The term "iron-carbon alloys" is used to refer to a large group of metal alloys, based on iron – steels. Despite the development in the last decade of new outstanding materials, iron alloys are still the most important and widely used metal materials, which are used in industrial practice. Their number all over the world, ten times the amount of all other metallic materials combined.
Iron-carbon alloys - steels
The popularity of steels can be attributed to several factors.:
1) Ores, from which iron is made, are among the most accessible minerals, found in the earth's crust. Near 4,2 % the earth's crust is made up of various iron ores. Iron can be extracted from these ores using relatively simple and cheap methods compared to other metals..
2) The melting point of pure iron is 1536 ° C. Below this temperature, iron exists in the form of various modifications - alpha iron, gamma iron and delta iron - in different temperature ranges. Therefore, there are many different thermal processes - and at not too high temperatures - that make it possible to advantageously modify the properties of iron alloys in a very wide range..
3) Below temperature 769 ° С - Curie points - iron becomes ferromagnetic, which makes it possible to use steel in numerous electrical applications.
Alloying elements as impurities
Iron alloys, except for iron itself - the base or base metal - always contain carbon, which is the main alloying element for them. Steels usually also contain small amounts of other elements.. During the production of iron and steel, all steels inevitably receive some manganese content., silicon, sulfur and phosphorus. These items include metal, as impurities from feedstock or combustion products. In some cases, these elements are deliberately added to the feedstock to obtain special alloy properties..
Therefore, we can conclude, that iron alloys are multicomponent metal alloys. but, until the quantities of the listed elements exceed the unavoidable levels of content, associated with the production of iron and steel (0,05-0,4 % silicon - 0,15-0,7 % magnesium - 0,035 % sulfur and phosphorus), they do not significantly affect the equilibrium phase diagram of the two-component alloy of the iron-carbon system. Therefore, for ordinary, unalloyed iron alloys are considered to be a two-component, that is, binary, iron-carbon equilibrium phase diagram.
Iron modifications – heat treatment lever
The melting point of pure iron is 1536 ° C. In the solid state, pure iron has three allotropic modifications, namely:
– delta iron is a body-centered cubic crystal structure ranging from 1392 to 1536 ° C;
– gamma iron is a face-centered cubic crystal structure ranging from 911 to 1392 ° C;
– alpha iron - body-centered cubic crystal structure up to 911 ° C.
Among all allotropic transformations, the alpha → gamma and gamma → alpha transformations play the most important role. It is these transformations that provide the theoretical basis for most heat treatment processes..
Iron forms solid solutions with many non-metallic elements. It forms substitution solutions with chromium, nickel, cobalt and vanadium, while with carbon it forms a very important interstitial solution.
Carbon and iron
The solubility of carbon in alpha iron - ferrite is very low - at room temperature only 0,006 %. In gamma iron - austenite - the solubility of iron is several orders of magnitude higher - 2,06 % at a temperature 1147 ° C.
Iron forms with carbon not only solid solutions, but metal connections. A substitute compound of iron and carbon is iron carbide - cementite - Fe3C. The main property of iron carbide is its high hardness (Vickers hardness approx. 900) and high fragility. Iron carbide practically does not lend itself to any deformation. Melting temperature of iron carbide - 1250 ° C. Iron carbide cannot be considered an equilibrium phase. Under certain conditions, it breaks down into its components - iron and carbon. This carbon - graphite - is already an equilibrium phase.