Iron making


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Iron making

  1. 1. IRON MAKINGSyllabus: Definition & classification ofmetallurgy,Extractive metallurgyClassification & composition of pig iron, castironManufacturing of pig iron,Principle, Construction & operation of blastfurnace.
  2. 2. 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.
  4. 4. • 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.
  5. 5. • 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.
  6. 6. • 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
  7. 7. • 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.
  8. 8. 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.
  9. 9. Pig ironPig iron is the intermediate product of smeltingiron ore with a high-carbon fuel such as coke, usuallywith limestone as a flux. Charcoal and anthracite havealso 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 directlyas a material except for limited applications.Pig iron, an intermediate form of iron produced fromiron ore and is subsequently worked into steel orwrought iron
  10. 10. Uses:Traditionally pig iron would be worked into wrought iron infinery forges, and later puddling furnaces, more recently into steel.In these processes, pig iron is melted and a strong current of air isdirected over it while it is being stirred or agitated. This causes thedissolved impurities (such as silicon) to be thoroughly oxidized. Anintermediate product of puddling is known as refined pig iron, finersmetal, or refined iron.[5]Pig iron can also be used to produce gray iron. This is achieved byremelting pig iron, often along with substantial quantities of steel andscrap iron, removing undesirable contaminants, adding alloys, andadjusting the carbon content. Some pig iron grades are suitable forproducing ductile iron.These are high purity pig irons and depending on the grade of ductile iron beingproduced these pig irons may be low in the elements silicon, manganese, sulfur andphosphorus. These types of pig irons are useful to dilute all elements in a ductile ironcharge (except carbon) which may be harmful to the ductile iron process.
  11. 11. Modern usesToday, pig iron is typically poured directly out of thebottom of the blast furnace through a trough into a ladlecar for transfer to the steel mill in mostly liquid form,referred to as hot metal.The hot metal is then charged into a steelmaking vesselto produce steel, typically with an electric arc furnace orbasic oxygen furnace, by burning off the excess carbon in acontrolled fashion and adjusting the alloy composition.Earlier processes for this included the finery forge, thepuddling furnace, the Bessemer process, andopen hearth furnace.
  12. 12. Classification of cast ironsWhite cast irons - hard and brittle wear resistant castirons consisting of pearlite and cementite.Grey cast irons - cast irons at slow cooling andconsisting of ferrite and dispersed graphite flakes.Malleable cast irons - cast irons, produced by heattreatment of white cast irons and consisting of ferriteand particles of free graphite.Nodular (ductile) cast irons - grey cast iron in whichGraphite particles are modified by magnesium added tothe melt before casting. Nodular cast iron consists ofspheroid nodular graphite particles in ferrite or pearlitematrix.
  13. 13.  Malleable cast iron cast irons produced by heat treatment ofwhite cast irons and consisting of ferrite and particles of freegraphite.Malleable cast irons have good ductility and machinability. Ferriticmalleable cast irons are more ductile and less strong and hard, thanpearlitic malleable cast irons.Applications of malleable cast irons: parts of power train ofvehicles, bearing caps, steering gear housings, agriculturalequipment, railroad equipment.Nodular (ductile) cast irons – grey cast iron, in which graphiteparticles are modified by magnesium added to the melt beforecasting. Nodular cast iron consists of spheroid nodular graphiteparticles 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 enginecrankshafts, heavy duty gears, military and railroad vehicles.
  14. 14. White cast irons – hard and brittle highly wear resistant cast ironsconsisting 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 castiron.Applications of white cast irons: brake shoes, shot blasting nozzles, millliners, crushers, pump impellers and other abrasion resistant parts.Grey cast irons – cast irons, produced at slow cooling and consisting offerrite and dispersed graphite flakes.Grey cast irons possess high compressing strength, fatigue resistance andwear resistance. Presence of graphite in grey cast irons impart them verygood vibration dumping capacity.Applications of grey cast irons: gears, flywheels, water pipes, enginecylinders, brake discs, gears.
  15. 15. Extractive metallurgy of ironThe 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.
  16. 16. Blast furnaceIt the shaft-type furnace consisting of asteel shell lined with refractory bricks.The top of the furnace is equipped withthe bell-like or other system, providingcorrect charging and distribution of theraw materials (ore, coke, limestone).Air heated to 2200°F (1200°C) is blownthrough the tuyeres at the bottom.Oxigen containing in air reacts with thecoke, producing carbon monoxide:2C + O2= 2COHot gases pass up through thedescending materials, causing reductionof the iron oxides to iron according to thefollwing reactions:3Fe2O3 + CO = 2Fe3O4 + CO2Fe3O4 + CO = 3FeO + CO2FeO + CO = Fe + CO2
  17. 17. Iron in form of a spongy mass moves down and itstemperature reaches the melting point at thebottom regions of the furnace where it melts andaccumulates.The gangue, ash and other fractions of ore andcoke are mixed by fluxes, formig slag which iscapable to absorb sulphure and other impurities.The furnace is periodically tapped andthe melt(pig iron) is poured into ladles, which aretransferred to steel making furnaces.Pig iron usually contains 3-4% of carbon, 2-4% ofsilicon, 1-2% of manganese and 1-1.2% ofphosphorous.
  18. 18. 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.
  19. 19. 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 byprocesses, involving reducing the amounts ofcarbon, silicon and phosphorous.There are various types of steels used invarious purposes
  20. 20. Carbon steels are iron-carbon alloys containingup to 2.06% of carbon, up to 1.65% of manganese,up to 0.5% of silicon and sulfur and phosphorus asimpurities.Carbon content in carbon steel determines itsstrength and ductility.The higher carbon content, the higher steelstrength and the lower its ductility.According to the steels classification there arefollowing 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%)
  21. 21. 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 behardened by quenchingApplications: rolls, axles, screws, cylinders, crankshafts, heat treated machineparts.High carbon steels (C > 0.55%)Properties: high strength, hardness and wear resistance, moderate ductility.Applications: rolling mills, rope wire, screw drivers, hammers, wrenches, bandsaws.Tool carbon steels (C>0.8%) – subgroup of high carbon steelsProperties: very high strength, hardness and wear resistance, poor weldabilitylow ductility.Applications: punches, shear blades, springs, milling cutters, knives, razors.
  22. 22. Alloy steels are iron-carbon alloys, to whichalloying elements are added with a purpose toimprove the steels properties as compared tothe Carbon steels.Due to effect of alloying elements, propertiesof alloy steels exceed those of plane carbonsteels.AISI/SAE classification divide alloy steels ontogroups according to the major alloyingelements:Low alloy steels (alloying elements ⇐ 8%);High alloy steels (alloying elements > 8%).
  23. 23. Tool and die steels are high carbon steels (eithercarbon or alloy) possessing high hardness, strength and wearresistance.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 thecomposition.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 forsmall 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;
  24. 24. S – Shock resistant low carbon tool steelsApplications: tools experiencing hot or cold impact.Properties: combine high toughness with good wearresistance.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.
  25. 25. Stainless steels are steels possessing high corrosionresistance due to the presence of substantial amount of chromium.Chromium forms a thin film of chromium oxide on the steelsurface. This film protects the steel from further oxidation, making itstainless.Most of the stainless steels contain 12% - 18% of chromium. Otheralloying elements of the stainless steels are nickel, molybdenum,Nitrogen, titanium and manganese.According to the AISI classification Stainless steels are divided ontogroups:Austenitic stainless steelsFerritic stainless steelsMartensitic stainless steelsAustenitic-ferritic (Duplex) stainless steelsPrecipitation hardening stainless steels
  26. 26. Austenitic stainless steelsAustenitic 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, weldabilityand ductility.Austenitic stainless steels retain their properties at elevated temperatures.At the temperatures 900-1400ºF (482-760ºC) chromium carbides form alongthe austenite grains. This causes depletion of chromium from the grains resultingin decreasing the corrosion protective passive film.This effect is called sensitization. It is particularly important in welding ofaustenitic 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 stabilizedaustenitic stainless steels containing carbide forming elements like titanium,niobium, tantalum, zirconium. Stabilization heat treatment of such steels resultsin preferred formation of carbides of the stabilizing elements instead ofchromium carbides.These steel are not heat treatable and may be hardened only by cold work.Applications of austenitic stainless steels: chemical equipment, foodequipment, kitchen sinks, medical devices, heat exchangers, parts of furnaces
  27. 27. Ferritic stainless steelsFerritic stainless steels (400 series) contain chromium only asalloying 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 bestmachinability. The steels are ferromagnetic. Ductility andformability of ferritic steels are low. Corrosion resistance andweldability are moderate. Resistance to the stress corrosioncracking is high.Ferritic steels are not heat treatable because of low carbonconcentration and they are commonly used in annealed state.Applications of ferritic steels: decorative and architecturalparts, automotive trims and exhausting systems, computerfloppy disc hubs, hot water tanks.
  28. 28. Martensitic stainless steelsMartensitic stainless steels (400 and 500 series) containchromium as alloying element and increased (as comparedto ferritic grade) amount of carbon.Due to increased concentration of carbon the steels fromthis group are heat treatable. The steels have austeniticstructure (FCC) at high temperature, which transforms tomartensitic structure (BCC) as a result of quenching .Martensitic steels have poor weldability and ductility.Corrosion resistance of these steels is moderate (slightlybetter than in ferritic steels).Applications of martensitic stainless steels: turbineblades, knife blades, surgical instruments, shafts, pins,springs.
  29. 29. Austenitic-ferritic (Duplex) stainless steelsAustenitic-ferritic (Duplex) stainless steels contain increasedamount of chromium (18% -28%) and decreased (as compared toaustenitic steels) amount of nickel (4.5% - 8%) as major alloyingelements. As additional alloying element molybdenum is used insome of Duplex steels.Since the quantity of nickel is insufficient for formation of fullyaustenitic structure, the structure of Duplex steels is mixed:austenitic-ferritic.The properties of Duplex steels are somewhere between theproperties of austenitic and ferritic steels. Duplex steels have highresistance to the stress corrosion cracking and to chloride ionsattack. These steels are weldable and formable and possess highstrength.Applications of austenitic-ferritic stainless steels: desalinationequipment, marine equipment, petrochemical plants, heatexchangers.
  30. 30. Precipitation hardening stainless steelsPrecipitation hardening stainless steels containchromium, nickel as major alloying elements.Precipitation hardening steels are supplied insolution treated condition. These steels may be eitheraustenitic or martensitic and they are hardened by heattreatment (aging). The heat treatment is conducted aftermachining, however low temperature of the treatmentdoes not cause distortions.Precipitation hardening steels have very high strength,good weldability and fair corrosion resistance. They aremagnetic.Applications of precipitation hardening stainless steels:pump shafts and valves, turbine blades, paper industryequipment, aerospace equipment
  31. 31. Creep resistant steels are steelsdesigned to withstand a constant load at hightemperatures.The most important application of creepresistant steels is components of steam powerplants operating at elevated temperatures(boilers, turbines, steam lines).Supercritical and Ultra Supercritical power plants9-12 Cr martensitic creep resistant steelsAustenitic creep resistant steels
  32. 32. The main steel making methods are:Basic Oxygen Process (BOP)Electric-arc furnaceLadle refining (ladle metallurgy, secondary refining)
  33. 33. The Basic Oxygen Process is the mostpowerful and effective method of steelmanufacturing.The scheme of theBasic Oxygen Furnace (BOF) (basic oxygenfurnace, basic oxygen converter) ispresented in the picture.Typical basic oxygen converter has avertical steel shell lined with refractorylining.The furnace is capable to rotate about itshorizontal axis on trunnions.This rotation is necessary for charging rawmaterials and fluxes, sampling the melt andpouring the steel and the slag out of thefurnace.The Basic Oxygen is equipped with thewater cooled oxygen lance for blowingoxygen into the melt.The basic oxygen converter uses noadditional fuel. The pig iron impurities(carbon, silicon, manganese and
  34. 34. The steel making process in the oxygen converterconsists of:Charging steel scrap.Pouring liquid pig iron into the furnace.Charging fluxes.Oxygen blowing.Sampling and temperature measurementTapping the steel to a ladle.De-slagging.The iron impurities oxidize, evolving heat, necessary forthe process.The forming oxides and sulfur are absorbed by the slag.The oxygen converter has a capacity up to 400 t andproduction cycle of about 40 min.
  35. 35. Electric-arc furnaceThe electric-arc furnace employs threevertical graphite electrodes for producingarcs, striking on to the charge and heatingit to the required temperature.As the electric-arc furnace utilizes theexternal origin of energy (electriccurrent), 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,limestoneLowering the electrodes and startingthe power (melting)Oxidizing stageAt this stage the heat, produced by thearcs, causes oxidizing phosphorous, siliconand manganese. The oxides are absorbedinto the slag. By the end of the stage theslag is removed.
  36. 36. De-slaggingReducing stageNew fluxes (lime and anthracite) are added at this stagefor formation of basic reducing slag.The function of this slag is refining of the steel fromsulfur and absorption of oxides, formed as a result ofdeoxidation.TappingLining maintenanceThe advantages of the electric-arc furnace are as follows:Unlimited scrap quantity may be melt;Easy temperature control;Deep desulfurization;Precise alloying.
  37. 37. Ladle refining (ladle metallurgy, secondary refining)Ladle refining is post steel making technological operations, performed in the ladle priorto casting with the purposes of desulfurization, degassing, temperature and chemicalhomogenization, deoxidation and others.Ladle refining may be carried out at atmospheric pressure, at vacuum, may involveheating, 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 iswidely 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 bythis method.
  38. 38. 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 itdecreases during cooling down and then drops sharply in Solidification reaching0.003% in solid steel.Oxygen liberated from the solid solution oxidizes the steel components (C, Fe,alloying elements) forming gas pores (blowholes) and non-metallic inclusionsentrapped within the ingot structure. Both blowholes and inclusions adverselyaffect the steel quality.In order to prevent oxidizing of steel components during solidification the oxygencontent should be reduced.Deoxidation of steel is a steel making technological operation, in whichconcentration (activity) of oxygen dissolved in molten steel is reduced to arequired level.
  39. 39. There are three principal deoxidation methods: Deoxidation by metallic deoxidizers Deoxidation by vacuum Diffusion deoxidation.Deoxidation by metallic deoxidizersThis is the most popular deoxidation method. It uses elements forming strong andstable 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: Equilibrium constant at Deoxidizer Reaction A B 1873 °K (2912°F, 1600°C) [Mn] + [O] = Manganese 12440 5.33 1.318 (MnO) [Si] + 2[O] = Silicone 30000 11.5 4.518 (SiO2) 2[Al] + 3[O] = Aluminum 62780 20.5 13.018 (Al2O3)
  40. 40. According to the degree of deoxidation Carbon steelsmay be subdivided into three groups:Killed steels - completely deoxidized steels,solidification of which does not cause formation ofcarbon monoxide (CO). Ingots and castings of killed steelhave homogeneous structure and no gas porosity(blowholes).Semi-killed steels - incompletely deoxidized steelscontaining some amount of excess oxygen, which formscarbon monoxide during last stages of solidification.Rimmed steels - partially deoxidized or non-deoxidized low carbon steels evolving sufficient amountof carbon monoxide during solidification. Ingots ofrimmed steels are characterized by good surface qualityand considerable quantity of blowholes.
  41. 41. Effect of alloying elements on steel propertiesAlloying is changing chemical composition of steel by adding elements withpurpose to improve its properties as compared to the plane carbon steel.The properties, which may be improvedStabilizing austenite – increasing the temperature range, in which austeniteexists.The elements, having the same crystal structure as that of austenite (cubic face centered – FCC), raise the A4 point (the temperature of formation ofaustenite 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 austeniteexists.The elements, having the same crystal structure as that of ferrite (cubic body centered – BCC), lower the A4 point and increase the A3temperature.These elements lower the solubility of carbon in austenite, causing increaseof amount of carbides in the steel.The following elements have ferrite stabilizing effect: chromium (Cr),
  42. 42. 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) formhard (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 breakingand 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 oxidefilm on the steel surface, protecting it from chemical attacks.
  43. 43. Characteristics of alloying elementsManganese (Mn) – improves hardenability, ductility and wear resistance. Mn eliminatesformation of harmful iron sulfides, increasing strength at high temperatures.Nickel (Ni) – increases strength, impact strength and toughness, impart corrosionresistance in combination with other elements.Chromium (Cr) – improves hardenability, strength and wear resistance, sharply increasescorrosion resistance at high concentrations (> 12%).Tungsten (W) – increases hardness particularly at elevated temperatures due to stablecarbides, refines grain size.Vanadium (V) – increases strength, hardness, creep resistance and impact resistance dueto formation of hard vanadium carbides, limits grain size.Molybdenum (Mo) – increases hardenability and strength particularly at hightemperatures 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 andmachinability.Copper (Cu) – improves corrosion resistance.Aluminum (Al) – deoxidizer, limits austenite grains growth.
  44. 44. Four types of furnaces are used to makecast 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.
  45. 45. The choice of furnace and melting practicedepends 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., whetherplain or alloyed, high or low carbon, etc.The raw materials available and the pricesthereofFuel or power costsThe, amount of capital to be invested Previous experience
  46. 46. Generally, the open-hearth furnace isused for large tonnages and largecastings, and the electric furnace forsmaller heats or where steels of widelydiffering analyses must be produced.Special steels or high alloy steels areoften produced in an induction furnace.
  47. 47.  The air used BASIC OPEN-HEARTH MELTING forcombustion purposesmust be preheated toobtain the hightemperatures requiredin an open hearth A companioncheckerwork, in themeantime, is beingheated by the outgoinggases. The cycle isreversed about every 15min to preventexcessive cooling of thecheckerwork bricks. The preheated air ismixed with theincoming oil or fuel gasat the burner pors. Thiscreates a flame over thehearth which heats thecharge and surroundingrefractories.
  48. 48. Fuels and Charge Materials Basic open-hearth furnaces are fired with either gas or oil. The oil may have to bepreheated before it is burned.The charge materials consist of pig iron, purchased scrap, foundryscrap returns, lime, andore. 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% maximumphosphorusThe manganese is kept high to aid in desulfurization and in controlling the slag. Silicon islimited because it is an acid component in slag and hence tends to require excess lime inthe charge and also may increase
  49. 49. Charging and MeltingSeveral methods are used in placing thematerials in the furnace, but the usualpractice is to cover the bottom with scrap,followed by the lime spread as evenly aspossible. This is followed by the pig iron.The lime addition will vary from 4 to 7 percent of the weight of the metal charge.The carbon content of the charge will varyfrom 1.0 to 1.75 per cent.
  50. 50. Oxidation and RefiningThe principle underlying the melting and refining of steelin open-hearth and electric furnaces is to create anoxidizing condition that will oxidize such elements ascarbon, manganese, silicon, and phosphorus. Theseoxides, with the exception of CO gas, dissolve in the slag.
  51. 51. The amount of the iron oxide addition is determined bythe meltdown carbon content of the bath and the desiredcarbon content at tapping. The addition of iron oxidecauses an evolution of CO from t.he reactionFeO + C 7 CO + FeThe bubbles of CO originate in the melt at the hearthbottom and create a "carbon boil" as they percolate up tothe surface. The carbon boil is an important part of refiningsince it aids in heat transfer by stirring the melt, cleansesthe metal of retained oxides by bringing them to the slag,hastens reactions at the gas-metal interface, and aids inremoving hydrogen and nitrogen. Hydrogen and nitrogendiffuse into the CO bubbles and are thereby flushed out ofthe liquid steel
  52. 52. The reaction actually is largely between oxygen dissolved in the steel and thecarbon. At the temperatures used in steel refining (around 2900 F), the steel iscapable of dissolving considerable oxygen as FeO.t This FeO is picked up from theslag or from a reaction between the added iron ore and the steel, such asF~08 +Fe ~3FeOschematic representation of the oxidation cycle in the open hearth.
  53. 53. 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 followingcomposition range:CaO,4O to 50% Si02 , 13 to 18% MnO, 7to 15% FeU, 12 to 16%Deoxidation and TappingWhen the heat is considered ready for tapping, deoxidizing agents are added. This step ofadding a deoxidizing agent is referred to as "blocking the heat" because it prevents anyfurther reaction between oxygen and carbon, the oxygen reacting with these additives toform non gaseous reaction products. The deoxidizers include spiegel, ferrosilicon,ferromanganese, and silicomanganese.
  54. 54. Electric furnaces BASIC ELECTRIC MELTINGFurnace ConstructionBasic electric furnaces are much smaller than open-hearth furnaces, ranging from 1/2 to 1/7tons 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 operatingfrom a three-phase circuit. The height of the electrodes above thebath is controlled electrically. Voltages are fairly low, and currentflow is high, necessitating large bus bars and heavy lead-in cablesfrom the transformers. Charging is usually done by removing thefurnace top. The roof of the furnace is silica brick, whereas the sidewalls are lined with magnesite brick or chrome-magnesit€ brick.Bottoms are rammed into place.
  55. 55. Cross-sectional view of an electric arc furnace showing an aucid lining (left) and a basic lining (right) .
  56. 56. Melting and Refining Although the general principles controlling the refining operation in the basicopen-hearth furnace also apply to the basic electric, certain modifications inoperation are possible which give the basic electric furnace greater flexibility.Unlike the open-hearth charge, the charge for the electric furnace may notnecessarily include pig iron, because not so much carbon is lost during melting asin the open-hearth.Once this slag is removed, a refining slag composed of lime and fluorspar isadded. The purpose of this so-called refining slag is to remove sulfur, which canbe accomplil>hed only by establishing basic and reducing conditions. The·necessary reducing agent in this case is carbon added to the top of theslag. The reaction is considered to beC (in steel) + CaO + FeS (in steel) ~CaS (in slag) +CO +FeRefining proceeds for about 1 to 1Yz hr, the completion of which isindicated by the appearance of a slag sample. The metal is usually tappedinto bottom-pour ladles. Since the composition was adjusted before andduring the refining period, no further additions are required at tapping.