The Production of Cast Iron - Essay Example

Cast Iron Cast irons contan 2% to 4% carbon, 1% to 3% silicon, and in most cases the remainder is iron, which means that they are iron-carbon-silicon alloys. The presence of the silicon in the alloy promotes the formation of graphite. If an iron-carbon alloy containing over 2% carbon is cooled very slowly it will result in graphite, which is pure carbon, and iron crystallising out, to form cast iron. At normal cooling rates the cemenite Fe3C is formed. Other element can be aded to the melt to produce desired properties before the final form is produced by casting. The production of cast iron involves remelting pig iron, steel scrap, foundry scrap and ferro-alloys to give the right composition. Undesirable contaminants such as phosphorous and sulphur are removed as best as possible so as not to cause undesirable effects in the finished metal. The most common melting unit used to do this is called a cupola, which is like a small blast furnace. Cold pig iron and scrap are charged from the top onto a bed of hot coke. Air is then blown through this. Alternatively, the metallic charge is melted in a coreless induction furnace, or a small electric-arc furnace. After melting is complete, the molten iron is ladled from the forehearth of the blast furnace in a method devised by the Chinese. These were the first to use cast iron, for production of weaponry. As time went on, the process began to be used for building bridges and other structures during the industrial revolution. However there were several instances of these structures collapsing years later, and many (especially in the UK and USA) have now been replaced by steel structures. Cast iron does still have sevral uses today, however, and the different methods of production can produce different types, and we will look at each in turn. Grey cast iron Composition This is the most widely used form of cast iron, and is the one usually implied when referring to just 'cast iron'. It contains 2.5% to 4% carbon and >2% silicon. The silicon promotes the formation of graphite from the unstable cementite, and it is essential to have high levels for production of grey cast iron as opposed to white cast iron (see below). Structure The structure of grey cast iron is ferrite (or pearlite) and graphite structure. In this type of cast iron, the graphite is in the form of irregular flakes, which when the metal is fractured, can be seen as the identifiable grey matrix. These result from the weak bonding between planes, which lead to a high activation energy for growth in that direction. This means that it is preferable for the structure to expand outwards rather than upwards between planes. The silicon causes the carbon to come out of the solution rapidly, which results in relatively pure, soft iron. Properties The properties of grey cast iron are highly dependent on the cooling rate of the casting, and also on the section thickness. For instance thin sections can have a reasonably high tensile strength, where as the section thickness increases, this tensile strength decreases. The properties are also highly dependent on the proportions of graphite present in the matrix. If full graphitisation occurs, which is where all of the carbon hs seperated from the molten metal, then the grey cast iron formed will have graphite flakes in a ferritic matrix. However if some carbon, around 0.5% to 0.8% remains in the form of Fe3C, in the molten metal, the resulting matrix will be pearlitic, and the cast iron will be both stronger and harder. Sulphur is present in cast iron, as is manganese. The levels of both are generally kept quite low. However the manganese will combine with the sulfur to form a precipitate, and this can reduce the formation of graphite. It is therefore important not to let the levels reach too high, as while a small restriction is good to increase the hardness and strength, too high a level would result in complete prevention of cast iron formation. Alloying additions of chromium, nickel and molybdenum can be used to increase strength in thicker sections that would usually be lacking the strengths required. Because of the presence of this graphite, cast iron is actually brittle unless treated before being used. The graphite content of this type of cast iron offers good corrosion resistance. Graphite also acts as a lubricant, which gives high wear resistance. Graphite has a high thermal conductivity, which is caused by a very high speed of sound within the structure. This is caused by the heavy atoms being weakly bonded. This also gives cast iron the property of being able to dampen vibrations, including sound, which helps machinery to run more smoothly. Machining of grey cast iron is easy, as the sharp edges of the graphite flakes tend to concentrate stress, so cracks can form easily. However this property can also be a downside to the material, since it means that the finished metal has less tensile strength and shock resistance than steel. It also makes the metal difficult to weld. Another property of cast iron that can be exploited is its high thermal conductivity, and also its specific heat capacity. Uses The heat conducting properties of cast iron discussed above mean that it is very useful as disc brake rotors, and is also useful for making cast iron cookware. Grey cast iron has a very high resistance to wear, making it ideal for uses such as cylinder bores, piston rings and slideways on machine tools. Bacause of the the graphite flakes present in the structure, grey cast iron also has very good resistance to galling and seizing, and has a low friction coefficient. It is one of the easiest iron alloys to machine, although the fine pearlite matrix version is a little harder to machine than the ferrite-graphite cast iron. Grey cast irons have an excellent capacity for absorbing vibration energy, and are therefore excellent for dampoening vibrations. This is most evident in fomrs with a high percentage of graphite flakes. White Cast Iron Composition and Structure While the composition of white cast iron is similar to that of grey cast iron, the way in which it is produced mean that it has a pearlite structure in a cementite matrix. Properties White cast iron is hard, brittle and almost impossible to machine. Uses Due to the properties dicsussed above, white cast iron finds uses only in limited areas of industry, such as extrusion dyes and cement mixer liners. It is however used to produce malleable cast iron. Malleable Cast Iron Composition Malleable cast iron is made from white cast iron, so the composition is the same. Structure Malleable cast iron is made of heat-treated forms of white cast iron, with improved ductility, but stil with high tensile strength. The white cast iron is heat-treated at temperatures of 870ºC for long periods of time, and then controllably cooled over long periods of time. The cementite loses carbon, which forms free nodules, and the final product is therefore a ferrite matrix with free nodules of carbon throughout. A pearlite structure can be obtained by heating the malleable cast iron at even higher temperatures of 970º for over 12 hours, and then air-cooling. This faster cooling produces less ferrite and a finer pearlite structure. If this is done at a slightly lower temperature of about 940ºC and then quenched in oil, it will result in a martensitic structure. There are three main types of malleable cast iron that result. Whiteheart is heat-treated white iron, with a ferrite outer layer and ferrite ir pearlite core. It is easy to cast into sections, which have a tough core. Blackheart is soaked at a high temoerature to break down the cementite then slowly cooled to give ferrite and graphite. Pearlite is similar to Blackheart, but is cooled faster to give the pearlite structure. Properties Malleable cast iron has superior mechanical properties to grey cast iron, except for its resistance to wear, which is lower. Increased strength and wear resistance can be obtained by converting the structure to a pearlite matrix with free nodules of carbon, however this also results in reduced ductility. Uses Malleable iron is preferable for thin-section castings and for components that require maximum machiniability, that must retain good impact resistance at low temperatures, and components requring wear resistance (for which martensitic malleable iron is used). Ductile Cast Iron Nodular feritic, or ductile iron is a more recent development in the manufacture of cast iron. Itis produced by adding magnesium before casting, which encourages the graphite to form spheres or nodules. These produce much improved mechanical advantages over the usual graphite flakes in grey cast iron. It is similar to grey cast iron in its low melting point, good fluidiity, castability, excellent machinibility and wewar resistance. Howveer it also has improved strength, ductiity toughness and hot workability. Ductile iron competes favourably with steel. References Cast Irons. 2006. 29th March 2006. . Houk C. and Post R. Chemistry: Concepts and Problems: A Self-Teaching Guide. USA: John Wiley and Sons, 1996. Yescas-Gonzalez M.A. Cast Irons. Unknown date. University of Cambridge. Read More