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Non-metallic inclusions in steel

Dr. Dmitri Kopeliovich

Non-metallic inclusions are chemical compounds of metals (Fe, Mn, Al, Si, Ca) with non-metals (O, S, C, H, N). Non-metallic inclusions form separate phases. The non-metallic phases containing more than one compound (eg. different oxides, oxide+sulfide) are called complex non-metallic inclusions (spinels, silicates, oxysulfides, carbonitrides).

Despite of small content of non-metallic inclusions in steel (0.01-0.02%) they exert significant effect on the steel properties such as:

Depending on the source, from which non-metallic inclusion are derived, they are subdivided into two groups: indigenous and exogenous inclusions.

Indigenous inclusions are formed in liquid, solidified or solid steel as a result of chemical reactions (deoxidation, desulfurization) between the elments dissolved in steel.

Exogenous inclusions are derived from external sources such as furnace refractories, ladle lining, mold materials etc. Amount of exogenous inclusions and their influence on the steel properties are insufficient.

Types of non-metallic inclusions

  • Oxides

FeO, Al2O3, SiO2, MnO, Cr2O3 etc.
Al2O3*SiO2, Al2O3*FeO, Cr2O3*FeO, MgO*Al2O3, MnO*SiO2 etc.

  • Sulfides

FeS, MnS, CaS, MgS, Ce2S3 etc.

  • Oxysulfides

MnS*MnO, Al2O3*CaS, FeS*FeO etc.

  • Carbides

Fe3C, WC, Cr3C2, Mn3C, Fe3W3C etc.

  • Nitrides

TiN, AlN, VN, BN etc.

  • Carbonitrides

Titanium carbonitrides, vanadium carbonitrides, niobium carbonitrides etc.

  • Phosphides

Fe3P, Fe2P, Mn5P2

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Formation of non-metallic inclusions

There are three stages of inclusions formation:

1. Nucleation
At this stage nuclei of new phase are formed as a result of supersaturation of the solution (liquid or solid steel) with the solutes (eg. Al and O) due to dissolution of the additives (deoxidation or desulfurization reagents) or cooling down of the metal.
The nucleation process is determined by surface tension on the boundary inclusion-liquid steel. The less the surface tension, the lower supersaturation is required for formation of the new phase nuclei.
The nucleation process is much easier in the presence of other phase (other inclusions) in the melt. In this case the new phase formation is determined by the wetting angle between a nucleus and the substrate inclusion. Wetting condition (low wetting angle) are favorable for the new phase nucleation.

2. Growth
Growth of a separate inclusion continues until the chemical equilibrium is achieved (no supersaturation).
Growth of inclusions in solid steel is very slow process therefore a certain level of non-equilibrium supersaturation may be retained (eg. martensite).

3. Coalescence and agglomeration
Motion of the molten steel due to thermal convection or forced stirring causes collisions of the inclusions, which may result in their coalescence (merging of liquid inclusions) or agglomeration (merging of solid inclusions). The coalescence/agglomeration process is driven by the energetic benefit obtained from decrease of the boundary surface between the inclusion and the liquid steel. Inclusions with higher surface energy have higher chance to merge when collide.
Large inclusions float up faster than the smaller ones. The floating inclusions are absorbed by the slag. The floating process may be intensified by moderate stirring. Vigorous stirring will result in breaking the large inclusions into small droplets/fragments. Gas bubbles moving up through the molten steel also promote the inclusions floating and absorption by the slag. Blowing inert gas in Ladle Furnace (LF) or Vacuum ladle degassing of steel in result in obtaining cleaner steel.

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Morphology of non-metallic inclusions

  • Globular shape of inclusions is preferable since their effect on the mechanical properties of steel is moderate. Spherical shape of globular inclusions is a result of their formation in liquid state at low content of aluminum. Examples of globular inclusions are manganese sulfides and oxysulfides formed during solidification in the spaces between the dendrite arms, iron aluminates and silicates.
  • Platelet shaped inclusions. Steels deoxidized by aluminum contain manganese sulfides and oxysulfides in form of thin films (platelets) located along the steel grain boundaries. Such inclusions are formed as a result of eutectic transformation during solidification. Platelet shaped inclusions are most undesirable. They considerably weaken the grain boundaries and exert adverse effect on the mechanical properties particularly in hot state (hot shortness).
  • Dendrite shaped inclusions. Excessive amount of strong deoxidizer (aluminum) results in formation of dendrite shaped oxide and sulfide inclusions (separate and aggregated). These inclusions have melting point higher than that of steel. Sharp edges and corners of the dendrite shaped inclusions may cause local concentration of internal stress, which considerably decrease of ductility, toughness and fatigue strength of the steel part.
  • Polyhedral inclusions. Morphology of dendrite shaped inclusions may be improved by addition (after deep deoxidation by aluminum) of small amounts of rare earth (Ce,La) or alkaline earth (Ca, Mg) elements. Due to their more globular shape polyhedral inclusions exert less effect on the steel properties than dendrite shape inclusions.

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Distribution of non-metallic inclusions

Besides of the shape of non-metallic inclusions their distribution throughout the steel grain structure is very important factor determining mechanical properties of the steel.

  • Homogeneous distribution of small inclusions is the most desirable type of distribution. In some steels microscopic carbides or nitrides homogeneously distributed in the steel are created by purpose in order to increase the steel strength.
  • Location of inclusions along the grain boundaries is undesirable since this type of distribution weakens the metal.
  • Clusters of inclusions are also unfavorable since they may result in local drop of mechanical properties such as toughness and fatigue strength.

Distribution of non-metallic inclusions may change as a result of metal forming (eg. Rolling). Ductile inclusions are deformed and elongated in the rolling direction. The less ductile inclusions are breaking forming chains of fragments. Elongated inclusions and chains result anisotropy of mechanical and other properties. Mechanical properties in transverse direction are lower than those parallel to the rolling direction.

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non-metallic_inclusions_in_steel.txt · Last modified: 2012/05/31 by dmitri_kopeliovich
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