Cast Iron - Essay Example

Running head: CAST IRON Cast Iron Name: Institution: Cast Iron Cast iron is one of the ferrous metals that have a wide range of properties produced through the process of casting. Cast iron has a crystalline structure and good compressive strength. However, it is very brittle and weak in tension. In addition, cast iron has excellent fluidity when molten, hence it is used for production of intricate shapes (Tata Steel, 2013a). Cast iron has high carbon content and its structure exhibits a rich carbon phase. Cast iron can be classified into two groups based on fracturing; white cast iron and gray cast iron. White cast iron has a white crystalline fracture surface since the fracture takes place along iron carbide plates. On the other hand, gray cast iron shows a gray surface after fracturing because the fracture takes place along the flakes or graphite plates (Key to metals, 2010). Moreover, cast iron contains more than two percent of carbon and 1-3 percent silicon, hence they are iron-carbon-silicon alloys. Therefore, the high content of silicon and carbon makes cast irons excellent casting alloys. Cast irons have low melting temperatures compared with steel, and they have relatively less reaction with molding materials. During cast iron production, it reduces in volume from the liquid state to solid thus making production of intricate castings possible. Cast iron is made by re-melting of pig iron, and possibly along with other scrap iron. The pig iron has many impurities such as iron carbide that makes the material very brittle and hard. The melting process ensures that the carbon content is between 2.4 and 4.0 percent (Tata Steel, 2013b). The production of cast iron is done through sand casting techniques although the process is enhanced by modern technology and materials. The production process often involves four stages; designing of the component to be cast, pattern making, making of mould, and casting. In the designing stage, the company provides a well detailed, complex, manufacturing drawing. The drawing shows the shape and precise dimensions of the component to be made. The next step is to make a pattern. A pattern is made using fiberglass, plastics, or wood. Patterns have the same shape and precise dimensions as the finished product because they are used to make sand moulds. A pattern can be re-used for many years to make more moulds. The pattern must give an allowance for metal shrinkage and create runners to allow the molten iron to flow into the mould. The risers are made to allow gases to escape when the molten iron is being poured. The next step is creation of a sand mould into which the molten iron will be poured. The pattern is put into the sand that is mixed with resin or clay. The pattern leaves the shape of the casting after the sand is well packed around it. Mostly, the moulds are made in two precise parts and are held in casing boxes for the pouring. When the two boxes are placed one on top of the other, a cavity into which the molten metal will flow is established. In case the casting is very large, moulds are created out of the sand on the foundry floor. The final stage involves pouring of the molten iron into the moulds. In this stage, the molten metal is at the temperatures of 1350 degrees centigrade and the process is potentially violent, hence safety is given the highest priority. First, the furnace used to melt the pig iron is loaded and the correct chemical characteristics of the iron grade needed are achieved. After the pig iron is melted, the molten iron is poured onto the moulds and it is now referred to as cast iron. The slag waste is put aside for disposal. The cast iron is given enough time to cool and then the moulds are broken out. The excess iron from the runners is cleaned through fettling process. The process involves shot blasting and grinding to attain a finished casting (Hargreaves Foundry, 2013). There are three types of cast irons; ductile, gray and white cast iron. Gray cast iron’s structure has graphite flakes. During solidification process, carbon in the iron separates to form graphite flakes, therefore properties of gray cast iron are determined by the amount, distribution and size of graphite flakes. If the molten iron has higher carbon and silicon contents and it is subjected to slower cooling rate, larger and more graphite flakes are developed. Graphite flakes makes gray cast iron to have unique properties such as excellent vibration damping, galling resistance, wear resistance, and excellent machinability. Ductile cast iron’s structure contains graphite that occurs as spheroids, unlike individual flakes in gray cast iron. Ductile cast irons are formed by adding appropriate amount of magnesium to molten metal before solidification. The added magnesium reacts with oxygen and sulfur to change the way the graphite is formed (Atlas Foundry Company, 2010). White cast iron or white iron is more brittle and harder than gray and ductile cast irons. White cast iron is formed by rapid cooling of the molten iron. After rapid cooling, carbon remains distributed all over the iron in form of cementite (Infoplease, 2013). After the final product is obtained, it is important to inspect it for any defects. Some of the most common defects in cast irons include dispersed shrinkage, flash, blow holes or pinholes, lustrous carbon, and axial shrinkage, among others. Dispersed shrinkage is characterized by cavities often perpendicular to the cast iron surface, with depths of less than 2 centimeters or 0.8 inches. This casting defect is caused by high nitrogen content or low carbon continent in the melt. Flash defects are projections that appear at the parting line when there is a clearance between two metal casting molds, hence allowing molten iron to get in and solidify. Blowholes or pinholes defect is characterized by appearance of smooth tiny holes on the surface of the cast iron. The interior walls of the pinholes are shiny, less or more oxidized compared with other regions, and they are covered with a thin graphite layer. Lustrous carbon appears as wrinkled or folded films that are distinctly outlined and occur within the walls of cat iron and causes a linear discontinuity of the structure. They are only noticeable when the cast is fractured. The defect occurs when the material from binders, core additives, or mould decompose and becomes part of the melt. Lastly, axial shrinkage defect results when metal at the central part of the casing takes more time to cool compared with the surrounding regions. This defect can be influenced by the pouring temperature and speed, alloy purity, and riser use (American Foundry Society, 2013). It is important to ensure that components with severe defects are re-melted again. Old broken cast iron scraps can be recycled to produce new items. The scrap materials can be collected from homes, garages, dustbins, old machines that have failed to operate such as lathes, finished components with defects, among others. To start with, the scrap cast iron scrap is broken down into small pieces by use of a hammer. Cast iron is very brittle and it breaks relatively easily. However, it is recommendable for the person carrying out the task to wear safety gloves, safety boots and glasses because pieces flying off can injure him or her. They are then put in a cupola furnace for re-melting. The slag is continuously removed as the melting continues to ensure the purity of the melt is attained. Then, the normal process of pouring takes place (Oliver, 2007). Metal fabrication involves building of metal structures by bending, cutting, and assembling of various parts. Cast iron is an excellent fabrication material. Cast iron can be assembled together with other parts to carry out a given function in a machine. One of the important fabrication processes is welding to assemble cast iron parts. The carbon content of carbon ranges from 2 to 4 percent, therefore it is ten times greater compared with other alloys such as steel or iron. The welding procedure can be done in two different ways; with preheat, and without preheat. Cast iron welding by preheating is done by applying heat on the whole object to be welded at temperatures ranging between 500 to 1200 degrees Celsius. The heating temperatures should not exceed 1400 degree because the metal starts melting. After the metal get hot, one should start welding by use of a low current. In addition, the weld should be minimized to approximately one inch long segment. The component must be left to cool gradually. Preheated welding is done at the normal temperature levels. Therefore, it is necessary to control the welding gun and make small welds of about one inch thick. The welds must be left to cool gradually and then pin out the extra welds deposited on the cast iron surface (The Lincoln Electric Company, 2013). Cast irons are still relevant today and have a wide range of both decorative and structural applications. This is because it is relatively cheap, easily cast into different shapes, and it is durable. The most typical uses of cast iron include; fences, firebacks and tools, piping, ordnance, decorative features, stairs, columns, balusters, historic plaques and markers, structural connectors in monuments and buildings, and hardware uses such as latches and hinges. The cast iron material used in these applications may appear to be similar, or the same, but the component composition, use, and exposure dictates the different treatments that should be used in every application (US General Services Administration, 2012). In conclusion, many people prefer to use cast iron because it is readily available and affordable. In addition it does not require complex technology to be modified to different and intricate shapes. Therefore, cast iron production process is cheaper compared to other metals such as steel. Additionally, cast iron creates a protective scale or film on its surface that makes it initially resistant to corrosion. The cast iron is prone to corrosion after being exposed on atmosphere for long, but different methods of preventing rusting such as use of paints, plating, and galvanizing can be used. References American Foundry Society (2013). Identifying Casting Defects. Retrieved from http://www.afsinc.org/content.cfm?ItemNumber=6944 Atlas Foundry Company (2010). Understanding cast iron. Retrieved from http://www.atlasfdry.com/cast-irons.htm Hargreaves Foundry (2013). Cast iron production. Retrieved from http://www.hargreavesfoundry.co.uk/userfiles/file/downloads/foundry/castironproduction.pdf Infoplease (2013). Iron. Retrieved from http://www.infoplease.com/encyclopedia/science/iron-production-refining.html Key to metals (2010). Classification of cast iron. Retrieved from http://www.keytometals.com/Articles/Art63.htm Oliver, L. (2007). Scrap iron recycling at the Lab. Retrieved from http://www.backyardmetalcasting.com/scrapiron02.html Tata Steel 2013. The properties of cast iron, wrought iron and steel. Retrieved from http://www.tatasteelconstruction.com/en/reference/teaching_resources/architectural_studio_reference/history/the_technology_of_iron_and_steel/the_properties_of_cast_iron,_wrought_iron_and_steel/ Tata Steel (2013b). Definitions of what is meant by cast iron, wrought iron and steel. Retrieved From http://www.tatasteelconstruction.com/en/reference/teaching_resources/architectural_studio_reference/history/the_technology_of_iron_and_steel/definitions_of_cast_iron,_wrought_iron_and_steel/ The Lincoln Electric Company (2013). Guidelines for welding cast iron. Retrieved from http://www.lincolnelectric.com/en-us/support/welding-how-to/Pages/welding-cast-iron-detail.aspx US General Services Administration (2012). Cast Iron:  Characteristics, Uses and Problems. Retrieved from http://www.gsa.gov/portal/content/111738 Read More