Iron

      Iron is a chemical element with the symbol Fe (from Latin: ferrum) and atomic number 26. It is a metal in the first transition series. It is the most common element (by mass) forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust. Iron's very common presence in rocky planets like Earth is due to its abundant production as a result of fusion in high-mass stars, where the production of nickel-56 (which decays to the most common isotope of iron) is the last nuclear fusion reaction that is exothermic. This causes radioactive nickel to become the last element to be produced before collapse of a supernova leads to the explosive events that scatter this precursor radionuclide of iron abundantly into space.
       Iron is a relatively abundant element in the universe. It is found in the sun and many types of stars in considerable quantity. Iron nuclei are very stable. Iron is a vital constituent of plant and animal life, and is the key component of haemoglobin.

       The pure metal is not often encountered in commerce, but is usually alloyed with carbon or other metals. The pure metal is very reactive chemically, and rapidly corrodes, especially in moist air or at elevated temperatures. Any car owner knows this. Iron metal is a silvery, lustrous metal which has important magnetic properties.

Basic information about and classifications of iron:
   *Name: Iron
   *Symbol: Fe
   *Atomic number: 26
   *Atomic weight: 55.845 (2)
   *Standard state: solid at 298 K
   *CAS Registry ID: 7439-89-6 Group in periodic table: 8
   *Group name: (none)
   *Period in periodic table: 4
   *Block in periodic table: d-block
   *Colour: lustrous, metallic, greyish tinge
   *Classification: Metallic

History:
          Iron was discovered by Known since ancient times at no data in not known. Origin of name: from the Anglo-Saxon word "iron" or "iren" (the origin of the symbol Fe comes from the Latin word "ferrum" meaning "iron"). Possibly the word iron is derived from earlier words meaning "holy metal" because it was used to make the swords used in the Crusades..

         Iron was known in prehistoric times. Genesis says that Tubal-Cain, seven generations from Adam, was "an instructor of every artificer in brass and iron." Smelted iron artifacts have been identified from around 3000 B.C. A remarkable iron pillar, dating to about A.D. 400, remains standing today in Delhi, India. This solid pillar is wrought iron and about 7.5 m high by 40 cm in diameter. Corrosion to the pillar has been minimal despite its exposure to the weather since its erection.

         Sometime prior to the autumn of 1803, the Englishman John Dalton was able to explain the results of some of his studies by assuming that matter is composed of atoms and that all samples of any given compound consist of the same combination of these atoms. Dalton also noted that in series of compounds, the ratios of the masses of the second element that combine with a given weight of the first element can be reduced to small whole numbers (the law of multiple proportions). This was further evidence for atoms. Dalton's theory of atoms was published by Thomas Thomson in the 3rd edition of his System of Chemistry in 1807 and in a paper about strontium oxalates published in the Philosophical Transactions. Dalton published these ideas himself in the following year in the New System of Chemical Philosophy. The symbol used by Dalton for iron is shown below. [See History of Chemistry, Sir Edward Thorpe, volume 1, Watts & Co, London, 1914.]

Iron orbital properties:
   *Ground state electron configuration: [Ar].3d6.4s2
   *Shell structure: 2.8.14.2
   *Term symbol: 5D4
   *Pauling electronegativity: 1.83 (Pauling units)
   *First ionisation energy: 762.5 kJ mol-1
   *Second ionisation energy: 1561.9 kJ mol-1

Isolation:
        Isolation: it is not normally necessary to make iron in the laboratory as it is available commercially. Small amounts of pure iron can be made through the purification of crude iron with carbon monoxide. The intermediate in this process is iron pentacarbonyl, Fe(CO)5. The carbonyl decomposes on heatingto about 250°C to form pure iron powder.

                                    Fe + CO → Fe(CO)5 (250°C) → Fe + 5CO

       The Fe(CO)5 is a volatile oily complex which is easily flushed from the reaction vessel leaving the impurities behind. Other routes to small samples of pure iron include the reduction of iron oxide, Fe2O3, with hyrogen, H2.

      Nearly all iron produced commercially is used in the steel industry and made using a blast furnace. Most chemistry text books cover the blast furnace process. In essence, iron oxide, Fe2O3, is reduced with with carbon (as coke) although in the furnace the actual reducing agent is probably carbon monoxide, CO.

                                              2Fe2O3 + 3C → 4Fe + 3CO2

      This process is one of the most significant industrial processes in history and the origins of the modern process are traceable back to a small town called Coalbrookdale in Shropshire (England) around the year 1773.

Properties:
              Iron is a lustrous, ductile, malleable, silver-gray metal found in Group 8 of the periodic table. It is known to exist in four distinct crystalline forms (see allotropy). The most common is the α-form, which is stable below about 770°C, and has a body-centered cubic crystalline structure; it is often called ferrite. Iron is attracted by a magnet and is itself easily magnetized (see magnetism). It is a good conductor of heat and electricity. It displaces hydrogen from hydrochloric or dilute sulfuric acid, but becomes passive (loses its normal chemical activity) when treated with cold nitric acid.

Iron Uses:
           Iron is the most widely used of all the metals, accounting for 95% of worldwide metal production.[citation needed] Its low cost and high strength make it indispensable in engineering applications such as the construction of machinery and machine tools, automobiles, the hulls of large ships, and structural components for buildings. Since pure iron is quite soft, it is most commonly combined with alloying elements to make steel.

Commercially available iron is classified based on purity and the abundance of additives. Pig iron has 3.5–4.5% carbon and contains varying amounts of contaminants such as sulfur, silicon and phosphorus. Pig iron is not a saleable product, but rather an intermediate step in the production of cast iron and steel. The reduction of contaminants in pig iron that negatively affect material properties, such as sulfur and phosphorus, yields cast iron containing 2–4% carbon, 1–6% silicon, and small amounts of manganese. It has a melting point in the range of 1420–1470 K, which is lower than either of its two main components, and makes it the first product to be melted when carbon and iron are heated together. Its mechanical properties vary greatly and depend on the form the carbon takes in the alloy.

"White" cast irons contain their carbon in the form of cementite, or iron-carbide. This hard, brittle compound dominates the mechanical properties of white cast irons, rendering them hard, but unresistant to shock. The broken surface of a white cast iron is full of fine facets of the broken iron-carbide, a very pale, silvery, shiny material, hence the appellation.

In gray iron the carbon exists as separate, fine flakes of graphite, and also renders the material brittle due to the sharp edged flakes of graphite that produce stress concentration sites within the material. A newer variant of gray iron, referred to as ductile iron is specially treated with trace amounts of magnesium to alter the shape of graphite to spheroids, or nodules, reducing the stress concentrations and vastly increasing the toughness and strength of the material.

Wrought iron contains less than 0.25% carbon but large amounts of slag that give it a fibrous characteristic.It is a tough, malleable product, but not as fusible as pig iron. If honed to an edge, it loses it quickly. Wrought iron is characterized by the presence of fine fibers of slag entrapped within the metal. Wrought iron is more corrosion resistant than steel. It has been almost completely replaced by mild steel for traditional "wrought iron" products and blacksmithing.

Published by Ravindra.K(Mechanical Engineering)