Iron and Steel - √دنیـــــــای متـــــــــــالورژی فقـــــــط از ایـــــــنجا
به "با سابقه ترين و محبوب ترين وبلاگ متالورژي در ايران" خوش آمديد

تاريخ : شنبه ۱٧ دی ۱۳۸٤

Iron and Steel

With the exception of aluminum  , iron is the most abundant of all metals. It is estimated that iron ore constitutes 4.44 percent, by weight , of the earth’s crust. Iron is a constituent of all rocks and minerals. It is never found in the pure state except as meteorites . Its presence  in matter is indicated by color. The color of given materials by the presence of iron may be black, brown , red, yellow ,or a variety of tints. Iron possesses greater magnetic qualities than any other metal. This quality has made it possible for engineers , by means of sensitive electrical instruments , to discover deposits rich in iron ore  easily and to plot their extent accurately .

Iron ores are found in may forms . those principally used in the production of iron are hematite , magnetite , and limonite .all ores used in the production of iron are graded according to their iron content , their physical properties , and the amount of impurities present. Most molecules of iron are found combined with phosphorus , sulphur , manganese , or silicon atoms in various combinations.

Deposits of Iron Ore

Most of the rich deposits of iron ore are believed to have been formed by the leaching  action of water on ore-bearing rock. After the soluble ore was loosened from the rock mass, it was carried by flowing water to depressions where the ore settled to the bottom to form a deposit. This process continued over thousands of years .Thus the great iron deposits were formed and found in the lake superior region of Minnesota , the Birmingham district of Alabama, and the cornwall district of Pennsylvania. The principal producing source in the united states is the lake superior region . from the mines in this region , fifty million tons or more of ore are taken each year.

Methods of Mining Iron ore

There are three methods of mining iron ore, namely .the open-pit method, the milling method, and the underground method . over half the ore mined in the united states is taken from the open pits. In  the lake superior region where this method is employed , great steam shovels strip away the earth above the deposit, while similar devices load the ore into railway cars. The ore is transported from the mine to docks where it is loaded by machinery into boats and shipped to lake ports where it is unloaded at steel mills. Some mills are located in inland cities in which case the ore is again loaded into railway cars and thus taken to its destination . whenever feasible the open-pit method of mining is preferred because of lower cost of production, even though the initial cost of opening the mine is much greater.

Milling Method

this method is practiced when the deposit is so located that tunnels can be constructed beneath it and shafts raised to it . the ore is then mined by hand or machinery and put into cars waiting in the tunnels below.

The Underground Method

When the ore is located at a considerable depth (several hundred feet) below the surface of the earth , the underground method of mining is employed .When this method is used shafts are sunk to the deposit and the ore hauled to the surface by hoists. As mining proceeds , tunnels are cut into the deposit.

As it is taken from the mine , iron ore varies in value , depending upon its purity and the costs involved in refining it. When purified and cast in pigs , the value is increased about five-fold ; in the from of basic steel it is worth about 50%  more than in pig form.

Reducing and Refining Iron Ore

In its natural state, iron ore contains many impurities. Until these are removed, relatively pure iron of the kind one sees every day cannot be obtained . the most important factors in refining iron ore are heat, oxygen, and a suitable flux.

The application of refined iron, determines the method of reduction employed. When used as a  cast iron only, the blast furance procedure is necessary . when used in the form of wrought iron or mild steel, both the blast and the open hearth processes are used.

Making Steel

There are two main kinds of iron used for making steel -‘basic’ iron and ‘acid’ iron . these kinds of iron can be called steel-making irons. Both contain small quantities of carbon, silicon, sulphur , phosphorus ,and manganese . these elements are called ‘impurities’ in the metal. How are they reduced ? the process by which they are reduced is called ‘oxidation’ . the two main processes used to turn iron into steel are called : the Bessemer process ,and the open hearth process.

The Bessemer Process

when molten iron is to be used in the manufacture of steel of steel , it is taken from the blast furance in ladles to a ‘Bessemer converter’ . this process of removing impurities from iron was invented in 1856 by sir Henry Bessemer .the molten iron is conveyed in a huge ladle from the blast furance to the converter. When the molten metal arrives at the converter station , the converter , which revolves on trunions , is tipped on its side. While in this position , the molten metal is poured into the converter . when the converter is loaded the hot air blast entering at the bottom is turned on . the hot air being under pressure of about 20 pounds , exert sufficient force to keep the metal from running into the blast openings. As the hot air works up through the molten mass of iron , most of the remaining carbon, silicon ,and manganese is burned out due to a rise in temperature resulting from oxidation of the carbon , manganese , and silicon .hot air is the only element used in the reducing process. when the metal has been refined to the extent desired , the slag formed in the oxidizing (burning) process is removed. In this condition the metal ,when cooled , does not have sufficient hardness or toughness. To restore these qualities , sufficient carbon and manganese or other alloys are added to give the steel the degree of hardness, toughness, ductility, and malleability desired . when the alloys have combined with the molten metal to give it the characteristics desired , the metal is again poured into the ladle and from there into ingot molds where the molten metal is formed into blocks or slab of iron about 20 inches square and 60 inches long.

Type by : M.H


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