Today there is almost no area of human life,wherever cast iron is used. This material is known to mankind for a long time and has perfectly proved itself from a practical point of view. Iron casting is the basis of a great many parts, assemblies and mechanisms, and in some cases even a self-contained product capable of performing the functions assigned to it. Therefore, in this article we will pay the closest attention to this iron-containing compound. Also find out what types of cast iron are, their physical and chemical features.
Definition
Cast iron is a truly unique alloy of iron and carbon, in which Fe is more than 90%, and C is not more than 6.67%, but not less than 2.14%. Also, carbon can be found in cast iron in the form of cementite or graphite.
Carbon gives the alloy a high enough hardnesshowever, at the same time, lowers the ductility and ductility. In this regard, cast iron is a fragile material. Also, special additives are added to certain cast iron grades that are able to give the compound certain properties. In the role of alloying elements can be: nickel, chromium, vanadium, aluminum. The density of cast iron is equal to 7200 kilograms per cubic meter. From which we can conclude that the weight of the iron - an indicator that can not be called small.
Historical reference
The smelting of cast iron has long been known to man. The first mentions of the alloy date back to the sixth century BC.
In China, in ancient times, iron was obtained fromfairly low melting point. In Europe, iron began to be produced around the 14th century, when they first started using blast furnaces. At that time, such iron casting was used to manufacture weapons, shells, parts for construction.
In Russia, pig iron production is activeIt began in the 16th century and then expanded rapidly. At the time of Peter the Great, the Russian Empire, in terms of pig iron production, was able to bypass all the states of the world, but after a hundred years, it began again to give up its position in the iron and steel industry.
Cast iron was used to createa variety of works of art in the Middle Ages. In particular, in the 10th century, Chinese masters cast a truly unique figure of a lion, whose weight exceeded 100 tons. Starting from the 15th century on the territory of Germany, and after and in other countries, casting from cast iron became widespread. Fences, grates, park sculptures, garden furniture, gravestones were made of it.
In the last years of the 18th century, iron castings are most involved in the architecture of Russia. And the 19th century was generally called the “cast-iron age”, since the alloy was very actively used in architecture.
Features
Существуют различные виды чугуна, однако средняя The melting point of this metal compound is about 1200 degrees Celsius. This figure is 250-300 degrees less than required for steel production. This difference is associated with a fairly high carbon content, which leads to its less close bonds with iron atoms at the molecular level.
В момент выплавки и последующей кристаллизации the carbon contained in the iron does not have time to completely penetrate the molecular lattice of iron, and therefore the iron as a result turns out to be quite fragile. In this regard, it does not apply where there are constant dynamic loads. But at the same time it is great for those parts that have increased requirements for strength.
Production technology
Абсолютно все виды чугуна производятся в доменной ovens. Actually, the smelting process itself is a rather time-consuming activity, requiring serious material investments. One ton of pig iron requires about 550 kilograms of coke and almost a ton of water. The volume of ore loaded into the furnace will depend on the iron content. Most often used ore in which iron is not less than 70%. A lower concentration of the element is undesirable because it would be unprofitable to use economically.
The first stage of production
Smelting of iron occurs as follows.First of all, ore is poured into the furnace, as well as coking coal grades, which serve to inject and maintain the required temperature inside the furnace shaft. In addition, these products are actively involved in the ongoing chemical reactions in the role of iron reducing agents in the combustion process.
At the same time, a flux serving as a catalyst is shipped to the furnace. It helps rocks melt faster, which contributes to the early release of iron.
Важно заметить, что руда перед загрузкой в печь subjected to special pretreatment. It is crushed in a crushing plant (fine particles melt faster). After it is washed to remove particles that do not contain metal. After that, the raw material is burned, due to this sulfur and other foreign elements are removed from it.
Second production phase
In loaded and ready for operation ovenNatural gas is supplied through special burners. Coke heats up raw materials. This releases carbon, which combines with oxygen and forms an oxide. This oxide subsequently takes part in the reduction of iron from the ore. Note that with an increase in the amount of gas in the furnace, the rate of flow of a chemical reaction decreases, and when a certain ratio is reached, it stops altogether.
Excess carbon penetrates into the melt and enterscompound with iron, forming ultimately cast iron. All those elements that are not melted, are on the surface and eventually removed. This waste is called slag. It can also be used for the production of other materials. The types of iron produced in this way are called foundry and pig iron.
Differentiation
The modern classification of cast iron provides for the separation of these alloys into the following types:
- White.
- Half-hearted.
- Gray with lamellar graphite.
- High strength nodular graphite.
- Malleable.
Let's look at each species separately.
White cast iron
Таким чугуном называется тот, у которого almost all carbon is chemically bound. In engineering this alloy is not used very often, because it is hard, but very fragile. It also can not be machined by various cutting tools, and therefore is used for casting parts that do not require any processing. Although this type of cast iron allows grinding with abrasive wheels. White cast iron can be both ordinary and alloyed. In this case, its welding causes difficulties, since it is accompanied by the formation of various cracks during cooling or heating, as well as due to the heterogeneity of the structure that forms at the welding point.
White wear-resistant cast iron is obtained byprimary crystallization of the liquid alloy with rapid cooling. Most often they are used for work in dry friction conditions (for example, brake pads) or for the production of parts with enhanced wear resistance and heat resistance (rolls of rolling mills).
By the way, white cast iron got its name.due to the fact that the appearance of its fracture is a light-crystalline, radiant surface. The structure of this iron is a combination of ledeburite, perlite and secondary cementite. If this cast iron is doped, then perlite is transformed into troostite, austenite or martensite.
Half cast iron
The cast iron classification will be incomplete if not to mention this type of metal alloy.
This cast iron is characterized by a combinationcarbide eutectic and graphite in its structure. In general, the full structure has the following form: graphite, perlite, ledeburit. If the cast iron is subjected to heat treatment or alloying, this will lead to the formation of austenite, martensite or needle troostite.
This type of cast iron is quite fragile, so its use is very limited. The very same alloy was named because its fracture is a combination of dark and light areas of a crystalline structure.
The most common engineering material
GOST 1412-85 gray cast iron contains about 3.5% carbon, from 1.9 to 2.5% silicon, up to 0.8% manganese, up to 0.3% phosphorus and less than 0.12% sulfur.
Graphite in such a cast iron has a plate shape. It does not require special modification.
Пластинки графита имеют сильно ослабляющее the effect and therefore gray cast iron is characterized by a very low impact strength and the almost complete absence of relative elongation (less than 0.5%).
Gray cast iron is well processed. The structure of the alloy may be as follows:
- Ferrite-graphite.
- Ferrite-perlite-graphite.
- Perlite graphite.
На сжатие серый чугун работает гораздо лучше, rather than stretching. It also welds quite well, but this requires preheating, and special cast iron rods with a high content of silicon and carbon should be used as filler material. Without preheating, welding will be difficult, since the pig iron will be bleached in the weld area.
Gray cast iron is used to produce parts that operate in the absence of a shock load (pulleys, covers, beds).
The designation of the cast iron occurs according to the following principle: MF 25-52. Two letters signal that it is gray cast iron, the number 25 is an indicator of tensile strength (in MPa or kgf / mm2), the number 52 is the ultimate strength at the time of bending.
High strength cast iron
Nodular cast irondiffers from its other "fellow" in that it contains spherical graphite. It is obtained by introducing special modifiers (Mg, Ce) into the liquid alloy. The number of graphite inclusions and their linear dimensions may vary.
What is spherical graphite good for? The fact that such a form minimally weakens the metal base, which, in turn, can be pearlitic, ferritic or pearlitic-ferritic.
Due to the use of heat treatment or doping, the base of the cast iron can be needle-like, martensitic, austenitic.
Brands of high-strength cast iron are different, but ingeneral form of its designation is: RF 40-5. It is easy to guess that HF is high-strength cast iron, the number 40 is an indicator of tensile strength (kgf / mm2), the number 5 is relatively elongation, expressed as a percentage.
Ductile iron
Структура ковкого чугуна заключается в наличии в there is graphite in flaky or spherical shape. In this case, flocculent graphite can have different dispersity and compactness, which, in turn, has a direct impact on the mechanical properties of cast iron.
In industry, malleable cast iron is often produced with a ferritic base that provides greater ductility.
The appearance of the fracture of ferritic malleable cast iron has a black velvety appearance. The higher the amount of perlite in the structure, the brighter the fracture becomes.
In general, ductile iron is obtained from white iron castings due to prolonged languor in furnaces heated to a temperature of 800–950 degrees Celsius.
To date, there are two ways to manufacture ductile iron: European and American.
The American method consists in pining the alloy in sand at a temperature of 800-850 degrees. In this process, graphite is located between the grains of the purest iron. As a result, cast iron acquires viscosity.
In the European method of casting languishing in ironore. The temperature is about 850-950 degrees Celsius. The carbon is converted into iron ore, due to which the surface layer of the castings is decarburized and becomes soft. Cast iron becomes ductile, and the core remains fragile.
Marking of malleable cast iron: KCh 40-6, where KCh is, of course, malleable cast iron; 40 is an indicator of tensile strength; 6 - relative elongation,%.
Other indicators
As for the separation of cast iron strength, the following classification applies:
- Normal strength: σ to 20 kg / mm2.
- Increased strength: σв = 20 - 38 kg / mm2.
- High strength: σв = 40 kg / mm2 and higher.
The ductility of cast iron is divided into:
- Non-plastic - relative elongation of less than 1%.
- Low plasticity - from 1% to 5%.
- Plastic - from 5% to 10%.
- Increased plasticity - more than 10%.
In conclusion, I would also like to be sure to note that even the shape and nature of the casting has a significant effect on the properties of any cast iron.
