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IRON MAKING Syllabus: Definition & classification of metallurgy, Extractive metallurgy Classification & composition of pig iron, cast iron Manufacturing of pig iron, Principle, Construction & operation of blast furnace.

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Page 1: Iron making

IRON MAKING

Syllabus: Definition & classification of metallurgy, Extractive metallurgy Classification & composition of pig iron, cast ironManufacturing of pig iron, Principle, Construction & operation of blast furnace.

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Definition

• The science that deals with procedures used in extracting metals from their ores, purifying, alloying and fabrication of metals, and creating useful objects from metals.

• The field of metallurgy may be divided into process metallurgy (production metallurgy, extractive metallurgy) and physical metallurgy. In this system metal processing is considered to be a part of process metallurgy and the mechanical behavior of metals a part of physical metallurgy.

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METALLURGY

PRODUCTION METALLURGY

EXTRACTIVEMETALLURGY

ELCTRO METALLURGY

PHYSICALMETALLURGY

HYDRO METALLURGY

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• Process metallurgy, The science and technology used in the production

of metals, employs some of the same unit operations and unit processes as chemical engineering.

These operations and processes are carried out with ores, concentrates, scrap metals, fuels, fluxes, slags, solvents, and electrolytes.

Different metals require different combinations of operations and processes, but typically the production of a metal involves two major steps.

The first is the production of an impure metal from ore minerals, commonly oxides or sulfides, and the second is the refining of the reduced impure metal, for example, by selective oxidation of impurities or by electrolysis.

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• Extractive metallurgy is the study of the processes used in the

separation and concentration (benefaction) of raw materials.

The field is an applied science, covering all aspects of the physical and chemical processes used to produce mineral-containing and metallic materials, sometimes for direct use as a finished product, but more often in a form that requires further physical processing which is generally the subject of physical metallurgy, ceramics, and other disciplines within the broad field of materials science.

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• Physical metallurgy investigates the effects of composition and treatment on the structure of metals and the relations of the structure to the properties of metals.

• Physical metallurgy is also concerned with the engineering applications of scientific principles to the fabrication, mechanical treatment, heat treatment, and service behavior of metals

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• Hydrometallurgy is concerned with processes involving aqueous solutions to extract metals from ores.

• The most common hydrometallurgical process is leaching, which involves dissolution of the valuable metals into the aqueous solution.

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Electrometallurgy• Electrometallurgy involves metallurgical processes that take place in some

form of electrolytic cell. • The most common types of electrometallurgical processes are

electrowinning and electro-refining. • Electrowinning is an electrolysis process used to recover metals in

aqueous solution, usually as the result of an ore having undergone one or more hydrometallurgical processes.

• The metal of interest is plated onto the cathode, while the anode is an inert electrical conductor. Electro-refining is used to dissolve an impure metallic anode (typically from a smelting process) and produce a high purity cathode.

• Fused salt electrolysis is another electrometallurgical process whereby the valuable metal has been dissolved into a molten salt which acts as the electrolyte, and the valuable metal collects on the cathode of the cell.

• The fused salt electrolysis process is conducted at temperatures sufficient to keep both the electrolyte and the metal being produced in the molten state. The scope of electrometallurgy has significant overlap with the areas of hydrometallurgy and (in the case of fused salt electrolysis) pyrometallurgy. Additionally, electrochemical phenomena play a considerable role in many mineral processing and hydrometallurgical processes.

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Pig iron Pig iron is the intermediate product of smelting iron ore with a high-carbon fuel such as coke, usually with limestone as a flux. Charcoal and anthracite have also been used as fuel. Pig iron has a very high carbon content, typically 3.5–4.5%, which makes it very brittle and not useful directly as a material except for limited applications.

Pig iron, an intermediate form of iron produced from iron ore and is subsequently worked into steel or wrought iron

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Uses: Traditionally pig iron would be worked into wrought iron in finery forges, and later puddling furnaces, more recently into steel.

In these processes, pig iron is melted and a strong current of air is directed over it while it is being stirred or agitated. This causes the dissolved impurities (such as silicon) to be thoroughly oxidized. An intermediate product of puddling is known as refined pig iron, finers metal, or refined iron.[5]

Pig iron can also be used to produce gray iron. This is achieved by remelting pig iron, often along with substantial quantities of steel and scrap iron, removing undesirable contaminants, adding alloys, and adjusting the carbon content. Some pig iron grades are suitable for producing ductile iron. These are high purity pig irons and depending on the grade of ductile iron being produced these pig irons may be low in the elements silicon, manganese, sulfur and phosphorus. These types of pig irons are useful to dilute all elements in a ductile iron charge (except carbon) which may be harmful to the ductile iron process.

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Modern usesToday, pig iron is typically poured directly out of the bottom of the blast furnace through a trough into a ladle car for transfer to the steel mill in mostly liquid form, referred to as hot metal. The hot metal is then charged into a steelmaking vessel to produce steel, typically with an electric arc furnace or basic oxygen furnace, by burning off the excess carbon in a controlled fashion and adjusting the alloy composition.

Earlier processes for this included the finery forge, the puddling furnace, the Bessemer process, and open hearth furnace.

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Classification of cast ironsWhite cast irons - hard and brittle wear resistant cast irons consisting of pearlite and cementite.Grey cast irons - cast irons at slow cooling and consisting of ferrite and dispersed graphite flakes.Malleable cast irons - cast irons, produced by heat treatment of white cast irons and consisting of ferrite and particles of free graphite.Nodular (ductile) cast irons - grey cast iron in which Graphite particles are modified by magnesium added to the melt before casting. Nodular cast iron consists of spheroid nodular graphite particles in ferrite or pearlite matrix.

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Malleable cast iron cast irons produced by heat treatment of white cast irons and consisting of ferrite and particles of free graphite. Malleable cast irons have good ductility and machinability. Ferritic malleable cast irons are more ductile and less strong and hard, than pearlitic malleable cast irons. Applications of malleable cast irons: parts of power train of vehicles, bearing caps, steering gear housings, agricultural equipment, railroad equipment.Nodular (ductile) cast irons – grey cast iron, in which graphite particles are modified by magnesium added to the melt before casting. Nodular cast iron consists of spheroid nodular graphite particles in ferrite or pearlite matrix. Ductile cast irons possess high ductility, good fatigue strength, wear resistance, shock resistance and high modulus of elasticity. Applications of nodular (ductile) cast irons: automotive engine crankshafts, heavy duty gears, military and railroad vehicles.

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White cast irons – hard and brittle highly wear resistant cast irons consisting of pearlite and cementite. White cast irons are produced by chilling some surfaces of the cast mold. Chilling prevents formation of Graphite during solidification of the cast iron. Applications of white cast irons: brake shoes, shot blasting nozzles, mill liners, crushers, pump impellers and other abrasion resistant parts.

Grey cast irons – cast irons, produced at slow cooling and consisting of ferrite and dispersed graphite flakes. Grey cast irons possess high compressing strength, fatigue resistance and wear resistance. Presence of graphite in grey cast irons impart them very good vibration dumping capacity. Applications of grey cast irons: gears, flywheels, water pipes, engine cylinders, brake discs, gears.

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Extractive metallurgy of iron

The following raw materials are involved in manufacturing of iron:

Iron ores (magnetite, hematite) – iron oxides with earth impurities;

Coke, which is both reducing agent and fuel, providing heat for melting the metal and slag.

Coke is produced from coking coals by heating them away from air.

Limestone – calcium silicate fluxes, forming a fluid slag for removal gangue from the ore.

Iron is produced in a blast furnace, schematically shown in the picture.

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It the shaft-type furnace consisting of a steel shell lined with refractory bricks. The top of the furnace is equipped with the bell-like or other system, providing correct charging and distribution of the raw materials (ore, coke, limestone). Air heated to 2200°F (1200°C) is blown through the tuyeres at the bottom. Oxigen containing in air reacts with the coke, producing carbon monoxide: 2C + O2= 2CO Hot gases pass up through the descending materials, causing reduction of the iron oxides to iron according to the follwing reactions: 3Fe2O3 + CO = 2Fe3O4 + CO2 Fe3O4 + CO = 3FeO + CO2 FeO + CO = Fe + CO2

Blast furnace

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Iron in form of a spongy mass moves down and its temperature reaches the melting point at the bottom regions of the furnace where it melts and accumulates. The gangue, ash and other fractions of ore and coke are mixed by fluxes, formig slag which is capable to absorb sulphure and other impurities. The furnace is periodically tapped andthe melt (pig iron) is poured into ladles, which are transferred to steel making furnaces. Pig iron usually contains 3-4% of carbon, 2-4% of silicon, 1-2% of manganese and 1-1.2% of phosphorous.

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Steel making (introduction)• Syllabus• Introduction to various types of steel.• Methods of steel making: Crucible process, Bessemer

converter: principle, Construction & operational details,

• Open hearth furnace: principle, Construction & operational details, Oxygen steel making: Basic oxygen or

• L.D. process, & kaldo process. Electric furnace for steel making: arc & induction furnace, Mer its & demerits of the various processes.

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Steel is an alloy of iron, containing up to 2% of carbon (usually up to 1%). Steel contains lower (compared to pig iron) quantities of impurities like phosphorous, sulfur and silicon. Steel is produced from pig iron by processes, involving reducing the amounts of carbon, silicon and phosphorous. There are various types of steels used in various purposes

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Carbon steels are iron-carbon alloys containing up to 2.06% of carbon, up to 1.65% of manganese, up to 0.5% of silicon and sulfur and phosphorus as impurities. Carbon content in carbon steel determines its strength and ductility. The higher carbon content, the higher steel strength and the lower its ductility. According to the steels classification there are following groups of carbon steels:Low carbon steels (C < 0.25%)Medium carbon steels (C =0.25% to 0.55%)High carbon steels (C > 0.55%)Tool carbon steels (C>0.8%)

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Low carbon steels (C < 0.25%)Properties: good formability and weldability, low strength, low cost. Applications: deep drawing parts, chain, pipe, wire, nails, some machine parts.

Medium carbon steels (C =0.25% to 0.55%)Properties: good toughness and ductility, relatively good strength, may be hardened by quenching Applications: rolls, axles, screws, cylinders, crankshafts, heat treated machine parts.High carbon steels (C > 0.55%)Properties: high strength, hardness and wear resistance, moderate ductility. Applications: rolling mills, rope wire, screw drivers, hammers, wrenches, band saws.

Tool carbon steels (C>0.8%) – subgroup of high carbon steelsProperties: very high strength, hardness and wear resistance, poor weldability low ductility. Applications: punches, shear blades, springs, milling cutters, knives, razors.

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Alloy steels are iron-carbon alloys, to which alloying elements are added with a purpose to improve the steels properties as compared to the Carbon steels. Due to effect of alloying elements, properties of alloy steels exceed those of plane carbon steels. AISI/SAE classification divide alloy steels onto groups according to the major alloying elements: Low alloy steels (alloying elements 8%);⇐High alloy steels (alloying elements > 8%).

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Tool and die steels are high carbon steels (either carbon or alloy) possessing high hardness, strength and wear resistance. Tool steels are heat treatable. In order to increase hardness and wear resistance of tool steels, alloying elements forming hard and stable carbides (chromium, tungsten, vanadium, manganese, molybdenum) are added to the composition. Designation system of one-letter in combination with a number is accepted for tool steels. The letter means: W - Water hardened plain carbon tool steelsApplications: chisels, forging dies, hummers, drills, cutters, shear blades, cutters, drills, razors. Properties: low cost, very hard, brittle, relatively low hardenabilityhardenability, suitable for small parts working at not elevated temperatures. O, A, D - Cold work tool steelsApplications: drawing and forging dies, shear blades, highly effective cutters. Properties: strong, hard and tough crack resistant. O -Oil hardening cold work alloy steels; A -Air hardening cold work alloy steels; D -Diffused hardening cold work alloy steels;

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S – Shock resistant low carbon tool steelsApplications: tools experiencing hot or cold impact. Properties: combine high toughness with good wear resistance. T,M – High speed tool steels (T-tungsten, M-molybdenum)Applications: cutting tools. Properties: high wear heat and shock resistance. H – Hot work tool steelsApplications: parts working at elevated temperatures, like extrusion, casting and forging dies. Properties: strong and hard at elevated temperatures. P – Plastic mold tool steelsApplications: molds for injection molding of plastics. Properties: good machinability.

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Stainless steels are steels possessing high corrosion resistance due to the presence of substantial amount of chromium. Chromium forms a thin film of chromium oxide on the steel surface. This film protects the steel from further oxidation, making it stainless. Most of the stainless steels contain 12% - 18% of chromium. Other alloying elements of the stainless steels are nickel, molybdenum, Nitrogen, titanium and manganese. According to the AISI classification Stainless steels are divided onto groups:Austenitic stainless steels Ferritic stainless steels Martensitic stainless steels Austenitic-ferritic (Duplex) stainless steelsPrecipitation hardening stainless steels

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Austenitic stainless steelsAustenitic stainless steels (200 and 300 series) contain chromium and nickel (7% or more) as major alloying elements. The crystallographic structure of the steels is austenitic with FCC crystal lattice. The steels from this group have the highest corrosion resistance, weldability and ductility. Austenitic stainless steels retain their properties at elevated temperatures. At the temperatures 900-1400ºF (482-760ºC) chromium carbides form along the austenite grains. This causes depletion of chromium from the grains resulting in decreasing the corrosion protective passive film. This effect is called sensitization. It is particularly important in welding of austenitic stainless steels. Sensitization is depressed in low carbon steels (0.03%) designated with suffix L (304L, 316L). Formation of chromium carbides is also avoided in stabilized austenitic stainless steels containing carbide forming elements like titanium, niobium, tantalum, zirconium. Stabilization heat treatment of such steels results in preferred formation of carbides of the stabilizing elements instead of chromium carbides. These steel are not heat treatable and may be hardened only by cold work. Applications of austenitic stainless steels: chemical equipment, food equipment, kitchen sinks, medical devices, heat exchangers, parts of furnaces and ovens.

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Ferritic stainless steelsFerritic stainless steels (400 series) contain chromium only as alloying element. The crystallographic structure of the steels is ferritic (BCC crystal lattice) at all temperatures. The steels from this group are low cost and have the best machinability. The steels are ferromagnetic. Ductility and formability of ferritic steels are low. Corrosion resistance and weldability are moderate. Resistance to the stress corrosion cracking is high. Ferritic steels are not heat treatable because of low carbon concentration and they are commonly used in annealed state. Applications of ferritic steels: decorative and architectural parts, automotive trims and exhausting systems, computer floppy disc hubs, hot water tanks.

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Martensitic stainless steelsMartensitic stainless steels (400 and 500 series) contain chromium as alloying element and increased (as compared to ferritic grade) amount of carbon. Due to increased concentration of carbon the steels from this group are heat treatable. The steels have austenitic structure (FCC) at high temperature, which transforms to martensitic structure (BCC) as a result of quenching . Martensitic steels have poor weldability and ductility. Corrosion resistance of these steels is moderate (slightly better than in ferritic steels). Applications of martensitic stainless steels: turbine blades, knife blades, surgical instruments, shafts, pins, springs.

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Austenitic-ferritic (Duplex) stainless steelsAustenitic-ferritic (Duplex) stainless steels contain increased amount of chromium (18% -28%) and decreased (as compared to austenitic steels) amount of nickel (4.5% - 8%) as major alloying elements. As additional alloying element molybdenum is used in some of Duplex steels. Since the quantity of nickel is insufficient for formation of fully austenitic structure, the structure of Duplex steels is mixed: austenitic-ferritic. The properties of Duplex steels are somewhere between the properties of austenitic and ferritic steels. Duplex steels have high resistance to the stress corrosion cracking and to chloride ions attack. These steels are weldable and formable and possess high strength. Applications of austenitic-ferritic stainless steels: desalination equipment, marine equipment, petrochemical plants, heat exchangers.

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Precipitation hardening stainless steelsPrecipitation hardening stainless steels contain chromium, nickel as major alloying elements. Precipitation hardening steels are supplied in solution treated condition. These steels may be either austenitic or martensitic and they are hardened by heat treatment (aging). The heat treatment is conducted after machining, however low temperature of the treatment does not cause distortions. Precipitation hardening steels have very high strength, good weldability and fair corrosion resistance. They are magnetic. Applications of precipitation hardening stainless steels: pump shafts and valves, turbine blades, paper industry equipment, aerospace equipment

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Creep resistant steels are steels designed to withstand a constant load at high temperatures. The most important application of creep resistant steels is components of steam power plants operating at elevated temperatures (boilers, turbines, steam lines). Supercritical and Ultra Supercritical power pla

nts9-12 Cr martensitic creep resistant steelsAustenitic creep resistant steels

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The Basic Oxygen Process is the most powerful and effective method of steel manufacturing. The scheme of the Basic Oxygen Furnace (BOF) (basic oxygen furnace, basic oxygen converter) is presented in the picture. Typical basic oxygen converter has a vertical steel shell lined with refractory lining. The furnace is capable to rotate about its horizontal axis on trunnions. This rotation is necessary for charging raw materials and fluxes, sampling the melt and pouring the steel and the slag out of the furnace. The Basic Oxygen is equipped with the water cooled oxygen lance for blowing oxygen into the melt. The basic oxygen converter uses no additional fuel. The pig iron impurities (carbon, silicon, manganese and phosphorous) serve as fuel.

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The steel making process in the oxygen converter consists of: Charging steel scrap.Pouring liquid pig iron into the furnace.Charging fluxes.Oxygen blowing.Sampling and temperature measurementTapping the steel to a ladle.De-slagging.The iron impurities oxidize, evolving heat, necessary for the process. The forming oxides and sulfur are absorbed by the slag. The oxygen converter has a capacity up to 400 t and production cycle of about 40 min.

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Electric-arc furnaceThe electric-arc furnace employs three vertical graphite electrodes for producing arcs, striking on to the charge and heating it to the required temperature. As the electric-arc furnace utilizes the external origin of energy (electric current), it is capable to melt up to 100% of steel scrap.The steel making process in the electric-arc furnace consists of: Charging scrap metal, pig iron, limestoneLowering the electrodes and starting the power (melting)Oxidizing stage At this stage the heat, produced by the arcs, causes oxidizing phosphorous, silicon and manganese. The oxides are absorbed into the slag. By the end of the stage the slag is removed.

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De-slaggingReducing stageNew fluxes (lime and anthracite) are added at this stage for formation of basic reducing slag. The function of this slag is refining of the steel from sulfur and absorption of oxides, formed as a result of deoxidation. TappingLining maintenanceThe advantages of the electric-arc furnace are as follows: Unlimited scrap quantity may be melt;Easy temperature control;Deep desulfurization;Precise alloying.

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Ladle refining (ladle metallurgy, secondary refining)Ladle refining is post steel making technological operations, performed in the ladle prior to casting with the purposes of desulfurization, degassing, temperature and chemical homogenization, deoxidation and others. Ladle refining may be carried out at atmospheric pressure, at vacuum, may involve heating, gas purging and stirring. Sulfur refining (desulfurization) in the ladle metallurgy is performed by addition of fluxes (CaO, CaF2 and others) into the ladle and stirring the steel together with the slag, absorbing sulfur. In the production of high quality steel the operation of vacuum treatment in ladle is widely used. Vacuum causes proceeding chemical reaction within the molten steel: [C] + [O] = {CO} This reaction results in reduction of the quantity of oxide inclusions. The bubbles of carbon oxide remove Hydrogen, diffusing into the CO phase. An example of ladle refining method is Recirculation Degassing (RH) vacuum degasser, which consists of a vacuum vessel with two tubes (snorkels), immersed in the steel. In one of the tubes argon is injected. Argon bubbles, moving upwards, cause steel circulation through the vacuum vessel. Additions of fluxes in the vacuum vessel permits conducting desulfurization treatment by this method.

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Deoxidation of steelThe main sources of Oxygen in steel are as follows:Oxygen blowing (example: Basic Oxygen Furnace (BOF));Oxidizing slags used in steel making processes(example: Electric-arc furnace;Atmospheric oxygen dissolving in liquid steel during pouring operation;Oxidizing refractories (lining of furnaces and ladles);Rusted and wet scrap.Solubility of oxygen in molten steel is 0.23% at 3090°F (1700°C). However it decreases during cooling down and then drops sharply in Solidification reaching 0.003% in solid steel.Oxygen liberated from the solid solution oxidizes the steel components (C, Fe, alloying elements) forming gas pores (blowholes) and non-metallic inclusions entrapped within the ingot structure. Both blowholes and inclusions adversely affect the steel quality.In order to prevent oxidizing of steel components during solidification the oxygen content should be reduced.Deoxidation of steel is a steel making technological operation, in which concentration (activity) of oxygen dissolved in molten steel is reduced to a required level.

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There are three principal deoxidation methods:

Deoxidation by metallic deoxidizersDeoxidation by vacuumDiffusion deoxidation.

Deoxidation by metallic deoxidizersThis is the most popular deoxidation method. It uses elements forming strong and stable oxides. Manganese (Mn), silicone (Si), aluminum (Al), cerium (Ce), calcium (Ca) are commonly used as deoxidizers.Deoxidation by an element (D) may be presented by the reaction:

Deoxidizer Reaction A B

Equilibrium constant at 1873 °K (2912°F, 1600°C)

Manganese [Mn] + [O] = (MnO) 12440 5.33 1.318

Silicone [Si] + 2[O] = (SiO2)

30000 11.5 4.518

Aluminum 2[Al] + 3[O] = (Al2O3)

62780 20.5 13.018

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According to the degree of deoxidation Carbon steels may be subdivided into three groups:Killed steels - completely deoxidized steels, solidification of which does not cause formation of carbon monoxide (CO). Ingots and castings of killed steel have homogeneous structure and no gas porosity (blowholes).Semi-killed steels - incompletely deoxidized steels containing some amount of excess oxygen, which forms carbon monoxide during last stages of solidification.Rimmed steels - partially deoxidized or non-deoxidized low carbon steels evolving sufficient amount of carbon monoxide during solidification. Ingots of rimmed steels are characterized by good surface quality and considerable quantity of blowholes.

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Effect of alloying elements on steel propertiesAlloying is changing chemical composition of steel by adding elements with purpose to improve its properties as compared to the plane carbon steel. The properties, which may be improvedStabilizing austenite – increasing the temperature range, in which austenite exists.The elements, having the same crystal structure as that of austenite (cubic face centered – FCC), raise the A4 point (the temperature of formation of austenite from liquid phase) and decrease the A3 temperature. These elements are nickel (Ni), manganese (Mn), cobalt (Co) and copper (Cu).Examples of austenitic steels: austenitic stainless steels, Hadfield steel (1%C, 13%Mn, 1.2%Cr). Stabilizing ferrite – decreasing the temperature range, in which austenite exists.The elements, having the same crystal structure as that of ferrite (cubic body centered – BCC), lower the A4 point and increase the A3 temperature. These elements lower the solubility of carbon in austenite, causing increase of amount of carbides in the steel. The following elements have ferrite stabilizing effect: chromium (Cr), tungsten (W), Molybdenum (Mo), vanadium (V), aluminum (Al) and silicon (Si).Examples of ferritic steels:transformer sheets steel (3%Si), F-Cr alloys.

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Carbide forming – elements forming hard carbides in steels.The elements like chromium (Cr), tungsten (W), molybdenum (Mo), vanadium (V), titanium (Ti), niobium (Nb), tantalum (Ta), zirconium (Zr) form hard (often complex) carbides, increasing steel hardness and strength.Examples of steels containing relatively high concentration of carbides: hot work tool steels, high speed steels.Carbide forming elements also form nitrides reacting with Nitrogen in steels. Graphitizing – decreasing stability of carbides, promoting their breaking and formation of free Graphite.The following elements have graphitizing effect: silicon (Si), nickel (Ni), cobalt (Co), aluminum (Al). Decrease of the eutectoid concentration.The following elements lower eutectoid concentration of carbon: titanium (Ti), molybdenum (Mo), tungsten (W), silicon (Si), chromium (Cr), nickel (Ni). Increase of corrosion resistance.Aluminum (Al), silicon (Si), and chromium (Cr) form thin an strong oxide film on the steel surface, protecting it from chemical attacks.

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Characteristics of alloying elementsManganese (Mn) – improves hardenability, ductility and wear resistance. Mn eliminates formation of harmful iron sulfides, increasing strength at high temperatures. Nickel (Ni) – increases strength, impact strength and toughness, impart corrosion resistance in combination with other elements. Chromium (Cr) – improves hardenability, strength and wear resistance, sharply increases corrosion resistance at high concentrations (> 12%). Tungsten (W) – increases hardness particularly at elevated temperatures due to stable carbides, refines grain size. Vanadium (V) – increases strength, hardness, creep resistance and impact resistance due to formation of hard vanadium carbides, limits grain size. Molybdenum (Mo) – increases hardenability and strength particularly at high temperatures and under dynamic conditions. Silicon (Si) – improves strength, elasticity, acid resistance and promotes large grain sizes, which cause increasing magnetic permeability. Titanium (Ti) – improves strength and corrosion resistance, limits austenite grain size. Cobalt (Co) – improves strength at high temperatures and magnetic permeability. Zirconium (Zr) – increases strength and limits grain sizes. Boron (B) – highly effective hardenability agent, improves deformability and machinability. Copper (Cu) – improves corrosion resistance. Aluminum (Al) – deoxidizer, limits austenite grains growth.

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Four types of furnaces are used to make cast steel: Open-hearth (acid and basic) Electric-arc (acid and basic) Converter (acid side-blow) Electric induction (acid and basic) Of these the first two contribute most of the tonnage.

The distinction between acid and basic practice is in regard to the “type of refractories” used in the construction and maintenance of the furnace.

Furnaces operated by the acid practice are lined with silica base (Si02 ) refractories, and the slags employed in the refining process have a relatively high silica content. Basic furnaces, on the other hand, use a basic refractory such as magnesite or dolomite base* and have a high lime (CaO) content in the slag.

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The choice of furnace and melting practice depends on many variables, including: The plant capacity or tonnage required The size of the castings The intricacy of the castings The type of steel to be produced, i.e., whether plain or alloyed, high or low carbon, etc. The raw materials available and the prices thereof Fuel or power costsThe, amount of capital to be invested Previous experience

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Generally, the open-hearth furnace is used for large tonnages and large castings, and the electric furnace for smaller heats or where steels of widely differing analyses must be produced.

Special steels or high alloy steels are often produced in an induction furnace.

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BASIC OPEN-HEARTH MELTING The air used for combustion purposes must be preheated to obtain the high temperatures required in an open hearth

A companion checkerwork, in the meantime, is being heated by the outgoing gases. The cycle is reversed about every 15 min to prevent excessive cooling of the checkerwork bricks.

The preheated air is mixed with the incoming oil or fuel gas at the burner pors. This creates a flame over the hearth which heats the charge and surrounding refractories.

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Fuels and Charge Materials Basic open-hearth furnaces are fired with either gas or oil. The oil may have to be preheated before it is burned. The charge materials consist of pig iron, purchased scrap, foundryscrap returns, lime, and ore. The pig iron has approximately the following composition: 3.50 to 4.40% carbon . 1.50 to 2.00% manganese 1.25% maximum silicon 0.06% maximum sulfur 0.35% maximum phosphorus The manganese is kept high to aid in desulfurization and in controlling the slag. Silicon is limited because it is an acid component in slag and hence tends to require excess lime in the charge and also may increase

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Charging and Melting Several methods are used in placing the materials in the furnace, but the usual practice is to cover the bottom with scrap, followed by the lime spread as evenly as possible. This is followed by the pig iron. The lime addition will vary from 4 to 7 per cent of the weight of the metal charge. The carbon content of the charge will vary from 1.0 to 1.75 per cent.

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Oxidation and Refining The principle underlying the melting and refining of steel in open-hearth and electric furnaces is to create an oxidizing condition that will oxidize such elements as carbon, manganese, silicon, and phosphorus. These oxides, with the exception of CO gas, dissolve in the slag.

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The amount of the iron oxide addition is determined by the meltdown carbon content of the bath and the desired carbon content at tapping. The addition of iron oxide causes an evolution of CO from t.he reaction FeO + C 7 CO + Fe The bubbles of CO originate in the melt at the hearth bottom and create a "carbon boil" as they percolate up to the surface. The carbon boil is an important part of refining since it aids in heat transfer by stirring the melt, cleanses the metal of retained oxides by bringing them to the slag, hastens reactions at the gas-metal interface, and aids in removing hydrogen and nitrogen. Hydrogen and nitrogen diffuse into the CO bubbles and are thereby flushed out of the liquid steel

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The reaction actually is largely between oxygen dissolved in the steel and the carbon. At the temperatures used in steel refining (around 2900 F), the steel is capable of dissolving considerable oxygen as FeO.t This FeO is picked up from the slag or from a reaction between the added iron ore and the steel, such as F~08 +Fe ~3FeO schematic representation of the oxidation cycle in the open hearth.

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it possible to standardize operations so that good-quality steel is produced in each heat. Slag analyses just prior to stopping the boil (blocking) will usually fall in the following composition range: CaO,4O to 50% Si02 , 13 to 18% MnO, 7to 15% FeU, 12 to 16% Deoxidation and Tapping When the heat is considered ready for tapping, deoxidizing agents are added. This step of adding a deoxidizing agent is referred to as "blocking the heat" because it prevents any further reaction between oxygen and carbon, the oxygen reacting with these additives to form non gaseous reaction products. The deoxidizers include spiegel, ferrosilicon, ferromanganese, and silicomanganese.

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Electric furnaces BASIC ELECTRIC MELTING Furnace Construction Basic electric furnaces are much smaller than open-hearth furnaces, ranging from 1/2 to 1/7 tons capacity. A cross-sectional sketch of an electric furnace is shown in Fig.

The arc furnace is heated from the arc struck between the charge, or bath, and three large electrodes of carbon or graphite operating from a three-phase circuit. The height of the electrodes above the bath is controlled electrically. Voltages are fairly low, and current flow is high, necessitating large bus bars and heavy lead-in cables from the transformers. Charging is usually done by removing the furnace top. The roof of the furnace is silica brick, whereas the side walls are lined with magnesite brick or chrome-magnesit€ brick. Bottoms are rammed into place.

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Cross-sectional view of an electric arc furnace showing an aucid lining (left) and a basic lining (right) .

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Melting and Refining Although the general principles controlling the refining operation in the basic open-hearth furnace also apply to the basic electric, certain modifications in operation are possible which give the basic electric furnace greater flexibility. Unlike the open-hearth charge, the charge for the electric furnace may not necessarily include pig iron, because not so much carbon is lost during melting as in the open-hearth.

Once this slag is removed, a refining slag composed of lime and fluorspar is added. The purpose of this so-called refining slag is to remove sulfur, which can be accomplil>hed only by establishing basic and reducing conditions. The ·necessary reducing agent in this case is carbon added to the top of the slag. The reaction is considered to be C (in steel) + CaO + FeS (in steel) ~CaS (in slag) +CO +Fe Refining proceeds for about 1 to 1Yz hr, the completion of which is indicated by the appearance of a slag sample. The metal is usually tapped into bottom-pour ladles. Since the composition was adjusted before and during the refining period, no further additions are required at tapping.