Free Ferrite

Definition - What does Free Ferrite mean?

Free ferrite is ferrite that is structurally separate and distinct, as may be formed without the simultaneous formation of carbide when cooling hypoeutectoid austenite into the critical temperature range.

Free ferrite is a relatively pure metallic iron phase present in steel or cast iron. In this phase, solid steel has a body-centered cubic crystal structure. Ferrite steel can hold only a minimal amount of carbon, is relatively soft and ductile and is magnetic up to 1414°F (768°C).

Applications of free ferrite include:

  • Beams for bridges
  • High-rise buildings
  • Plates for ships
  • Reinforcing bars for roadways

Free ferrite is also known as proeutectoid ferrite.

Corrosionpedia explains Free Ferrite

Free ferrite is formed directly from the decomposition of hypoeutectoid austenite during cooling, before eutectoid transformation. It forms without the simultaneous formation of cementite.

Ferrite tends to grow along the austenite grain boundaries. When the eutectoid temperature is reached, the remaining austenite transforms into a mixture of ferrite and cementite, with the distinctive lamellar bands of pearlite dispersed between the boundaries of the free ferrite. The amount and distribution of pearlite formation depends on the amount of carbon alloying and carbon enrichment of the austenite during the formation of free ferrite.

Steels with less than 0.76% carbon are called hypoeutectoid steels. Free ferrite is a microconstituent of hypoeutectoid steel that is formed above the eutectoid temperature. Eutectoid ferrite is one of the constituents of pearlite that is formed at a temperature below the eutectoid.

Free ferrite has the advantage of being able to be produced with a wide range of properties, with varying amounts of ferrite and iron carbide, depending on carbon content.

There are some commercial ultralow-carbon steels, which have a completely ferritic structure. The most common structural steels produced have a mixed ferrite-pearlite microstructure. These steels are relatively inexpensive and are produced in large volumes.

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