The ferrite is white after attack by nitric acid solution, the pearlite is black, the martensite (M) is a supersaturated solid solution of carbon in α-Fe, elongated sliver or needles in metallographic observation shape.
Refers to the chemical composition of the metal structure, the same crystal structure and physical properties, including solid solution, metal compounds and pure substance.
Metallography that is metallography, is the study of the internal structure of the metal or alloy science. Not only that, it also studies the effect on the internal structure of metals or alloys when external conditions or intrinsic factors change.
The so-called external conditions refers to the temperature, deformation, pouring and so on. The so-called internal factors mainly refers to the chemical composition of the metal or alloy. Metallography is to reflect the specific shape of the metal metallurgy, such as martensite, austenite, ferrite, pearlite and so on.
Austenite – a solid solution of carbon and alloying elements dissolved in γ-fe, maintains the face-centered cubic lattice of γ-fe. Grain boundaries are relatively straight, regular polygons; hardened steel residual austenite distribution in the martensite gap
Ferrite – A solid solution of carbon and alloying elements dissolved in α-fe. The slow-cooling ferrite in the sub-eutectoid steel is massive and the grain boundary is relatively smooth. When the carbon content is close to the eutectoid composition, the ferrite precipitates along the grain boundaries.
Cementite – A compound of carbon and iron. In the liquid iron-carbon alloy, the first cementite alone (primary cementite) is massive, the angle is not sharp, eutectic cementite was skeletal. Eutectoid steel cooling along the acm line precipitated carbide (secondary cementite) was net-like, eutectoid cementite flake. When the iron-carbon alloy is cooled to below ar1, cementite is precipitated from the ferrite (cubic cementite), and discontinuous flakes are formed on the secondary cementite or at the grain boundary.
Pearlite – iron-carbon alloy eutectoid reaction formed by the mechanical mixture of ferrite and cementite.
The pearlite interlaminar distance depends on the degree of undercooling during austenite decomposition. The higher the undercooling, the smaller the distance between the formed pearlite sheets. The pearlite layer formed at a1 ~ 650 ℃ is thicker and more than 400 times larger than that under the microscope. It can be seen that parallel wide ferrite and fine-grained cementite are called coarse pearlite, lamellar pearlite, Referred to as pearlite. The pearlite formed at 650-600C was magnified 500 times with a metallurgical microscope. Only one black line was seen from the cementite of the pearlite. Only the layer that was magnified 1000 times to distinguish it was called sorbite. The pearlite formed at 600-550 ° C was magnified 500 times with a metallurgical microscope, the pearlite layer could not be distinguished and only the black pellet-like structure was seen. Only the layer that can be resolved by magnifying 10000 times with an electron microscope was called Tu’s body.
Martensite – supersaturated solid solution of carbon in a-fe.
Lamellar martensite: formed in low, medium carbon steel and stainless steel, consisting of a number of parallel slats forming a slab bundle. One austenite grain can be transformed into several slats (usually 3 to 5 ).
Flake martensite (acicular martensite): Common in high, medium carbon steel and high Ni Fe-Ni alloy, there is a needle in the stitch martensite is divided into two halves, due to different orientation Was acicular or massive, needle and needle at 120 ° angle arrangement, high-carbon martensite cones grain boundaries clear, fine needle-like martensitic cloth-like, known as cryptocrystalline martensite.
Secondary-martensite– Tempering of the workpiece during the cooling process of retained austenite formed martensite transformation.
Tempering martensitic – martensitic decomposition of very fine transitional carbide and supersaturated (low carbon) a-phase mixed tissue which is formed by the martensite at 150 ~ 250 ℃ tempering.
This tissue is extremely susceptible to corrosion. Dark microscopic needle-like tissue (keeping the quenched martensite oriented) is very similar to lower bainite, and very fine carbide dots are only visible under high power electron microscopy.