Ferrite, Austenite, Cementite, Pearlite, Bainite, Widmanstatten Structure, Martensite, Ledeburite... All in One Article!

Modern materials can be divided into four categories-metals, polymers, ceramics and composite materials. Despite the rapid development of polymer materials, steel in metal materials is still the most widely used and most important material in engineering technology. So what factors determine the dominance of steel materials. Here is a detailed introduction for the gold fans.

Steel is refined from iron ore, with abundant sources and low prices. Steel, also known as iron-carbon alloy, is composed of iron (Fe), carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S) and other small amounts of elements (Cr, V, etc.) Alloy. By adjusting the content of various elements in the steel and the heat treatment process (four fires: quenching, annealing, tempering, and normalizing), a variety of metallographic structures can be obtained, so that the steel has different physical properties.

The steel is sampled, polished, polished, and finally corroded with a specific corrosive agent. The structure observed under a metallurgical microscope is called the metallurgical structure of steel. The secrets of steel materials are hidden in these organizational structures.

In the Fe-Fe3C system, a variety of iron-carbon alloys with different compositions can be formulated. Their equilibrium structures at different temperatures are different, but they are composed of several basic phases (ferrite F, austenite A and cementite Fe3C) composition. These basic phases are combined in the form of a mechanical mixture to form a colourful metallographic structure in steel. There are eight common metallographic structures as follows:

1. Ferrite

The interstitial solid solution formed by dissolving carbon in the interstices of the α-Fe lattice is called ferrite, which has a bcc structure and has an equiaxed polygonal grain distribution, which is represented by the symbol F. Its structure and properties are similar to those of pure iron, with good plasticity and toughness, but low strength and hardness (30-100 HB).

In alloy steel, it is a solid solution of carbon and alloying elements in α-Fe. The amount of carbon dissolved in α-Fe is very low. At AC1 temperature, the maximum amount of carbon dissolved is 0.0218%, but the solubility decreases to 0.0084% as the temperature decreases. Therefore, the ferrite grain boundary will be Cementite appears three times. As the carbon content in steel increases, the amount of ferrite is relatively reduced, and the number of pearlite increases. At this time, the ferrite is network-shaped and crescent-shaped.

 2. Austenite

The interstitial solid solution formed by dissolving carbon in the interstices of the γ-Fe lattice is called austenite, which has a face-centred cubic structure and is a high-temperature phase, which is represented by the symbol A.

Austenite has a maximum solubility of 2.11%C at 1148℃, and a solid solution of 0.77%C at 727℃; higher strength and hardness than ferrite, good plasticity and toughness, and non-magnetic, specific mechanical properties and carbon content and grains The size is related, generally 170~220 HBS, =40~50%.

TRIP steel (plasticized steel) is a steel developed on the basis of good austenite plasticity and flexibility. It uses the strain-induced transformation and transformation-induced plasticity of retained austenite to increase the plasticity of the steel plate and improve the forming of the steel plate. performance. The austenite in carbon or alloy structural steel transforms into other phases during the cooling process. Only after high-carbon steel and carburized steel are carburized and quenched at high temperature, can austenite remain in the gaps of martensite. The metallographic structure is white because it is not easily corroded.

3. Cementite

Cementite is a metal compound synthesized by carbon and iron in a certain ratio. It is represented by the molecular formula Fe3C. Its carbon content is 6.69% and forms (Fe, M)3C in the alloy. Cementite is hard and brittle, plasticity and impact toughness is almost zero, brittleness is very high, hardness is 800HB. It is often distributed in-network, semi-network, flake, needle flake and granular form in steel.

 4. Pearlite

The mechanical mixture composed of ferrite and cementite is called pearlite and is represented by the symbol P. Its mechanical properties are between ferrite and cementite, with high strength, moderate hardness and a certain degree of plasticity.

Pearlite is the eutectoid transformation product of steel, and its form is that ferrite and cementite are alternately shaped like fingerprints, and are arranged in layers. According to the distribution of carbides, it can be divided into two types: flake pearlite and spherical pearlite.

(1) Flake pearlite: It can be divided into three types: coarse flake, medium flake and fine flake.

(2) Spherical pearlite: obtained by spheroidizing annealing, cementite is distributed into spheroids on the ferrite matrix; the size of cementite spheroids depends on the spheroidizing annealing process, especially the cooling rate. Spherical pearlite can be divided into four types of pearlite: coarse sphere, sphere, fine sphere and dot.

 5. Bainite

It is the product of the transformation of steel austenite below the pearlite transformation zone and above the Ms point in the medium temperature zone. Bainite is a mechanical mixture of ferrite and cementite, a structure between pearlite and martensite, represented by the symbol B.

According to the different formation temperatures, it is divided into granular bainite, upper bainite (upper B) and lower bainite (lower B).

Granular bainite has low strength but good toughness; lower bainite has both high strength and good toughness; granular bainite has the worst toughness. The shape of bainite is changeable. From the perspective of shape characteristics, bainite can be divided into three types: feather-like, needle-like and granular.

(1) Upper bainite: The characteristic of upper bainite is that the ferrite strips are arranged roughly in parallel, and there is thin strips (or short rod-shaped) cementite parallel to the ferrite needle axis, which are feathers. shape.

(2) Lower bainite: It is in the shape of thin needles and has a certain orientation, and is more susceptible to corrosion than quenched martensite, very similar to tempered martensite, which is extremely difficult to distinguish under a light microscope, and easy to distinguish under an electron microscope; Carbides are precipitated in the acicular ferrite, and their alignment is at an angle of 55 to 60 degrees with the long axis of the ferrite sheet. There are no twins in the lower bainite, and there are more dislocations.

(3) Granular bainite: the shape is equivalent to polygonal ferrite, and there are many irregular island-like structures inside. When the austenite of the steel is cooled to slightly higher than the upper bainite formation temperature, a part of the carbon atoms in the precipitated ferrite will migrate from the ferrite and through the ferrite/austenite phase boundary into the austenite, so that The austenite is unevenly rich in carbon, so that the transformation from austenite to ferrite is suppressed. These austenite regions are generally shaped like islands, which are granular or elongated and distributed on the ferrite matrix. During the continuous cooling process, according to the composition of the austenite and the cooling conditions, the austenite in the grains can occur Several changes are as follows.

1) Fully or partly decomposed into ferrite and carbide. Under the electron microscope, granular, rod-shaped or small lumps of carbides distributed in multiple directions can be seen;

2) Part of it is transformed into martensite, which is yellowish under the light microscope;

3) Still maintaining carbon-rich austenite.

The ferrite matrix in the granular bainite is covered with granular carbides (the small island structure was originally carbon-rich austenite, which decomposed into ferrite and carbides when cooled or transformed into martensite or still rich Carbon austenite particles). Feather-like bainite, the matrix is ferrite, and strip-like carbides are precipitated on the edge of the ferrite sheet. Lower bainite, acicular ferrite is covered with small flaky carbides, and the long axis of the flaky carbides is approximately 55 to 60 degrees on the ferrite.

6. Wisdom Organization

It is a kind of superheated structure, which is formed by inserting ferrite needle pieces intersecting each other at an angle of about 60 degrees into a steel matrix. The coarse Widmanstatten structure reduces the plasticity and toughness of the steel and increases the brittleness. The hypereutectoid steel forms coarse crystals due to overheating when heated and precipitates quickly when cooled. Therefore, in addition to the net-like precipitation along the austenite grain boundary, a part of the ferrite is cut from the grain boundary to the inside of the grain. The deformation mechanism is formed and arranged in needles to separate out. This kind of distributed organization is called the Widmanstatten organization. When the superheated hypereutectoid steel is cooled, the cementite will also form needles extending from the grain boundary to the grain to form the Widmanstatten structure.

7. Martensite

The supersaturated solid solution of carbon in α-Fe is called martensite. Martensite has high strength and hardness, but its plasticity is very poor, almost zero. It is indicated by the symbol M and cannot bear impact load. Martensite is the product of the rapid cooling of undercooled austenite and the transformation of the shear mode between the Ms and Mf points.

At this time, carbon (and alloying elements) does not have time to diffuse, but the lattice (face centre) of γ-Fe is transformed into the lattice (body centre) of α-Fe, that is, the solid solution (austenite) of carbon in γ-Fe is transformed It is a solid solution of carbon in α-Fe, so martensite transformation is "non-diffusion". According to the metallographic characteristics of martensite, it can be divided into lath martensite (low carbon) and acicular martensite.

(1) Lath martensite: also known as low carbon martensite. The fine martensite strips of roughly the same size are aligned in parallel to form a martensite bundle or martensite field; there is a large difference in orientation between fields, and several different orientations can be formed in a single original austenite grain field.

Since lath martensite is formed at a higher temperature, self-tempering will inevitably occur during the cooling process, and carbides are precipitated inside the formed martensite, so it is susceptible to erosion and darkening

(2) Acicular martensite: also known as flaky martensite or high-carbon martensite, its basic characteristics are: the first martensite sheet formed in an austenite grain is relatively coarse and often Through the entire grain, the austenite grains are divided to limit the size of the martensite formed later, so the size of the flaky martensite is different and the distribution is irregular.

Needle-shaped martensite is formed in a certain orientation. There is a middle ridge in the martensite needles. The higher the carbon content, the more obvious the martensite. At the same time, there is white retained austenite between the martensite.

(3) The martensite formed after quenching can also form three special metallographic structures after tempering:

1) Tempered martensite: refers to the flaky martensite formed during quenching (the crystal structure is body-centred and tetragonal), which is decomposed in the first stage of tempering-the carbon in it is desolvated in the form of transitional carbides-formed, In the solid solution matrix (the crystal structure has become body-centred cubic), there is a complex structure of extremely fine transitional carbide flakes (the interface with the matrix is a coherent interface);

Under a metallographic (optical) microscope, the internal structure of this structure cannot be distinguished even if it is magnified to the maximum magnification. Only black needles (the shape of the black needles and the flaky martensite formed during quenching (also known as The white needles of “α martensite” are basically the same), and this black needle is called “tempered martensite”.

2) Tempered troostite: the product of quenched martensite after intermediate temperature tempering, which is characterized by: the acicular form of martensite will gradually disappear, but it is still faintly visible (chromium-containing alloy steel, the re-alloyed ferrite The crystallization temperature is high, so it still maintains the needle-like shape). The precipitated carbides are small and difficult to distinguish under the light microscope. Only under the electron microscope can the carbide particles be seen, which is very easy to be eroded and the structure becomes black.

If the tempering temperature is higher than the upper limit or the retention time is slightly longer, the needles will appear white; at this time, the carbides are concentrated on the edges of the needles, and the hardness of the steel will be lower and the strength will decrease.

3) Tempered sorbate: the product of quenched martensite after high-temperature tempering. Its characteristic is: the sorbate matrix is covered with fine granular carbides, which can be clearly distinguished under a light microscope. This kind of organization is also called quenched and tempered organization, it has a good combination of strength and toughness. The finer the fine granular carbide on the ferrite, the higher the hardness and strength, and the worse the toughness; on the contrary, the lower the hardness and strength, and the higher the toughness.

 8. ledeburite

The eutectic mixture in the iron-carbon alloy, that is, the liquid iron-carbon alloy with a mass fraction of carbon (carbon content) of 4.3%. At 1480℃, the mechanical mixture of austenite and cementite crystallized from the liquid at the same time is called It is ledeburite, represented by the symbol Ld.

Since austenite transforms into pearlite at 727°C, ledeburite is composed of pearlite and cementite at room temperature. For the sake of distinction, the ledeburite above 727°C is called high-temperature ledeburite (Ld), and the ledeburite below 727°C is called low-temperature ledeburite (L'd). The properties of ledeburite are similar to cementite, with high hardness and poor plasticity.

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