Steelsarealloysofironand other elements, primarilycarbon, widely used in construction and other applications because of their high tensile strengthsand low costs. Carbon, other elements, and inclusions within iron act as hardening agents that prevent the movement of dislocationsthat otherwise occur in thecrystal latticesof iron atoms.The carbon in typical steel alloys may contribute up to 2.1% of its weight. Varying the amount of alloying elements, their formation in the steel either as solute elements, or as precipitated phases, retards the movement of those dislocations that make iron comparatively ductile and weak, and thus controls qualities such as thehardness,ductility, and tensile strength of the resulting steel. Steel's strength compared to pure iron is only possible at the expense ofductility, of which iron has an excess.Although steel had been produced inbloomeryfurnaces for thousands of years, steel's use expanded extensively after more efficient production methods were devised in the 17th century forblister steeland thencrucible steel. With the invention of theBessemer process in the mid-19th century, a new era ofmass-producedsteel began. This was followed bySiemens-Martin processand thenGilchrist-Thomas processthat refined the quality of steel. With their introductions, mild steel replacedwrought iron.Further refinements in the process, such asbasic oxygen steelmaking(BOS), largely replaced earlier methods by further lowering the cost of production and increasing the quality of the metal. Today, steel is one of the most common materials in the world, with more than 1.3 billion tons produced annually. It is a major component in buildings, infrastructure, tools, ships,automobiles, machines, appliances, and weapons. Modern steel is generally identified by various grades defined by assortedstandards organizations.Definitions and related materialsThe carbon content of steel is between 0.002% and 2.1% by weight for plain iron-carbon alloys. These values vary depending onalloying elementssuch asmanganese, chromium,nickel,iron,tungsten,carbonand so on. Basically, steel is an iron-carbon alloy that does not undergoeutectic reaction. In contrast,cast irondoes undergo eutectic reaction, suddenly solidifying into solid phases at exactly the same temperature. Too little carbon content leaves (pure) iron quite soft, ductile, and weak. Carbon contents higher than those of steel make an alloy, commonly calledpig iron, that is brittle (not malleable). While iron alloyed with carbon is called carbon steel,alloy steelis steel to which other alloying elements have been intentionally added to modify the characteristics of steel. Common alloying elements include:manganese,nickel,chromium,molybdenum,boron,titanium,vanadium,tungsten,cobalt, andniobium.Additional elements are also important in steel:phosphorus,sulphur,silicon, and traces ofoxygen,nitrogen, andcopper.Alloys with a higher than 2.1% carbon content, depending on other element content and possibly on processing, are known ascast iron. Cast iron is not malleable even when hot, but it can be formed bycastingas it has a lowermelting pointthan steel and goodcastabilityproperties. Certain compositions of cast iron, while retaining the economies of melting and casting, can be heat treated after casting to makemalleable ironorductile ironobjects. Steel is also distinguishable fromwrought iron(now largely obsolete), which may contain a small amount of carbon but large amounts ofslag.Material properties Iron-carbonphase diagram, showing the conditions necessary to form different phasesIron is commonly found in the Earth'scrustin the form of anore, usually an iron oxide, such asmagnetite, hematiteetc. Iron is extracted fromiron oreby removing the oxygen through combination with a preferred chemical partner such as carbon that is lost to the atmosphere as carbon dioxide. This process, known assmelting, was first applied to metals with lowermeltingpoints, such astin, which melts at approximately 250C (482F) andcopper, which melts at approximately 1,100C (2,010F). In comparison, cast iron melts at approximately 1,375C (2,507F).[2]Small quantities of iron were smelted in ancient times, in the solid state, by heating the ore buried in acharcoalfire and welding the metal together with a hammer, squeezing out the impurities. With care, the carbon content could be controlled by moving it around in the fire.All of these temperatures could be reached with ancient methods that have been used since theBronze Age. Since the oxidation rate of iron increases rapidly beyond 800C (1,470F), it is important that smelting take place in a low-oxygen environment. Unlike copper and tin, liquid or solid iron dissolves carbon quite readily. Smelting, using carbon to reduce iron oxides, results in an alloy (pig iron) that retains too much carbon to be called steel.[2]The excess carbon and other impurities are removed in a subsequent step.Other materials are often added to the iron/carbon mixture to produce steel with desired properties.Nickel andmanganesein steel add to its tensile strength and make theausteniteform of the iron-carbon solution more stable,chromiumincreases hardness and melting temperature, andvanadiumalso increases hardness while making it less prone tometal fatigue. To inhibit corrosion, at least 11% chromium is added to steel so that a hardoxideforms on the metal surface; this is known asstainless steel. Tungsten interferes with the formation ofcementite, allowingmartensiteto preferentially form at slower quench rates, resulting inhigh speed steel. On the other hand, sulphur,nitrogen, andphosphorusmake steel more brittle, so these commonly found elements must be removed from the steel melt during processing. Thedensityof steel varies based on the alloying constituents but usually ranges between 7,750 and 8,050kg/m3(484 and 503lb/cuft), or 7.75 and 8.05g/cm3(4.48 and 4.65oz/cuin). Even in a narrow range of concentrations of mixtures of carbon and iron that make a steel, a number of different metallurgical structures, with very different properties can form. Understanding such properties is essential to making quality steel. Atroom temperature, the most stable form of pure iron is thebody-centered cubic(BCC) structure called ferriteor -iron. It is a fairly soft metal that can dissolve only a small concentration of carbon, no more than 0.005% at 0C (32F) and 0.021 wt% at 723C (1,333F). At 910C pure iron transforms into aface-centered cubic(FCC) structure, calledausteniteor -iron. The FCC structure of austenite can dissolve considerably more carbon, as much as 2.1%[5](38 times that of ferrite) carbon at 1,148C (2,098F), which reflects the upper carbon content of steel, beyond which is cast iron. When steels with less than 0.8% carbon (known as a hypoeutectoid steel), are cooled, theausteniticphase (FCC) of the mixture attempts to revert to the ferrite phase (BCC). The carbon no longer fits within the FCC structure, resulting in an excess of carbon. One way for carbon to leave theausteniteis for it toprecipitateout of solution ascementite, leaving behind a surrounding phase of BCC iron that is low enough in carbon to take the form of ferrite, resulting in a ferrite matrix with cementite inclusions. Cementite is a hard and brittleintermetallic compoundwith thechemical formulaof Fe3C. At theeutectoid, 0.8% carbon, the cooled structure takes the form ofpearlite, named for its resemblance to mother. On a larger scale, it appears as a lamellar structure of ferrite and cementite. For steels that have more than 0.8% carbon, the cooled structure takes the form of pearlite and cementite. Perhaps the most importantpolymorphic formof steel ismartensite, a metastable phase that is significantly stronger than other steel phases. When the steel is in an austenitic phase and thenquenchedrapidly, it forms into martensite, as the atoms "freeze" in place when the cell structure changes from FCC to a distorted form of BCC as the atoms do not have time enough to migrate and form the cementite compound. Depending on the carbon content, the martensitic phase takes different forms. Below approximately 0.2% carbon, it takes an ferrite BCC crystal form, but at higher carbon content it takes abody-centered tetragonal(BCT) structure. There is no thermalactivation energyfor the transformation from austenite to martensite. Moreover, there is no compositional change so the atoms generally retain their same neighbours. Martensite has a lower density than does austenite, so that the transformation between them results in a change of volume. In this case, expansion occurs. Internal stresses from this expansion generally take the form ofcompressionon the crystals of martensite andtensionon the remaining ferrite, with a fair amount ofshearon both constituents. If quenching is done improperly, the internal stresses can cause a part to shatter as it cools. At the very least, they cause internalwork hardeningand other microscopic imperfections. It is common for quench cracks to form when steel is water quenched, although they may not always be visible. Heat treatmentThere are many types ofheat treatingprocesses available to steel. The most common areannealing, quenching, andtempering. Annealing is the process of heating the steel to a sufficiently high temperature to soften it. This process goes through three phases:recovery,recrystallization, andgrain growth. The temperature required to anneal steel depends on the type of annealing to be achieved and the constituents of the alloy. Quenching and tempering first involves heating the steel to theaustenitephase then quenching it inwateroroil. This rapid cooling results in a hard but brittle martensitic structure. The steel is then tempered, which is just a specialized type of annealing, to reduce brittleness. In this application the annealing (tempering) process transforms some of the martensite into cementite, orspheroiditeand hence reduces the internal stresses and defects. The result is a more ductile and fracture-resistant steel. Steel productionWhen iron is smelted from its ore, it contains more carbon than is desirable. To become steel, it must be reprocessed to reduce the carbon to the correct amount, at which point other elements can be added. In the past, steel facilities would cast the raw cast iron product into ingots which would be stored until use in further refinement processes that resulted in the final product. In modern facilities, the initial product is close to the final composition and iscontinuously castinto long slabs, cut and shaped into bars and extrusions and heat treated to produce a final product. Today only a small fraction iscastintoingots. Approximately 96% of steel is continuously cast, while only 4% is produced as ingots. The ingots are then heated in a soaking pit andhot rolledinto slabs,blooms, orbillets. Slabs are hot orcold rolledintosheet metalor plates. Billets are hot or cold rolled into bars, rods, and wire. Blooms are hot or cold rolled intostructural steel, such asI-beamsand rails. In modern steel mills these processes often occur in oneassembly line, with ore coming in and finished steel products coming out. Sometimes after steels final rolling it is heat treated for strength, however this is relatively rare. Carbon steel can be prepared at the point of origin for use in various building and metalworking applications in several different ways and there are subtle structural and mechanical differences between cast steel, forged steel and wrought steel. These differences can help to determine what applications the metals can safely be used in, as well as what strengths and weaknesses the metals will have. Most people know that cast metals are metals that are made molten and then poured into a mold, which leads to their final finished shapes, but most people do not know the difference between wrought steel and forged steel.Wrought Steel Starts as Cast Steel Wrought steel begins life as cast steel that is poured into ingots or moulds. After it has cooled, the steel is reheated and rolled into a final finished shape. This means that wrought steel has some of the structural properties of cast steel, such as additional weight of metal required to have the same structural integrity as a similar piece of forged steel. Wrought steel is also less likely to be used in high-tension applications and it may be harder and more brittle than forged steel.Forged Steel Can Be More Durable According to Steel Forge.com, forged steel is more durable in certain applications because, although it begins life as a casting as well, it is hammer forged using large hydraulic hammers that force the atoms and molecules of the steel into alignment as they hit it. Wrought steel does not undergo this same process, which makes forged steel harder and less likely to crack when struck. Striking tools and axes are often made of forged steel because they are used to hit things, and the brittle nature of a cast steel would lead them to breaking rapidly if they were not forged.Carbon Mixtures The amount of carbon used in the mixture determines the final strength of a piece of wrought or forged steel. There are as many possible blends for carbon mixtures as there are applications for the finished metal. This is true for both wrought and forged steel, which can be made either harder or softer depending on the particular blend chosen. However, forged steel tends to be more flexible than wrought steel in the final product regardless of the blend, which can also contribute to making it less susceptible to breakage.Fine Steel versus Course Steel Fine grained steels are tougher than coarse grained steels when it comes to the hardness. The graining of the steel depends on the method by which the steel was forged or wrought and the composition of the steel. Either forged or wrought steel can be made with either hard or coarse grains. Forged steel may be, but isn't necessarily always, more fine grained, in part because of the hammer-forging process.

Definitions of what is meant by cast iron, wrought iron and steelCast iron, wrought iron and steel are all essentially alloys of iron and carbon.Although the actual situation is much more complex, cast iron, wrought iron and steel can all be thought of as alloys, principally of iron and carbon. To complicate matters, though, it is worth mentioning that there are no precise definitions of the relative make -up of the three types of steel.Iron is extracted from naturally occurring ores and we can think of these ores as providing the source material, iron oxide (FeO).In the early days of iron and steel production iron oxide ores were mined but the sources of iron oxide have now been worked out.When iron oxide is heated at high temperatures it becomes transformed into iron.When iron oxide is heated at high temperatures of 1600 to 3000F the oxide is reduced to the metal and the resulting reaction can be expressed as:Iron Oxide + Carbonheated along with a blast of air yieldsIron + Carbon Monoxide (a gas released into the air)i.e. FeO + C > Fe + COIn practice, this process does not yield pure iron, but an impure product called pig iron. This pig iron contains impurities such as Iron Carbide (Fe3C) which make the material hard and brittle. It is, however, the raw material from which cast iron, wrought iron and steel can be produced.Pig iron, which is essentially cast iron, derives its name from the shape of the casting beds which resemble piglets being suckled by the sow and the pigs were the billets as supplied to the foundries.Cast iron is the material produced by remelting this iron (known as pig iron), possibly along with some scrap iron.The remelting of pig iron, and scrap iron, whilst blowing air into the molten mass until the Carbon content is between 2.4 and 4.0% produces Contemporary Cast Iron which can exist in two forms: grey (Graphite) cast iron and white (Iron Carbide) cast iron.In the early days of cast iron production, it was difficult to control the level of carbon and other impurities such as sulphur (which has a particularly detrimental effect on the properties of iron). This means that the strength and properties of the material were very much a hit-and-miss affair. Nor was it possible to be sure that the molten material had been able to flow through all of the mould before setting. Consequently, the early cast iron structural elements were often load tested before being used in a building. And putty was sometimes used to plug holes in large section such as circular columns.Wrought iron is achieved by simple reprocessing of cast iron.The strength deficiencies of cast iron were eventually partly addressed by the development of a process termed "puddling". This involved reheating cast iron and manually mixing air in with the molten mass. Because of the nature of the puddling process the volumes that could be produced by this process at any one time were small. This, in turn, limited the size of structural components that could be made of this type of iron.The material produced this way had reasonably high tensile strengths and was much more ductile than cast iron.The process of producing wrought iron improves the tensile strength. This made it suitable for beams, and the ductility meant that its behaviour in column elements was more predictable than cast iron. However, its use in columns was rare due to the comparative cost of cast and wrought iron.The production of true wrought iron in Britain ceased in 1973, so what is marketed today under that name is either old material recycled or a type of mild steel.The invention of the Bessemer process allowed the oxidisation process after remelting to be carefully controlled and the carbon content could therefore be held at a particular level, providing good tensile strength and ductility.In what we refer to today as steel the carbon content will typically be below 1%. For most structural steel the actual value will be in the region of 0.2%. It is the addition of elements such as silicon and manganese that allow the carbon levels to be controlled with some accuracy, and the manganese also has the beneficial effect of neutralising the otherwise harmful effects of sulphur.The resulting material has equally high tensile and compressive strengths along with a high degree of ductility.The rolling process for shaping steel sectionsThere is a wide range of steels which can be classified in various ways.The terminology relating to the classification of different steel types is not precise. Broadly speaking steels are described in the following table.Mild Steelsup to 0.3% Carbon

Medium Carbon Steels(or simply Carbon Steels)0.3 to 0.6 % carbon

High Carbon Steelsover 0.6% Carbon

To form steel into the kind of sections used in structures ingots are heated and then forged or rolled repeatedly, each repetition getting closer to the desired cross sectional shape.

Carbon Steel vs Stainless SteelSteel is an alloy made out of iron and carbon. The carbon percentage can vary depending on the grade, and mostly it is between 0.2% and 2.1% by weight. Though carbon is the main alloying material for iron some other elements like Tungsten, chromium, manganese can also be used for the purpose. Different types and amounts of alloying element used determine the hardness, ductility and tensile strength of steel. Alloying element is responsible for maintaining the crystal lattice structure of steel by preventing dislocation of iron atoms. Thus, it acts as the hardening agent in steel. The density of steel varies between 7,750 and 8,050 kg/m3, and this is affected by the alloying constituents too. Heat treatment is a process which changes the mechanical properties of steels. This will affect the ductility, hardness and electrical and thermal properties of steel.There are different types of steel as carbon steel, mild steel, stainless steel, etc. Steel is mainly used for construction purposes. Buildings, stadiums, railway tracks, bridges are few places among many where steel is heavily used. Other than that, they are used in vehicles, ships, planes, machines, etc. Most of the daily used house appliances are also made by steel. Now, most furniture is also being replaced by steel products. When steel is used for these applications, it is important to ensure their durability. One drawback in using steel is its tendency to corrode, and there have been various measures taken to reduce or eliminate corrosion of steel. Stainless steel and galvanized steel are two examples of steel which are capable of fighting corrosion successfully.Carbon SteelCarbon steel is used to denote steel with carbon as the main alloying element. In carbon steel, the properties are mainly defined by the amount of carbon it has. For this alloy, the amounts of other alloying elements like chromium, manganese, cobalt, tungsten are not defined.There are four types of carbon steel. This categorization is based on the carbon content. Mild and low carbon steel contain very low carbon percentages. There are three other types of carbon steel as medium carbon steel, high carbon steel and ultra high carbon steel. In the higher carbon steels, the carbon level varies between 0.301.70 % by weight. Medium carbon steel has 0.300.59% carbon content, whereas the high steel has 0.6-0.99%. Ultra high carbon steel has 1.0-2.0% of carbon content. They can undergo heat treatment successfully. Therefore, normally these are very strong and hard. However, the ductility can be low.Stainless SteelStainless steel is different from other steel alloys because it doesnt corrode or rust. Other than this, it has other basic properties of steel, as mentioned above. Stainless steel is different from carbon steel due to the amount of chromium present. It contains minimum 10.5% to 11% chromium amount by mass. So it forms a chromium oxide layer which is inert. This is the reason for non corrosion ability of stainless steel. Therefore, stainless steel is used for many purposes such as in buildings, monuments, automobile, machinery, etc.Carbon Steel vs Stainless SteelCarbon steel can corrode whereas stainless steel is protected from corrosion.Stainless steel is different from carbon steel due to the amount of chromium present. Stainless steel contains minimum 10.5% to 11% chromium amount by mass.There is an in built chromium oxide layer in stainless steel, which is not present in carbon steel.

What is Stainless Steel?Stainless Steel is a highly durable alloy containing the following major ingredients:10 - 30 per cent of chromium by mass giving excellent corrosion and oxidation resistance to it.Rest about more than 50 percent iron.Through varying chromium content and by addition, substitution of other alloying elements like nickel, molybdenum, copper, titanium, aluminium, silicon, niobium, sulphur, selenium etc., resistance to corrosion, oxidation, abrasion, hardness and a variety of other distinct properties are either created or enhanced.

Difference between Stainless Steel and Carbon Steel or Alloy SteelThe basic difference between stainless steel and conventional alloy steel or carbon steel is thatStainless Steelcontains a very high percentage of chromium (11 26 percent) and nickel (3.5 22 percent). On the other hand, steel that contains carbon up to about 1.7 percent as an essential alloying constituent and has properties and structure made up mostly of the element carbon is better known asCarbon Steel. Most of the steel produced in the world is carbon steel. Unprotected carbon steel rusts when exposed to air or moisture but stainless steel is almost immune to rusting and ordinary corrosion.Alloy Steelis an iron based mixture containing manganese greater than 1.65%, silicon over 0.5%, copper above 0.6%, or other minimum quantities of different alloying elements such as chromium, nickel, molybdenum, or tungsten are present. Austenitic Stainless Steel, the largest among all categories of stainless steel production,offers the most resistance to corrosion among allCategories of Stainless Steel, owing to its substantial nickel content and higher levels of chromium. Excellent weldability, superior performance in very low-temperature services and ductility is exceptional for the austenitic stainless steels. Carbon is usually present at 0.03 percent to over 1.0 percent in someMartensitic grades of Stainless Steel. It has a high strength to weight ratio and excellent fatigue resistance properties.

Properties of Stainless SteelThe main properties of stainless steel may be summarized as follows:Excellent resistance to abrasion, corrosion, oxidation and sulphidation.Excellent fatigue resistance and fire resistance with critical temperature above 800OC.Strength elongation and formability properties.Availability in wide range of surface finishes.Easy cladding on carbon steel (a method of applying a stainless steel coating to carbon steel or lower alloy steel).Easy to clean and suitable for hygienic uses.Totally recyclable.Excellent energy absorbing property.High tensile strength.Magnetic properties.Retention of cutting edge.Retention of strength at elevated temperature.Thermal conductivity and ductility.Cryogenic strength.Ferritic These steels contain less than 0.10% carbon and are magnetic. The fact that they cant be hardened via heat treatment and dont weld to a high standard limits the use of these metals somewhat, but they are still suitable for a wide range of applications.Austenitic This is the most common type of stainless steel, accounting for up to 70% of all stainless steel production. Its versatility is in large part down to the fact that it can be formed and welded with successful results.Martensitic This type of steel shares some characteristics with ferritic, but boasts higher levels of carbon, up to a full 1%. This means that they can be tempered and hardened and are thus highly useful in situations where the strength of the steel is more important than its resistance to corrosion.Duplex Put simply,Duplex steelsare a combination of ferritic and austenitic steels, a structure which renders duplex steel stronger than both.Precipitation Hardening With the addition of elements such as Aluminium, Copper and Niobium, these steels become extremely strong. They can be machined and worked into a wide variety of shapes without becoming distorted and, in terms of corrosion, have the same resistance levels as austenitic steels.