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Steel is an alloy of iron with typically a few tenths of a percent of carbon to improve its strength and fracture resistance compared to iron. Many other elements may be present or added. Because of its high tensile strength and low cost, steel is used in buildings , infrastructure , tools , ships , trains , Model Boat Building Materials Pdf cars , machines , electrical appliances , and weapons.
Iron is the base metal of steel. Depending on the temperature, it can take two crystalline forms allotropic forms : body-centred cubic and face-centred cubic. The interaction of the allotropes of iron with the alloying elements, primarily carbon, gives steel and cast iron their range of unique properties. In pure iron, the crystal structure has relatively little resistance to the iron atoms slipping past one another, and so pure iron is quite ductile , or soft and easily formed.
In steel, small amounts of carbon, other elements, and inclusions within the iron act as hardening agents that prevent the movement of dislocations. The carbon in typical steel alloys may contribute up to 2. These qualities include the hardness , quenching behaviour , need for annealing , tempering behaviour , yield strength , and tensile strength of the resulting steel.
The increase in steel's strength compared to pure iron is possible only by reducing iron's ductility. Steel was produced in bloomery furnaces for thousands of years, but its large-scale, industrial use began only after more efficient production methods were devised in the 17th century, with the introduction of the blast furnace and production of crucible steel.
This was followed by the open-hearth furnace and then the Bessemer process in England in the midth century. With the invention of the Bessemer process, a new era of mass-produced steel began. Mild steel replaced wrought iron. The German states saw major steel prowess over Europe by way of cheap exports in the 19th century.
Further refinements in the process, such as basic oxygen steelmaking BOS , largely replaced earlier methods by further lowering the cost of production and increasing the quality of the final product. Today, steel is one of the most common manmade materials in the world, with more than 1. Modern steel is generally identified by various grades defined by assorted standards organisations. The carbon content of steel is between 0.
Too little carbon content leaves pure iron quite soft, ductile, and weak. Carbon contents higher than those of steel make a brittle alloy commonly called pig iron. Alloy steel is 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 , and niobium.
Additional elements, most frequently considered undesirable, are also important in steel: phosphorus , sulfur , silicon , and traces of oxygen , nitrogen , and copper. Plain carbon-iron alloys with a higher than 2. With modern steelmaking techniques such as powder metal forming, it is possible to make very high-carbon and other alloy material steels, but such are not common.
Cast iron is not malleable even when hot, but it can Model Boat Building Materials 800 be formed by casting as it has a lower melting point than steel and good castability properties. Steel is distinguishable from wrought iron now largely obsolete , which may contain a small amount of carbon but large amounts of slag. Iron is commonly found in the Earth's crust in the form of an ore , usually an iron oxide, such as magnetite or hematite.
Iron is extracted from iron ore by removing the oxygen through its combination with a preferred chemical partner such as carbon which is then lost to the atmosphere as carbon dioxide. With care, the carbon content could be controlled by moving it around in the fire.
Unlike copper and tin, liquid or solid iron dissolves carbon quite readily. All of these temperatures could be reached with ancient methods used since the Bronze Age. Smelting, using carbon to reduce iron oxides, results in an alloy pig iron that retains too much carbon to be called steel.
Nickel and manganese in steel add to its tensile strength and make the austenite form of the iron-carbon solution more stable, chromium increases hardness and melting temperature, and vanadium also increases hardness while making it less prone to metal fatigue. Tungsten slows the formation of cementite , keeping carbon in the iron matrix and allowing martensite to preferentially form at slower quench rates, resulting in high-speed steel.
The addition of lead and sulfur decrease grain size, thereby making the steel easier to turn , but also more brittle and prone to corrosion. Such alloys are nevertheless frequently used for components such as nuts, bolts, and washers in applications where toughness and corrosion resistance are not paramount.
For the most part, however, p-block elements such as sulfur, nitrogen , phosphorus , and lead are considered contaminants that make steel more brittle and are therefore removed from the steel melt during processing. Even in a narrow range of concentrations of mixtures of carbon and iron that make steel, several different metallurgical structures, with very different properties can form. Understanding such properties is essential to making quality steel.
It is a fairly soft metal that can dissolve only a small concentration of carbon, no more than 0. The inclusion of carbon in alpha iron is called ferrite. The inclusion of carbon in gamma iron is called austenite. The more open FCC structure of austenite can dissolve considerably more carbon, as much as 2.
When steels with exactly 0. The carbon no longer fits within the FCC austenite structure, resulting in an excess of carbon. One way for carbon to leave the austenite is for it to precipitate out of solution as cementite , leaving behind a surrounding phase of BCC iron called ferrite with a small percentage of carbon in solution. The two, ferrite and cementite, precipitate simultaneously producing a layered structure called pearlite , named for its resemblance to mother of pearl.
In a hypereutectoid composition greater than 0. For steels that have less than 0. No large inclusions of cementite will form at the boundaries in hypoeuctoid steel. As the rate of cooling is increased the carbon will have less time to migrate to form carbide at the grain boundaries but will have increasingly large amounts of pearlite of a finer and finer structure within the grains; hence the carbide is more widely dispersed and acts to prevent slip of defects within those grains, resulting in hardening of the steel.
At the very high cooling rates produced by quenching, the carbon has no time to migrate but is locked within the face-centered austenite and forms martensite. Martensite is a highly strained and stressed, supersaturated form of carbon and iron and is exceedingly hard but brittle. Depending on the carbon content, the martensitic phase takes different forms. Below 0. There is no thermal activation energy for the transformation from austenite to martensite.
Martensite has a lower density it expands during the cooling 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 of compression on the crystals of martensite and tension on the remaining ferrite, with a fair amount of shear on 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 internal work hardening and other microscopic imperfections. It is common for quench cracks to form when steel is water quenched, although they may not always be visible. There are many types of heat treating processes available to steel.
The most common are annealing , quenching , and tempering. Heat treatment is effective on compositions above the eutectoid composition hypereutectoid of 0. Hypoeutectoid steel does not benefit from heat treatment. Annealing is the process of heating the steel to a sufficiently high temperature to relieve local internal stresses. It does not create a general softening of the product but only locally relieves strains and stresses locked up within the material.
Annealing goes through three phases: recovery , recrystallization , and grain growth. The temperature required to anneal a particular steel depends on the type of annealing to be achieved and the alloying constituents. Quenching involves heating the steel to create the austenite phase then quenching it in water or oil. This rapid cooling results in a hard but brittle martensitic structure.
In this application the annealing tempering process transforms some of the martensite into cementite, or spheroidite and hence it reduces the internal stresses and defects. The result is a more ductile and fracture-resistant steel. When Model Boat Building Materials 3d 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 steel product into ingots which would be stored until use in further refinement processes that resulted in the finished product.
In modern facilities, the initial product is close to the final composition and is continuously cast into long slabs, cut and shaped into bars and extrusions and heat-treated to produce a final product. The ingots are then heated in a soaking pit and hot rolled into slabs, billets , or blooms.
Slabs are hot or cold rolled into sheet metal or plates. Billets are hot or cold rolled into bars, rods, and wire. Blooms are hot or cold rolled into structural steel , such as I-beams and rails. In modern steel mills these processes often occur in one assembly line , with ore coming in and finished steel products coming out.
Steel was known in antiquity and was produced in bloomeries and crucibles. The reputation of Seric iron of South India wootz steel grew considerably in the rest of the world. The Chinese of the Warring States period � BC had quench-hardened steel, [23] while Chinese of the Han dynasty BC � AD created steel by melting together wrought iron with cast iron, thus producing a carbon-intermediate steel by the 1st century AD.
The manufacture of what came to be called Wootz, or Damascus steel , famous for its durability and ability to hold an edge, may have been taken by the Arabs from Persia, who took it from India. It was originally created from several different materials including various trace elements , apparently ultimately from the writings of Zosimos of Panopolis. In BC, Alexander the Great was rewarded by the defeated King Porus , not with gold or silver but with 30 pounds of steel.
The ancient Sinhalese managed to extract a ton of steel for every 2 tons of soil, [41] a remarkable feat at the time.
One such furnace was found in Samanalawewa and archaeologists were able to produce steel as the ancients did. Crucible steel , formed by slowly heating and cooling pure iron and carbon typically in the form of charcoal in a crucible, was produced in Merv by the 9th to 10th century AD. Since the 17th century, the first step in European steel production has been the smelting of iron ore into pig iron in a blast furnace. In these processes pig iron was refined fined in a finery forge to produce bar iron , which was then used in steel-making.
The production of steel by the cementation process was described in a treatise published in Prague in and was in use in Nuremberg from A similar process for case hardening armor and files was described in a book published in Naples in
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