Melting Furnaces in Casting MDPN • What is Furnace? • Heating device used for heating and melting. • The furnace may be heated by fuel as in many furnaces coke or oil is used as a fuel. • some are operated by electrical energy (Induction & electric arc furnace). Furnaces for Casting Processes • • • • • Cupola Direct fuel-fired furnaces Crucible furnaces Electric-arc furnaces Induction furnaces Cupola furnace Cupola Furnace • Cupola was made by Rene-Antoine around 1720. • Used in foundries for production of all types of cast iron and Bronzes. • Its charge is Coke , Metal , Flux. • Scrap of blast furnace is re melted in cupola. • Large cupolas may produce up to 100 tons/hour of hot iron. Construction • Cupola is a cylindrical in shape and placed vertical with shell made of steel. • Its size is expressed in diameters and can range from 0.5 to 4.0 m. • It is supported by four legs. • Internal walls are lined with refectory bricks. • Its lining is temporary. Cupola Parts • • • • • • Spark arrester. Charging door. Air box. Tuyeres. Tap hole. Slag hole. Zones • Well • The space between the bottom of the Tuyeres and the sand bed. • Molten metal collected in this portion. • • • • Combustion zone Also known as oxidizing zone . Combustion take place in this Zone and it is located between well and melting zone. • Height of this zone is normally from 15cm to 30cm. Zones • In this zone the temperature is 1540°C to 1870°C. • The exothermic reactions takes place in this zone these are as following . • C + O2 → CO2 + Heat • Si + O2 → SiO2 + Heat • 2Mn + O2 → 2MnO + Heat • Reducing zone • Located between upper level of combustion zone and upper level of coke bed. Zones • In this zone temperature is about 1200°C and CO2 change into CO. • CO2 + C (coke) → 2CO • Melting zone • In this zone the melting is done and It is located between preheating zone and combustion zone. • The following reaction take place in this zone. • 3Fe + 2CO → Fe3C + CO2 Zones • Preheating zone • This zone is starts from the upper end of the melting zone and continues up to the bottom level of the charging door . • Objective of this zone is preheat the charges from room temperature to about 1090°C before entering the metal charge to the melting zone. • Stack • The empty portion of cupola above the preheating zone is called as stack. It provides the passage to hot gases to go to atmosphere from the cupola furnace. Charging of Cupola Furnace • • • • • • • • • • • • • Before blower is started, the furnace is pre heated and the metal & coke charges, lying in alternate layers, are sufficiently heated up. The height of coke charge in the cupola in each layer varies generally from 10 to 15 cm. The requirement of flux to metal charge depend upon the quality of the charged metal & scrap, the composition of the coke and the amount of ash content present in the coke. About 40 Kg to 50 Kg of limestone, in the form of flux, per metric ton of the metal is used. The excess of amount of flux affects the acid lining of cupola. Less amount of the flux results in the loss of molten metal. First charge received of molten metal is allowed to drain out or used for rough castings. For having desired composition of casting, it is essential to control the proportion of its various constituents at the stage of raw material requirement for melting. During the process of melting ,number of losses & gains of different constituents take place inside the cupola. The losses and gains in composition during melting as identified are given as under : 1. Iron - Loss of about 4% 2. Carbon - Gain of about 0.1 to 0.15%. 3. Silicon - Loss of about 10% 4. Manganese - Loss of about 15% to 20% 5. Phosphorus - Practically no change. 6. Sulphur - gain of about 0.03 to 0.05% Working of Cupola Furnace • Its charge consist of scrap, coke and flux. • The charge is placed layer • by layer; the first layer is coke, second is flux and third is metal. • Air enter through the bottom tuyeres. This increases the energy efficiency of the furnace. • Coke is consumed. Working of Cupola Furnace • The hot exhaust gases rise up through the charge, preheating it. • The charge is melted; As the material is consumed, additional charges can be added to the furnace. • A continuous flow of iron emerges from the bottom of the furnace. • The slag is removed from slag hole. • The molten metal achieved by tap hole. Operation of Cupola • • • • • • Preparation of cupola. Firing the cupola. Soaking of iron. Opening of air blast. Pouring the molten metal. Closing the cupola. Preparation of cupola • Slag and metal adhere to the cupola lining from the previous run is removed and lining of cupola is re made. • The bottom plates are swung to closing position supported by prob. • The sand bed is then prepared with molding sand such that its slopes to towards the tap hole. Firing of cupola • The cupola is fired by kindling wood at the bottom. • This should be done 2.5 to 3 hours before the molten metal is required. • On the top of the kindling wood a bed of coke is built. • The height of the coke bed is may be vary from 50cm to 125cm according to the size of cupola. Soaking of Iron • When the furnace is charged fully it is maintain for about 45 minutes. • The charge is slowly heated. • During the stage the air blast is shut off and iron is soaked. Opening of blast air • At the end of the soaking period the air blast is opened. • The taping hole is closed by a plug when the melting proceeds and molten metal is collect at the bottom. Pouring of molten metal • When the sufficient amount of metal has collected in the hearth the slag hole is opened and the slag is removed. • Then taping hole is opened and molten metal is flows out in the table. • The same procedure is repeated until the charge is melted and the operation is over. Closing the cupola • When the operation is over the air blast is shut off. • The bottom of furnace is opened by removing the prop. Advantages • It is simple and economical to operate . • Cupola can refine the metal charge, removing impurities out to the slag. • High melt rates . • Ease of operation . • Adequate temperature control . • Chemical composition control . • Efficiency of cupola varies from 30 to 50%. Disadvantages • Since molten iron and coke are in contact with each other, certain elements like si , Mn are lost and others like Sulphur are picked up. This changes the final chemical analysis of molten metal. • Close temperature control is difficult to maintain Direct Fuel-Fired Furnaces • Small open-hearth in which charge is heated by natural gas fuel burners located on side of furnace • Furnace roof assists heating action by reflecting flame down against the charge • At the bottom of hearth is a tap hole to release molten metal • Generally used for nonferrous metals such as copper-base alloys and aluminum Crucible Furnaces • Metal is melted without direct contact with burning fuel mixture • Sometimes called indirect fuel-fired furnaces • Container (crucible) is made of refractory material or high-temperature steel alloy • Used for nonferrous metals such as bronze, brass, and alloys of zinc and aluminum • Three types used in foundries: (a) lift-out type, (b) stationary, (c) tilting Crucible Furnaces • Metal is melted without direct contact with burning fuel mixture • Sometimes called indirect fuel-fired furnaces • Container (crucible) is made of refractory material or high-temperature steel alloy • Used for nonferrous metals such as bronze, brass, and alloys of zinc and aluminum • Three types used in foundries: (a) lift-out type, (b) stationary, (c) tilting Crucible Furnaces • Three types of crucible furnaces: (a) lift-out crucible, (b) stationary pot, from which molten metal must be ladled, and (c) tilting-pot furnace. Electric Arc Furnaces • Charge is melted by heat generated from an electric arc • High power consumption, but electric-arc furnaces can be designed for high melting capacity • Used primarily for melting steel Electric Arc Furnaces • Electric Arc Furnace is a furnace that heats the charged material by mean of an electric arc. • Arc Furnace range in size from small units of approximately one ton capacity up to 400 tons; industrial arc furnace can be heat up to 1800°C. • Arc Furnace is different from induction furnace because charge material is directly exposed to an electric arc furnace, and the current in the furnace terminals passes through the charged material. Electric Arc Furnaces • Construction: • The furnace consists of a spherical hearth (bottom), cylindrical shell and a swinging water-cooled domeshaped roof. • The roof has three holes for consumable graphite electrodes held by a clamping mechanism. • This mechanism provides independent lifting and lowering of each electrode Electric Arc Furnaces • Construction: Furnace is split into three sections • The “shell”, which consist of the sidewalls and lower steel bowl. • The “Hearth”, Which consist of the refractory that lines the lower bowl. • The “roof”, which may be refractory lined or water cooled, and can be shaped as a section of sphere, or as a conical section. Electric Arc Furnaces Operation: The electric arc furnace operates as a batch melting process. • • • • Furnace Charging Melting Tapping Furnace turn-around Electric Arc Furnaces Furnace Charging: • The first step is “charging”. The roof and electrode are raised and are swung to the side of the furnace to allow the material to be charged. • When the charging is complete the roof and electrodes swing back into place over the furnace. The roof is lowered and then the electrodes are lowered to strike an arc on the charged material. • The heat produce by electrode is primarily dependent on volume and density of charge. Electric Arc Furnaces Melting: • The melting period is a heart of Electric arc furnace. The EAF has evolved into a highly efficient melting apparatus and modern design are focused on maximizing the supplied energy to the furnace interior. This energy can be electrical or chemical. • Electrical energy is supplied via graphite electrodes and is usually the largest contributor in melting operations. Initially, an intermediate voltage tap is selected until the electrodes bore into the scrap. Usually light scrap is placed on top of the charge to accelerate bore-in. approximately 15% of scrap is melted during the initial bore-in period. Electric Arc Furnaces Melting: • After a few minutes, the electrodes will have penetrated the material sufficiently so that a long arc tap can be used without fear of radiation damage to the roof. • The long arc maximizing the transfer of power to the material and a liquid pool of a metal will form in the furnace hearth. At the start of melting the arc is unstable. As the atmosphere of furnace is heated up the arc stabilizes and once the molten pool is formed, the arc become stable and the average power input increases. • Chemical energy is supplied via several sources including oxyfuel burners and oxygen lances. Oxy-fuel burners burn natural gas using oxygen or a blend of oxygen and air. Electric Arc Furnaces Melting: • Heat is transferred to charge material by flame radiation and convection by the hot products of combustion. Heat is transferred within the charged material by conduction. • Large pieces of scrap take longer time to melt into the bath than smaller pieces. In some operations oxygen is injected via a consumable pipe lance to “cut” the charged material and burns iron to produce intense heat. • This oxygen will react with several components in the bath including, aluminum , silicon , manganese , phosphorous , carbon , and iron all these reactions are exothermic. Electric Arc Furnaces Melting: • Heat is transferred to charge material by flame radiation and convection by the hot products of combustion. Heat is transferred within the charged material by conduction. • Large pieces of scrap take longer time to melt into the bath than smaller pieces. In some operations oxygen is injected via a consumable pipe lance to “cut” the charged material and burns iron to produce intense heat. • This oxygen will react with several components in the bath including, aluminum , silicon , manganese , phosphorous , carbon , and iron all these reactions are exothermic. Electric Arc Furnaces Tapping: • Once the charged material has been melted, the tap hole of furnace is opened, the furnace is tilted, and molten metal is pours into a ladle. Furnace turned around: • It is the period after completion of tapping until the furnace is recharged for the next heat. • During this period the electrodes and roof are raised and furnace lining is inspected for refractory damage. Electric Arc Furnaces Advantages: • It can be used for melting. • EAF is used for production of steel making from pig iron • Electric arc furnace provides flexibility, EAFs can be rapidly started and stopped. Disadvantages: • high electricity consumption. Induction Furnaces • Uses alternating current passing through a coil to develop magnetic field in metal • Induced current causes rapid heating and melting • Electromagnetic force field also causes mixing action in liquid metal • Since metal does not contact heating elements, environment can be closely controlled to produce molten metals of high quality and purity • Melting steel, cast iron, and aluminum alloys are common applications in foundry work Induction Furnaces Definition • Electric induction furnace is type of melting furnace that uses electric current to melt the metal. • Once molten, the high frequency magnetic field can also be used to stir the hot metal, which is useful in ensuring that alloying additions are fully mixed into the melt. • Induction furnaces are used in most modern foundries as a cleaner method of melting metals than cupola. • Sizes range from kilograms to hundred tones capacity. Induction Furnaces • Metals which are melted in an induction furnace include iron and steel, copper, aluminum, and precious metals because it is clean and noncontact process which can be used in vacuum or inert atmosphere. • Vacuum furnaces make use of induction heating for production of specialty steels and other alloys that would oxidize if heated in the presence of air. • The basic principle of induction furnace is the induction heating. Induction Furnaces Principle of induction heating is mainly based on two well known physical phenomenas. 1. Electro-magnetic Induction 2. The Joule Effect • Induction heating relies on the unique characteristics of radio frequency energy that portion of the electro magnetic spectrum below infrared and microwave energy. E α h γ • The Joule heating or Ohmic Heating produced is proportional to the square of the current multiplied by the electrical resistance of the wire. Q α I²Rt Induction Furnaces • Advantages • Higher Yield The absence of combustion sources reduces oxidation losses and increase the yield. • Faster Startup Full power from power supply is available instantaneously; thus shorter time to reach working temperature. • Energy Conservation Overall energy efficiency in induction melting ranges from 55 to 75 percent. • Flexibility No molten metal is necessary to start • Natural Stirring Medium frequency units can give a strong stirring action resulting in a homogeneous melt. • Cleaner Melting No bi-product are produced due to cleaner melting environment. • Compact Installation High melting rates can be obtained from small furnaces. Induction Furnaces • Disadvantages • The one major drawback of induction furnace usage in foundry is the lack of refining capacity; charge materials must be clean of oxidation products and of known composition, and some alloying elements may be lost due to oxidation. • Removal of S & P is limited, so selection of charges with less impurity is required. Melting Furnaces MDPN Thanks Any Questions?????
