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Pearlite is an iron alloy that's is characterized by the formation of distinct bands of ferrite

andcementite. It contains around 88% ferrite and 12% cementite. It only forms under specialized

conditions which must be controlled to create it rather than another one. Pearlite is known for

being tough, thanks to the way in which it forms, and it may be used in a variety of applications.

The formation process of pearlite involves the creation of a euctetic mixture. In a euctectic

mixture, two molten materials crystallize at the same time. This creates the distinctive bandingassociated with pearlite and also adds to the strength of the metal. In order for a euctetic mixture

to form, the components of the alloy must appear in the right balance. Pearlite also requires slow

cooling. If the mixture cools rapidly, it can transition intobainite, a different iron alloy phase which

is slightly harder.

People can identify pearlite by studying the structure of the alloy. Especially under a microscope,

it has a very distinctive appearance created by the lamellar bands. Pearlite is around the middle

of the chart in terms of strength when compared to other iron alloys. People who work with iron

and steel need to know about the different phases and the factors which can influence iron alloy

formation to understand the materials they are working with, and how those materials can be

applied.

The term pearlite is a reference to the material's appearance under the microscope. Itresembles mother of pearl, a natural lamellar structure seen among some shellfish. Mother of

pearl is created through the natural deposition of successive layers, rather than as the result of

special treatment of a euctectic mixture, but it shares the trait of hardness and strength created

by the layers of material.

Steel may sometimes be advertised as pearlite-free. This type ofsteel tends to be less prone to

cracking and metal fatigue, which makes it popular for certain types of applications. Brittle

fracture of steel is a concern in some situations and pearlite-free steel may be preferred in these

cases. Its level can also be adjusted to meet varying needs, and theproperties of the steel can

also be influenced by the use of different alloy materials, depending on how and where

the steel is going to be used.

Bainite is a microstructural crystalline pattern that forms in steel during heating. It is named after

Edgar C. Bain, a US metallurgist who worked on the alloying andheat treatmentof steel in

Pittsburgh, Pennsylvania, in the United States. Bainite is formed whenausteniteis cooled

rapidly. Austenite is a allotrope, or form of iron known as gamma iron, that contains carbon and a

cubical lattice structure when between 1,670 to 2,552 Fahrenheit (910 to 1,400 Celsius).

Two unique temperature conditions have to exist for the bainite microstructureto form. Austenite

must be cooled rapidly enough so thatpearlitedoes not form. Pearlite is a alternating layered

structure in steel of ferrite andcementitethat forms when the steel is slowly cooled, and falls

below a temperature of 1,341 Fahrenheit (727 Celsius). Cooling in the austenite must also be

delayed long enough to preventmartensitefrom forming. Martensite is a very hard, brittlecrystalline byproduct of austenite production.

If the processing of austenite is done correctly and bainitic steel is formed, it displays some of the

characteristics of both pearlite and martensite. It possesses some of the extreme hardness of

martensite, as well as the tough structure of pearlite. The bainitic microstructure consists of

ferrite, like in pearlite, and a minute dispersion of cementite also.

Uses for bainitic steel varieties are included in the power generation industry because of their

unique quality of creep resistance. They are less likely to deform under stress than other steel

types. This quality is enhanced by alloying the steel with chromium andmolybdenumto increase

hardness.

Another variation on bainitic steel manufacturing is to infuse it with nonmetallic particles, whichproduce a more disorganized microstructure. This is called nucleated bainite, or acicular ferrite,
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and it has a greater ability to deflect cracks than traditional bainite. Uses for this variety include in

large structural applications that undergo frequent stress, such as oil rigs and bridges.

Variations on the types of bainitic steel produced are often categorized as upper bainite or lower

bainite. The upper range is produced during the cooling process at a temperature of between

1,022 to 752 Fahrenheit (550 to 400 Celsius) and resembles a form of steel known asWidmanstatten ferrite. Lower bainitic steel is produced at a temperature cooling level of 752 to

482 Fahrenheit (400 to 250 Celsius), where it resembles the acicular morphology. Though

lower bainite is not specifically nucleated bainite, it is somewhere between upper bainite and

martensite structures in composition.

Austenite is a metallic, nonmagnetic solid steel consisting of carbon, iron, nickel and chromium.

When steel is heated above 1350 degrees Fahrenheit (732 Celsius), atoms change to formaustenite. This solid solution is easily manipulated at extreme temperatures and

resistscorrosion. These properties make it suitable for manufacturing food-service equipment,

architectural applications and medical instruments.

Austenistic stainless steel is one of five classes of metallurgical structures. Austenite stainless

steels use chromium and nickel. Sometimes,manganeseand nitrogen are added. If the mix is 18

percent chromium and 8 percent nickel, it is called 18-8. An iron, chromium and nickel

combination is included in the 300 series. Surgical steel, Type 304 in the series, contains 18 to

10 percent nickel and 18 to 20 percent chromium.

Temperatures above 1350 Fahrenheit (732 Celsius) cause iron to transform into a face-

centered cubic (FCC) crystal configuration. When forging this steel, austenite is pliable enough to

shape and hammer out imperfections.Annealingis the process of steadily heating the metal andthen putting it through a gradual cooling process. Usually, stainless steel is sold annealed, or in

its soft condition. Austenistic grades of steel are hardened by cold working as opposed to the

heat treatment used for carbon steels.

Cold working is the shaping of metal at a temperature lower than the molten state of that metal.

Room temperature is fine for cold working austenite. Cold-work tool steels are used in dies, steel

cutting shapes, that form metal at lower temperatures. An air-hardeningtool steelis often used to

shape molds.

Molybdenumis added to the nickel-chromium mix to help with corrosion resistance to chlorides.

Corrosive chlorides include sea water or the de-icing solutions used during snowy and icy

weather. Residents in coastal areas and cold climates benefit from these rust resisting

components of stainless steels.Austenite was named after Sir William Chandler Roberts-Austen, an Englishmetallurgist.

Roberts-Austen, who died inLondonin 1902 at age 59, studied impurities in pure metals. His

research and procedural improvements were used in a variety of applications and widely affected

the industrialized world.

Stainless steels are recyclable, making all types and mixes a natural, environmentally friendly

choice. During recycling, the steel is re-melted then formed into new stainless steel. Type 304

austenistic stainless steel is used for today's popular stainless steel kitchen appliances and vent

hoods. Austenistic stainless steels have also been used in conventional and nuclear power

plants' superheaters and heating components

Cementite is a chemical compound whose inclusion hardens steel. Each molecule is made of

three iron atoms bonded to one carbon atom (Fe3C) to form a crystal lattice structure called
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orthorhombic, where multiple rectangular prisms arise from the same base structure and

intersect at 90 degree angles. The result is a very hard and brittle substance called iron carbide,

or cementite.

In its purest form, cementite is classified as a non-oxide ceramic. It is solid and inert, and can

withstand crushing force, chemical erosion, abrasion, and temperatures up to 3000 degrees F

(1600 C). It forms naturally by the melting of whitecast iron, where it precipitates out of the ironas carbon to form large particles. It sometimes appears this way in phase withaustenite, an

allotrope of iron, which can sometimes cool to formmartensite, a steel with a very strong crystal

lattice.

Steel is tempered to increase hardness and reduce brittleness by creating cementite. The first

step in the tempering process is called austenizing, when the steel is melted into a solution of

iron and carbon, or austenite. The steel is rapidly cooled, and martensite forms from the

austenite. It is then heated again, and cooled slowly in a controlled manner, and cementite is

formed. It is impossible to produce enough energy to run the reaction to completion, so the

cementite is usually mixed with small amounts of unconverted martensite,bainite, which is also

Fe3C, but with a differentcrystal structure, and ferrite(iron).

Cementite is ferromagnetic, which means it displays magnetic characteristics with or without amagnetic field, like a refrigerator magnet. At 480K (404 F, 207 C), however, the atomic poles

begin to move around and are no longer aligned. The spins of the molecules become

randomized, and magnetization ceases. The substance becomes paramagnetic, which means it

is only magnetized if the field is applied by an outside source. Even then, the magnetization will

be weak because it relies upon induced dipoles, and no outside force can induce every dipole in

every molecule, crystalline structure or not. In fact, it is the non-linear attraction that gives

ferromagnets their strength.

There is a substance very similar to cementite called cohenite. It is also Fe 3C, except it forms a

rod-like crystal and contains trace amounts of nickel andcobalt. It occurs naturally in meteorites,

and on Earth is places with very high iron deposits, like volcanic magma flow trails that happen

over coal deposits.

The term "martensite" usually refers to a form of steel with a distinctive atomic structure created

through a process called martensitic transformation. Martensite is very hard, meaning that it

won't dent or scratch easily; this makes it a popular choice for tools, such as hammers and

chisels, as well as swords. It is brittle, however, so it will break rather than bend when put under

too much pressure. Martensite is made fromaustenite, a solid solution of iron with a small

amount of carbon in it.

Phase Changes

Austenite has a particular crystalline structure known as face-centered cubic (FCC). This means

that each cubic unit has a lattice point in the center of each side as well as at each corner; with

the lattice points connected, the crystal would look like a square box with an X on each side. This

type of steel begins to form at temperatures of about 1,350F (732C). Austenite can hold more

carbon than other forms of iron. If allowed to cool naturally, austenite turns into ferrite (alpha iron

or pure iron) andcementite(iron carbide).

Martensitic transformation occurs when the austenite is rapidly cooled in a process known as

quenching. The rapid drop in temperature traps the carbon atoms inside the crystal structures of

the iron atoms. This causes the crystals to change from FCC to body-centered tetragonal (BCT);

the crystals are stretched so that they are square on each end but longer on the sides (like a

shoe box), and the lattice points that were in the center of each face are now joined together at
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one point in the center of the crystal. This new structure is what greatly increases the hardness of

the steel.

Tempering

The resulting martensitic steel is extremely hard, meaning that it won't scratch, but very brittle, soit will break under stress. To address thisweakness, martensite is heated in a process called

tempering, which causes the martensite to transform partially into ferrite and cementite. This

tempered steel is not quite as hard, but becomes tougher (less likely to break) and more

malleable, and thus better suited for industrial use.

Uses

Tempered martensite's hardness makes it a good material for tool steels, since resistance to

abrasion and deformation is important in such applications. It is a common component in

machine parts and forging dies. Tempered steels containing silicon are often used forspring

steel, which can be used to make springs, musical instrument strings, and components on model

trains and other toys. Spring steel can be twisted or bent without permanent deformation, making

it a good choice for components that require the steel to move repeatedly without degradation.

Stainless steel, which contains chromium as well as iron and carbon, can also be made with a

martenistic crystalline structure. This form is less resistant tocorrosionthan other forms of

stainless steel, but it is also stronger and more easily machined in most cases. One method of

making it, called precipitation hardening (or age hardening), adds impurities like chromium and

nickel during a process of extendedheat treatment; precipitation-hardened martensitic stainless

steel has even greater strength along with high corrosion resistance. Such steel is often used in

military and aerospace applications.

Displacive Transformation

The martensitic transformation is the best-known example of displacive transformation, a type of

phase change in which the atoms of a material move short distances in unison rather than

diffusing individually over longer distances. A phase change occurs when a substance changes

from one state, like a solid, to another, like a liquid. Because they are so well known as a type of

displacive transformation, the terms "martensite" or "martensitic" are sometimes used in a

broader sense to describe any material produced by displacive transformation.

Hardenability is a term used to describe a material's ability to be hardened when it is exposed to

heat and then quenched, or cooled rapidly. It should not be confused with hardness, which refersto a material's strength and ability to resist damage. Instead, hardenability determines whether

an object can be made harder, or whether it is resistant to hardening. This term is used only to

refer to metal objects, including steel and metallic alloys, and is not applied to plastics or other

materials.

The primary type of hardenability test is known as the Jominy or "quench" test. To perform this

test, a steel rod is heated until it crystallizes into a face-centered cubic structure called austenite.

After the heat source is removed, one end of the austenite rod is immediately subjected to a

water spray to cool it to room temperature. This process of cooling is called quenching.

A very quick quench will causemartensite a very strong material to form. If the quench is

not quick enough, a different material will form which is not as strong. The hardness of the rod is
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measured at specific increments from the quenched end. The further from the quenched end, the

slower the cooling rates, making it less likely for martensite to form.

A material that forms martensite with lower cooling rates is easier to harden. A material that

needs very quick quenches to form martnsite will be more difficult to harden. As a result, the

greater the difference in hardness between the two ends, the lower the hardenability.

The hardenability of steels and other metals depends on both the composition of the object andits shape or geometry. The thicker an object, the slower the cooling rates in the center, making it

more difficult to harden the material within. This means that thicker objects, or those with little

surface area, will have a lower level of hardenability than smaller or thinner objects made from

the same material. In a thin object, the heat has very little distance to travel, so the cooling rates

can be fast and increase its level of hardness.

In general, the higher the carbon content of a steel product, the greater the steel's hardenability

will be. Common elements added to steel to increase its hardenability include

boron,manganese, chromium, andmolybdenum. The addition of alloys should be carefully

performed to avoid changing the properties of the steel or affecting its ability to be hardened.

Hardenability of steel and its ability to be welded are inversely related. The more hardenable thesteel, the harder it will be to weld; the lower the ability to be hardened, the easier it is to weld. A

hardenability test is often used in welding applications to determine whether two materials can

successfully be welded. It may also help welders choose electrodes and welding equipment or

settings.

Austempering is a form ofheat treatmentused onferrous metals, such as iron and steel, to

improve the metal'smechanical properties. The metal is heated until it reaches an austenitic

state and then rapidly cooled, or quenched, but kept at a temperature high enough to prevent the

formation ofmartensitefor an extended period. Austempered metals have improved strength,toughness, and resistance to distortion, wear, andhydrogen embrittlement, and are often used in

machine parts.

In the first part of the austempering process, the metal is heated to a temperature of between

1,350 F (about 732 C) and 2,462 F (about 1,394 C). This causes it to undergo a phase

transition that changes the crystalline structure where the iron atoms are arranged, turning it into

austenite. The austenite is then quenched, usually in a bath of molten nitrate salt, and cooled to

a temperature of between 459 F and 750 F (about 232 C and 399 C). It is then kept at that

temperature for a time period ranging from several minutes to several hours. The amount of time

the metal is kept in the salt bath and the precise temperatures used in both phases vary

according to the composition of the ferrous metal and the mechanical properties desired in the

final product.

The austempering process differs from conventional heat treating, which rapidly quenches the

austenite in water or oil, usually to room temperature. This produces a form of steel called

martensite. Martensite is quite hard but highly brittle and requires further heat treating, a process

called tempering, to become ductile enough to use.

Generally, the result of austempering depends on the material used. Austempered steel

becomes a form of steel calledbainite, which is more ductile than martensite and does not

require additional tempering. It is also stronger, tougher, and more resistant to wear for a given

hardness than martensitic steels. Austemperedductile ironresults in a structure called ausferrite,

which has greater strength relative to itsductilitythan the products of standard heat treatment.
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The austempering process was patented by E.C. Bain and E.S. Davenport in 1933. It produced

high-quality steel, but the process was originally quite expensive and not cost-effective for most

uses. This limited its use to the production of high-performance parts that required extreme

toughness and resistance to distortion, such as gun components. It was not until the 1960s,

when technological advances in steel working greatly reduced production costs, that

austempering became an economically viable means of producing steel for large-scalecommercial use.

Ferrites are a class of compounds composed of oxidized iron and other metals in a brittle

ceramic state. They are polycrystalline, meaning that they are composed of large amounts of

minute crystals, and they exhibit strong magnetic properties. A common use for ferrites is in the

suppression of electromagnetic interference (EMI) andradio frequency(RF) interference in

electrical circuits, where they are often referred to as magnetic insulators.

Two general categories for the composition of ferrites exists. Soft ferrite compounds are a

mixture of iron and lightweight metals such as nickel, aluminum, or manganese, where they are

used in electrical transformers and other devices that require the ability for the magnetic field to

be easily reversed. Hard ferrite compounds are composed of iron and harder metals, suchascobalt,barium, andstrontium. Barium ferrite compounds have uses as magnetic insulators

and where permanent magnets are required in consumer applications, such as magnetic door

latches.

The use of ferrite material is widespread, as they are easy and inexpensive to manufacture. Their

main attraction is that they demonstrate large magnetic flux densities compared to the small

magnetizing forces applied to them. Their frequent use leads to various trade names for diverse

applications, with EMI suppression ferrites often being called magic beads, due to the lump-like

appearance that they can have when attached toelectrical wiring.

In RF and ambient electrical signal suppression, ferrites are most effective at bandwidth levels

above 100 megahertz, where they replace decoupling capacitors that begin to exhibit circuit

resonance problems in noise filtering above 75 megahertz. They can be designed to impede lowfrequencies below 10 megahertz as well. This makes ferrites useful as both alternating current

(AC) and direct current (DC) noise filters.

Ferrite cores are designed to be as thick and long as is practical for the wiring or device into

which they are built. This requires that, when used on electrical cable, they be encased in plastic

or heat-shrink tubing that prevents them from breaking up under stress due to their fragile

ceramic nature. The ferrites used for EMI suppression also tend to be of the hard type, which

makes them more prone to breakage than their soft counterparts. A ferrite core often is employed

to shield 100 Base-T cabling used in computer networking, which can be subject to a significant

amount of shock during installation and maintenance. Small amounts of damage to

a ferrite shield, however, will not degrade its ability to filter out noise.

Ferrite, also known as -ferrite (-Fe) oralpha iron, is amaterials scienceterm foriron, or asolid

solutionwith iron as the main constituent, with abody-centered cubiccrystal structure. It is this

crystalline structure which givessteelandcast irontheirmagneticproperties, and is the classic

example of aferromagneticmaterial. Practically speaking, it can be considered pure iron.[1]

It has a strength of 280 N/mm2[citation needed]

and a hardness of approximately 80Brinell.[2]

Mild steel(carbon steel with up to about 0.2 wt% C) consist mostly of ferrite, with increasing amounts

ofpearlite(a fine lamellar structure of ferrite andcementite) as the carbon content is increased.

Sincebainite(shown as ledeburite on the diagram at the bottom of this page) and pearlite each have

ferrite as a component, any iron-carbon alloy will contain some amount of ferrite if it is allowed to

reachequilibriumat room temperature. The exact amount of ferrite will depend on the coolingprocesses the iron-carbon alloy undergoes as it cools from liquid state.
http://www.wisegeek.com/what-is-a-radio-frequency.htmhttp://www.wisegeek.com/what-is-a-radio-frequency.htmhttp://www.wisegeek.com/what-is-a-radio-frequency.htmhttp://www.wisegeek.com/what-is-cobalt.htmhttp://www.wisegeek.com/what-is-cobalt.htmhttp://www.wisegeek.com/what-is-cobalt.htmhttp://www.wisegeek.com/what-is-barium.htmhttp://www.wisegeek.com/what-is-barium.htmhttp://www.wisegeek.com/what-is-barium.htmhttp://www.wisegeek.com/what-is-strontium.htmhttp://www.wisegeek.com/what-is-strontium.htmhttp://www.wisegeek.com/what-is-strontium.htmhttp://www.wisegeek.com/what-are-the-different-types-of-electrical-wiring.htmhttp://www.wisegeek.com/what-are-the-different-types-of-electrical-wiring.htmhttp://www.wisegeek.com/what-are-the-different-types-of-electrical-wiring.htmhttp://www.wisegeek.com/what-are-the-most-common-applications-for-ac-current.htmhttp://www.wisegeek.com/what-are-the-most-common-applications-for-ac-current.htmhttp://www.wisegeek.com/what-are-the-most-common-applications-for-ac-current.htmhttp://en.wikipedia.org/wiki/Materials_sciencehttp://en.wikipedia.org/wiki/Materials_sciencehttp://en.wikipedia.org/wiki/Materials_sciencehttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Solid_solutionhttp://en.wikipedia.org/wiki/Solid_solutionhttp://en.wikipedia.org/wiki/Solid_solutionhttp://en.wikipedia.org/wiki/Solid_solutionhttp://en.wikipedia.org/wiki/Body-centered_cubichttp://en.wikipedia.org/wiki/Body-centered_cubichttp://en.wikipedia.org/wiki/Crystal_structurehttp://en.wikipedia.org/wiki/Crystal_structurehttp://en.wikipedia.org/wiki/Crystal_structurehttp://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Cast_ironhttp://en.wikipedia.org/wiki/Cast_ironhttp://en.wikipedia.org/wiki/Cast_ironhttp://en.wikipedia.org/wiki/Magnetichttp://en.wikipedia.org/wiki/Magnetichttp://en.wikipedia.org/wiki/Magnetichttp://en.wikipedia.org/wiki/Ferromagnetismhttp://en.wikipedia.org/wiki/Ferromagnetismhttp://en.wikipedia.org/wiki/Ferromagnetismhttp://en.wikipedia.org/wiki/Ferrite_(iron)#cite_note-1http://en.wikipedia.org/wiki/Ferrite_(iron)#cite_note-1http://en.wikipedia.org/wiki/Ferrite_(iron)#cite_note-1http://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Brinell_scalehttp://en.wikipedia.org/wiki/Brinell_scalehttp://en.wikipedia.org/wiki/Ferrite_(iron)#cite_note-2http://en.wikipedia.org/wiki/Ferrite_(iron)#cite_note-2http://en.wikipedia.org/wiki/Ferrite_(iron)#cite_note-2http://en.wikipedia.org/wiki/Mild_steelhttp://en.wikipedia.org/wiki/Mild_steelhttp://en.wikipedia.org/wiki/Pearlitehttp://en.wikipedia.org/wiki/Pearlitehttp://en.wikipedia.org/wiki/Pearlitehttp://en.wikipedia.org/wiki/Cementitehttp://en.wikipedia.org/wiki/Cementitehttp://en.wikipedia.org/wiki/Cementitehttp://en.wikipedia.org/wiki/Bainitehttp://en.wikipedia.org/wiki/Bainitehttp://en.wikipedia.org/wiki/Bainitehttp://en.wikipedia.org/wiki/Chemical_equilibriumhttp://en.wikipedia.org/wiki/Chemical_equilibriumhttp://en.wikipedia.org/wiki/Chemical_equilibriumhttp://en.wikipedia.org/wiki/Chemical_equilibriumhttp://en.wikipedia.org/wiki/Bainitehttp://en.wikipedia.org/wiki/Cementitehttp://en.wikipedia.org/wiki/Pearlitehttp://en.wikipedia.org/wiki/Mild_steelhttp://en.wikipedia.org/wiki/Ferrite_(iron)#cite_note-2http://en.wikipedia.org/wiki/Brinell_scalehttp://en.wikipedia.org/wiki/Wikipedia:Citation_neededhttp://en.wikipedia.org/wiki/Ferrite_(iron)#cite_note-1http://en.wikipedia.org/wiki/Ferromagnetismhttp://en.wikipedia.org/wiki/Magnetichttp://en.wikipedia.org/wiki/Cast_ironhttp://en.wikipedia.org/wiki/Steelhttp://en.wikipedia.org/wiki/Crystal_structurehttp://en.wikipedia.org/wiki/Body-centered_cubichttp://en.wikipedia.org/wiki/Solid_solutionhttp://en.wikipedia.org/wiki/Solid_solutionhttp://en.wikipedia.org/wiki/Ironhttp://en.wikipedia.org/wiki/Materials_sciencehttp://www.wisegeek.com/what-are-the-most-common-applications-for-ac-current.htmhttp://www.wisegeek.com/what-are-the-different-types-of-electrical-wiring.htmhttp://www.wisegeek.com/what-is-strontium.htmhttp://www.wisegeek.com/what-is-barium.htmhttp://www.wisegeek.com/what-is-cobalt.htmhttp://www.wisegeek.com/what-is-a-radio-frequency.htm


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In pure iron, ferrite is stable below 910 C (1,670 F). Above this temperature theface-centred

cubicform of iron,austenite(gamma-iron) is stable. Above 1,390 C (2,530 F), up to themelting

pointat 1,539 C (2,802 F), the body-centred cubic crystal structure is again the more stable form

ofdelta-ferrite (-Fe). Ferrite above the critical temperature A2(Curie temperature) of 771

C (1,044 K; 1,420 F), where it is paramagnetic rather than ferromagnetic, isbeta ferriteor beta iron

(-Fe). The term beta iron is seldom used because it is crystallographically identical to, and its phase

field contiguous with, -Fe.

Only a very small amount ofcarboncan be dissolved in ferrite; the maximumsolubilityis about 0.02

wt% at 723 C (1,333 F) and 0.005% carbon at 0 C (32 F).[3]

This is because carbon dissolves in

iron interstitially, with the carbon atoms being about twice the diameter of the interstitial "holes", so

that each carbon atom is surrounded by a strong localstrain field. Hence theenthalpyof mixing is

positive (unfavourable), but the contribution ofentropyto thefree energyofsolutionstabilises the

structure for low carbon content.723 C (1,333 F) also is the minimum temperature at which iron-

carbon austenite (0.8 wt% C) is stable; at this temperature there is aeutectoidreaction between

ferrite, austenite and cementite.
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icc phases

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