Alloy cast iron is a material that is produced by smelting in blast furnaces. It may contain a different amount of carbon. Depending on the quantitative content of this substance, two types of cast iron are distinguished. The first is called conversion, or white, and the second - gray, or foundry.
Description of types of cast iron
The first type is pig iron. This is the name of the material in which carbon is presented in the form of a substance such as cementite. At the break, it has a white color, from where its name came from. Such cast iron is characterized by high hardness and brittleness. It is very difficult to machine. At the same time, about 80% of all cast iron is white. The main purpose of this type of material is further melting into steel.
Gray alloyed cast iron is a metal in which carbon is presented in the form of ductile graphite. At the break, its color is gray, which also determined its name. The fragility and hardness of such cast iron is less than that of white, but at the same time it is much better amenable to mechanical processing.
The properties of this type of alloyed cast iron are as follows:
- Firstly, it perfectly resists compressive loads.
- Secondly, this metal is insensitive to defects of the surface type, and also differs in that it resists fatigue failure quite well.
However, cast alloy steel has a fairly low toughness, as well as low ductility. Due to these two drawbacks, it is rather difficult to use such material for structural purposes.
General description of grades of gray cast iron
Today, there are such grades of alloyed cast iron of gray type: MF 10, MF 15, MF 18, MF 20 and several others. The letters MF in the marking indicate that it is gray cast iron, and the following two digits indicate the ultimate load that the metal can withstand when stretched. The tensile strength in this case is measured in MPa.
Varieties of gray cast iron
Foundry alloyed cast iron has several subtypes. Malleable cast iron became one of them . This conditional name was given to the material, which differs in that it is softer and more viscous than gray. Get it from white cast iron. To do this, use the annealing procedure, which lasts long enough. The marking here is about the same, for example KCH 30-6, KCh 33-8, KCh 37-12. The letters indicate that this cast iron is malleable, and the following two digits determine the tensile strength. But as for the last one or two digits, they characterize the maximum elongation, which is measured in percent.
Another subtype of cast iron is modified. In order to get it, you need to add special elements to the gray. Such modifiers are added before the substance is cast. As additives, aluminum, silicon, calcium and others can be used. These additives significantly increase the number of crystallization centers. In other words, they contribute to significant grinding of graphite.
Thanks to such additives, special alloyed cast iron is characterized by higher strength characteristics, less brittleness, and increased resistance to cracking. It is worth adding that all the best grades of this alloy are obtained precisely from such a modified material.
Types of Alloy Alloy
What does alloy cast iron mean? Doping is the operation of introducing various impurities into the composition of a material that can improve its characteristics. For cast iron, such additives are titanium, chromium, vanadium and others. The introduction of alloying elements can increase such characteristics as strength, hardness, wear resistance, corrosion resistance and many others.
Today, depending on the number of alloying elements of cast iron, three types can be distinguished:
- If additives are contained in an amount up to 2.5% of the total mass, then this is low-alloyed cast iron.
- Medium-alloyed materials are those whose content is in the range from 2.5 to 10%.
- The latter type is highly alloyed if the content of modifiers exceeds 10% in total.
Alloying agents, labeling
According to GOST, alloyed cast iron must contain a certain amount of substances for use in a certain field. In addition, labeling is also standard. For example, ChN15D7X is a high-strength alloy that contains 15% nickel, 7% copper and about 1% chromium. As you can see, in the marking, alloying elements are marked with a single letter, followed by a figure indicating the quantitative content of the additive. However, it may be that the figure is missing, as after chromium. This means that the content of the substance in the composition is about 1%.
As for the manufacture of such cast iron, it is quite inexpensive. At the same time, the final product has sufficiently high operational properties. Due to these two factors, the scope of the described material is constantly increasing.
Types of Alloy Alloy
Naturally, the addition of any substance to the composition will increase any particular characteristic. Therefore, there are several classes of alloyed materials.
So, cast iron can be wear resistant. The material belonging to this group is characterized in that it has an increased resistance to abrasion, which occurs during surface friction. This category includes anti-friction and friction cast iron. The first of these is characterized by a very low coefficient of friction. Due to this, the main application of alloyed iron of the named type is the production of such parts as sliding bearings, liners for them and other parts with similar requirements.
Friction material, on the contrary, is characterized by a rather high coefficient of friction, and therefore it is most often used in the manufacture of braking devices for a wide variety of mechanisms, equipment, etc.
Stainless cast iron
Many people know that there is a metal called stainless. In fact, this is not an exact definition. Just such alloys are distinguished by the fact that their corrosion resistance is quite high. The most common named cast iron received in shipbuilding. If more than 12% of chromium is introduced into the composition of iron as an alloying element and the carbon content is minimized, then this alloy will be obtained as a result.
Its most common brands were ChNKhT, ChN1KhMD, ChN15D7Kh2. They are distinguished by the fact that they maintain high corrosion resistance even in aggressive environments, have good resistance to cavitation and wear in a steam-water environment.
A small, but still quite popular group is heat-resistant cast iron. The main advantage of the material lies in the fact that it is quite poorly oxidized, and also perfectly retains its qualities at elevated temperatures.
Copper introduction
Currently, more and more often begin to use cast iron with the addition of copper. The introduction of this additive in the alloy significantly improves its casting quality. This is best reflected in the fluidity of the material. In addition, the tendency to cracking and shrinkage porosity is significantly reduced.
The introduction of 0.5% Cu (copper) makes cast iron suitable enough to cast parts from it that have a wall thickness of 10 to 25 mm. If it is necessary to increase the wall thickness of future elements, then it will be necessary to increase the quantitative content of copper, as well as its complexes. It is worth noting that the effect of adding copper can be enhanced if elements such as antimony or bismuth are introduced into the alloy.
If the carbon equivalent increases, then the effect of copper on the crystallization of graphite decreases. Alloying cast iron with copper can also prevent bleaching in the surface layers, and also significantly increases hardness towards the middle. This is quite noticeable in the smelting of cylinder liners, cylinder heads of cast iron and other elements.
Complex alloyed cast iron
In order to successfully cast iron, which can be used for casting cylinder liners, it is necessary to use not only a variety of ferroalloys, but also an induction furnace. In order to cast such parts, the brand IChHN4, ChN1KhMD and ChNMSh and several other types are used. Their distinctive feature is that the smelting of all parts is carried out in a thin-walled or massive chill mold.
Needle structure and high strength material
A small variety of gray cast iron, which belongs to the group of wear-resistant, is a needle-shaped material. The degree of alloying of such a material is quite low. The content of silicon and carbon in it is also quite low. In this case, the quantitative content of substances such as copper, molybdenum, nickel and some other modifiers increases depending on the required wall thickness, and also depending on the casting method to be used.
Another variety is high-strength material. This is a small variety of gray cast iron, which is distinguished by the content of spherical graphite. In order to obtain such a structure, magnesium, cerium and bismuth must be introduced into the alloy. The addition of these three alloying elements makes the plastic graphite of the foundry material spherical. This type differs in that its mechanical properties are significantly higher than that of other types of cast iron. Today, about 10 various alloy grades of this category are produced. Such material can be perfectly used in shipbuilding instead of the malleable type. And the most common type of ductile iron is magnesium (with a large amount of magnesium in the composition).