Explorando as estruturas cristalinas de aços austeníticos, ferríticos, cimentíticos e martensíticos

Exploring the crystal structures of austenitic, ferritic, cementitic and martensitic steels

Solid matter in nature can be classified into two categories: crystalline and amorphous.

A crystal is a solid with a regular geometric shape formed through the process of crystallization. In a crystal, atoms or molecules are periodically and repeatedly arranged in space according to a certain rule.

An amorphous solid corresponds to a crystal with atoms or molecules arranged irregularly, without periodicity or symmetry. Glass is an example of an amorphous solid.

Solid metals and alloys are mainly crystals. The crystalline structure of metals and alloys is one of the fundamental factors that determine their physical, chemical and mechanical properties.

Iron and steel are alloy systems with iron and carbon as basic elements.

Related reading: Steel vs Iron

Within the Fe-C system, when the carbon content is less than 0.02%, the material is classified as pure iron. If the carbon content exceeds 2.0%, it is called pig iron, while the range between these two limits is classified as steel.

Pure iron, or wrought iron, is characterized by four crystalline structures: α, β, γ and δ. Three of these structures, namely α, β and δ, exhibit cubic core structures, while the fourth, c, has a face cubic core structure.

Pure elemental iron crystallizes at 1538℃ to form a cubic core structure known as δ-iron. As it cools to 1394℃, it transforms into a face-centered cubic structure called γ iron. Further cooling to 912℃ results in the formation of a central cubic structure known as α-iron.

Steel has four main phases: austenite, ferrite, cementite and martensite.

Related Reading: Pig Iron vs Wrought Iron

(a) Austenite

Austenite is an interstitial compound of carbon in γ-Fe. The ratio of Fe atoms to C atoms is 27:1, which means that only one C atom is present in every 6-7 face-centered cubic cells. The concentration of dissolved carbon in γ-Fe is 2.11% at 1148°C and 0.77% at 727°C.

The characteristics of austenite are that its strength and hardness are higher compared to ferrite, while its plasticity and toughness are better. Furthermore, its grain has a polygonal shape and its grain boundary is straighter than that of ferrite.

(b) Ferrite

Ferrite is a solid solution of carbon in α-Fe, with a carbon content close to that of pure iron, approximately 0.02%.

Ferrite has characteristics similar to those of pure iron, including low strength and hardness, as well as good plasticity and toughness. Its microstructure is characterized by shiny polygonal grains.

(c) Cementite

Cementite is a compound composed of iron and carbon in a 3:1 ratio, known as Fe 3 C. It belongs to the orthogonal crystalline system and has a complex crystalline structure. Each cementite cell consists of 12 Fe atoms and 4 C atoms.

The characteristics of cementite include high hardness, low plasticity and toughness. Its δ and Akk values ​​are close to zero and it presents great fragility.

(d) Martensite

When austenitic steel is quenched to a temperature below 150°C, it transforms into martensite, which is extremely hard. Martensite can be considered a supersaturated solid solution that consists of 1.6% carbon in α-Fe and has a tetragonal crystal structure.

There are two types of martensite: high-carbon martensite (lath martensite) and low-carbon martensite (lamellar martensite).

Martensite is characterized by being hard and brittle, having low toughness, high internal tension and being prone to cracking.

The stability of the four phases varies. Ferrite and cementite are crystalline forms that are stable at room temperature, while austenite is stable at high temperatures.

When carbon steel is quenched, mainly martensite is obtained, which is an unstable crystalline form. Alloy steels with different compositions such as Mn, Ni and Cr can be made for different purposes.

Non-researchers in the stainless steel industry are primarily exposed to austenite, ferrite and martensite, with cementite being less commonly encountered.

Stainless steel is a typical alloy with special properties that are obtained by adding alloying components to the base phase.

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1 comment

Muy buena presentación, simple y concreta, soy catedrático de facultad y veo la explicación eficiente para mis alumnos, Muchas gracias.

Gerardo Ayala

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