where did the steel begin? and what was its origin?

Answer 1

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Answer 2

iron ore is heated to the maximum. Then the fragments of the metal are combined with other chemicals. Then the heated ore called hot ingots are rolled into shape, then once it is rolled into shape, the shapes are rolled thinner and thinner into sheets. The sheets are then rolled into different thicknesses each thickness is a different guage of steel, used for a different purpose. then the sheets are rolled into rolls of steel.

Answer 3

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Answer 4

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Answer 5

Iron and Steel Manufacture
I INTRODUCTION

Iron and Steel Manufacture, technology 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 steel are sometimes confusing because of the nomenclature used. Steel in general is an alloy of iron and carbon, often with an admixture of other elements. Some alloys that are commercially called irons contain more carbon than commercial steels. Open-hearth iron and wrought iron contain only a few hundredths of 1 percent of carbon. Steels of various types contain from 0.04 percent to 2.25 percent of carbon. Cast iron, malleable cast iron, and pig iron contain amounts of carbon varying from 2 to 4 percent. A special form of malleable iron, containing virtually no carbon, is known as white-heart malleable iron. A special group of iron alloys, known as ferroalloys, is used in the manufacture of iron and steel alloys; they contain from 20 to 80 percent of an alloying element, such as manganese, silicon, or chromium.

II HISTORY

The exact date at which people discovered the technique of smelting iron ore to produce usable metal is not known. The earliest iron implements discovered by archaeologists in Egypt date from about 3000 bc, and iron ornaments were used even earlier; the comparatively advanced technique of hardening iron weapons by heat treatment was known to the Greeks about 1000 BC.

The alloys produced by early iron workers, and, indeed, all the iron alloys made until about the 14th century ad, would be classified today as wrought iron. They were made by heating a mass of iron ore and charcoal in a forge or furnace having a forced draft. Under this treatment the ore was reduced to the sponge of metallic iron filled with a slag composed of metallic impurities and charcoal ash. This sponge of iron was removed from the furnace while still incandescent and beaten with heavy sledges to drive out the slag and to weld and consolidate the iron. The iron produced under these conditions usually contained about 3 percent of slag particles and 0.1 percent of other impurities. Occasionally this technique of ironmaking produced, by accident, a true steel rather than wrought iron. Ironworkers learned to make steel by heating wrought iron and charcoal in clay boxes for a period of several days. By this process the iron absorbed enough carbon to become a true steel.

After the 14th century the furnaces used in smelting were increased in size, and increased draft was used to force the combustion gases through the “charge,” the mixture of raw materials. In these larger furnaces, the iron ore in the upper part of the furnace was first reduced to metallic iron and then took on more carbon as a result of the gases forced through it by the blast. The product of these furnaces was pig iron, an alloy that melts at a lower temperature than steel or wrought iron. Pig iron (so called because it was usually cast in stubby, round ingots known as pigs) was then further refined to make steel.

Modern steelmaking employs blast furnaces that are merely refinements of the furnaces used by the old ironworkers. The process of refining molten iron with blasts of air was accomplished by the British inventor Sir Henry Bessemer who developed the Bessemer furnace, or converter, in 1855. Since the 1960s, several so-called minimills have been producing steel from scrap metal in electric furnaces. Such mills are an important component of total U.S. steel production. The giant steel mills remain essential for the production of steel from iron ore.

III PIG-IRON PRODUCTION

The basic materials used for the manufacture of pig iron are iron ore, coke, and limestone. The coke is burned as a fuel to heat the furnace; as it burns, the coke gives off carbon monoxide, which combines with the iron oxides in the ore, reducing them to metallic iron. This is the basic chemical reaction in the blast furnace; it has the equation: Fe2O3 + 3CO = 3CO2 + 2Fe. The limestone in the furnace charge is used as an additional source of carbon monoxide and as a “flux” to combine with the infusible silica present in the ore to form fusible calcium silicate. Without the limestone, iron silicate would be formed, with a resulting loss of metallic iron. Calcium silicate plus other impurities form a slag that floats on top of the molten metal at the bottom of the furnace. Ordinary pig iron as produced by blast furnaces contains iron, about 92 percent; carbon, 3 or 4 percent; silicon, 0.5 to 3 percent; manganese, 0.25 to 2.5 percent; phosphorus, 0.04 to 2 percent; and a trace of sulfur.

A typical blast furnace consists of a cylindrical steel shell lined with a refractory, which is any nonmetallic substance such as firebrick. The shell is tapered at the top and at the bottom and is widest at a point about one-quarter of the distance from the bottom. The lower portion of the furnace, called the bosh, is equipped with several tubular openings or tuyeres through which the air blast is forced. Near the bottom of the bosh is a hole through which the molten pig iron flows when the furnace is tapped, and above this hole, but below the tuyeres, is another hole for draining the slag. The top of the furnace, which is about 27 m (about 90 ft) in height, contains vents for the escaping gases, and a pair of round hoppers closed with bell-shaped valves through which the charge is introduced into the furnace. The materials are brought up to the hoppers in small dump cars or skips that are hauled up an inclined external skip hoist.

Blast furnaces operate continuously. The raw material to be fed into the furnace is divided into a number of small charges that are introduced into the furnace at 10- to 15-min intervals. Slag is drawn off from the top of the melt about once every 2 hr, and the iron itself is drawn off or tapped about five times a day.

The air used to supply the blast in a blast furnace is preheated to temperatures between approximately 540° and 870° C (approximately 1000° and 1600° F). The heating is performed in stoves, cylinders containing networks of firebrick. The bricks in the stoves are heated for several hours by burning blast-furnace gas, the waste gases from the top of the furnace. Then the flame is turned off and the air for the blast is blown through the stove. The weight of air used in the operation of a blast furnace exceeds 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 furnace vents, the pressure within the furnace may be built up to 1.7 atm or more. The pressurizing technique makes possible better combustion of the coke and higher output of pig iron. The output of many blast furnaces can be increased 25 percent by pressurizing. Experimental installations have also shown that the output of blast furnaces can be increased by enriching the air blast with oxygen.

The process of tapping consists of knocking out a clay plug from the iron hole near the bottom of the bosh and allowing the molten metal 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 car capable of holding as much as 100 tons of metal. Any slag that may flow from the furnace with the metal is skimmed off before it reaches the container. The container of molten pig iron is then transported to the steelmaking shop.

Modern-day blast furnaces are operated in conjunction with basic oxygen furnaces and sometimes the older open-hearth furnaces as part of a single steel-producing plant. In such plants the molten pig iron is used to charge the steel furnaces. The molten metal from several blast furnaces may be mixed in a large ladle before it is converted to steel, to minimize any irregularities in the composition of the individual melts.

IV OTHER METHODS OF IRON REFINING

Although almost all the iron and steel manufactured in the world is made from pig iron produced by the blast-furnace process, other methods 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 this process 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. The secondary reactions that occur in a blast furnace, however, do not occur, 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 electric current through a solution of ferrous chloride. Neither the direct nor the electrolytic processes has yet achieved any great commercial significance.

V OPEN-HEARTH PROCESS

Essentially the production of steel from pig iron by any process consists of burning out the excess carbon and other impurities present in the iron. One difficulty in the manufacture of steel is its high melting point, about 1370° C (about 2500° F), which prevents the use of ordinary fuels and furnaces. To overcome this difficulty the open-hearth furnace was developed; this furnace can be operated at a high temperature by regenerative preheating of the fuel gas and air used for combustion in the furnace. In regenerative preheating, the exhaust gases from the furnace are drawn through one of a series of chambers containing a mass of brickwork and give up most of their heat to the bricks. Then the flow through the furnace is reversed and the fuel and air pass through the heated chambers and are warmed by the bricks. Through this method open-hearth furnaces can reach temperatures as high as 1650° C (approximately 3000° F).

The furnace itself consists typically of a flat, rectangular brick hearth about 6 m by 10 m (about 20 ft by 33 ft), which is roofed over at a height of about 2.5 m (about 8 ft). In front of the hearth a series of doors opens out onto a working floor in front of the hearth. The entire hearth and working floor are one story above ground level, and the space 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.

The furnace is charged with a mixture of pig iron (either molten or cold), scrap steel, and iron ore that provides additional oxygen. Limestone is added for flux and fluorspar to make the slag more fluid. The proportions of the charge vary within wide limits, but a typical charge might 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) of limestone, 900 kg (2000 lb) of iron ore, and 230 kg (500 lb) of fluorspar. After the furnace has been charged, the furnace is lighted and the flames play back and forth over the hearth as their direction is reversed by the operator to provide heat regeneration.

Chemically the action of the open-hearth furnace consists of lowering the carbon content of the charge by oxidization and of removing such impurities as silicon, phosphorus, manganese, and sulfur, which combine with the limestone to form slag. These reactions take place while the metal in the furnace is at melting heat, and the furnace is held between 1540° and 1650° C (2800° and 3000° F) for many hours until the molten metal has the desired carbon content. Experienced open-hearth operators can often judge the carbon content of the metal by its appearance, but the melt is usually tested by withdrawing a small amount of metal from the furnace, cooling it, and subjecting it to physical examination or chemical analysis. When the carbon content of the melt reaches the desired level, the furnace is tapped through a hole at the rear. The molten steel then flows through a short trough to a large ladle set below the furnace at ground level. From the ladle the 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 material for all forms of fabricated steel, weigh approximately 2.25 metric tons in this size. Recently, methods have been put into practice for the continuous processing of steel without first having to go through the process of casting ingots.

VI BASIC OXYGEN PROCESS

The oldest process for making steel in large quantities, the Bessemer process, made use of a tall, pear-shaped furnace, called a Bessemer converter, that could be tilted sideways for charging and pouring. Great quantities of air were blown through the molten metal; its oxygen united chemically with the impurities and carried them off.

In the basic oxygen process, steel is also refined in a pear-shaped furnace that tilts sideways for charging and pouring. Air, however, has been replaced by a high-pressure stream of nearly pure oxygen. After the furnace has been charged and turned upright, an oxygen lance is lowered into 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 to requirements. Thousands of cubic meters of oxygen are blown into the furnace at supersonic speed. The oxygen combines with carbon and other unwanted elements and starts a high-temperature churning reaction that rapidly burns out impurities from the pig iron and converts it into steel. The refining process takes 50 min or less; approximately 275 metric tons of steel can be made in an hour.

VII ELECTRIC-FURNACE STEEL

In some furnaces, electricity instead of fire supplies the heat for the melting and refining of steel. Because refining conditions in such a furnace can be regulated more strictly than in open-hearth or basic oxygen furnaces, electric furnaces are particularly valuable for producing stainless steels and other highly alloyed steels that must be made to exacting specifications. Refining takes place in a tightly closed chamber, where temperatures and other conditions are kept under rigid control by automatic devices. During the early stages of this refining process, high-purity oxygen is injected through a lance, raising the temperature of the furnace and decreasing the time needed to produce the finished steel. The quantity of oxygen entering the furnace can always be closely controlled, thus keeping down undesirable oxidizing reactions.

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 dry lime, 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 the electrodes, 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. See Electric Furnace.

VIII FINISHING PROCESSES

Steel is marketed in a wide variety of sizes and shapes, such as rods, pipes, railroad rails, tees, channels, and I-beams. These shapes are produced at steel mills by rolling and otherwise forming heated ingots to the required shape. The working of steel also improves the quality of the steel by refining its crystalline structure and making the metal tougher.

The basic process of working steel is known as hot rolling. In hot rolling the cast ingot is first heated to bright-red heat in a furnace called a soaking pit and is then passed between a series of pairs of metal rollers that squeeze it to the desired size and shape. The distance between the rollers diminishes for each successive pair as the steel is elongated and reduced in thickness.

The first pair of rollers through which the ingot passes is commonly called the blooming mill, and the square billets of steel that the ingot produces are known as blooms. From the blooming mill, the steel is passed on to roughing mills and finally to finishing mills that reduce it to the correct cross section. The rollers of mills used to produce railroad rails and such structural shapes as I-beams, H-beams, and angles are grooved to give the required shape.

Modern manufacturing requires a large amount of thin sheet steel. Continuous mills roll steel strips and sheets in widths of up to 2.4 m (8 ft). Such mills process thin sheet steel rapidly, before it cools and becomes unworkable. A slab of hot steel over 11 cm (about 4.5 in) thick is fed through a series of rollers which reduce it progressively in thickness to 0.127 cm (0.05 inc) and increase its length from 4 m (13 ft) to 370 m (1210 ft). Continuous mills are equipped with a number of accessory devices including edging rollers, descaling devices, and devices for coiling the sheet automatically when it reaches the end of the mill. The edging rollers are sets of vertical rolls set opposite each other at either side of the sheet to ensure that the width of the sheet is maintained. Descaling apparatus removes the scale that forms on the surface of the sheet by knocking it off mechanically, loosening it by means of an air blast, or bending the sheet sharply at some point in its travel. The completed coils of sheet are dropped on a conveyor and carried away to be annealed and cut into individual sheets. A more efficient way to produce thin sheet steel is to feed thinner slabs through the rollers. Using conventional casting methods, ingots must still be passed through a blooming mill in order to produce slabs thin enough to enter a continuous mill.

By devising a continuous casting system that produces an endless steel slab less than 5 cm (2 in) thick, German engineers have eliminated any need for blooming and roughing mills. In 1989, a steel mill in Indiana became the first outside Europe to adopt this new system.

IX PIPE

Cheaper grades of pipe are shaped by bending a flat strip, or skelp, of hot steel into cylindrical form and welding the edges to complete the pipe. For the smaller sizes of pipe, the edges of the skelp are usually overlapped and passed between a pair of rollers curved to correspond with the outside diameter of the pipe. The pressure on the rollers is great enough to weld the edges together. Seamless pipe or tubing is made from solid rods by passing them between a pair of inclined rollers that have a pointed metal bar, or mandrel, set between them in such a way that it pierces the rods and forms the inside diameter of the pipe at the same time that the rollers are forming the outside diameter.

X TIN PLATE

By far the most important coated product of the steel mill is tin plate for the manufacture of containers. The “tin” can is actually more than 99 percent steel. In some mills steel sheets that have been hot-rolled and then cold-rolled are coated by passing them through a bath of molten tin. The most common method of coating is by the electrolytic process. Sheet steel is slowly unrolled from its coil and passed through a chemical solution. Meanwhile, a current of electricity is passing through a piece of pure tin into the same solution, causing the tin to dissolve slowly and to be deposited on the steel. In electrolytic processing, less than half a kilogram of tin will coat more than 18.6 sq m (more than 200 sq ft) of steel. For the product known as thin tin, sheet and strip are given a second cold rolling before being coated with tin, a treatment that makes the steel plate extra tough as well as extra thin. Cans made of thin tin are about as strong as ordinary tin cans, yet they contain less steel, with a resultant saving in weight and cost. Lightweight packaging containers are also being made of tin-plated steel foil that has been laminated to paper or cardboard.

Other processes of steel fabrication include forging, founding, and drawing the steel through dies (see Die).

XI WROUGHT IRON

The process of making the tough, malleable alloy known as wrought iron differs markedly from other forms of steel making. Because this process, known as puddling, required a great deal of hand labor, production of wrought iron in tonnage quantities was impossible. The development of new processes using Bessemer converters and open-hearth furnaces allowed the production of larger quantities of wrought iron.

Wrought iron is no longer produced commercially, however, because it can be effectively replaced in nearly all applications by low-carbon steel, which is less expensive to produce and is typically of more uniform quality than wrought iron.

The puddling furnace used in the older process has a low, arched roof and a depressed hearth on which the crude metal lies, separated by a wall from the combustion chamber in which bituminous coal is burned. The flame in the combustion chamber surmounts the wall, strikes the arched roof, and “reverberates” upon the contents of the hearth. After the furnace is lit and has become moderately heated, the puddler, or furnace operator, “fettles” it by plastering the hearth and walls with a paste of iron oxide, usually hematite ore. The furnace is then charged with about 270 kg (about 600 lb) of pig iron and the door is closed. After about 30 min the iron is melted and the puddler adds more iron oxide or mill scale to the charge, working the oxide into the iron with a bent iron bar called a raddle. The silicon and most of the manganese in the iron are oxidized and some sulfur and phosphorus are eliminated. The temperature of the furnace is then raised slightly, and the carbon starts to burn out as carbon-oxide gases. As the gas is evolved the slag puffs up and the level of the charge rises. As the carbon is burned away the melting temperature of the alloy increases and the charge becomes more and more pasty, and finally the bath drops to its former level. As the iron increases in purity, the puddler stirs the charge with the raddle to ensure uniform composition and proper cohesion of the particles. The resulting pasty, spongelike mass is separated into lumps, called balls, of about 80 to 90 kg (about 180 to 200 lb) each. The balls are withdrawn from the furnace with tongs and are placed directly in a squeezer, a machine in which the greater part of the intermingled siliceous slag is expelled from the ball and the grains of pure iron are thoroughly welded together. The iron is then cut into flat pieces that are piled on one another, heated to welding temperature, and then rolled into a single piece. This rolling process is sometimes repeated to improve the quality of the product.

The modern technique of making wrought iron uses molten iron from a Bessemer converter and molten slag, which is usually prepared by melting iron ore, mill scale, and sand in an open-hearth furnace. The molten slag is maintained in a ladle at a temperature several hundred degrees below the temperature of the molten iron. When the molten iron, which carries a large amount of gas in solution, is poured into the ladle containing the molten slag, the metal solidifies almost instantly, releasing the dissolved gas. The force exerted by the gas shatters the metal into minute particles that are heavier than the slag and that accumulate in the bottom of the ladle, agglomerating into a spongy mass similar to the balls produced in a puddling furnace. After the slag has been poured off the top of the ladle, the ball of iron is removed and squeezed and rolled like the product of the puddling furnace.

XII CLASSIFICATIONS OF STEEL

Steels are grouped into five main classifications.

A Carbon Steels

More than 90 percent of all steels are carbon steels. They contain varying amounts of carbon and not more than 1.65 percent manganese, 0.60 percent silicon, and 0.60 percent copper. Machines, automobile bodies, most structural steel for buildings, ship hulls, bedsprings, and bobby pins are among the products made of carbon steels.

B Alloy Steels

These steels have a specified composition, containing certain percentages of vanadium, molybdenum, or other elements, as well as larger amounts of manganese, silicon, and copper than do the regular carbon steels. Automobile gears and axles, roller skates, and carving knives are some of the many things that are made of alloy steels.

C High-Strength Low-Alloy Steels

Called HSLA steels, they are the newest of the five chief families of steels. They cost less than the regular alloy steels because they contain only small amounts of the expensive alloying elements. They have been specially processed, however, to have much more strength than carbon steels of the same weight. For example, freight cars made of HSLA steels can carry larger loads because their walls are thinner than would be necessary with carbon steel of equal strength; also, because an HSLA freight car is lighter in weight than the ordinary car, it is less of a load for the locomotive to pull. Numerous buildings are now being constructed with frameworks of HSLA steels. Girders can be made thinner without sacrificing their strength, and additional space is left for offices and apartments.

D Stainless Steels

Stainless steels contain chromium, nickel, and other alloying elements that keep them bright and rust resistant in spite of moisture or the action of corrosive acids and gases. Some stainless steels are very hard; some have unusual strength and will retain that strength for long periods at extremely high and low temperatures. Because of their shining surfaces architects often use them for decorative purposes. Stainless steels are used for the pipes and tanks of petroleum refineries and chemical plants, for jet planes, and for space capsules. Surgical instruments and equipment are made from these steels, and they are also used to patch or replace broken bones because the steels can withstand the action of body fluids. In kitchens and in plants where food is prepared, handling equipment is often made of stainless steel because it does not taint the food and can be easily cleaned.

E Tool Steels

These steels are fabricated into many types of tools or into the cutting and shaping parts of power-driven machinery for various manufacturing operations. They contain tungsten, molybdenum, and other alloying elements that give them extra strength, hardness, and resistance to wear.

XIII STRUCTURE OF STEEL

The physical properties of various types of steel and of any given steel alloy at varying temperatures depend primarily on the amount of carbon present and on how it is distributed in the iron. Before heat treatment most steels are a mixture of three substances: ferrite, pearlite, and cementite. Ferrite is iron containing small amounts of carbon and other elements in solution and is soft and ductile. Cementite, a compound of iron containing about 7 percent carbon, is extremely brittle and hard. Pearlite is an intimate mixture of ferrite and cementite having a specific composition and characteristic structure, and physical characteristics intermediate between its two constituents. The toughness and hardness of a steel that is not heat treated depend on the proportions of these three ingredients. As the carbon content of a steel increases, the amount of ferrite present decreases and the amount of pearlite increases until, when the steel has 0.8 percent of carbon, it is entirely composed of pearlite. Steel with still more carbon is a mixture of pearlite and cementite. Raising the temperature of steel changes ferrite and pearlite to an allotropic form of iron-carbon alloy known as austenite, which has the property of dissolving all the free carbon present in the metal. If the steel is cooled slowly the austenite reverts to ferrite and pearlite, but if cooling is sudden, the austenite is “frozen” or changes to martensite, which is an extremely hard allotropic modification that resembles ferrite but contains carbon in solid solution.

XIV HEAT TREATMENT OF STEEL

The basic process of hardening steel by heat treatment consists of heating the metal to a temperature at which austenite is formed, usually about 760° to 870° C (about 1400° to 1600° F) and then cooling, or quenching, it rapidly in water or oil. Such hardening treatments, which form martensite, set up large internal strains in the metal, and these are relieved by tempering, or annealing, which consists of reheating the steel to a lower temperature. Tempering results in a decrease in hardness and strength and an increase in ductility and toughness.

The primary purpose of the heat-treating process is to control the amount, size, shape, and distribution of the cementite particles in the ferrite, which in turn determines the physical properties of the steel.

Many variations of the basic process are practiced. Metallurgists have discovered that the change from austenite to martensite occurs during the latter part of the cooling period and that this change is accompanied by a change in volume that may crack the metal if the cooling is too swift. Three comparatively new processes have been developed to avoid cracking. In time-quenching the steel is withdrawn from the quenching bath when it has reached the temperature at which the martensite begins to form, and is then cooled slowly in air. In martempering the steel is withdrawn from the quench at the same point, and is then placed in a constant-temperature bath until it attains a uniform temperature throughout its cross section. The steel is then allowed to cool in air through the temperature range of martensite formation, which for most steels is the range from about 288° C (about 550° F) to room temperature. In austempering the steel is quenched in a bath of metal or salt maintained at the constant temperature at which the desired structural change occurs and is held in this bath until the change is complete before being subjected to the final cooling.

Other methods of heat treating steel to harden it are used. In case hardening, a finished piece of steel is given an extremely hard surface by heating it with carbon or nitrogen compounds. These compounds react with the steel, either raising the carbon content or forming nitrides in its surface layer. In carburizing, the piece is heated in charcoal or coke, or in carbonaceous gases such as methane or carbon monoxide. Cyaniding consists of hardening in a bath of molten cyanide salt to form both carbides and nitrides. In nitriding, steels of special composition are hardened by heating them in ammonia gas to form alloy nitrides.