Metallographic structure refers to the internal arrangement of metals and alloys, which can be observed using metallographic techniques. These structures are generally classified into two main categories: macroscopic structure and microscopic structure. Metallography is the scientific study of these internal structures, focusing not only on their forms but also on how external conditions or internal factors influence them.
External conditions such as temperature, mechanical deformation, and casting processes can significantly affect the microstructure of a material. Internal factors like the chemical composition of the alloy also play a crucial role in determining its final structure. Different phases, such as martensite, austenite, ferrite, and pearlite, are formed based on these factors and their interactions during processing.
Austenite is a solid solution where carbon and alloying elements are dissolved in γ-iron, maintaining a face-centered cubic lattice. The grain boundaries are usually straight and exhibit a regular polygonal shape. In quenched steel, retained austenite often appears in the spaces between martensite.
Ferrite is a solid solution of carbon and alloying elements in α-iron. In hypoeutectoid steel, slow-cooled ferrite tends to form large, massive grains with smooth grain boundaries. When the carbon content approaches the eutectoid composition, ferrite may precipitate along the grain boundaries.
Cementite is a compound formed by iron and carbon. In liquid iron-carbon alloys, primary cementite crystallizes first in a block-like form, while eutectic cementite has a bone-like appearance. Secondary cementite, which precipitates during cooling in hypereutectoid steels, appears as a network. Eutectoid cementite forms as thin sheets, and when cooled below the A1 temperature, three-dimensional cementite may form, creating discontinuous flakes.
Pearlite is a mechanical mixture of ferrite and cementite formed through a reaction in iron-carbon alloys. The interlamellar spacing depends on the degree of supercooling during austenite decomposition. Coarser pearlite forms at higher temperatures (A1 to 650°C), appearing as wide ferrite and thin cementite layers under a 400x magnification. At lower temperatures (650–600°C), it appears as black lines under 500x magnification, known as sorbite. At even lower temperatures (600–550°C), it becomes indistinguishable under optical microscopy and is referred to as troostite.
Upper bainite is a mixture of supersaturated acicular ferrite and cementite, forming at medium temperatures (350–550°C). It typically consists of parallel ferrite needles with carbide particles aligned along their length. Its morphology can vary from feathery to needle-like, depending on the alloy composition.
Lower bainite is similar but contains carbide within the ferrite needles. It forms at temperatures below 350°C and appears as lenticular ferrite with fine carbide plates. Unlike tempered martensite, it has a different etching response and color.
Granular bainite is composed of large or strip-shaped ferrite and small islands of carbon-rich austenite. It forms in the upper part of the bainite transformation range and can contain retained austenite that may later transform into pearlite or bainite.
Carbide-free bainite, also known as ferrite bainite, consists solely of ferrite and is found in low-carbon steels or those with high silicon or aluminum content.
Martensite is a carbon-supersaturated solid solution in α-iron. Lath martensite forms in low and medium carbon steels, consisting of parallel slats arranged in bundles. Flaky martensite, common in high-carbon steels, exhibits a needle-like structure with a suture dividing it into halves.
Tempered martensite results from the decomposition of martensite at 150–250°C, forming very fine carbides and a lower carbon α-phase. It appears dark and needle-like under optical microscopy.
Tempered troostite is a mixture of fine carbide particles and ferrite, formed at 350–500°C. Its microstructure is characterized by granular carbides distributed in a ferrite matrix.
Tempered sorbite is a multiphase structure of equiaxed ferrite and fine carbides, formed at 500–650°C. It lacks the original martensitic morphology.
Ledeburite is a eutectic mixture of austenite and cementite, typically found in cast irons. Granular pearlite consists of ferrite and granular carbides, formed through spheroidizing annealing.
Weiss structure refers to a microstructure where pro-eutectoid phases appear as needle-like or flaky forms mixed with pearlite, commonly seen in hypoeutectoid and hypereutectoid steels. This structure develops when austenite grains are coarse and the cooling rate is suitable.
41022 Green Tea,Green Tea 41022,Azawad Tea Alfata Ould Mohamed,Zahra Tea Nouveau Soleil Sarl
DIYAS development Limited , https://www.diyastea.com