Subject: Manufacturing Processes Class: 3rd Mechanical Engineering Department Tikrit University Prepared by: Assistant Prof.Dr.Farouk Mansour Mahdi Lecture No.1 Week No.1 No. of hours: 2 theoretical and 1 tutorial Metal Casting Metal Casting 1- Casting Furnaces ( Melting Furnaces ) Melting furnaces used in the foundry industry are of many diverse configurations. The selection of the melting unit is one of the most important decisions foundries must make. Several important factors must be considered for proper selection, these includes: 1. The temperature required to melt the metal or alloy. 2. The melting rate and quantity of molten metal required. 3. The required quality of the melt and subsequent final product. 4. The economy of installation, operation and maintenance. 5. Environmental and waste disposal requirements. Furnaces can be classified according to the type of lining: 1- Acidic lined furnaces ( e.g. SiO2 ) 2- Basic lined furnaces ( e.g. MgO, MgCO3 , CaO) Lining materials are characterized by: 1-Refractoriness. 2-High wear resistance. 3-Low coefficient of thermal expansion. 4-High resistance to thermal shock. 5-Heat insulation. 6-passive towards molten metal, furnace gases and slag ( chemically inert). 1-1- Cupola Furnaces coke type cupola furnace Description: A cupola or cupola furnace is a melting device used in foundries to melt cast iron, some bronzes and even aluminum when attention is paid to keep the temperature low. The construction of a conventional cupola consists of a vertical steel shell which is lined with a refractory brick. The size of a cupola is expressed in diameters and can range from 0.5 to 4.0 m while the stack height is between 6 to 11 m. The bottom of the cylinder is fitted with doors which swing down and out. The top, where gases escape can be opened or fitted with a cap to prevent rain from entering the cupola. Operation: To begin a production run, called a 'cupola campaign', the furnace is charged with layers of coke and ignited with torches. When the coke is ignited, air is introduced to the coke bed through ports in the sides called tuyeres. When the coke is very hot, solid pieces of metal are charged into the furnace through the charging door. The metal is alternated with additional layers of fresh coke. Limestone ( CaCO3 ) is added to act as a flux. As the heat rises within the stack the metal is melted. It drips down through the coke bed to collect in a pool at the bottom, just above the bottom doors. Some of the carbon is picked up by the falling droplets of molten metal which raises the carbon content of the iron. Additions to the molten iron such as ferro-manganese, ferro-silicon, silicon carbide and other alloying agents are used to alter the molten iron to conform the required composition. When the metal level is sufficiently high in the well, the cupola operator opens the "tap hole" to let the metal flow into a ladle or other container to hold the molten metal. When slag will rise to the top of the formed iron pool. The slag hole is opened to permit the slag flow out. Advantages: 1. Lower initial cost on a small floor area comparing with those furnaces with the same capacity. 2. The cupolas is the only continuous melting method. 3. High melt rates. 4. Relatively low operating costs. 5. Ease of operation. 6. From a life-cycle perspective, cupolas are more efficient and less harmful to the environment than electric furnaces. This is because they derive energy directly from coke rather than from electricity that first has to be generated. 7. Adequate temperature control. 8. Adequate Chemical composition control. 9. Efficiency of cupola varies from 30 to 50%. Disadvantages: 1- Since molten iron, coke and oxygen are in contact with each other, certain elements like Si and Mn are lost and others like S and C are picked up. This changes the final analysis of molten metal. 2- Close temperature control is difficult to maintain. 3- Accurate control of chemical composition is not possible. 1-2 Reverberatory Furnaces reverberatory furnace ( Pb remelting) Reverberatory furnaces which are also called "air furnaces" are used for smelting (refining) or melting processes, in which the fuel is not in direct contact with the contents but heats it by a flame and hot combustion gases blown over it from another chamber. Such furnaces are used in copper, tin, nickel production, and in aluminum recycling. In steelmaking, this process is called the open-hearth process ( which will be explained later ). The basic idea of a reverberatory furnace is to use the heat reflecting off a surface, usually brick, to heat the metal. Operation: The material to be heated is placed on the hearth and is heated by the hot gases or flame produced by the burning of fuel. The waste gases escape out of the chimney. This way, the metal does not come into direct contact with the fuel or the flame. By placing the metal in a shallow depression and then directing an intense flame over that depression and to the wall, the heat rebounds (radiates back or reverberates) to melt the metal. Many casters will adjust the length of the flame since a longer path will mean that the heat will be more intense. Reverberatory furnaces are available with capacities of up to 150 tons of molten aluminum. Advantages: 1-Low operating and maintenance costs. 2-High volume processing rate. 3-Adequate temperature control. 4-Adequate Chemical composition control. Disadvantages: 1-High initial cost. 2-The reverberatory process is a batch type. 3-Large floor space requirements. 4-Precious control of melt temperature cannot be made (Wide Metal Temperature Variations +/- 50º F). 5-Accurate control of chemical composition cannot be satisfied. 6-Limited control of furnace atmosphere. 7- Typical aluminum reverberatory furnaces have melting efficiencies of 15 - 39%. 8- Greater hydrogen gas pick up in aluminum melting. 1-3 Open hearth furnace Description: It is an alternative steelmaking process in which natural gas, oil, atomized heavy oils, tar, or pulverized coal are used as fuel. Both air and fuel are preheated to about 800o C before combustion. A flame temperature of about 2,000° C could be obtained, and this is sufficient to melt the charge. Initially, charges of 10 tons were made, but furnace capacity gradually increased to 100, 300 and eventually to 600 tons. Operation: In case of re-melting of steel scrap, the furnace is charged with light scrap, such as sheet metal, shredded vehicles or waste metal. When light scrap has melted, heavy scrap, such as building, construction or steel milling scrap is added, together with pig iron from blast furnaces. After all steel has been melted, slag forming agents, such as limestone, are added. The oxygen in iron oxide and other impurities decarburize the pig iron by burning the carbon away, forming steel. To increase the oxygen content of the hearth, iron ore can be added. Preparing a hearth usually takes 8 h to 8 h and 30 minutes. Additions can be made to the steel to produce the desired composition. After a period of time, the direction of air and fuel flow is reversed. The chambers heated from the previous cycle, in turn, heat the incoming fuel and air. Most open hearth furnaces are chemically basic. The basic furnaces can remove phosphorous, sulfur, silicon, carbon, and manganese from the charge metal. The furnace is tapped through a tap hole located at the side of the hearth and liquid steel is let to flow out. Once all the steel has been tapped, the slag is skimmed away. The tapped steel may be cast into ingots or it may be used in continuous casting for the rolling mill. Advantages: 1-The great advantage of the open hearth was its flexibility: the charge could be all molten pig iron, all cold scrap, or any combination of pig iron and scrap. 2-Basic open hearth furnaces are capable of processing iron of almost any chemical composition. 3-The process is suited to handle any amount of low cost steel scrap. 4-Open hearth furnaces can operate on any kind of fuel. 5-The quality of open hearth furnace is the highest among commercial steel making processes ( Bessemer, Thomas and oxygen converter techniques ). 6-Adequate temperature control. 7-Adequate Chemical composition control. Disadvantages: 1-High initial cost. 2-Large floor space requirements. 3- The open hearth process is a batch type. 4-Low productivity as compared with oxygen converter process. 5-The necessity of providing fluxes and regenerators ( such as alloying elements) raises the costs of construction and running of open hearth furnaces. 6-Precious control of melt temperature cannot be made. 7-Accurate control of chemical composition cannot be satisfied. 8-Limited control of furnace atmosphere. 1-4 Electric Arc Furnaces Electric arc furnaces may be categorized as direct arc and indirect arc. Both types of units are suited for the melting of high melting point alloys such as steels. They may be lined with acid or basic refractories. The main advantage of the Electric Arc Furnaces over the Basic Oxygen Furnaces (BOF) is their capability to treat charges containing up to 100% of scrap. About 33% of the crude steel in the world is made in the Electric Arc Furnaces Direct arc furnaces are very popular for the melting of alloy steels and range in size from a few kilograms, for laboratory units, to about 400 tons per batch. Typical units found in foundries are in the range of 1 to 10 tons. The furnace generally consists of steel shell lined with acid or basic refractories.