Why do metallographic experiments?- Trojan (Suzhou) material technology Co., Ltd.

Metallographic structure, the internal structure of metals and alloys observed by metallographic methods. It can be divided into: 1. Macrostructure. 2. Microstructure.

Metallography is the science of studying the internal structure of metals or alloys. Not only that, but it also studies the effect on the internal structure of metals or alloys when external conditions or internal factors change. The so-called external conditions refer to temperature, processing deformation, pouring conditions, etc. The so-called intrinsic factors mainly refer to the chemical composition of metals or alloys. The metallographic structure reflects the specific forms of metallographic phases such as martensite, austenite, ferrite, and pearlite.

1. Austenitic carbon and alloying elements are dissolved in the γ-Fe solid solution, and the face-centered cubic lattice of γ-Fe is still maintained. The grain boundaries are relatively straight and regular polygons; the retained austenite in the quenched steel is distributed in the voids between the martensites.

2. Ferritic A solid solution of carbon and alloying elements dissolved in iron. The slow-cooled ferrite in hypoeutectoid steel is massive and the grain boundaries are smooth. When the carbon content is close to the eutectoid composition, ferrite precipitates along the grain boundaries.

3. Cementite - A compound formed from carbon and iron. In the liquid iron-carbon alloy, the cementite (primary cementite) that first crystallizes alone is in the form of a block, the angle is not sharp, and the eutectic cementite is in the shape of a bone. The carbides (secondary cementite) precipitated along the acm line during the cooling process of the hypereutectoid steel are in the form of a network, and the eutectoid cementite is in the form of sheets. When the iron-carbon alloy is cooled below ar1, cementite (three-dimensional cementite) is precipitated from the ferrite, forming discontinuous sheets at the secondary cementite or grain boundaries.

4. The mechanical reaction of pearlite-iron-carbon alloy is formed by the mixed reaction of ferrite and cementite.

The interplate distance of pearlite depends on the degree of undercooling during the decomposition of austenite. The greater the degree of undercooling, the smaller the distance between the formed pearlite sheets. The pearlite layer formed at a1~650°C is thicker, and the magnifying glass is magnified more than 400 times, and parallel wide strip ferrite and thin strip cementite can be distinguished, which are called coarse pearlite and flaky pearlite. It is called pearlite. The pearlite formed at 650~600°C is magnified 500 times under a metallographic microscope. Only a black line can be seen on the cementite of pearlite, and only the 1000-fold soluble flakes are called sorbite. The pearlite formed at 600~550°C is magnified 500 times with a metallographic microscope. The pearlite layer cannot be resolved. Only black globular structures were seen. Only the flakes that can be distinguished by 10,000 times magnification with an electron microscope are called troostites.

5. Upper bainite - a mixture of supersaturated acicular ferrite and cementite with cementite between the ferrite acicularities. The medium temperature (350~550℃) supercooled austenite phase transformation product is usually a bundle of ferrite laths with a misorientation of 6~8od, distributed along the lath. Short rods or small pieces of cemented carbide arranged in the direction of the long axis; typical bainite is feathery, and the grain boundaries are the axis of symmetry. Depending on the orientation, the feathers may be symmetrical or asymmetrical, and the ferritic feathers may be needles, points or blocksIf it is high-carbon and high-alloy steel, the needle-like feathers are invisible; for medium-carbon alloy steel, the needle-like feathers are clear; for low-carbon low-alloy steel, the feathers are clear and the needles are thick. During the transformation process, upper bainite is formed at the grain boundary, and interpenetration does not occur after growth.

6. Lower Bainite - As above, but cementite is needle-like in ferrite. The transition product of supercooled austenite at 350°C ~ s, the typical form is a lens body containing supersaturated carbon ferrite, and there are unidirectionally arranged carbide flakes in the lens body; it is needle-shaped in the crystal, needle Does not cross, but can be shifted. Different from the tempered martensite, the martensite has a layer division, and the lower bainite has the same color, and the lower bainite has a carbide point thicker than the tempered martensite, which is easy to be corroded and blackened. The body is lighter in color and less prone to erosion. The carbide dispersion of high-carbon and high-alloy steel is higher than that of low-carbon and low-alloy steel, and the needle tip is thinner than that of low-carbon and low-alloy steel.

7. Granular Bainite - a complex phase of large or bar ferrite distributed in many small islands. In the bainite transformation temperature region, the transformation product of supercooled austenite in the upper part of the mouth. It consists of carbon-rich island austenite formed by the combination of massive ferrite and strip ferrite. Carbon-rich austenite may remain as retained austenite during subsequent cooling. It is also possible to partially or completely decompose into a mixture of ferrite and cementite (pearlite or bainite); Zui can be partially transformed into martensite and partially retained to form a two-phase mixture called ma structure.

8. A carbide-free bainite-plate ferrite single-phase structure, also known as ferritic bainite. The upper part of the Zui having a temperature in the bainite transformation temperature region is formed. Ferritic ferrite is carbon-rich austenite, and carbon-rich austenite undergoes a similar transformation during subsequent cooling. Carbide-free bainite is commonly found in low carbon steels and is also readily formed in steels with high silicon and aluminum content.

9. Martensite - a supersaturated solid solution of carbon in iron.

Lath martensite: formed in low and medium carbon steel and stainless steel, composed of many parallel laths to form a lath bundle, one austenite grain can be transformed into multiple laths (usually 3 to 5) .

Flaky Martensite (Acicular Martensite): Commonly found in high and high carbon steels and high iron alloys. There is a stitch on the needle that divides the martensite in half. It is needle-shaped or block-shaped, and the needles are arranged at an angle of 120 degrees. High-carbon martensite has clear grain boundaries, and fine acicular martensite is cloth-like, which is called cryptocrystalline martensite.

10. Tempered martensite-Martensite decomposes to form fine transitional carbides and a supersaturated (low carbon) a-phase mixed structure formed by tempering martensite at 150~250 °C.

This type of structure is very easy to be corroded, and it shows a dark black needle-like structure under an optical microscope (maintaining the quenched martensite orientation), which is very similar to lower bainite, and only very small carbonized material points can be seen under a high-power electron microscope .

11. Mixture of tempered troostite carbides and phase A.

It is formed by tempering martensite at 350~500°C. Its microstructure is characterized by very fine granular carbides distributed in the ferrite matrix. The needle-like morphology gradually disappeared, but was still faintly visible. Carbides cannot be resolved under an optical microscope. Only dark tissue can be observed, which can only be observed under an electron microscope. The obvious distinction between the two phases indicates that the carbide particles grow significantly.

12. Tempered sorbite - a ferrite matrix with evenly distributed carbide particles on the matrix.It is formed by tempering martensite at a high temperature of 500~650°C. Its microstructure is characterized by a multiphase structure composed of equiaxed ferrite and fine-grained carbides. Traces of martensitic flakes have disappeared. The shape of cementite is clear, but it is difficult to distinguish under the optical microscope. Under the electron microscope, it can be seen that the cementite particles are relatively large.

13. Ledeburite-eutectic mixture of austenite and cementite. Dendritic austenite is distributed on the matrix of cementite.

14. Granular Pearlite - Composed of ferrite and granular carbides.

It is formed by spheroidizing annealing or martensitic tempering in the temperature range of 650 ° C ~ a1. It is characterized by the distribution of carbides on ferrite in granular form.

15. Widmanstatten structure - if the austenite grains are thicker and the cooling rate is more appropriate, the pre-eutectoid phase may be acicular (flaky) phase, which contains flaky pearlite, which is called Weidmanstatten structure. The Weiss structure ferrite in hypoeutectic steel is flaky, feathery or triangular, and the coarse ferrite is parallel or triangular. Grain growth occurs in austenite grain boundaries. In hypereutectoid steel, the cementite of the Weiss structure is needle-like or rod-like, appearing inside the austenite grains.