Non-metallic inclusions

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

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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. They include oxides, sulphides, 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 or accidental.

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

Purity of steel or Non-metallic inclusions are divided by chemical and mineralogical content, by stableness and 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. sulphides (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 sulphides 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 presented 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 stableness non-metallic inclusions are stable and unstable. Unstable are inclusions which dissolve in dilute acids (less than 10%concentration). Unstable are iron and manganese sulphides and also some free oxides.

Present-day level of steel production allow to move off from the metal different inclusions. However its general content in different steels vary in 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 have no opportunities to get completely pure metal without any non-metallic inclusions. More or less inclusions are presented in any steel in order with it mixture and condition of production. Usually amount of non-metallic inclusions in steel is not higher than 0,1%. However, number of inclusions in metal is very high because of its extremely small size.

Non-metallic inclusions in steel are foreign substances. They break the homogeneity of structure, so its influence on the mechanical and other properties could be considerable. During deformation which occur in the process of flatting, forging and stamping non-metallic inclusions can lead to crack and to fatigue failure of steel.

While investigate influence of non-metallic inclusions to the quality of steel the great importance is its properties: size, shape, chemical and physical characteristics. All this properties depend on chemical content of steel, method of smelting and for certain steel grade. It can change in wide limits even for one and the same mode of production.

Different methods for non-metallic inclusions analysis are developed and used now. It allows to determine content, structure and amount of non-metallic inclusions in steel and alloys with high exactness.