Today, there is almost no sphere of human life where cast iron is not used. This material has been known to mankind for a long time and has excellently proven 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 close attention to this iron-containing compound. We also find out what types of cast iron are, their physical and chemical characteristics.
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 also not less than 2.14%. Also, carbon can be in cast iron in the form of cementite or graphite.
Carbon gives the alloy a fairly high hardness, however, at the same time, it reduces ductility and ductility. In this regard, cast iron is a brittle material. Also, special additives are added to certain grades of cast iron, which can give the compound certain properties. The role of alloying elements may be: nickel, chromium, vanadium, aluminum. The density index of cast iron is 7200 kilograms per cubic meter. From which we can conclude that the weight of cast iron is an indicator that can not be called small.
Historical reference
Iron smelting has long been known to man. The first mention of the alloy dates back to the sixth century BC.
In ancient times, China received cast iron with a rather low melting point. In Europe, pig iron began to be produced around the 14th century, when blast furnaces were first used. At that time, such an iron cast went to the production of weapons, shells, and parts for construction.
On the territory of Russia, pig iron production began actively in the 16th century and then expanded rapidly. At the time of Peter the Great, the Russian Empire was able to bypass all the states of the world in terms of pig iron production, but after a hundred years it began to lose ground again in the iron and steel industry.
Iron casting was used to create a variety of artistic works back in the Middle Ages. In particular, in the 10th century, Chinese craftsmen cast a truly unique figure of a lion, whose weight exceeded 100 tons. Starting from the 15th century in Germany, and after in other countries, casting from cast iron has gained widespread distribution. Fences, trellises, park sculptures, garden furniture, gravestones were made from it.
In the last years of the 18th century, iron casting was maximally involved in the architecture of Russia. And the 19th century was generally called the βcast-iron ageβ, since the alloy was very actively used in architecture.
Features
There are various types of cast iron, however, the average melting point of this metal compound is about 1200 degrees Celsius. This figure is 250-300 degrees less than that required for steelmaking. This difference is associated with a fairly high carbon content, which leads to its less close bonds with iron atoms at the molecular level.
At the time of smelting and subsequent crystallization, the carbon contained in the cast iron does not have time to completely penetrate the molecular lattice of iron, and therefore the cast iron as a result is rather brittle. In this regard, it is not applied where there are constant dynamic loads. But at the same time, it is great for those parts that have high strength requirements.
Production technology
Absolutely all types of cast iron are produced in a blast furnace. Actually, the smelting process itself is a rather labor-intensive activity, requiring serious material investments. One ton of cast iron requires approximately 550 kilograms of coke and almost a ton of water. The amount of ore loaded into the furnace will depend on the iron content. The most commonly used ore in which iron is at least 70%. A lower concentration of the element is undesirable, since it will be unprofitable to use it economically.
First stage of production
Iron smelting is as follows. First of all, ore is added to the furnace, as well as coking coal grades, which serve to pump and maintain the required temperature inside the furnace shaft. In addition, these products during the combustion process actively participate in the ongoing chemical reactions as iron reducing agents.
At the same time, a flux serving as a catalyst is shipped to the furnace. It helps the rocks to melt faster, which contributes to the speedy release of iron.
It is important to note that the ore is subjected to special pre-treatment before being loaded into the furnace. It is crushed in a crushing plant (small particles melt faster). After it is washed to remove particles that do not contain metal. After which the raw materials are fired, due to this, sulfur and other foreign elements are removed from it.
Second stage of production
A loaded and ready-for-use stove is supplied with natural gas through special burners. Coke heats raw materials. In this case, carbon is released, which combines with oxygen and forms an oxide. This oxide subsequently takes part in the reduction of iron from ore. Note that with an increase in the amount of gas in the furnace, the rate of the chemical reaction decreases, and when a certain ratio is reached, it completely stops.
Excess carbon enters the melt and enters the compound with iron, eventually forming cast iron. All those elements that have not melted are on the surface and eventually removed. This waste is called slag. It can also be used to produce other materials. The types of cast iron obtained in this way are called foundry and foundry.
Differentiation
The modern classification of cast irons provides for the separation of these alloys into the following types:
- White.
- Half.
- Gray with lamellar graphite.
- High strength with spherical graphite.
- Ductile.
Let's look at each view separately.
White cast iron
Such cast iron is called one in which almost all carbon is chemically bonded. In mechanical engineering, this alloy is not used very often, because it is hard, but very fragile. It is also not amenable to mechanical processing 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 either ordinary or alloyed. At the same time, it is difficult to weld, since it is accompanied by the formation of various cracks during cooling or heating, as well as due to the heterogeneity of the structure formed at the weld point.
White wear-resistant cast irons are obtained due to the primary crystallization of a liquid alloy during flash cooling. Most often, they are used for working in dry friction conditions (for example, brake pads) or for the production of parts with increased 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 cast iron is a combination of ledeburite, perlite and secondary cementite. If this cast iron is subjected to alloying, then perlite is transformed into troostite, austenite or martensite.
Half cast iron
The classification of cast irons will be incomplete if not to mention this kind of metal alloy.
The indicated cast iron is characterized by a combination of carbide eutectic and graphite in its structure. In general, a full-fledged structure has the following form: graphite, perlite, ledeburite. If 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 alloy itself got its name because its fracture is a combination of dark and light areas of the 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 cast iron has a plate shape. In this case, no special modification is required.
Graphite plates have a very weakening effect and therefore gray cast iron is characterized by a very low impact strength and almost complete absence of elongation (the index is less than 0.5%).
Gray cast iron is well processed. The alloy structure may be as follows:
- Ferrite graphite.
- Ferrite-perlite-graphite.
- Perlite-graphite.
Compression cast iron works much better than tensile. 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 preliminary heating, welding will be difficult, since the bleaching of cast iron in the weld zone will occur.
From gray cast iron, parts are produced that work in the absence of an impact load (pulleys, covers, beds).
The designation of this cast iron occurs according to this principle: SCH 25-52. Two letters indicate that it is gray cast iron, the number 25 is an indicator of the tensile strength (in MPa or kgf / mm 2 ), the number 52 is the tensile strength at the time of bending.
Ductile iron
Spheroidal graphite iron is fundamentally different from its other "brethren" 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 can be different.
What is spherical graphite good for? The fact that this form minimally weakens the metal base, which, in turn, can be pearlitic, ferritic or pearlitic-ferritic.
Through the use of heat treatment or alloying, the base of cast iron can be needle-troostitic, martensitic, austenitic.
Brands of ductile iron are different, but in general terms its designation is as follows: RF 40-5. It is easy to guess that HF ββis high-strength cast iron, the number 40 is an indicator of the tensile strength (kgf / mm 2 ), the number 5 is the relative elongation, expressed as a percentage.
Ductile iron
The structure of malleable cast iron is the presence of graphite in it in a flocculent or spherical shape. At the same time, flocculent graphite can have different dispersion and compactness, which, in turn, has a direct effect on the mechanical properties of cast iron.
In industry, malleable cast iron is often produced with a ferritic base, which provides greater ductility.
The appearance of a fracture of malleable cast iron has a black and velvety appearance. The higher the amount of perlite in the structure, the brighter the break will become.
In general, malleable cast iron is obtained from castings of white cast iron due to prolonged languishing in furnaces heated to a temperature of 800β950 degrees Celsius.
Today, there are two ways to make malleable cast iron: European and American.
The American method consists in languishing the alloy in the sand at a temperature of 800-850 degrees. In this process, graphite is located between the grains of pure iron. As a result, cast iron acquires viscosity.
In the European method, castings languish in iron ore. The temperature is about 850-950 degrees Celsius. Carbon passes into iron ore, due to which the surface layer of the castings is decarburized and becomes soft. Cast iron becomes malleable, and the core remains brittle.
Marking of malleable cast iron: 40-6, where - is, of course, malleable cast iron; 40 - an indicator of tensile strength; 6 - elongation,%.
Other indicators
As for the separation of cast iron by strength, the following classification is applied here:
- Normal strength: Ο up to 20 kg / mm 2 .
- Increased strength: Ο = 20 - 38 kg / mm 2 .
- High strength: Ο = 40 kg / mm 2 and higher.
By ductility, cast irons are divided into:
- Non-ductile - elongation of less than 1%.
- Low-plastic - from 1% to 5%.
- Plastic - from 5% to 10%.
- Increased ductility - more than 10%.
In conclusion, I would also like to note that even the shape and nature of the casting have a significant effect on the properties of any cast iron.