FERROUS METALS: STRUCTURE, TYPES, CHARACTERISTICS - CHEMISTRY - 2024

The ferrous metals are those containing iron (Fe) as well as small amounts of other metals that are added to give certain beneficial properties to its alloys. Although iron can exist in various oxidation states, +2 (ferrous) and +3 (ferric) are the most common.

However, the term "ferrous" refers to the presence of iron regardless of its oxidation state in the material. Iron is the fourth most abundant element in the earth's crust, but globally it is the main earth element. Therefore, historically and industrially, ferrous metals have participated in the evolution of man.

This has been the case due to its great abundance and modifiable properties. These ferrous metals start from the extraction of iron from mineralogical sources, such as: hematite (Fe ₂ O ₃), magnetite (Fe ₃ O ₄) and siderite (FeCO ₃). Due to performance, these oxides are most desired in iron processing.

The top image shows an incandescent cast iron “tongue of fire”. Of all the ferrous metals, the most important consists of an iron alloy with small amounts of carbon added: steel.

Structure

Because iron is the main component of ferrous metals, their structures consist of crystalline deformations of their pure solid.

Consequently, ferrous alloys like steel are nothing more than the interstitial inclusion of other atoms in the crystal arrangement of iron.

What is this arrangement? Iron forms allotropes (different solid structures) depending on the temperature to which it is exposed, changing its magnetic properties. Thus, at room temperature it presents a bcc arrangement, also known as alpha-iron (the cube on the left, top image).

On the other hand, in a range of high temperatures (912-1394 (ºC)), it shows the ccp or fcc arrangement: the iron-gamma (the cube on the right). Once this temperature is exceeded, the iron returns to the bcc form to finally melt.

This change in alpha-gamma structure is known as phase transformation. The gamma phase is capable of "trapping" carbon atoms, while the alpha phase is not.

Thus, in the case of steel, its structure can be visualized as sets of iron atoms surrounding a carbon atom.

In this way, the structure of ferrous metals depends on the distribution of the phases of iron and the atoms of other species in the solid.

Characteristics and properties

Pure iron is a soft and very ductile metal, highly sensitive to corrosion and oxidation from external factors. However, when it includes different proportions of another metal or carbon, it acquires new characteristics and properties.

In fact, it is these changes that make ferrous metals useful for myriad applications.

Ferrous alloys are generally strong, durable, and tough, with bright grayish colors and magnetic properties.

Examples

Wrought iron or sweet

It has a carbon content of less than 0.03%. It is silver in color, rusts easily and cracks internally. In addition, it is ductile and moldable, a good conductor of electricity and difficult to weld.

It is the type of ferrous metal that man first used in the manufacture of weapons, utensils and buildings. It is currently used in plates, rivets, trellises, etc. As it is a good electrical conductor, it is used in the core of electromagnets.

Rough iron or cast iron

In the initial blast furnace product, it contains 3-4% carbon and traces of other elements such as silicon, magnesium and phosphorus. Its main use is to intervene in the production of other ferrous metals.

Pure iron

It is a grayish-white metal with magnetic properties. Despite its hardness, it is fragile and brittle. Its melting point is high (1500 ºC.) And it oxidizes quickly.

It is a good electrical conductor, which is why it is used in electrical and electronic components. Otherwise it is of little use.

Cast or cast iron (foundries)

They have a high carbon content (between 1.76% and 6.67%). They are harder than steel but more brittle. They melt at a lower temperature than pure iron, around 1100ºC.

Because it is moldable, it can be used to manufacture pieces of different sizes and complexity. In this type of iron, gray type cast iron is used, which gives it stability and moldability.

They have a higher resistance to corrosion than steel. Also, they are cheap and dense. They present fluidity at relatively low temperatures, being able to fill the molds.

They also have good compression properties, but they are brittle and break before bending, so they are not suitable for very elaborate pieces.

Gray iron

It is the most common cast iron, its gray hue being due to the presence of graphite. It has a carbon concentration between 2.5% and 4%; additionally, it contains 1-3% silicone to stabilize the graphite.

It has many of the attributes of basic cast iron, being highly fluid. It is inflexible and bends shortly before breaking.

Ductil iron

Carbon is added, in the form of spherical granite, at a concentration between 3.2% and 3.6%. Graphite's spherical shape gives it greater impact resistance and malleability than gray iron, allowing it to be used in detailed and edged designs.

Steels

Carbon content between 0.03% and 1.76%. Among its qualities are hardness, tenacity and resistance to physical efforts. In general they rust easily. They are weldable and can be processed in the forging or mechanically.

They also have greater hardness and less fluidity than cast irons. For this reason they need high temperatures to flow in the molds.

Steel and its applications

There are several types of steel, each with different applications:

Carbon or construction steel

The carbon concentration can vary, establishing four forms: mild steel (0.25% carbon), semi-sweet steel (0.35% carbon), semi-hard steel (0.45% carbon) and hard (0.5%).

It is used in the manufacture of tools, steel sheets, railway vehicles, nails, screws, automobiles and boats.

Silicon Steel

Also called electrical steel or magnetic steel. Its silicon concentration varies between 1% and 5%, Fe varies between 95% and 99%, and carbon has 0.5%.

In addition, minor amounts of manganese and aluminum are added. It has great hardness and high electrical resistance. It is used in the manufacture of magnets and electrical transformers.

Galvanized steel

It is coated with a zinc coating that protects it from rust and corrosion. Therefore, it is useful for the manufacture of pipe parts and tools.

Stainless steel

It has a composition of Cr (14-18%), Ni (7-9%), Fe (73-79%) and C (0.2%). It is resistant to rust and corrosion. It is used in the production of cutlery as well as cutting material.

Manganese steel

Its composition is Mn (10-18%), Fe (82-90%) and C (1.12%). It is hard and resistant to wear. Used on train rails, safes, and armor.

Invar steel

It has 36% Ni, 64% Fe and 0.5% carbon. It has a low coefficient of expansion. It is used in the construction of indicator scales; for example: tape measures.

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