Modern materials can be divided into four main 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. So what factors determine the dominance of steel materials? The following is a detailed introduction to the characteristics of 8 common metallographic structures. Steel is smelted from iron ore, which is abundant in source and low in price. Steel, also known as iron-carbon alloy, is composed of iron (Fe) and carbon (C), silicon (Si), manganese (Mn), phosphorus (P), sulfur (S) and other small amounts of elements (Cr, V, etc.). alloy. By adjusting the content of various elements in steel and the heat treatment process (the main four fires: quenching, annealing, tempering, normalizing), various metallographic structures can be obtained, so that steel has different physical properties. The steel is sampled, polished, polished, and finally corroded with a specific etchant. The structure observed under the metallographic microscope is called the metallographic structure of steel. The secrets of steel materials are hidden in these organizational structures. In the Fe-Fe3C system, a variety of iron-carbon alloys with different compositions can be prepared. Their equilibrium structures at different temperatures are different, but they are composed of several basic phases (ferrite F, austenite A and cementite). Fe3C) composition. These basic phases combine in the form of mechanical mixtures to form the colorful metallographic structure in steel. There are eight common metallographic structures: 1. The interstitial solid solution formed by ferritic carbon dissolved in the α-Fe lattice gap is called ferrite, which belongs to the bcc structure and has an equiaxed polygonal grain distribution. The symbol F is used. express. Its structure and properties are similar to those of pure iron, with good plasticity and toughness, but lower strength and hardness (30-100 HB). In alloy steel, it is a solid solution of carbon and alloying elements in α-Fe. The dissolved amount of carbon in α-Fe is very low. At AC1 temperature, the maximum dissolved amount of carbon is 0.0218%, but the solubility decreases to 0.0084% with the decrease of temperature. Cementite appears three times. As the carbon content in steel increases, the amount of ferrite decreases relatively, and the amount of pearlite increases, and the ferrite is network-like and crescent-like at this time. 2. The interstitial solid solution formed by dissolving austenite carbon in the γ-Fe lattice gap is called austenite, which has a face-centered cubic structure and is a high-temperature phase, which is represented by the symbol A. Austenite has a maximum solubility of 2.11%C at 1148℃, and can dissolve 0.77%C at 727℃; its strength and hardness are higher than that of ferrite, its plasticity and toughness are good, and it is non-magnetic. The specific mechanical properties are related to carbon content and grain size. Size is related, generally 170~220 HBS. TRIP steel (plastic-transformation steel) is a steel developed on the basis of austenite plasticity and good flexibility. It uses the strain-induced transformation and transformation-induced plasticity of retained austenite to improve the plasticity of the steel plate and improve the forming of the steel plate. performance. Austenite in carbon or alloy structural steel is transformed into other phases during cooling. Only after high-carbon steel and carburized steel are carburized and quenched at high temperature, austenite can remain in the gap of martensite. The metallographic structure is white because it is not easily corroded. 3. Cementite Cementite is a metal compound synthesized by carbon and iron in a certain proportion. It is represented by the molecular formula Fe3C, and its carbon content is 6.69%, which forms (Fe, M)3C in the alloy. Cementite is hard and brittle, with almost zero plasticity and impact toughness, very brittle, with a hardness of 800HB. It is often distributed in the form of network, semi-net, flake, needle flake and granular in steel. 4. Pearlite The mechanical mixture composed of ferrite and cementite is called pearlite, which 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, and its morphology is that ferrite and cementite interphase with each other like fingerprints and are arranged in layers. According to the distribution of carbides, it can be divided into two types: flaky pearlite and spherical pearlite. (1) Flake pearlite: It can be divided into three types: coarse flake, medium flake and fine flake. (2) Spherical pearlite: obtained by spheroidizing annealing, the cementite is distributed on the ferrite matrix in a spherical shape; the size of the cementite spheroid depends on the spheroidizing annealing process, especially the cooling rate. Spherical pearlite can be divided into coarse spherical, spherical, fine spherical and point-like four kinds of pearlite. 5. Bainite is the product of the transformation of the austenite of the steel below the pearlite transformation zone and the medium temperature zone above the Ms point. Bainite is a mechanical mixture of ferrite and cementite, a structure between pearlite and martensite, represented by the symbol B. According to the different forming temperature, it is divided into granular bainite, upper bainite (B upper) and lower bainite (B lower). Granular bainite has low strength but good toughness; lower bainite has both high strength and good toughness; granular bainite has the worst toughness. Bainite has a variety of shapes. From the perspective of shape characteristics, bainite can be divided into three categories: feather-like, needle-like and granular.