What is the difference between steel and cast iron?

Iron and steel products of 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 in its pure form it is impossible to exploit it, 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 pure iron that is used, but its derivatives, iron and steel. What is the difference between steel and cast iron?

Iron and steel production

Cast iron and steel are alloys of iron and carbon.

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

Gradation of steel and cast iron

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Steel

Steel production scheme

Steel production scheme.

To produce steel, iron is fused with carbon and various impurities. A prerequisite is a carbon content of not more than 2% (it increases strength), and iron is not less than 45%. The remaining part consists of alloying binding components (chromium, molybdenum, nickel, etc.). Chromium increases the strength of steel, its hardness and resistance to wear. Nickel increases strength, toughness and hardness, increases its anti-corrosion properties and hardenability. Silicon adds strength, hardness and elasticity to steel, reduces its viscosity. Manganese improves weldability and hardenability. Metallurgists emit different types of steel. Classify them according to the volume of the leaving elements. For example, the content of more than 11% of alloying metals gives high-alloy steel. There is also:

  1. Low alloy steel - up to 4%.
  2. Medium alloyed steel - up to 11%.
Mechanical properties of steel

Mechanical properties of steel.

By 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 a metal into:

  • ordinary;
  • quality;
  • high quality;
  • especially high-grade steel.

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

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Cast iron

In the production of iron, iron and carbon are also alloyed. The main difference of iron from 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 fragile than steel, and collapses without visible deformation. Carbon in metal is represented by graphite or cementite, while the volume and shape of the element give the definition of the alloy:

Iron production

Cast iron production.

  1. White cast iron, in which the entire volume of carbon is represented by cementite. At the break, this material is white, very hard, but fragile. It is easy to process and is used for the production of forging varieties.
  2. Gray - carbon is represented by graphite, which gives the material plasticity. Soft, prone to cutting, with a low melting point.
  3. Ductile, which is obtained from white cast iron by special annealing (languishing) it in special heating furnaces at a temperature of 950-1000 ° C. At the same time, excessive brittleness and hardness characteristic of white cast iron are much reduced. Ductile iron is not forged, and the name only indicates its plasticity.
  4. High-strength cast iron containing nodular graphite formed during the crystallization process.

The amount of carbon in an alloy determines its melting point (the higher the element content, the lower the temperature and the higher the fluidity during heating). Therefore, cast iron is a flowable, non-ductile, brittle and difficult to process material with a melting point of from 1150 to 1250 ° C.

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Corrosion Resistance

Both alloys are corrosive, and improper operation accelerates this process.

Getting iron from ore

Getting iron from ore.

Cast iron in the process of use is covered on top with dry rust. 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 equally susceptible to corrosion, just 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 Brearly made a discovery in the field of metallurgy. He found that steel with a high chromium content has good resistance to acid corrosion. This is how stainless steel appeared. She also has her own gradation:

  1. Corrosion-resistant steel has resistance to corrosion in elementary industrial and domestic conditions (oil and gas, light, engineering industry, surgical instruments, household stainless utensils).
  2. Heat resistant steel is resistant to high temperatures and corrosive environments (chemical industry).
  3. Heat resisting steel differs in the increased mechanical durability in the conditions of high temperatures.
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Thermal shock and impact resistance

Comparative performance of iron and steel

Comparative indicators of iron and steel.

Cast iron and steel are often used in the manufacture of heating boilers. In this case, the issue of resistance to thermal shock becomes particularly important. If cold water gets into the unheated pig-iron boiler, it may crack. Steel products thermal shock is not terrible. Steel is more elastic and tolerates temperature differences. But large and frequent temperature drops 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 brittleness of the cast iron inevitably leads to the formation of cracks upon impact or warp.

Gray cast iron has a more uniform structure, enhanced plasticity and anti-corrosion properties, and is capable of withstanding large temperature surges.

Findings:

  1. Cast iron is less durable and hard than steel.
  2. Steel is heavier and has a higher melting point.
  3. The lower carbon content in steel, in contrast to cast iron, allows it to be processed more easily (boil, cut, forge).
  4. For a similar reason, cast iron products are produced only by the casting method, while steel can be forged and welded.
  5. Steel products are less porous than cast iron, and therefore their thermal conductivity is much higher.
  6. Products made of cast iron, as a rule, have a black color and a matte surface, while those made of steel are light with a shiny surface.

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How to distinguish cast iron from steel?

Ways to distinguish:

  1. According to the density of the product. It is necessary to weigh the object and determine how much water it will displace. The density of steel lies in the range of 7.7-7.9 g / cm ³, gray iron - does not exceed 7.2 g / cm ³. This method is not particularly reliable because white cast iron has a density of between 7.6 and 7.8 g / cm³.
  2. With the help of a magnet. Cast iron is magnetically worse than steel. The disadvantage of this method is that steel with a high nickel content practically does not attract a magnet.
  3. The most accurate method is to determine cast iron using a grinding machine and the type of chips that are formed. It should take a file with a small notch and hold on the surface of the object several times. The formed sawdust must be collected on paper, folded twice and vigorously rubbed. Cast iron noticeably stains the paper, the steel will leave almost no marks.

You can draw conclusions about the material size, shape and color of sparks that appear during grinding. The more carbon there is, the brighter and stronger the bundle of light yellow sparks will be. As we already know, cast iron contains carbon more than steel. Also, when drilling a product with a thin drill, you can determine the material by the type of chips. Cast iron shavings will literally turn into dust in the eyes, steel - will take the form of a twisted spring.

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