Understand the elements in iron and steel materials: Austenite, Martensite, Cementite, Pearlite, Bainite, Widmanstatten, Ferrite, Ledeburite

Modern materials can be divided into four categories - metals, polymers, ceramics and composites. Despite the rapid development of polymer materials, steel in metal materials is still the most widely used and important material in engineering technology. What factors determine the dominance of steel materials. Let's introduce it in detail below.

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

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

In Fe-Fe3C system, a variety of iron carbon alloys with different components can be prepared. Their equilibrium structures are different at different temperatures, but they are composed of several basic phases (Ferrite F, Austenite A and Cementite Fe3C). These basic phases are combined in the form of mechanical mixture to form a rich and colorful metallographic structure in iron and steel. There are eight common metallographic structures:

1

Ferrite

Carbon dissolved in α-Fe the interstitial solid solution formed in the Fe lattice gap is called ferrite, which belongs to bcc Structure and is distributed in equiaxed polygonal grains, represented by the symbol F. Its microstructure 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, carbon and alloy elements α-Fe Solid solution in Fe. Carbon in α- the dissolution of Fe is very low. At AC1 temperature, the maximum dissolution of carbon is 0.0218%, but the solubility decreases to 0.0084% with the decrease of temperature. Therefore, three times of cementite will appear at the ferrite grain boundary under slow cooling conditions. With the increase of carbon content in steel, the amount of ferrite decreases relatively and the amount of pearlite increases. At this time, the ferrite is network and crescent.

Left ) Ferrite 500x Rolling electrical pure iron    

Right ) Ferrite 500x Annealed state

2

Austenite

Carbon dissolved in γ- The interstitial solid solution formed in the Fe lattice gap is called Austenite, which has face centered cubic structure and is a high temperature phase, represented by symbol A.

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

TRIP steel is a steel developed on the basis of good austenite plasticity and flexibility. The plasticity of the steel plate and the formability of the steel plate are improved by using the strain induced transformation and transformation induced plasticity of residual austenite. Austenite in carbon or alloy structural steel changes into other phases during cooling. Only after carburizing and high-temperature quenching of high carbon steel and carburized steel can austenite remain in the gap of martensite, and its metallographic structure is white because it is not easy to be eroded.

Left ) Austenite 200x  

Middle ) Residual A after quenching of T12 steel 500x  

Right ) 20CrMo M + Residual A + K  400x

3

Cementite

Cementite is a metal compound synthesized by carbon and iron in a certain proportion. It is expressed by the molecular formula Fe3C. Its carbon content is 6.69%, and(Fe,M)3C is formed in the alloy. Cementite is hard and brittle, with almost zero plasticity and impact toughness, great brittleness and hardness of 800HB. It is often distributed in network, semi network, flake, needle flake and granular in iron and steel.

Left ) Reticular cementite 200x  

Middle ) Acicular cementite( Widmanstatten structure ) 200x  

Right ) Reticular, granular and tertiary cementite 500x

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 certain plasticity.

Pearlite is the eutectoid transformation product of steel. Its morphology is that ferrite and cementite alternate with each other, such as fingerprint, in layered arrangement. According to the distribution of carbide, it can be divided into flake pearlite and spherical pearlite.

Left ) Lamellar pearlite 200x  

Middle ) Spherical pearlite 500x  

Right ) Spherical pearlite 1000x  

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

(2) Spherical pearlite: obtained by spheroidizing annealing, and cementite is distributed on the ferrite matrix as spherical particles; The size of cementite spheres depends on the spheroidizing annealing process, especially the cooling rate. Globular pearlite can be divided into four types: coarse globular, globular, fine globular and punctate pearlite.

5

Bainite

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

According to the formation temperature, it is divided into granular bainite, upper bainite (above B) and lower bainite (below B).

Granular bainite has low strength but good toughness; Lower bainite has both high strength and good toughness; The toughness of granular bainite is the worst. The morphology of bainite is changeable. From the shape characteristics, bainite can be divided into feather, needle and granular.

Left ) Feathery bainite 500x  

Middle ) Lower bainite、Martensite、retained austenite and small amount of feathery martensite 200x  

Right ) Granular bainite 200x  

(1) Upper bainite: the upper bainite is characterized in that the strip ferrite is roughly arranged in parallel, and there are fine strip (or fine short rod) cementite parallel to the ferrite needle axis, which is feathery.

(2) Lower bainite: it is fine needle flake with certain orientation. It is easy to be eroded compared with quenched martensite and very similar to tempered martensite. It is very difficult to distinguish under light microscope and electron microscope; Carbide precipitates in acicular ferrite, and its arrangement orientation forms an angle of 55 ~ 60 degrees with the long axis of ferrite sheet. There are no twins in lower bainite and more dislocations.

(3) Granular bainite: ferrite with polygonal shape and many irregular island like structures. When the austenite of the steel is cooled to slightly higher than the upper bainite formation temperature, part of the carbon atoms of the precipitated ferrite migrate from the ferrite and through the ferrite / austenite phase boundary to the austenite, which makes the austenite unevenly rich in carbon, so that the transformation from austenite to ferrite is inhibited. These austenite areas are generally islands, granular or strip-shaped, distributed on the ferrite matrix. During continuous cooling, the austenite in the granular shell can change as follows according to the composition and cooling conditions of austenite.

1) It is completely or partially decomposed into ferrite and carbide. Under the electron microscope, granular, rod-shaped or small block carbides with dispersed multidirectional distribution can be seen;

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

3) Carbon rich austenite remains.

Granular carbides are distributed on the ferrite matrix in granular bainite (the small island structure is originally carbon rich austenite, which decomposes into ferrite and carbide when cooling, or transforms into martensite or still carbon rich austenite particles). Feathery bainite, the matrix is ferrite, and strip carbides precipitate at the edge of ferrite sheet. Small flake carbides are distributed on the lower bainite and acicular ferrite, and the angle of flake carbides on the long axis of ferrite is about 55 ~ 60 degrees.

Left ) Granular bainite 10000x

Middle ) Feathery bainite 8000x

Right ) Lower bainite 8000x

6

Widmanstatten structure

It is a kind of superheated structure, which is formed by inserting ferrite needles at an angle of about 60 degrees into the steel matrix. The coarse widmanstatten structure reduces the plasticity and toughness of steel and increases the brittleness. During heating, hypoeutectoid steel forms coarse grains due to overheating and precipitates rapidly during cooling. Therefore, in addition to the network precipitation of ferrite along the austenite grain boundary, a part of ferrite is formed from the grain boundary to the grain according to the shear mechanism and precipitated independently in needle shape. This distribution structure is called widmanstatten structure. When the superheated hypereutectoid steel is cooled, the cementite will also form needle shape, extend from the grain boundary to the grain and form widmanstatten structure.

Left ) Coarse grained widmanstatten structure 200x

Middle ) Coarse grained widmanstatten ferrite 200x

Right ) Widmanstatten structure 200x  

7

Martensite

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

At this time, carbon (and alloy elements) have no time to diffuse, but only by γ- The lattice (face center) of Fe is transformed into α- The lattice (body center) of Fe, that is, carbon in γ- The solid solution (austenite) in Fe is transformed into carbon α- Because of the solid solution in Fe, the martensite transformation is "non diffusion". According to the metallographic characteristics of martensite, it can be divided into strip martensite (low carbon) and needle martensite.

(1) Lath martensite: also known as low carbon martensite. Thin martensite strips with roughly the same size are arranged in parallel to form martensite bundles or martensite fields; The orientation difference between domains is large, and several domains with different orientations can be formed in one original austenite grain.

(2) Due to the high formation temperature of strip martensite, self tempering is bound to occur in the cooling process, and carbide precipitates in the formed martensite, so it is easy to be eroded and darkened.

Left ) Low carbon martensite of 20# steel 630x

Middle ) Low carbon martensite grains intersect at a certain angle 10000x

Right ) Low carbon martensite 600x  

(2) Acicular martensite: also known as sheet martensite or high carbon martensite, its basic feature is that the first martensite sheet formed in an austenitic grain is relatively large, often runs through the whole grain, and the austenitic grain is divided, so that the size of martensite formed later is limited. Therefore, the size of sheet martensite is different and the distribution is irregular.

Acicular martensite is formed in a certain orientation. There is a middle ridge in the martensite needle. The higher the carbon content, the more obvious, and the sharper the martensite. At the same time, there is white retained austenite between the martensites.

Left ) Coarse acicular martensite + Remnant A + Granular K 500x

Middle ) Superheated coarse martensite + Retained austenite there are middle ridges and small cracks in coarse needle martensite 1000x

Right ) T8 steel shape 600x  

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

1) Tempered martensite: refers to the sheet martensite formed during quenching (the crystal structure is body centered tetragonal) decomposed in the first stage of tempering - in which the carbon is desolved in the form of transition carbide - and extremely fine transition carbide sheets are dispersed and distributed in the solid solution matrix (the crystal structure has changed to body centered cubic) (the interface with the matrix is a coherent interface) Multiphase structure of;

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

2) Tempered troostite: the product of medium temperature tempering of quenched martensite, which is characterized in that the acicular form of martensite will gradually disappear, but it is still vaguely visible (the recrystallization temperature of alloy ferrite in chromium containing alloy steel is high, so it still maintains acicular form). The precipitated carbide is small, which is difficult to distinguish clearly under light microscope, and carbide particles can be seen only under electron microscope, It is easily eroded and blackens the tissue.

If the tempering temperature is higher than the upper limit or the retention time is slightly longer, the needle leaves will be white; At this time, the carbides are biased at the edge of the needle, and the hardness of the steel is slightly lower and the strength decreases.

Left ) Tempered acicular troostite 500x   

Right ) Tempered troostite,fine grained cementite is distributed under electron microscope 20000x  

3) Tempered sorbite: the product of quenched martensite after high temperature tempering. The utility model is characterized in that fine granular carbides are distributed on the sorbite matrix, which can be clearly distinguished under the light microscope. This kind of structure, also known as quenched and tempered structure, has a good combination of strength and toughness. The finer the fine granular carbide on ferrite, the higher the hardness and strength, and the worse the toughness; On the contrary, the hardness and strength are lower, while the toughness is higher.

Left ) Sorbite maintaining martensite orientation 500x   

Middle )  Tempered sorbite with maintaining martensite orientation 500x

Right ) Matrix ferrite of tempered sorbite with martensite orientation 15000x                 

8

Ledeburite

Eutectic mixture in iron carbon alloy, i.e. liquid iron carbon alloy with carbon mass fraction (carbon content) of 4.3%, the mechanical mixture of austenite and cementite crystallized from the liquid at 1480 ℃ is called ledeburite, which is represented by the symbol Ld.

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