Description of the composition of the cast iron alloy and its difference from steel. How to distinguish cast iron from steel at home without special equipment

Cast iron and steel products from the metallurgical industry are used both in everyday life and in production. Both materials are unique alloys of iron and carbon. Everyone knows that iron is mined from the depths of the earth in huge quantities. But it is impossible to use it in its pure form; this element is too soft and therefore unsuitable for the manufacture of high-strength products. Therefore, for industrial, construction and household purposes, it is not iron in its pure form that is used, but its derivatives - cast iron and steel. What is the difference between steel and cast iron?

Cast iron and steel are alloys of iron and carbon.

Their difference is manifested in many qualities, and the commonality of elements during production does not give the material identical characteristics.

Gradation of steel and cast iron

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Steel

To produce steel, iron is alloyed with carbon and various impurities. A prerequisite is that the carbon content is no more than 2% (it increases strength), and the iron content is no less than 45%. The remainder consists of alloying binding components (chromium, molybdenum, nickel, etc.). Chromium increases the strength of steel, its hardness and wear resistance. Nickel increases strength, toughness and hardness, improves its anti-corrosion qualities and hardenability. Silicon adds strength, hardness and elasticity to steel and reduces its toughness. Manganese improves weldability and hardenability. Metallurgists distinguish different types of steel. They are classified depending on the volume of the remaining elements. For example, a content of more than 11% alloy metals produces high-alloy steel. There is also:

Low alloy steel - up to 4%.
Medium alloy steel - up to 11%.

Based on the amount of carbon, steel is classified into:

low-carbon metal - up to 0.25% C;
medium carbon metal - up to 0.55% C;
high-carbon metal - up to 2% C.

The composition of non-metallic elements (phosphides, sulfides) classifies the metal into:

regular;
quality;
high quality;
especially high quality steel.

As a result, all types of steel are a strong, wear-resistant and deformation-resistant alloy with a melting point from 1450 to 1520 °C.

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Cast iron

Iron production also involves the fusion of iron and carbon. The main difference between cast iron and steel is the content of the latter in the mixture. It should be more than 2%. In addition, the mixture contains impurities: silicon, manganese, phosphorus, sulfur and alloying metals. Cast iron is more brittle than steel and breaks without visible deformation. Carbon in the metal is represented by graphite or cementite, while the volume and shape of the element determine the types of alloy:

White cast iron, in which all the carbon is represented by cementite. When broken, this material is white, very hard, but at the same time fragile. It is easy to process and is used to produce the malleable variety.
Gray - carbon is represented by graphite, which gives the material plasticity. Soft, easy to cut, with a low melting point.
Malleable, which is obtained from white cast iron by special annealing (simmering) in special heating furnaces at a temperature of 950-1000 ° C. At the same time, the excessive brittleness and hardness characteristic of white cast iron are greatly reduced. Malleable cast iron cannot be forged, and the name only indicates its ductility.
Ductile iron containing nodular graphite formed during the process of crystallization.

The amount of carbon in the alloy determines its melting point (the higher the content of the element, the lower the temperature and the higher the fluidity when heated). Therefore, cast iron is a fluid, non-plastic, brittle and difficult to process material with a melting point of 1150 to 1250 °C.

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Corrosion resistance

Both alloys are susceptible to corrosion, and improper use will accelerate this process.

Cast iron becomes covered with dry rust during use. This is the so-called chemical corrosion. Wet (electrochemical) corrosion affects cast iron more slowly than steel. Initially, the conclusion suggests itself that the anti-corrosion characteristics of cast iron are much higher. In fact, both of these alloys are susceptible to corrosion to the same extent; it’s just that for cast iron products, due to thick walls, the process takes longer. This, for example, can explain the difference in the service life of boilers: steel - from 5 to 15 years, cast iron - from 30 years.

In 1913, Harry Brearley made a discovery in the field of metallurgy. He discovered that steel with a high chromium content had good resistance to acid corrosion. This is how stainless steel was born. It also has its own gradation:

Corrosion-resistant steel is resistant to corrosion in basic industrial and domestic conditions (oil and gas, light industry, engineering industry, surgical instruments, household stainless steel utensils).
Heat-resistant steel is resistant to high temperatures and aggressive environments (chemical industry).
Heat-resistant steel has increased mechanical strength at high temperatures.

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Thermal shock and shock resistance

Cast iron and steel are often used in the manufacture of heating boilers. In this case, the issue of resistance to thermal shocks becomes especially important. If cold water gets into a cast iron boiler that has not cooled down, it may crack. Thermal shock is not dangerous for steel products. Steel is more elastic and tolerates temperature differences well. But large and frequent temperature changes in steel contribute to the appearance of “tired” zones and, as a result, cracks in places that are weakened by welding.

Good ductility makes steel products resistant to mechanical damage. The fragility of cast iron inevitably leads to the formation of cracks due to impacts or distortions.

Gray cast iron has a more uniform structure, increased ductility and anti-corrosion properties, and is able to withstand large temperature changes.

Cast iron is less durable and hard than steel.
Steel is heavier and has a higher melting point.
The lower carbon content in steel, unlike cast iron, makes it easier to process (cook, cut, forge).
For a similar reason, cast iron products are produced only by casting, while steel products can be forged and welded.
Products made of steel are less porous than those made of cast iron, and therefore their thermal conductivity is much higher.
Products made of cast iron are usually black in color and have a matte surface, while those made of steel are light-colored with a shiny surface.

Cast iron is an alloy of iron and carbon. The percentage of iron contained is more than 90%. The amount of carbon ranges from 2.14-6.67%. Thanks to this element, the material has high hardness, but becomes brittle. This entails a deterioration in ductility and ductility. In some types, alloying elements are added to improve characteristics: aluminum, chromium, vanadium, nickel.

Characteristics of types of carbon metal

The iron-carbon diagram shows what cast iron is made of. In addition to iron, carbon is present in the form of graphite and cementite.

The composition of the cast iron alloy has varieties:

Individual properties of metal

The material is characterized by certain characteristics. These include:

Depending on the presence of impurities, a difference in the properties of the material appears.

These elements include sulfur, phosphorus, silicon, manganese:

Sulfur reduces the fluidity of the metal.
Phosphorus reduces strength, but allows the manufacture of products of complex shapes.
Silicon increases the fluidity of the material, lowering its melting point.
Manganese gives strength, but reduces fluidity.

Differences between cast iron and steel

To understand the difference between steel and cast iron, you need to consider their characteristics. A distinctive feature of cast iron is the amount of carbon. Its minimum content is 2.14%. This is the main indicator by which this material can be distinguished from steel.

Only chemical analysis can determine the percentage of impurities. If we compare the melting point of cast iron and steel, then for cast iron it is lower and amounts to 1150-1250 degrees. For steel - around 1500.

To distinguish the material, you need to do the following:

The product is lowered into water and the volume of displaced water is determined. Cast iron has a lower density. It is 7.2 g/cm3. For steel - 7.7−7.9 g/cm3.
A magnet is applied to the surface, which attracts the steel better.
The chips are rubbed using a grinder or file. Then it is collected in paper and wiped on it. Steel will not leave marks.

Pros and cons of the material

Like any material, cast iron has positive and negative sides. Positive qualities include:

Question: March 28, 2009
What is the difference between cast iron and steel, and why?

Answer:
Oddly enough, despite the abundance of specialized literature on this topic, we are often asked the following question: How is cast iron different from steel? Briefly and in general terms, we can say that in composition cast iron differs from steel in its higher carbon content, in technological properties - better casting qualities and low ability for plastic deformation. Cast iron is generally cheaper than steel.
And if in more detail, then read the classics, dear ones! Many volumes are devoted to materials science and metallurgy of ferrous alloys. As an example, I give an excerpt from the fundamental work of A.P. Gulyaev. "Metal science":
“Steel is an iron-carbon alloy containing less than 2.14% carbon. However, the indicated limit (2.14% C) applies only to double iron-carbon alloys or alloys containing a relatively small number of impurities. The question of the boundary between steels and cast irons in high-alloy iron-carbon alloys, i.e. containing even greater amounts of elements other than iron and carbon is controversial.
In the light of modern technology, iron-based alloys are known and have recently become widespread, in which there is very little carbon and it is even a harmful element; however, such alloys are also called steels. To avoid terminological confusion, it is customary to consider alloys containing more than 50% iron as steels (cast iron) and not to call them alloys, but to call alloys containing less than 50% iron. It’s not scientifically rigorous, but it’s technically clear.”

The metallurgical industry products often used in everyday life are cast iron and steel. Both materials are a unique alloy of iron and carbon. But the use of identical components in production does not endow the materials with similar properties. Cast iron and steel are two different materials. What are their differences?

Steel

To make steel, you need to fuse iron, carbon and impurities. In this case, the carbon content in the mixture should not exceed 2%, and the iron content should not be less than 45%. The remaining percentage in the mixture can be alloying elements (substances that bind the mixture, for example, molybdenum, nickel, chromium and others). Thanks to carbon, iron acquires strength and extreme hardness. Without his participation, a viscous and plastic substance would be obtained.

Cast iron

Iron and carbon are also fused together in the production of cast iron. Only the content of the latter in the mixture is more than 2%. In addition to the listed components, the mixture contains permanent impurities: silicon, manganese, phosphorus, sulfur and alloying additives.

Differences

In metallurgy, a fairly large number of varieties of steel are distinguished. Their classification depends on the amount of one or another component in the mixture. For example, a high content of binding elements produces high-alloy (more than 11%) steel. In addition there are:

low alloyed – up to 4% binding components;
medium alloyed - up to 11% of connecting elements.

low-carbon metal – up to 0.25% C;
medium carbon metal – up to 0.55% C;
high carbon – up to 2% C.

And finally, depending on the content of non-metallic inclusions that are formed as a result of reactions (for example, oxides, phosphides, sulfides), classification is carried out according to physical properties:

especially high quality;
high quality;
quality;
ordinary steel.

This is far from a complete classification of steel. The types are also distinguished by the structure of the material, production method, and so on. But no matter how the main components are fused, the result is a hard, durable, wear-resistant and deformation-resistant material with a specific gravity of 7.75 (up to 7.9) G/cm 3 . The melting point of steel is from 1450 to 1520°C.

Unlike steel, cast iron is more fragile; it is distinguished by its ability to collapse without noticeable residual deformation. In this case, the carbon itself in the alloy is presented in the form of graphite and/or cementite; their shape and, accordingly, quantity determine the types of cast iron:

white - all the necessary carbon is contained in the form of cementite. The material is white when broken. Very hard, but fragile. It can be processed and is mainly used to produce the malleable variety;
gray – carbon in the form of graphite (plastic form). It is soft, easily processed (can be cut) and has a low melting point;
malleable - obtained after prolonged annealing of a white appearance, resulting in the formation of graphite. Heating (over 900°C) and the cooling rate of graphite negatively affect the properties of the material. This makes welding and processing difficult;
high-strength - contains spherical graphite formed as a result of crystallization.

Conclusions website

Steel is stronger and harder than cast iron.
Cast iron is lighter than steel and has a lower melting point.
Due to its lower carbon content, steel is easier to process (welding, cutting, rolling, forging) than cast iron.
For the same reason, cast iron products are made only by casting.
Products made of cast iron are more porous (due to casting) than those made of steel, and therefore their thermal conductivity is much lower.
Typically, art products made from cast iron are black and matte, while those made from steel are light and shiny.
Cast iron has low thermal conductivity, while steel has higher thermal conductivity.
Cast iron is the primary product of ferrous metallurgy, and steel is the final product.
Cast iron is not hardened, but some types of steel must be subjected to a hardening procedure.
Products made of cast iron are only cast, and products made of steel are forged and welded.

"Strong as steel." How many times have you heard this? But we have never heard: “Strong as cast iron.” The centuries-old history of cast iron has led metallurgists, foundry workers and designers to the belief that cast iron only withstands compressive loads well. It works well for compression. Where high tensile strength is needed, cast iron is unsuitable. The reason for the low strength of cast iron is explained by its nature. After all, cast iron differs from steel in its higher carbon content. Only part of it dissolves in iron. The rest is formed by graphite inclusions. In the structure of ordinary gray cast iron, these inclusions look like elongated plates cutting the iron base (Fig. 13, a). They are clearly visible under a microscope. Inclusions of lamellar graphite can be likened to cracks in metal filled with soft material - graphite. Graphite inclusions are stress concentrators. As soon as a tensile force appears, cast iron products are easily torn. And the reason for such weak strength is the inclusion of flake graphite.

Is it possible to reduce these graphite inclusions in size? Should we give them not an elongated (elongated) shape, but make these inclusions more compact? Bring them closer to a spherical shape?

Malleable cast iron that cannot be forged. Experience has shown that if a product is cast from white cast iron (in which all the carbon is in a bound state in the form of iron carbide) and then this casting is subjected to prolonged annealing at high temperatures (above 1000 ° C), then the graphite inclusions take on a completely different shape. Their shape becomes flocculent (Fig. 13.6). These inclusions are more compact, they cut less into the metal base, and such cast iron turns out to be much more durable. He can already work in tension. This type of cast iron is called “malleable”, although in reality it is not yet malleable enough to be forged.

Now let's make a quantitative comparison. Tensile strength is measured in kilograms per square millimeter of cross-section. Let's imagine a wire with a cross section of 1 mm 2. Let's hang one end of it from the ceiling, and we'll hang weights from the downward end. What is the maximum load the wire can bear before it breaks? Pure iron wire can withstand approximately 25, and steel up to 70 kgf/mm 2 .

Gray cast iron, as we have already said, does not work well in tension and can withstand only 12-15 kgf/mm 2. But malleable cast iron turns out to be stronger - it can withstand from 30 to 60 kgf/mm 2. But malleable cast iron is expensive. It is expensive due to the fact that the cast parts still need to be packed in boxes, sprinkled with either coke or ore, immersed in a furnace and kept at a temperature of 950-1050 ° C for at least a day. Previously, the annealing cycle lasted 4-5 days. Now they have managed to reduce it to 24-20 hours, but this also significantly increases the cost of cast iron.

Is cast iron stronger than steel? Due to what did the strength of malleable cast iron increase by 2-3 times? Only by changing the shape of graphite inclusions. Instead of long plates, carbon after annealing white cast iron took on a more compact form (in the form of flakes), their sizes were significantly reduced compared to the sizes of carbon plates in gray cast iron.

Is it possible to make the size of carbon inclusions even smaller and their shape even more compact?

It turned out that this is possible. This was achieved by modifying cast iron, that is, introducing small additives of substances into liquid cast iron that ensure the release of graphite in the cast iron in the form of tiny balls (Fig. 13, c). Cast iron with spherical graphite is obtained by introducing magnesium, cerium, yttrium, and barium into it. Silicon, calcium and some other elements contribute to this.