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  1. 1. Iron and Steel ManufactureTechnology related to the production of iron and its alloys,particularly those containing a small percentage of carbon.The differences between the various types of iron and steelare sometimes confusing because of the nomenclature used.Steel in general is an alloy of iron and carbon, often with anadmixture of other elements. Some alloys that arecommercially called irons contain more carbon thancommercial steels. Open-hearth iron and wrought ironcontain only a few hundredths of 1 percent of carbon. Steelsof various types contain from 0.04 percent to 2.25 percent ofcarbon. Cast iron, malleable cast iron, and pig iron containamounts of carbon varying from 2 to 4 percent. A specialform of malleable iron, containing virtually no carbon, isknown as white-heart malleable iron. A special group of ironalloys, known as ferroalloys, is used in the manufacture ofiron and steel alloys; they contain from 20 to 80 percent of analloying element, such as manganese, silicon, or chromium. (c) Coláiste Lorcain 1
  2. 2. HistoryThe exact date at which people discovered the technique of smelting ironore to produce usable metal is not known. The earliest iron implementsdiscovered by archaeologists in Egypt date from about 3000 BC, and ironornaments were used even earlier; the comparatively advanced techniqueof hardening iron weapons by heat treatment was known to the Greeksabout 1000 BC.The alloys produced by early iron workers, and, indeed, all the iron alloysmade until about the 14th century AD, would be classified today as wroughtiron. They were made by heating a mass of iron ore and charcoal in a forgeor furnace having a forced draft. Under this treatment the ore was reducedto the sponge of metallic iron filled with a slag composed of metallicimpurities and charcoal ash. This sponge of iron was removed from thefurnace while still incandescent and beaten with heavy sledges to drive outthe slag and to weld and consolidate the iron. The iron produced underthese conditions usually contained about 3 percent of slag particles and 0.1percent of other impurities. Occasionally this technique of ironmakingproduced, by accident, a true steel rather than wrought iron. Ironworkerslearned to make steel by heating wrought iron and charcoal in clay boxesfor a period of several days. By this process the iron absorbed enoughcarbon to become a true steel. (c) Coláiste Lorcain 2
  3. 3. After the 14th century the furnaces used in smelting were increased in size, andincreased draft was used to force the combustion gases through the ”charge,” themixture of raw materials. In these larger furnaces, the iron ore in the upper part of thefurnace was first reduced to metallic iron and then took on more carbon as a result ofthe gases forced through it by the blast. The product of these furnaces was pig iron, analloy that melts at a lower temperature than steel or wrought iron. Pig iron (so calledbecause it was usually cast in stubby, round ingots known as pigs) was then furtherrefined to make steel.Modern steelmaking employs blast furnaces that are merely refinements of thefurnaces used by the old ironworkers. The process of refining molten iron with blasts ofair was accomplished by the British inventor Sir Henry Bessemer who developed theBessemer furnace, or converter, in 1855. Since the 1960s, several so-called minimillshave been producing steel from scrap metal in electric furnaces. Such mills are animportant component of total U.S. steel production. The giant steel mills remainessential for the production of steel from iron ore. (c) Coláiste Lorcain 3
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  6. 6. The basic materials used for the manufacture of pig iron are iron ore, coke, andlimestone. The coke is burned as a fuel to heat the furnace; as it burns, the coke givesoff carbon monoxide, which combines with the iron oxides in the ore, reducing them tometallic iron. This is the basic chemical reaction in the blast furnace; it has theequation: Fe2O3 + 3CO = 3CO2 + 2Fe. The limestone in the furnace charge is used asan additional source of carbon monoxide and as a “flux” to combine with the infusiblesilica present in the ore to form fusible calcium silicate. Without the limestone, ironsilicate would be formed, with a resulting loss of metallic iron. Calcium silicate plusother impurities form a slag that floats on top of the molten metal at the bottom of thefurnace. Ordinary pig iron as produced by blast furnaces contains iron, about 92percent; carbon, 3 or 4 percent; silicon, 0.5 to 3 percent; manganese, 0.25 to 2.5percent; phosphorus, 0.04 to 2 percent; and a trace of sulfur. (c) Coláiste Lorcain 6
  7. 7. The basic materials used for the manufacture of pig iron are iron ore, coke, andlimestone. The coke is burned as a fuel to heat the furnace; as it burns, the coke givesoff carbon monoxide, which combines with the iron oxides in the ore, reducing them tometallic iron. This is the basic chemical reaction in the blast furnace; it has theequation: Fe2O3 + 3CO = 3CO2 + 2Fe. The limestone in the furnace charge is used asan additional source of carbon monoxide and as a “flux” to combine with the infusiblesilica present in the ore to form fusible calcium silicate. Without the limestone, ironsilicate would be formed, with a resulting loss of metallic iron. Calcium silicate plusother impurities form a slag that floats on top of the molten metal at the bottom of thefurnace. Ordinary pig iron as produced by blast furnaces contains iron, about 92percent; carbon, 3 or 4 percent; silicon, 0.5 to 3 percent; manganese, 0.25 to 2.5percent; phosphorus, 0.04 to 2 percent; and a trace of sulfur. (c) Coláiste Lorcain 7
  8. 8. The air used to supply the blast in a blast furnace is preheated to temperaturesbetween approximately 540° and 870° C (approximately 1000° and 1600° F). Theheating is performed in stoves, cylinders containing networks of firebrick. The bricks inthe stoves are heated for several hours by burning blast-furnace gas, the waste gasesfrom the top of the furnace. Then the flame is turned off and the air for the blast isblown through the stove. The weight of air used in the operation of a blast furnaceexceeds the total weight of the other raw materials employed.An important development in blast furnace technology, the pressurizing of furnaces,was introduced after World War II. By “throttling” the flow of gas from the furnacevents, the pressure within the furnace may be built up to 1.7 atm or more. Thepressurizing technique makes possible better combustion of the coke and higheroutput of pig iron. The output of many blast furnaces can be increased 25 percent bypressurizing. Experimental installations have also shown that the output of blastfurnaces can be increased by enriching the air blast with oxygen. (c) Coláiste Lorcain 8
  9. 9. The process of tapping consists of knocking out a clay plug fromthe iron hole near the bottom of the bosh and allowing the moltenmetal to flow into a clay-lined runner and then into a large, brick-lined metal container, which may be either a ladle or a rail carcapable of holding as much as 100 tons of metal. Any slag thatmay flow from the furnace with the metal is skimmed off before itreaches the container. The container of molten pig iron is thentransported to the steelmaking shop.Modern-day blast furnaces are operated in conjunction with basicoxygen furnaces and sometimes the older open-hearth furnaces aspart of a single steel-producing plant. In such plants the molten pigiron is used to charge the steel furnaces. The molten metal fromseveral blast furnaces may be mixed in a large ladle before it isconverted to steel, to minimize any irregularities in the compositionof the individual melts. (c) Coláiste Lorcain 9
  10. 10. Other Methods of Iron RefiningAlthough almost all the iron and steel manufactured in the world is made from pig iron produced by the blast-furnace process, othermethods of iron refining are possible and have been practiced to a limited extent. One such method is the so-called direct method of making iron and steel from ore, without making pig iron. In thisprocess iron ore and coke are mixed in a revolving kiln and heated to a temperature of about 950° C (about 1740° F). Carbon monoxide is given off from the heated coke just as in the blast furnace and reduces the oxides of the ore to metallic iron. Thesecondary reactions that occur in a blast furnace, however, do notoccur, and the kiln produces so-called sponge iron of much higher purity than pig iron. Virtually pure iron is also produced by means of electrolysis (see Electrochemistry), by passing an electriccurrent through a solution of ferrous chloride. Neither the direct nor the electrolytic processes has yet achieved any great commercial significance. (c) Coláiste Lorcain 10
  11. 11. Open-Hearth ProcessEssentially the production of steel from pig iron by any processconsists of burning out the excess carbon and other impuritiespresent in the iron. One difficulty in the manufacture of steel is itshigh melting point, about 1370° C (about 2500° F), which preventsthe use of ordinary fuels and furnaces. To overcome this difficultythe open-hearth furnace was developed; this furnace can beoperated at a high temperature by regenerative preheating of thefuel gas and air used for combustion in the furnace. In regenerativepreheating, the exhaust gases from the furnace are drawn throughone of a series of chambers containing a mass of brickwork andgive up most of their heat to the bricks. Then the flow through thefurnace is reversed and the fuel and air pass through the heatedchambers and are warmed by the bricks. Through this methodopen-hearth furnaces can reach temperatures as high as 1650° C (c) Coláiste Lorcain 11
  12. 12. The furnace itself consists typically of a flat, rectangular brick hearth about 6 m by 10m (about 20 ft by 33 ft), which is roofed over at a height of about 2.5 m (about 8 ft). Infront of the hearth a series of doors opens out onto a working floor in front of thehearth. The entire hearth and working floor are one story above ground level, and thespace under the hearth is taken up by the heat-regenerating chambers of the furnace.A furnace of this size produces about 100 metric tons of steel every 11 hr. (c) Coláiste Lorcain 12
  13. 13. The furnace is charged with a mixture of pig iron (eithermolten or cold), scrap steel, and iron ore that providesadditional oxygen. Limestone is added for flux andfluorspar to make the slag more fluid. The proportions ofthe charge vary within wide limits, but a typical chargemight consist of 56,750 kg (125,000 lb) of scrap steel,11,350 kg (25,000 lb) of cold pig iron, 45,400 kg(100,000 lb) of molten pig iron, 11,800 kg (26,000 lb) oflimestone, 900 kg (2000 lb) of iron ore, and 230 kg (500lb) of fluorspar. After the furnace has been charged, thefurnace is lighted and the flames play back and forthover the hearth as their direction is reversed by theoperator to provide heat regeneration. (c) Coláiste Lorcain 13
  14. 14. Chemically the action of the open-hearth furnace consists of lowering thecarbon content of the charge by oxidization and of removing such impuritiesas silicon, phosphorus, manganese, and sulphur, which combine with thelimestone to form slag. These reactions take place while the metal in thefurnace is at melting heat, and the furnace is held between 1540° and1650° C (2800° and 3000° F) for many hours until the molten metal has thedesired carbon content. Experienced open-hearth operators can often judgethe carbon content of the metal by its appearance, but the melt is usuallytested by withdrawing a small amount of metal from the furnace, cooling it,and subjecting it to physical examination or chemical analysis. When thecarbon content of the melt reaches the desired level, the furnace is tappedthrough a hole at the rear. The molten steel then flows through a shorttrough to a large ladle set below the furnace at ground level. From the ladlethe steel is poured into cast-iron molds that form ingots usually about 1.5 m(about 5 ft) long and 48 cm (19 in) square. These ingots, the raw materialfor all forms of fabricated steel, weigh approximately 2.25 metric tons in thissize. Recently, methods have been put into practice for the continuousprocessing of steel without first having to go through the process of castingingots. (c) Coláiste Lorcain 14
  15. 15. Basic Oxygen ProcessThe oldest process for making steel in large quantities, the Bessemerprocess, made use of a tall, pear-shaped furnace, called a Bessemerconverter, that could be tilted sideways for charging and pouring. Greatquantities of air were blown through the molten metal; its oxygen unitedchemically with the impurities and carried them off.In the basic oxygen process, steel is also refined in a pear-shaped furnacethat tilts sideways for charging and pouring. Air, however, has beenreplaced by a high-pressure stream of nearly pure oxygen. After thefurnace has been charged and turned upright, an oxygen lance is loweredinto it. The water-cooled tip of the lance is usually about 2 m (about 6 ft)above the charge although this distance can be varied according torequirements. Thousands of cubic meters of oxygen are blown into thefurnace at supersonic speed. The oxygen combines with carbon and otherunwanted elements and starts a high-temperature churning reaction thatrapidly burns out impurities from the pig iron and converts it into steel. Therefining process takes 50 min or less; approximately 275 metric tons ofsteel can be made in an hour. (c) Coláiste Lorcain 15
  16. 16. Electric-Furnace SteelIn some furnaces, electricity instead of fire supplies the heat for themelting and refining of steel. Because refining conditions in such afurnace can be regulated more strictly than in open-hearth or basicoxygen furnaces, electric furnaces are particularly valuable forproducing stainless steels and other highly alloyed steels that mustbe made to exacting specifications. Refining takes place in a tightlyclosed chamber, where temperatures and other conditions are keptunder rigid control by automatic devices. During the early stages ofthis refining process, high-purity oxygen is injected through a lance,raising the temperature of the furnace and decreasing the timeneeded to produce the finished steel. The quantity of oxygenentering the furnace can always be closely controlled, thus keepingdown undesirable oxidizing reactions. (c) Coláiste Lorcain 16
  17. 17. Most often the charge consists almost entirely of scrap. Before it is ready to be used, the scrap must first be analyzed and sorted, because its alloy content will affect the composition of the refined metal. Other materials, such as small quantities of iron ore and drylime, are added in order to help remove carbon and other impurities that are present. The additional alloying elements go either into the charge or, later, into the refined steel as it is poured into the ladle. After the furnace is charged, electrodes are lowered close to the surface of the metal. The current enters through one of theelectrodes, arcs to the metallic charge, flows through the metal, and then arcs back to the next electrode. Heat is generated by the overcoming of resistance to the flow of current through the charge.This heat, together with that coming from the intensely hot arc itself, quickly melts the metal. In another type of electric furnace, heat is generated in a coil. (c) Coláiste Lorcain 17
  18. 18. (c) Coláiste Lorcain 18
  19. 19. Steel is marketed in a wide variety of sizes andshapes, such as rods, pipes, railroad rails, tees,channels, and I-beams. These shapes are produced atsteel mills by rolling and otherwise forming heatedingots to the required shape. The working of steel alsoimproves the quality of the steel by refining itscrystalline structure and making the metal tougher.The basic process of working steel is known as hotrolling. In hot rolling the cast ingot is first heated tobright-red heat in a furnace called a soaking pit and isthen passed between a series of pairs of metal rollersthat squeeze it to the desired size and shape. Thedistance between the rollers diminishes for eachsuccessive pair as the steel is elongated and reducedin thickness. (c) Coláiste Lorcain 19
  20. 20. The first pair of rollers through which the ingot passes iscommonly called the blooming mill, and the square billets of steelthat the ingot produces are known as blooms. From the bloomingmill, the steel is passed on to roughing mills and finally to finishingmills that reduce it to the correct cross section. The rollers of millsused to produce railroad rails and such structural shapes as I-beams, H-beams, and angles are grooved to give the requiredshape.Modern manufacturing requires a large amount of thin sheet steel.Continuous mills roll steel strips and sheets in widths of up to 2.4m (8 ft). Such mills process thin sheet steel so rapidly, before itcools and becomes unworkable. A slab of hot steel over 11 cm(about 4.5 in) thick is fed through a series of rollers which reduceit progressively in thickness to 0.127 cm (0.05 inc) and increaseits length from 4 m (13 ft) to 370 m (1210 ft). (c) Coláiste Lorcain 20
  21. 21. Continuous mills are equipped with a number ofaccessory devices including edging rollers, descalingdevices, and devices for coiling the sheet automaticallywhen it reaches the end of the mill. The edging rollersare sets of vertical rolls set opposite each other ateither side of the sheet to ensure that the width of thesheet is maintained. Descaling apparatus removes thescale that forms on the surface of the sheet byknocking it off mechanically, loosening it by means ofan air blast, or bending the sheet sharply at some pointin its travel. The completed coils of sheet are droppedon a conveyor and carried away to be annealed and cutinto individual sheets. (c) Coláiste Lorcain 21
  22. 22. . A more efficient way to produce thin sheet steel is tofeed thinner slabs through the rollers. Usingconventional casting methods, ingots must still bepassed through a blooming mill in order to produceslabs thin enough to enter a continuous mill.By devising a continuous casting system that producesan endless steel slab less than 5 cm (2 in) thick,German engineers have eliminated any need forblooming and roughing mills. In 1989, a steel mill inIndiana became the first outside Europe to adopt thisnew system. (c) Coláiste Lorcain 22
  23. 23. PipeCheaper grades of pipe are shaped by bending a flatstrip, or skelp, of hot steel into cylindrical form andwelding the edges to complete the pipe. For the smallersizes of pipe, the edges of the skelp are usuallyoverlapped and passed between a pair of rollers curvedto correspond with the outside diameter of the pipe. Thepressure on the rollers is great enough to weld theedges together. Seamless pipe or tubing is made fromsolid rods by passing them between a pair of inclinedrollers that have a pointed metal bar, or mandrel, setbetween them in such a way that it pierces the rods andforms the inside diameter of the pipe at the same timethat the rollers are forming the outside diameter. (c) Coláiste Lorcain 23
  24. 24. Tin PlateBy far the most important coated product of the steelmill is tin plate for the manufacture of containers. The“tin” can is actually more than 99 percent steel. In somemills steel sheets that have been hot-rolled and thencold-rolled are coated by passing them through a bathof molten tin. The most common method of coating isby the electrolytic process. Sheet steel is slowly unrolledfrom its coil and passed through a chemical solution.Meanwhile, a current of electricity is passing through apiece of pure tin into the same solution, causing the tinto dissolve slowly and to be deposited on the steel. Inelectrolytic processing, less than half a kilogram of tinwill coat more than 18.6 sq m (more than 200 sq ft) ofsteel. (c) Coláiste Lorcain 24
  25. 25. For the product known as thin tin, sheet and stripare given a second cold rolling before being coatedwith tin, a treatment that makes the steel plate extratough as well as extra thin. Cans made of thin tinare about as strong as ordinary tin cans, yet theycontain less steel, with a resultant saving in weightand cost. Lightweight packaging containers are alsobeing made of tin-plated steel foil that has beenlaminated to paper or cardboard.Other processes of steel fabrication include forging,founding, and drawing the steel through dies. (c) Coláiste Lorcain 25
  26. 26. Wrought IronThe process of making the tough, malleable alloyknown as wrought iron differs markedly from otherforms of steel making. Because this process, known aspuddling, required a great deal of hand labour,production of wrought iron in tonnage quantities wasimpossible. The development of new processes usingBessemer converters and open-hearth furnacesallowed the production of larger quantities of wroughtiron.Wrought iron is no longer produced commercially,however, because it can be effectively replaced innearly all applications by low-carbon steel, which is lessexpensive to produce and is typically of more uniformquality than wrought iron. (c) Coláiste Lorcain 26
  27. 27. The puddling furnace used in the older process has alow, arched roof and a depressed hearth on which thecrude metal lies, separated by a wall from thecombustion chamber in which bituminous coal isburned. The flame in the combustion chambersurmounts the wall, strikes the arched roof, and“reverberates” upon the contents of the hearth. After thefurnace is lit and has become moderately heated, thepuddler, or furnace operator, “fettles” it by plastering thehearth and walls with a paste of iron oxide, usuallyhematite ore. The furnace is then charged with about270 kg (about 600 lb) of pig iron and the door is closed.After about 30 min the iron is melted and the puddleradds more iron oxide or mill scale to the charge,working the oxide into the iron with a bent iron barcalled a raddle. (c) Coláiste Lorcain 27
  28. 28. The silicon and most of the manganese in the iron areoxidized and some sulfur and phosphorus areeliminated. The temperature of the furnace is thenraised slightly, and the carbon starts to burn out ascarbon-oxide gases. As the gas is evolved the slagpuffs up and the level of the charge rises. As thecarbon is burned away the melting temperature of thealloy increases and the charge becomes more andmore pasty, and finally the bath drops to its formerlevel. As the iron increases in purity, the puddler stirsthe charge with the raddle to ensure uniformcomposition and proper cohesion of the particles. Theresulting pasty, spongelike mass is separated intolumps, called balls, of about 80 to 90 kg (about 180 to200 lb) each. (c) Coláiste Lorcain 28
  29. 29. The balls are withdrawn from the furnace with tongsand are placed directly in a squeezer, a machine inwhich the greater part of the intermingled siliceous slagis expelled from the ball and the grains of pure iron arethoroughly welded together. The iron is then cut intoflat pieces that are piled on one another, heated towelding temperature, and then rolled into a singlepiece. This rolling process is sometimes repeated toimprove the quality of the product. (c) Coláiste Lorcain 29
  30. 30. The modern technique of making wrought iron usesmolten iron from a Bessemer converter and moltenslag, which is usually prepared by melting iron ore, millscale, and sand in an open-hearth furnace. The moltenslag is maintained in a ladle at a temperature severalhundred degrees below the temperature of the molteniron. When the molten iron, which carries a largeamount of gas in solution, is poured into the ladlecontaining the molten slag, the metal solidifies almostinstantly, releasing the dissolved gas. (c) Coláiste Lorcain 30
  31. 31. The force exerted by the gas shatters the metal intominute particles that are heavier than the slag andthat accumulate in the bottom of the ladle,agglomerating into a spongy mass similar to theballs produced in a puddling furnace. After the slaghas been poured off the top of the ladle, the ball ofiron is removed and squeezed and rolled like theproduct of the puddling furnace. (c) Coláiste Lorcain 31
  32. 32. Classifications of SteelSteels are grouped into five main classifications.Carbon SteelsMore than 90 percent of all steels are carbonsteels. They contain varying amounts of carbonand not more than 1.65 percent manganese, 0.60percent silicon, and 0.60 percent copper.Machines, automobile bodies, most structural steelfor buildings, ship hulls, bedsprings, and bobbypins are among the products made of carbonsteels. (c) Coláiste Lorcain 32
  33. 33. Alloy SteelsThese steels have a specified composition, containingcertain percentages of vanadium, molybdenum, orother elements, as well as larger amounts ofmanganese, silicon, and copper than do the regularcarbon steels. Automobile gears and axles, rollerskates, and carving knives are some of the many thingsthat are made of alloy steels. (c) Coláiste Lorcain 33
  34. 34. High-Strength Low-Alloy SteelsCalled HSLA steels, they are the newest of the five chieffamilies of steels. They cost less than the regular alloy steelsbecause they contain only small amounts of the expensivealloying elements. They have been specially processed,however, to have much more strength than carbon steels ofthe same weight. For example, freight cars made of HSLAsteels can carry larger loads because their walls are thinnerthan would be necessary with carbon steel of equal strength;also, because an HSLA freight car is lighter in weight than theordinary car, it is less of a load for the locomotive to pull.Numerous buildings are now being constructed withframeworks of HSLA steels. Girders can be made thinnerwithout sacrificing their strength, and additional space is leftfor offices and apartments. (c) Coláiste Lorcain 34
  35. 35. Stainless SteelsStainless steels contain chromium, nickel, and other alloyingelements that keep them bright and rust resistant in spite ofmoisture or the action of corrosive acids and gases. Somestainless steels are very hard; some have unusual strengthand will retain that strength for long periods at extremely highand low temperatures. Because of their shining surfacesarchitects often use them for decorative purposes. Stainlesssteels are used for the pipes and tanks of petroleumrefineries and chemical plants, for jet planes, and for spacecapsules. Surgical instruments and equipment are made fromthese steels, and they are also used to patch or replacebroken bones because the steels can withstand the action ofbody fluids. In kitchens and in plants where food is prepared,handling equipment is often made of stainless steel becauseit does not taint the food and can be easily cleaned. (c) Coláiste Lorcain 35
  36. 36. Tool SteelsThese steels are fabricated into many types of tools or intothe cutting and shaping parts of power-driven machinery forvarious manufacturing operations. They contain tungsten,molybdenum, and other alloying elements that give themextra strength, hardness, and resistance to wear. (c) Coláiste Lorcain 36
  37. 37. Structure of SteelThe physical properties of various types of steel and ofany given steel alloy at varying temperatures dependprimarily on the amount of carbon present and on how itis distributed in the iron. Before heat treatment moststeels are a mixture of three substances: ferrite, pearlite,and cementite. Ferrite is iron containing small amounts ofcarbon and other elements in solution and is soft andductile. Cementite, a compound of iron containing about 7percent carbon, is extremely brittle and hard. Pearlite isan intimate mixture of ferrite and cementite having aspecific composition and characteristic structure, andphysical characteristics intermediate between its twoconstituents. (c) Coláiste Lorcain 37
  38. 38. The toughness and hardness of a steel that is not heattreated depend on the proportions of these threeingredients. As the carbon content of a steel increases,the amount of ferrite present decreases and the amountof pearlite increases until, when the steel has 0.8 percentof carbon, it is entirely composed of pearlite. Steel withstill more carbon is a mixture of pearlite and cementite.Raising the temperature of steel changes ferrite andpearlite to an allotropic form of iron-carbon alloy known asaustenite, which has the property of dissolving all the freecarbon present in the metal. If the steel is cooled slowlythe austenite reverts to ferrite and pearlite, but if coolingis sudden, the austenite is “frozen” or changes tomartensite, which is an extremely hard allotropicmodification that resembles ferrite but contains carbon insolid solution. (c) Coláiste Lorcain 38
  39. 39. Heat Treatment of SteelThe basic process of hardening steel by heat treatmentconsists of heating the metal to a temperature at whichaustenite is formed, usually about 760° to 870° C (about1400°) and then cooling, or quenching, it rapidly in wateror oil. Such hardening treatments, which form martensite,set up large internal strains in the metal, and these arerelieved by tempering, or annealing, which consists ofreheating the steel to a lower temperature. Temperingresults in a decrease in hardness and strength and anincrease in ductility and toughness. (c) Coláiste Lorcain 39
  40. 40. The primary purpose of the heat-treating process is tocontrol the amount, size, shape, and distribution of thecementite particles in the ferrite, which in turn determinesthe physical properties of the steel. (c) Coláiste Lorcain 40
  41. 41. Many variations of the basic process are practiced.Metallurgists have discovered that the change from austeniteto martensite occurs during the latter part of the coolingperiod and that this change is accompanied by a change involume that may crack the metal if the cooling is too swift.Three comparatively new processes have been developed toavoid cracking. In time-quenching the steel is withdrawn fromthe quenching bath when it has reached the temperature atwhich the martensite begins to form, and is then cooledslowly in air. In martempering the steel is withdrawn from thequench at the same point, and is then placed in a constant-temperature bath until it attains a uniform temperaturethroughout its cross section. (c) Coláiste Lorcain 41
  42. 42. The steel is then allowed to cool in air through thetemperature range of martensite formation, which formost steels is the range from about 288° C (about550° F) to room temperature. In austempering thesteel is quenched in a bath of metal or salt maintainedat the constant temperature at which the desiredstructural change occurs and is held in this bath untilthe change is complete before being subjected to thefinal cooling. (c) Coláiste Lorcain 42
  43. 43. Other methods of heat treating steel to harden it are used.In case hardening, a finished piece of steel is given anextremely hard surface by heating it with carbon ornitrogen compounds. These compounds react with thesteel, either raising the carbon content or forming nitridesin its surface layer. In carburizing, the piece is heated incharcoal or coke, or in carbonaceous gases such asmethane or carbon monoxide. Cyaniding consists ofhardening in a bath of molten cyanide salt to form bothcarbides and nitrides. In nitriding, steels of specialcomposition are hardened by heating them in ammoniagas to form alloy nitrides. (c) Coláiste Lorcain 43