived from ferroalloys and ores that do not float to the surface of the liquid metal or dissolve in it. Exoendogenous inclusions are exogenous inclusions that undergo a change in composition as a result of reactions taking place at the inclusion-metal boundary.

The size of nonmetallic inclusions ranges from a few millimeters to fractions of a micron. A distinction is made between macroinclusions ( > 1 mm) and microinclusions (≦ 1 mm). Upon deformation of the metal, some inclusions or their aggregates fracture and become elongated, forming defects called hairline cracks. Methods of metallography, X-ray spectral micrography, petrography, microchemistry, X-ray structural analysis, electron microscopy, and ultrasound are used to determine the composition and structure of nonmetallic inclusions.

The most effective means of removing nonmetallic inclusions from steels and alloys are refining remelting (by the electroslag, vacuum-arc, and electron-beam methods). Such methods are used to produce metals for critical applications.