Crystal Structure of Austenitic, Ferritic, Cementite and Martensitic Steels

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

Crystal is a solid with regular geometric shape formed through crystallization process;

In a crystal, atoms or molecules are arranged periodically and repeatedly in space according to a certain rule.

A solid corresponding to a crystal, with atoms or molecules arranged irregularly and without periodicity or symmetry, is called an amorphous.

Glass is known as amorphous.

Solid metals and alloys are mostly crystals.

The crystal structure of metals and alloys is one of the basic factors that determine their physical, chemical and mechanical properties.

Iron and steel is a kind of alloy system with iron and carbon as basic elements.

Related reading: Steel vs Iron

In the Fe-C system, when the carbon content is less than 0.02%, it is called pure iron; when the carbon content is greater than 2.0%, it is called pig iron; when the carbon content is in the middle, it is called steel.

Pure iron, also known as wrought iron, has four crystal structures: α, β, γ and δ. Among them, α, β and δ are cubic center structures, and c is cubic face center structures.

Crystal Structure of Austenitic, Ferritic, Cementite and Martensitic Steels 1

Pure elemental iron crystallizes at 1538 ℃ to form a cubic core structure, which is called δ-iron;

When it is cooled to 1394 ℃, it transforms into cubic face center structure, which is called γ iron;

When it continues to cool to 912 ℃, it becomes a cube core structure, called α-iron.

There are four main phases in steel: 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, that is, only one C atom is contained in 6-7 cubic face centered cells.

The amount of carbon dissolved in γ-Fe is 2.11% at 1148 ℃ and 0.77% at 727 ℃.

Crystal Structure of Austenitic, Ferritic, Cementite and Martensitic Steels 2

Characteristics of austenite: strength and hardness are higher than ferrite, and plasticity and toughness are better.

The grain is polygonal and the grain boundary is straighter than ferrite.

(b) Ferrite·

Ferrite is the solid solution of carbon in α-Fe, and the carbon content is close to pure iron, about 0.02%.

Crystal Structure of Austenitic, Ferritic, Cementite and Martensitic Steels 3

Characteristics of ferrite: its performance is similar to that of pure iron, that is, its strength and hardness are very low, and its plasticity and toughness are good. The microstructure is bright polygonal grains.

(c) Cementite·

Cementite is a compound Fe3C composed of iron and carbon in a ratio of 3:1, which belongs to orthogonal crystal system.

Each cell contains 12 Fe atoms and 4 C atoms, and has a complex crystal structure.

Crystal Structure of Austenitic, Ferritic, Cementite and Martensitic Steels 4

Characteristics of cementite: high hardness, poor plasticity and toughness, δ and Akk close to zero, and great brittleness.

(d) Martensite·

Austenitic steel is quenched to below 150 °C and becomes very hard martensite.

Martensite can be regarded as a supersaturated solid solution containing 1.6% C in α-Fe, which is a tetragonal system.

Martensite is divided into high carbon martensite (lath martensite) and low carbon martensite (lamellar martensite).

Crystal Structure of Austenitic, Ferritic, Cementite and Martensitic Steels 5

Characteristics of martensite: hard and brittle, poor toughness, large internal stress, easy to crack.

The stability of the four phases is different.

Ferrite and cementite are stable crystal forms at room temperature, while austenite is stable at high temperature.

Carbon steel quenching mainly obtains martensite, which is an unstable crystal form.

Alloying steels with different compositions such as Mn, Ni and Cr can be made for different purposes.

Non researchers are mainly exposed to austenite, ferrite and martensite in the stainless steel flow field, while cementite is less exposed.

Stainless steel is a typical alloy with special properties by adding alloy components in the basic phase.

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