Materials science
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- 4- - laboratory work Study of microstructures of cast iron The purpose of the work
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What is microslicing? How to polish and polish the sample? What changes are observed in the composition of carbon steels under the influence of a standard reagent? Name the mirror inclusions found in steels and cast irons. How to determine the carbon content of an alloy? 4- - laboratory work Study of microstructures of cast iron The purpose of the work : to acquire the qualification of studying the microstructures of white, gray, malleable and high-precision cast iron; evaluate the mechanical properties of these cast irons; to determine their areas of use. Theoretical information The amount of carbon and permanent additives in cast iron is higher than in steel. Although the mechanical properties of cast irons are lower than those of steels, their technological properties are much higher (low permeability, liquefiable at relatively low temperatures, good cutting ability, corrosion resistance...). Therefore, cast iron is considered a suitable structural material for the preparation of various details. The cast iron produced in the blast furnace is mostly white cast iron. The carbon in its composition is in the form of cementite (Fe 3 S). Therefore, it is very hard and brittle. According to its structure, white cast iron is divided into the following types: a) pre-eutectic white cast irons (2.14...4.3% S). The structure consists of pearlite, ledeburite and secondary cementite (Fig. 10.1, a). Perlite-black spots, ledeburite-black dots, cementite flow bands; b) the structure of eutectic white cast iron (4.3% С ) consists of ledeburite (a mechanical mixture of pearlite and cementite) (Fig. 10.1, b). c) the structure of post-eutectic white cast irons (4.3...6.67% S ) consists of primary cementite and ledeburite (Fig. 10.1, c). Figure 10.1. Microstructure of white cast iron: a – pre-eutectic cast iron; b – eutectic cast iron; v – cast iron after eutectic. Due to the fact that the composition consists mainly of cementite and ledeburite, the brittleness and hardness of white cast iron are very high, and they are very difficult to cut. Therefore, white cast iron is rarely used in engineering. Balls for grinding ores, asterisks for cleaning cast parts, rolling mills, and cast iron wheels are made from them. They are mainly used for steel production and for obtaining malleable cast iron. In machine building, various details are made from gray (SCH) , malleable (KCH) and high precision (VCH) cast iron. The structure of these cast irons consists of a metal base (ferrite, pearlite, ferrite+perlite) and carbon-graphite in a free state. The metal base of cast iron affects its hardness, precision, corrosion resistance. The limit of tensile strength, plasticity, and corrosion resistance of cast iron depends on the shape, size, amount, and location of the graphite. Gray cast iron. If the cast iron is cooled slowly, and if its content is high in silicon and low in manganese, gray cast iron is formed. Most or all of the carbon in this cast iron is in the form of lamellar graphite. The plate-like form of graphite can be considered as cracks existing in the metal base of cast iron (Fig. 10.1-a,b). This form of graphite reduces the fineness of cast iron. Table 10.1 Characteristics of gray cast iron
of cast iron with high precision consists of spherical graphite grains with ferrite + pearlite or pearlite (Fig. 10.3, d). To obtain such cast iron, ferrosilicon ( to increase the number of graphitization centers) and magnesium or cerium (to bring graphite into a spherical shape) are added to cold cast iron before it is placed in a mold. Since the graphite is spherical, the integrity of the metal base of the cast iron is almost not broken, which increases the precision and plasticity of the cast iron. Such cast irons can also be used instead of cast steel. Details such as crankshafts and distribution shafts of car engines, cylinder liners are made from them. VCH 60-2, VCH 45-5 cast iron is used more often in mechanical engineering . Fig. 1 0 .2. Forms of graphite found in cast iron: α-plate-like; b-spherical; v-bodrognuskha (paga-paga). Figure 10.3. Structures of cast iron: g – gray cast iron with pearlite; d – gray cast iron with ferrite-perlite; iron-ferrite malleable cast iron; j-ferrite-pearlite malleable cast iron; z-ferrite-pearlite cast iron with high precision 1 0 . Table 2 Description of high precision cast iron
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