Non-metallic inclusions

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Non-metallic inclusions are chemical compounds of metal with nonmetal which are present in steel and alloys like separated parts.

Sources of inclusions formation

Non-metallic inclusions arise because of many physical-chemical effects which proceed in molten and consolidated metal during its production.

Non-metallic inclusions which arise because of different reactions during metal production are called natural or indigenous. They include oxides, sulfides, nitrides and phosphides.

Apart from natural inclusions there are also parts of slag, refractories, material of a casting mould (the material which the metal contacts during production) in the metal. Such non-metallic inclusions are called foreign, accidental or exogenous.

Most inclusions in the reduction smelting of metal are inclusions which were formed because of admixture dissolubility decreasing during cooling and consolidation.

The present-day level of steel production technology allows the elimination of most natural and foreign inclusions from the metal. However its general content in different steels can vary between wide limits and has a big influence on the metal properties.

Classification of non-metallic inclusions

Non-metallic inclusions, the presence of which defines purity of steel, are classified by chemical and mineralogical content, by stability and by origin. By chemical content non-metallic inclusions are divided into the following groups:

  1. oxides (simple — FeO, MnO, Cr2O3, SiO2, Al2O3, TiO2 and others; compound — FeO·Fe2O3, FeO·Al2O3, FeO·Cr2O3, MgO·Al2O3, 2FeO·SiO2 and others;
  2. sulfides (simple — FeS, MnS, Al2S3, CaS, MgS, Zr2S3 and others; compound — FeS·FeO, MnS·MnO and others);
  3. nitrides (simple — ZrN, TiN, AlN, CeN and others; compound — Nb(C, N), V(c, N) and others), which can be found in alloyed steel and has strong nitride-generative elements in its content: titanium, aluminium, vanadium, cerium and others;
  4. phosphides (Fe3P, Fe2P and others)

The majority of inclusions in metals are oxides and sulfides since the content of phosphorus is very small. Usually nitrides present in special steels which has an element with a high relationship to nitrogen in its content.

By mineralogical content, oxygen inclusions are divided into the following main groups:

  1. free oxides — FeO, MnO, Cr2O3, SiO2 (quartz), Al2O3 (corundum) and others;
  2. spinels — compound oxides which are formed by bi and trivalent elements. Ferrites, chromites and aluminates are in this group.
  3. 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 which 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 occur in the process of flatting, forging and stamping non-metallic inclusions can lead to cracks and to fatigue failure of 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

home | This article is licensed under the GNU Free Documentation License. See full license termsIt uses material from the Wikipedia article "non-metallic_inclusions ". | compliance | March 21st 2010