Iron and Types of Iron...

Iron and Types of Iron Iron is a chemical element with symbol Fe (from Latin word Ferrum). Its atomic number is 26 and atomic mass is 55.85. It has a melting point of 1538 deg C and boiling point of 2862 deg C. The density of iron is 7.87 grams/cu cm. It is a metal in the first transition series. Like the elements of other group 8 elements (ruthenium and osmium), iron exists in a wide range of oxidation states, ?2 to +6, although +2 and +3 are the most common. Iron as a common metal is mostly confused with other metals such as different types of steels. Iron is by mass the most common element on the earth, forming much of earth’s outer and inner core. It is the fourth most common element and the second most common metal in the earth crust. Steels contain over 95 % Fe. Elemental iron occurs in meteoroids and other low oxygen environments, but is reactive to oxygen and water. Fresh iron surfaces appear lustrous silvery-gray, but oxidize in normal air to give hydrated iron oxides, commonly known as rust. Unlike the metals which form passivating oxide layers, iron oxides occupy more volume than the metal and thus flake off, exposing fresh surfaces for corrosion. Iron objects have been found in Egypt dating from around 3500 BCE (Before Common Era). They contain around 7.5 % nickel, which indicates that they were of meteoric origin. The ancient Hittites of Asia Minor (today’s Turkey) were the first to smelt iron from its ores around 1500 BCE. The ‘Iron Age’ had begun at that time. The first person to explain the various types of iron was René Antoine Ferchault de Réaumur who wrote a book on the subject in 1722. This explained how steel, wrought iron, and cast iron, were to be distinguished by the amount of charcoal (carbon) they contained. The...

Corrosion of Cast Irons...

Corrosion of Cast Irons Cast iron is a standard term which is used for a large family of alloys of ferrous materials. Cast iron is mainly alloy of iron (Fe) which contains higher than 2 % of carbon (C) and more than 1 % of silicon (Si). Low cost of raw materials and relative ease of production make cast iron the last cost engineering material. Cast iron can be cast into intricate shapes since it has excellent fluidity and comparatively low melting point. It can also be alloyed for improvement of corrosion resistance and strength. With suitable alloying, the corrosion resistance of cast iron can equal to or exceed that of stainless steel and nickel (Ni) based alloy. Since outstanding properties are obtained with this low cost engineering material, cast iron finds extensive use in atmospheres which need good corrosion resistance. Services in which cast iron can be used for its good corrosion resistance include water, soils, acids, alkalis, saline solutions, organic compounds, sulphur compounds, and liquid metals. In some cases, alloyed cast iron is the only economical choice for the equipment manufacture. Cast iron and the basic metallurgy The metallurgy of cast iron is similar to that of steel except that Si in sufficient quantities is present to necessitate use of the Fe-Si-C ternary phase diagram rather than the simple Fe-C binary diagram. A section of the Fe- Fe3C (iron carbide)-Si ternary diagram at 2 % Si is shown in Fig 1. Iron carbide is also known as cementite. The eutectic and eutectoid points in the Fe-Si-C diagram are both affected with the introduction of Si into the system. With normal Si in the range of 1 % to 3 % in cast irons, eutectic C percentage is related to Si percentage as...

Cast irons and their Classification...

Cast irons and their Classification  The term ‘cast iron’ represents a large family of ferrous alloys. Cast irons are multi-component ferrous alloys, which solidify with a eutectic. The major elements of cast irons are iron, carbon (2 % or more), silicon (1 % to 3 %), minor elements (less than 0.1 %), and often alloying elements (less than 0.1%). Cast iron has higher carbon and silicon contents than steel. The structure of cast iron displays a richer carbon phase than that of steel because of its higher carbon content. Cast iron can solidify according to the thermodynamically metastable Fe-Fe3C (iron carbide) system or the stable iron-graphite system depending principally on composition, cooling rate, and melt treatment. Cast iron in its basic form is a brittle material which has a very little impact strength. It has a little or practically no toughness when compared to low carbon steels.  It has a fraction of the tensile strength of low carbon steels.  When a cast iron piece fails it does not deform in a noticeable way and appears to snap apart or break in a manner consistent with a snap.  There is no early warning of a failure. The graphite phase which is pure carbon acts as a natural defect in the material.  The iron is so saturated with carbon that graphite forms (free carbon) and causes the cast iron to be weaker.  Much smaller amounts of carbon is combined with iron (Fe) in the form of iron carbide (Fe3C, cementite) which is hard and brittle. During the solidification process, when the metastable route is followed, the rich carbon phase in the eutectic is the iron carbide and when the stable solidification route is followed, the rich carbon phase is graphite. Referring only to the binary Fe-Fe3C or...

Malleable Cast Iron

Malleable Cast Iron  Malleable cast iron is essentially white cast iron which has been modified by heat treatment. It is formed when white cast iron is heated to around 920 deg C and then left to cool very slowly. Graphite separates out much more slowly in this case, so that surface tension has time to form it into spheroidal particles rather than flakes. Due to their lower aspect ratio, spheroids are relatively short and far from one another, and have a lower cross section vis-a-vis a propagating crack. They also have blunt boundaries, as opposed to flakes, which alleviates the stress concentration problems faced by the gray cast iron. In general, the properties of malleable cast iron are more like mild steel. There is a limit to how large a part can be cast in malleable cast iron, since it is made from white cast iron. The white cast iron is converted to malleable cast iron by a two stage heat treatment process to a condition having most of its carbon content in the form of irregularly shaped nodules of graphite, called temper carbon. The structure of malleable cast iron consists of ferrite, pearlite and tempered carbon as compared to the fracture inducing lamellar structure of gray cast iron. Malleable cast irons are a class of cast irons with mechanical strength properties that are intermediate to those of gray or ductile cast irons. The microstructure provides it properties that make malleable cast irons ideal for applications where toughness and machinability are required, and for components that are required to have some ductility or be malleable so that they can be bent or flexed into position without cracking. Malleable cast iron besides less sensitive to cracking has a range of features, such as higher values of...

Ductile Cast Iron

Ductile Cast Iron  Ductile cast iron also known as nodular cast iron, spheroidal graphite iron or SG iron, and spherulitic cast iron. The ductile iron process was developed by The International Nickel Company in 1948. As the name ductile iron suggests this grade of cast iron has a degree of ductility. The main characteristic of this material is the structure of the graphite. Ductile iron is a family of cast graphitic irons which possess high strength, ductility and resistance to shock. Annealed cast ductile iron can be bent, twisted or deformed without fracturing. Its strength, toughness and ductility duplicate many grades of steel and far exceed those of standard gray irons. Yet it possesses the advantages of design flexibility and low cost casting procedures similar to gray iron. The difference between ductile iron and gray iron is in the graphite formation. Ordinary gray iron is characterized by a random flake graphite pattern in the metal. In ductile iron the addition of a few hundredths of 1 % of magnesium or cerium causes the graphite to form in small spheroids rather than flakes. These create fewer discontinuities in the structure of the metal and produce a stronger, more ductile iron. This nodular graphite structure inhibits the creation of linear cracks hence the ability to withstand distortion. Fig 1 shows typical micro structure of ductile iron. Fig 1 Typical micro structure of ductile iron  With ductile iron, the safety and reliability of process equipment is improved. The improved mechanical properties increase its resistance to breakage from physical load, or mechanical and thermal shock far above that of gray iron. The corrosion resistance of ductile iron is equal or superior to gray cast iron and to cast steel in many corrosives. Its wear resistance is comparable to...