Steel


(المهندس) #1

[CENTER][SIZE=4][COLOR=blue]Steel is a metal alloy whose major component is iron, with carbon being the primary alloying material. Carbon acts as a hardening agent, preventing iron atoms, which are naturally arranged in a crystal lattice, from sliding past one another (dis********). Varying the amount of carbon and its distribution in the alloy controls qualities such as the hardness, elasticity, ductility, and tensile strength of the resulting steel. Steel with increased carbon ******* can be made harder and stronger than iron, but is also more brittle. One classical definition is that steels are iron–carbon alloys with up to 2.1 percent carbon by weight; alloys with higher carbon ******* than this are known as cast iron. Steel is also to be distinguished from wrought iron with little or no carbon. It is common today to talk about ‘the iron and steel industry’ as if it were a single thing; it is today, but historically they were separate products.

Currently there are several classes of steels in which carbon is replaced with other alloying materials, and carbon, if present, is undesired. A more recent definition is that steels are iron-****d alloys that can be plastically formed (pounded, rolled, etc.).

Iron and steel

Iron, like most metals, is not found in the Earth’s crust in a native state. Iron can be found in the crust only in combination with oxygen or sulfur. Typically Fe2O3—the form of iron oxide (rust) found as the mineral hematite, and FeS2—Pyrite (fools gold). Iron oxide is a soft sandstone-like material with limited uses on its own. Iron is extracted from ore by removing the oxygen by combining it with a preferred chemical partner such as carbon. This process, known as smelting, was first applied to metals with lower melting points. Copper melts at just over 1000 °C, while tin melts around 250 °C. Both temperatures could be reached with ancient methods that have been used for at least 6000 years (since the Bronze Age). Since the oxidation rate itself increases rapidly beyond 800 °C, it is important that smelting take place in a fairly oxygen-free environment. Unlike copper and tin, liquid iron dissolves carbon quite readily, so that smelting results in an alloy containing too much carbon to be called steel.

Even in the narrow range of concentrations that make up steel, mixtures of carbon and iron can form into a number of different structures, or allotropes, with very different properties; understanding these is essential to making quality steel. At room temperature, the most stable form of iron is the body-centered cubic (BCC) structure ferrite or α-iron, a fairly soft metallic material that can dissolve only a small concentration of carbon (no more than 0.021 wt% at 910 °C). Above 910 °C ferrite undergoes a phase transition from body-centered cubic to a face-centered cubic (FCC) structure, called austenite or γ-iron, which is similarly soft and metallic but can dissolve considerably more carbon (as much as 2.04 wt% carbon at 1146°C). As carbon-rich austenite cools, the mixture attempts to revert to the ferrite phase, resulting in an excess of carbon. One way for carbon to leave the austenite is for cementite to precipitate out of the mix, leaving behind iron that is pure enough to take the form of ferrite, and resulting in a cementite-ferrite mixture. Cementite is a stoichiometric phase with the chemical formula of Fe3C. Cementite forms in regions of higher carbon ******* while other areas revert to ferrite around it. Self-reinforcing patterns often emerge during this process, leading to a patterned layering known as pearlite due to its pearl-like appearance, or the similar but less beautiful bainite.

Perhaps the most important allotrope is martensite, a chemically metastable substance with about four to five times the strength of ferrite. A minimum of 0.4 wt% of carbon is needed in order to form Martensite. When the Austenite is quenched to form Martensite the carbon is “frozen” in place when the cell structure changes from FCC to BCC. The carbon atom are much too large to fit in the interstatial vaccancies and thus distort the cell structure into a Body Centered Tetragonal (BCT) structure. Martensite and Austenite have an identical chemical composition. As such, it requires extremely little thermal activation energy to form.

The heat treatment process for most steels involves heating the alloy until austenite forms, then quenching the hot metal in water or oil, cooling it so rapidly that the transformation to ferrite or pearlite does not have time to take place. The transformation into martensite, by contrast, occurs almost immediately, due to a lower activation energy.

Martensite has a lower density than 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, these 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 water quenched, although they may not always be visible.

At this point, if its carbon ******* is high enough to produce a significant concentration of martensite, and is extremely hard but very brittle. Often, steel undergoes further heat treatment at a lower temperature to destroy some of the martensite (by allowing enough time for cementite, etc., to form) and help settle the internal stresses and defects. This softens the steel, producing a more ductile and fracture-resistant metal. Because time is so critical to the end result, this process is known as tempering, source of the term tempered steel.

Other materials are often added to the iron-carbon mixture to tailor the resulting properties. Nickel and manganese in steel add to its tensile strength and make austenite more chemically stable, chromium increases the hardness and melting temperature, and vanadium also increases the hardness while reducing the effects of metal fatigue. Large amounts of chromium and nickel (often 18% and 8%, respectively) are added to stainless steel so that a hard oxide forms on the metal surface to inhibit corrosion. Tungsten interferes with the formation of cementite, allowing martensite to form with slower quench rates, resulting in high speed steel. On the other hand sulfur, nitrogen, and phosphorus make steel more brittle, so these commonly found elements must be removed from the ore during processing.

When iron is smelted from its ore by commercial processes, it contains more carbon than is desirable. To become steel, it must be melted and reprocessed to remove the correct amount of carbon, at which point other elements can be added. Once this liquid is cast into ingots, it usually must be “worked” at high temperature to remove any cracks or poorly mixed regions from the solidification process, and to produce shapes such as plate, sheet, wire, etc. It is then heat-treated to produce a desirable crystal structure, and often “cold worked” to produce the final shape. In modern steelmaking these processes are often combined, with ore going in one end of the assembly line and finished steel coming out the other. These can be streamlined by a deft control of the interaction between work hardening and tempering.

Types of steel
Alloy steels were known from antiquity, being nickel-rich iron from meteorites hot-worked into useful products. In a modern sense, alloy steels have been made since the invention of furnaces capable of melting iron, into which other metals could be thrown and mixed.

Historic types
Damascus steel, which was famous in ancient times for its durability and ability to hold an edge, was created from a number of different materials (some only in traces), essentially a complicated alloy with iron as main component.
Blister steel - steel produced by the cementation process
Crucible steel - steel produced by Benjamin Huntsman’s crucible technique
Styrian Steel, also called ‘German steel’ or ‘Cullen steel’ (being traded through Cologne) was made in Styria in Austria by fining cast iron from certain manganese-rich ores.
Shear steel was blister steel that was broekn up, faggotted, heated and welded to produce a more homogeneous product.

Contemporary Steel
Carbon steel, of which mild steel is one type.
Stainless steels and surgical stainless steels contain a minimum of 10.5% chromium, often combined with nickel, to resist corrosion (rust). Some stainless steels are nonmagnetic.
Tool steels
HSLA Steel (High Strength, Low Alloy)
Advanced High Strength Steels
Complex Phase Steel
Dual Phase Steel
TRIP steel
TWIP steel
Maraging steel
Ferrous superalloys
Hatfield steel or Manganese steel, this contains 12-14% manganese which when abraded forms an incredibly hard skin which resists wearing. Some examples are tank tracks, bulldozer blade edges and cutting blades on the jaws of life.
Though not an alloy, there exists also galvanized steel, which is steel that has gone through the chemical process of being hot-dipped or electroplated in zinc for protection against rust. Finished steel is steel that can be sold without further work or treatment.[/align:2ba0857679]

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(mohamed awad) #2

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