Cast iron – solid, corrosion-resistant, but brittle iron-alloy with the content of carbon C in the range from 2.14 to 6.67 %. Despite the presence of specific deficiencies, has a variety of types, properties, applications. Widely used is ductile cast iron.
This material has been known since the IV century BC, His Chinese roots are in the VI century BC In Europe the first mention of industrial alloy production date back to XIV, and in Russia – XVI century. But the production technology of ductile iron has been patented in Russia in the XIX century. After developed A. D. Annosum.
As the gray cast irons are limited in use because of low mechanical properties as steel – expensive and have a low hardness and longevity, the question arose about creating a metal reliable, durable, solid, at the same time having a high strength and a certain plasticity.
Wrought iron is impossible, however, due to its plastic characteristics, it lends itself to some types of forming (e.g., stamping).
The Main way – smelting in blast furnaces.
Source products for blast processing:
The Stages of the blast furnace:
At the end of the working cycle of the blast furnace to get pig iron, slag and flue gases.
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Depending on the cooling rate, microstructure, saturation with carbon and additives, you might receive several types of cast irons
Ductile iron obtained by heat treatment of pig.
With the ferum carbon can form several different types of alloys according to the type of the crystal lattice, which is displayed at the option of the microstructure.
For cast irons has a distinctive microstructure. Graphite can be bound and form the above-mentioned structure, and can stay in a free state in the form of various inclusions. On properties are affected as the main grain, so these formations. Graphite fractions in the metal are plates, flakes or balls.
Leaf shape characteristic of gray iron-carbon alloys. It leads to their fragility and unreliability.
The inclusion of flake are ductile iron, than a positive effect on their mechanical performance.
The structure of the Spherical graphite further improves the quality of the metal, affecting the increase in the hardness, reliability, endurance to significant loads. Such are the characteristics of high strength cast iron. Ductile iron properties of their causes ferritic or pearlitic bases with the presence of flake graphite inclusions.
It is produced from the white pig zaevtektoidnoj low-carbon alloy by annealing the ingots with carbon 2.4 to 2.8% and corresponding to the presence of additives (Mn, Si, S, P). The thickness of the annealed parts should not be more than 5 cm For casting very thick graphite has the shape of the plates and the desired properties are not achieved.
In Order to obtain ductile cast iron with ferritic basis, the metal is placed in special boxes and pour sand. Tightly closed containers is placed in a heating furnace. Carry out the following steps after annealing:
This process is called annealing gravityslider.
After the microstructure of a material is a ferrite of flaky granules of graphite. This type is called “chemosurgery”, as the fracture is black.
This is a variation of the iron-carbon alloy, which also arises from zaevtektoidnoj white, but the carbon content is increased: 3-3,6 %. To produce castings with perlite they are placed in boxes and pour the crushed powdery iron ore or mill scale. The procedure of annealing is simplified.
Due to languor under the influence of heat in a metal environment diffusion occurs: the allocated cementation in the collapse of the graphite partly leaves the surface layer of the annealed parts, settling on the surface of the ore or dross. Get softer, sticky and plastic top layer “beloserdechnykh” malleable cast iron with a solid middle.
This is called incomplete. It provides the breakdown of cementite and ledeburite on lamellar pearlite with the appropriate graphite. If necessary granular pearlitic ductile cast iron with high toughness and ductility, used for additional heating of the material up to 720 C. the formed grains of pearlite with graphite flake inclusions.
Long “yearning” of the metal in the furnace is the result of complete disintegration of the cementite and the ferrite ledeburite. Thanks to the technological tricks, get alloy with high carbon content – ferritic structure, typical of low carbon steel. However, carbon itself is not going away – it proceeds from the associated with iron status in free. The effects of temperature changes the shape of the graphite inclusions to flake.
Ferritic structure causes the decrease in hardness, increase in strength values, the presence of characteristics such as toughness and ductility.
Marking ductile cast irons ferritic: KCH30-6, KCH33-8, KCH35-10, KCH37-12 where:
KCH – symbol of the variety – ductile;
30, 33, 35, 37: σIn, 300, 330, 350, 370 N/mm2 – maximum load which it can withstand without rupturing;
6, 8, 10, 12 – elongation δ, % – indicator of plasticity (the higher the value, the more metal bending pressure).
Hardness – about 100-160 NV.
This material is for its parameters takes a middle position between such as steel and iron-alloy grey. Malleable cast iron ferritic pearlitic basis is inferior in terms of wear resistance, corrosion and fatigue strength, however, higher mechanical endurance, ductility, casting properties. Due to the low price is widely used in industry for the manufacture of parts operating under low and medium load: gears, housings, rear axles, plumbing.
Due to incomplete annealing of primary, secondary cementite and ledeburite time to fully dissolve in the austenite, which is at a temperature of 720 C is converted to perlite. The latter is a mechanical mixture of grains of ferrite and tertiary cementite. In fact, some of the carbon remains in the bound form, determines the structure, and part – “exempt” in flake graphite. The pearlite can be a lamellar or granular. Thus is formed pearlitic ductile cast iron. Properties due to the rich harder and less supple structure.
These, in comparison with ferritic, have higher anti-corrosion, wear resistant properties, their strength is much higher, but below casting characteristics and plasticity. Susceptibility to mechanical stress increased the surface, keeping the hardness and toughness of the core product.
Marking cast iron malleable pearlitic class: KCH45-7, KCH50-5, КЧ56-4, KCH60-3, KCH65-3, KCH70-2, КЧ80 in 1.5.
The First digit – symbol of strength: 450, 500, 560, 600, 650, 700 and 800 N/mm2, respectively.
The Second – symbol of ductility: elongation δ, % – 7, 5, 4, 3, 3, 2 1.5.
Pearlitic malleable cast iron has found application in machine building and instrument making for designs operating at high loads both static and dynamic: camshafts, crankshafts, clutch parts, pistons, rods.
Material resulting from thermal treatment, namely annealing can then be subjected to techniques of temperature influences. Their main goal – even greater strength, wear resistance, corrosion resistance and aging.
Heat a cast-iron products may be conducted locally or comprehensively. For local high-frequency currents are used either an acetylene flame (holding hardening). For a comprehensive – a heating furnace. By local heating is tempered only the upper layer with increased hardness and strength, but retains the ductility and toughness of the core.
It is important to specify that wrought iron is impossible not only because of insufficient mechanical characteristics, but also because of its high sensitivity to abrupt temperature changes, which is inevitable when tempering with water cooling.
This kind of relates to ductile and doped, they are grey (ASF), malleable (achk) and high strength (ACHV). For the production of achk use ductile iron which subjected to annealing or normalization. Processes are carried out with the aim of improving its mechanical properties and the formation of new features – wear resistance at friction with other items.
Marked: ABC-1, ABC-2. Used for the production of crankshafts, gears, bearings.
In addition to the iron-base and graphite they are composed of other components that also determine the properties of cast iron: manganese, silicon, phosphorus, sulfur, some alloying elements.
Mangan increases the fluidity of molten metal, corrosion resistance and wear resistance. It contributes to hardness and strength, the binding of carbon with iron in the chemical formula Fe3C, the formation of granular pearlite.
Silicon also has a positive effect on the fluidity of the liquid alloy, promotes dissolution of cementite and separation of the graphite inclusions.
Brimstone – negative, but an inevitable component. It reduces the mechanical and chemical properties, stimulates the formation of cracks. However, the rational relationship of its content with other elements (e.g., manganese) allows you to adjust the microstructural processes. Thus, when the ratio of Mn-S 0,8-1,2 perlite is maintained at all time temperature effects. While increasing the ratio to 3, it appears possible to obtain any desired structure depending on the specified parameters.
Phosphorus fluidity changes for the better, affect the strength, reduces toughness and ductility, affects the duration of graphitization.
Chromium and molybdenum hinder the formation of graphite flakes, some of the contents contribute to the formation of granular pearlite.
Tungsten increases wear resistance when working in areas of high temperatures.
Aluminum, Nickel, copper promote graphitization.
Adjusting the number of chemical elements included in the iron-carbon alloy, and their ratio, can influence the final properties of cast iron.
Ductile iron– a material with wide use in the art. Its main advantages:
The disadvantages can be attributed to individual characteristics:
Despite the existing shortcomings, ductile iron has an important place in metallurgy and mechanical engineering. Are produced from it are important parts, like crankshafts, parts brake pads, gears, pistons, rods. Having a small variety of brands, individual niche in the industry is ductile cast iron. Its application is typical of those loads under which the use of other materials is unlikely.
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Alin Trodden - author of the article, editor
"Hi, I'm Alin Trodden. I write texts, read books, and look for impressions. And I'm not bad at telling you about it. I am always happy to participate in interesting projects."
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