Have you noticed how some materials rust over time, due to prolonged exposure to the environment? Those materials invariably contain iron and are called ferrous metals. Ferrous metals are those that contain considerable amounts of iron. (The Latin name of iron is Ferrum, and hence, metals containing iron are ‘ferrous’).
Did you know? Iron is the most abundant element on earth. The earth’s core consists of an iron-nickel alloy that makes up about 35% of the mass of the earth.
Pure iron is very soft, appears greyish-white and has limited uses. Iron is rarely found in its pure form (except in meteorites). It exists in nature along with other elements, as minerals. The most common mineral is iron oxide. Minerals with significant iron content are called iron ores – commonly found iron ores are hematite and magnetite.
How are iron and other ferrous metals produced?
Iron is obtained from iron ore through smelting, in a blast furnace. The iron thus obtained is called pig iron, which constitutes 92-94% iron and 3-5% carbon, along with small amounts of other elements. Pig iron is brittle and has limited uses. It is therefore used to manufacture different kinds of ferrous metals.
Molten pig iron is usually poured into furnaces (electric arc, induction, open hearth or blast furnaces) and combined with other elements to form various ferrous metals. The molten material is either directly cast into the required shape or rolled (while hot, warm or cold, depending on the application). This further undergoes heat treatment (annealing, tempering, quenching etc.) to improve its mechanical properties and prevent corrosion.
Ferrous metals have enhanced physical properties like strong durability, ductility, and high tensile strength, and find a wide variety of commercial and industrial applications. The most common ferrous metals are mild steel, carbon steel, alloy steel, stainless steel, wrought iron and cast iron.
Types of Ferrous Metals
Mild steel and Carbon steel: Iron is combined with carbon to obtain mild steel and carbon steel. Mild steel has a lesser amount of carbon (about 0.16% to 0.3%, by weight) while carbon steel has a relatively higher amount of carbon (up to 2%). In general, the greater the amount of carbon, the harder the metal is.
Mild steel, being relatively softer, is used to make steel plates, while carbon steel, being harder, is used in shafts and pulleys. Additionally, carbon steel is used for cutting other metals and is used in machine tools, blades, and drills.
Alloy steel: Alloy steel is obtained by adding elements like nickel, titanium, chromium, tungsten and vanadium to pig iron. This is mainly done to enhance strength, durability, ductility, corrosion resistance etc. Alloy steel is used in electrical components and in construction.
Stainless steel: It is a kind of alloy steel – it has a large amount of chromium (anywhere from 10.5% to 27%). The amount of chromium present has a bearing on its toughness and resistance to corrosion. Depending on the amount of chromium present, stainless steel is classified into various categories (austenitic, ferritic, martensitic and precipitation-hardening martensitic) and has diverse applications, including pumps, pipes, exhaust systems, bearings, cutlery, medical instruments, turbine and aircraft components.
Cast iron: Cast iron is obtained by adding silicon to molten pig iron and allowing it to cool. It is hard and brittle and has high resistance to wear. Most common uses are in engines, pistons, machine tools, stoves, and pipes.
Wrought iron: Wrought iron is almost pure iron, as it has very little carbon content (less than 0.08%). Pig iron is mixed with slag in a furnace to obtain wrought iron. The slag added during manufacturing helps make the metal more resistant to oxidation (rusting) and corrosion. It is easy to weld and bends easily when hot or cold. Typical uses include railings, nails, chains, furniture, ornaments, and agricultural equipment.
In addition to the diverse applications listed above, ferrous metals have various advantages over non-ferrous metals.
- They have magnetic properties, and are, therefore, used in electrical appliances and large motors. This is especially important, as the magnetic properties of the stator, are what attract the rotor, to facilitate the proper working of the motor.
- They offer thermal resistance i.e., they are able to handle more heat, compared to non-ferrous metals. This is particularly useful in machine tools, to help improve the life of the tool and prevent damage.
- Ferrous metals can be easily made less corrosive, by adding other elements or by cathodic protection.
Compared to non-ferrous metals (e.g. aluminium), which do not have iron, ferrous metals are prone to rust, when exposed to even slight amounts of moisture. However some grades of stainless steel are an exception, because of the presence of chromium.
Ferrous metals can be readily combined with other elements to improve their corrosion resistance. This property, coupled with strong durability, high strength, and ductility, make ferrous metals very versatile and suitable for a wide variety of applications across various industries.