Initially, cast iron casting technology was first mastered in China in the 10th century, after which it was widely used in other countries of the world. The basis of cast iron is an alloy of iron with carbon and other components. A distinctive feature is that in its composition cast iron contains more than 2% carbon in the form of cementite, which is not found in other metals. A striking representative of such an alloy can be called white cast iron, which is used in mechanical engineering for the manufacture of parts, in industry and in everyday life.
Appearance
The alloy has a white color at the fracture and a characteristic metallic luster. The structure of white cast iron is fine-grained.
The properties
Compared with other metals, an iron-carbon alloy has the following characteristics and properties:
- high fragility;
- increased hardness;
- high resistivity;
- low casting properties;
- low workability;
- good heat resistance;
- high shrinkage (up to 2%) and poor filling of molds ;
- low impact resistance;
- high wear resistance.
The metal mass has a high corrosion resistance in hydrochloric or nitric acid. If there are free carbides in the structure, then corrosion will be observed when cast iron is placed in sulfuric acid.
White cast irons, which contain a lower percentage of carbon, are considered more resistant alloys to high temperatures. Due to the increased mechanical strength and viscosity that appear when exposed to high temperatures, the formation of cracks in castings is minimized.
Structure
Iron-carbon alloy is considered a cheaper material compared to steel. White cast iron contains iron and carbon, which are in a chemically bound state. Excess carbon, which is not present in the solid solution of iron, is contained in the combined state in the form of iron carbides (cementite), and in alloyed cast iron in the form of special carbides.
Kinds
Depending on the amount of carbon content in white iron, it is divided into the following types:
- The hypereutectic contains from 2.14% to 4.3% of carbon and, after complete cooling, acquires the structure of perlite, secondary cementite and ledeburite.
- The eutectic contains 4.3% carbon and has a structure in the form of a light background of cementite, which is dotted with dark perlite grains.
- Hypereutectic has from 4.3% to 6.67% carbon in its composition.
Application
Based on the above properties, we can conclude that it makes no sense to practice the thermal and mechanical treatment of white cast iron. The alloy found its main application only in the form of castings. Therefore, the best properties are obtained by white cast iron only if all conditions of the casting are met. This processing method is actively used if it is necessary to manufacture massive products that must have high surface hardness.
In addition, white iron is annealed, as a result of which malleable cast iron is obtained, which are used for the manufacture of thin-walled castings, for example:
- automotive parts;
- products for agriculture;
- parts for tractors, combines, etc.
The alloy is also used for the manufacture of plates with a ribbed or smooth surface, and is also actively used for the production of steel and gray cast iron.
The use of white cast iron in agriculture in the form of structural metal is quite limited. Most often, an iron-carbon alloy is used for the manufacture of parts for hydraulic machines, sand guns and other mechanisms that can function in conditions of increased abrasive wear.
Bleached cast irons
This alloy is considered a kind of white cast iron. It is possible to achieve bleached by 12-30 mm by quickly cooling the surface of the iron-carbon alloy. Material structure: the surface part is made of white, gray cast iron in the core. Wheels, balls for mills, rolling rolls, which are mounted in machines for processing sheet metal , are made from such material.
Alloy alloy elements
Specially introduced alloying agents added to the composition of white cast iron are able to give greater wear resistance and strength, corrosion resistance and heat resistance. Depending on the amount of added substances, these types of cast iron are distinguished:
- low alloy alloy (up to 2.5% of auxiliary substances);
- medium alloyed (from 2.5% to 10%);
- highly alloyed (more than 10%).
Alloying elements can be added to the alloy:
- chromium;
- sulfur;
- nickel;
- copper;
- molybdenum;
- titanium;
- vanadium,
- silicon;
- aluminum;
- manganese.
Alloy white cast iron has improved properties and is often used for casting turbines, blades, mills, parts for cement and conventional furnaces, blades of pumping machines, etc. Iron-carbon alloy is processed in two furnaces, which allows you to bring the material to a specific chemical composition:
- in the cupola;
- in electric furnaces.
Castings made of white cast iron are annealed in furnaces to stabilize the required dimensions and relieve internal stress. Annealing temperature can rise up to 850 degrees. The process of heating and cooling must necessarily be carried out slowly.
The marking or designation of white cast iron with impurities begins with the letter H. Which alloying elements are contained in the alloy can be determined by the following letters of the marking. The name may contain numbers that indicate the amount in percentage terms of additional substances that fit in white cast iron. If the marking has the designation , then this means that the structure of the alloy has spherical graphite.
Types of Annealing
For the formation of white iron in industry, rapid cooling of the alloy is used. Today, the following main types of annealing of carbon alloys are actively used:
- softening annealing is mainly used to increase the composition of cast iron ferrite;
- annealing to relieve internal stresses and minimize phase transformations;
- graphitizing annealing, as a result of which it is possible to obtain ductile iron ;
- normalization at a temperature regime of 850-960 degrees, as a result of which graphite and perlite are obtained, and also wear resistance and strength are increased.
Additional information
Today it is proved that there is no direct relationship between the wear resistance and hardness of a carbon alloy. Only due to the structure, namely the arrangement of carbides and phosphides in the form of a regular grid or in the form of uniform inclusions, is increased wear resistance achieved.
The strength of white cast iron is most intensively influenced by the amount of carbon, and the hardness depends on carbides. The most durable and hard are those cast irons that have a martensitic structure.