Why do metallographic experiments? Metallographic microscope principle

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

Metallography, 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, and the like. The so-called intrinsic factor mainly refers to the chemical composition of a metal or alloy. Metallographic structures reflect the specific forms of metallographic phases such as martensite, austenite, ferrite, pearlite and the like.

1. Austenite-carbon and alloy elements dissolved in γ-fe solid solution, still maintain the face-centered cubic lattice of γ-fe. The grain boundary is relatively straight and has a regular polygon; the retained austenite in the quenched steel is distributed in the void between the martensite
2. A solid solution in which ferrite-carbon and alloying elements are dissolved in a-fe. The slow-cooled ferrite in the hypoeutectoid steel is massive and the grain boundary is relatively smooth. When the carbon content is close to the eutectoid component, the ferrite precipitates along the grain boundary.
3. Cementite - a compound formed by carbon and iron. In the liquid iron-carbon alloy, the cementite (primary cementite) which is first crystallized separately is in the form of a block, the angle is not sharp, and the eutectic cementite is bone-like. The carbide (secondary cementite) precipitated along the acm line during the cooling of the hypereutectoid steel is net-like, and the eutectoid cementite is in the form of a sheet. When the iron-carbon alloy is cooled to below ar1, cementite (three-dimensional cementite) is precipitated from the ferrite, and discontinuous flakes are formed on the secondary cementite or at the grain boundary.
4. Pearlite-iron-carbon alloy Chinese mechanical reaction mixture of ferrite and cementite formed by the reaction.
The inter-plate distance of the pearlite depends on the degree of subcooling when the austenite is decomposed. The greater the degree of subcooling, the smaller the distance between the formed pearlite sheets. The pearlite layer formed at a1~650°C is thicker, and the magnified glass is magnified 400 times or more to distinguish parallel wide strip ferrite and thin strip cementite, which is called coarse pearlite and flaky pearlite. Referred to as pearlite. The pearlite formed at 650~600°C is magnified 500 times by a metallographic microscope. Only a black line is seen from the cementite of the pearlite, and only the sheet which can be resolved by 1000 times is called soxite. The pearlite formed at 600~550°C is magnified 500 times with a metallographic microscope. The pearlite layer cannot be distinguished. Only the black pellet-like structure is seen. Only the sheet that can be resolved by magnifying 10000 times with an electron microscope is called Qu. body.
5. A mixture of upper bainite-supersaturated acicular ferrite and cementite, cementite between ferrite needles. The phase transition product of supercooled austenite at medium temperature (about 350~550 °C) is typically a bundle of ferrite slats with a substantially parallel orientation difference of 6~8od, and distributed along the slabs. Carbide short rods or small pieces arranged in the direction of the long axis; typically, the bainite is feathery, the grain boundary is a symmetry axis, and the feathers may be symmetrical or asymmetrical due to different orientations, and the ferrite feathers may be needle-like or dotted. Blocky. If it is high-carbon high-alloy steel, needle-like feathers are not visible; medium-carbon alloy steel, needle-like feathers are clear; low-carbon low-alloy steel, feathers are clear, and needles are thick. During the transformation, upper bainite is formed at the grain boundary, and grows into the crystal without interpenetrating.
6. Lower bainite - same as above, but cementite is in the ferrite needle. The transition product of supercooled austenite at 350 ° C ~ ms. The typical form is a lenticular lens containing supersaturated carbon ferrite, and therein is arranged with unidirectionally arranged carbide flakes; it is needle-like in the crystal, and the needles do not cross, but can be transferred. Unlike tempered martensite, martensite has a layer of division, lower bainite has the same color, lower bainite has a carbonized material point that is thicker than tempered martensite, and is susceptible to erosion and blackening. The body is lighter in color and less susceptible to erosion. High carbon high alloy steel has higher carbide dispersion than low carbon low alloy steel, and the needle is thinner than low carbon low alloy steel.
7. Granular bainite - large or strip-shaped ferrite is distributed in the complex phase of many small islands. The transformation product of supercooled austenite in the upper part of zui in the bainite transformation temperature zone. It is composed of massive ferrite and island-like carbon-rich austenite formed by the combination of strip ferrite. The carbon-rich austenite may remain as retained austenite during the subsequent cooling process. It may also be partially or completely decomposed into a mixture of ferrite and cementite (pearlite or bainite); zui may be partially converted to martensite and partially retained to form a two-phase mixture called ma structure.
8. A structure free of carbide bainite-slab ferrite single phase, also known as ferrite bainite. An upper portion of the Zui having a temperature in the bainite transformation temperature region is formed. The ferritic ferrite is carbon-rich austenite, and the carbon-rich austenite has a similar transformation in the subsequent cooling process. Carbide-free bainite is generally found in low carbon steel and is also easily formed in steels with high silicon and aluminum contents.
9. Martensite-carbon supersaturated solid solution in a-fe.
Lath martensite: formed in low, medium carbon steel and stainless steel, consisting of a number of parallel slats forming a slat bundle, an austenite grain can be converted into several slats (usually 3 to 5 ).
Flaky martensite (needle martensite): common in high- and medium-carbon steels and high-fee-fe alloys. There is a suture in the needle to divide the martensite into two halves. It is needle-like or block-shaped, and the needle and needle are arranged at an angle of 120o. The high-carbon martensite has a clear grain boundary, and the fine-needle martensite is cloth-like, called cryptocrystalline martensite.
10. Tempered martensite-martensitic decomposition results in a very fine transitional carbide and supersaturated (lower carbon) a-phase mixed structure which is formed by martensite tempering at 150~250 °C.
This type of tissue is highly susceptible to corrosion, with a dark black needle-like structure under the optical microscope (maintaining the quenched martensite orientation), which is very similar to the lower bainite, and only a very small carbonized material point can be seen under a high power electron microscope.
11. A mixture of tempered troostite-carbide and a-phase.
It is formed by martensite tempering at 350~500 °C. Its microstructure is characterized by the distribution of very fine granular carbides in the ferrite matrix. The needle-like morphology has gradually disappeared, but it is still faintly visible. Carbides cannot be distinguished under the optical microscope. Only dark tissues are observed, which can only be observed under electron microscope. Clearly distinguishing the two phases shows that the carbide particles have grown significantly.
12. Tempered Sorbite - With ferrite as the matrix, uniform carbide particles are distributed on the substrate.
It is formed by high temperature tempering of martensite at 500~650 °C. Its structural feature is a multiphase structure composed of equiaxed ferrite and fine-grained carbides. The traces of martensite flakes have disappeared. The shape of cementite is clear, but it is difficult to distinguish under light microscope. The cementite particles visible under electron microscopy are large.
13. Leysite - a eutectic mixture of austenite and cementite. The dendritic austenite is distributed on the matrix of the cementite.
14. Granular pearlite - consists of ferrite and granular carbide.
It is formed by spheroidizing annealing or martensite tempering in the temperature range of 650 ° C ~ a1. It is characterized in that the carbides are distributed in a granular form on the ferrite.
15. Wei's organization - If the austenite grains are coarser and the cooling rate is more suitable, the pro-eutectoid phase may be in the form of a needle-like (flaky) form mixed with flaky pearlite, called Wei's structure. In the hypoeutectic steel, the ferrite of the Weiss structure has a flaky, feathery or triangular shape, and the coarse ferrite is parallel or triangular. It appears in the austenite grain boundary while growing into the crystal. In the hypereutectoid steel, the morphology of the cementite of the Weiss structure is needle-like or rod-shaped, which appears inside the austenite grains.
 

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