Dövülebilir Malzemeler İngilizce

Hazırlayan Ferdi ÖNEN

ANKARA 2012

1.

Steel is an alloy that consists mostly of and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten.Carbon and other elements act as a agent, preventing dislocations in the iron atom crystal lattice from sliding past one another. Varying the amount of alloying elements and the form of their presence in the steel (solute elements, precipitated phase) controls qualities such as the hardness, ductility, and tensile strength of the resulting steel. Steel with increased carbon content can be made harder and stronger than iron, but such steel is also less ductile than iron.

Alloys with a higher than 2.1% carbon content are known as because of their lower melting point and good castability.Steel is also distinguishable from , which can contain a small amount of carbon, but it is included in the form of inclusions. Two distinguishing factors are steel's increased rust resistance and better weldability.

Though steel had been produced by various inefficient methods long before the Renaissance, its use became more common after more-efficient production methods were devised in the 17th century. With the invention of the in the mid-19th century, steel became an inexpensive mass-produced material. Further refinements in the process, such as basic oxygen (BOS), lowered the cost of production while 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 assorted standards organizations.

Iron is found in the Earth's crust only in the form of an ore, i.e., combined with other elements such as oxygen or sulfur.Typical iron-containing minerals include Fe2O3—the form of iron oxide found as the mineral hematite, and FeS2—pyrite (fool's gold).Iron is extracted from ore by removing oxygen and combining the ore with a preferred chemical partner such as carbon. This process, known as , was first applied to metals with lower melting points, such as tin, which melts at approximately 250 °C (482 °F) and copper, which melts at approximately 1,100 °C (2,010 °F). In comparison, cast iron melts at approximately 1,375 °C (2,507 °F). All of these temperatures could be reached with ancient methods that have been used since the Bronze Age. Since the oxidation rate itself increases rapidly beyond 800 °C (1,470 °F), it is important that smelting take place in a low- oxygen environment. Unlike copper and tin, liquid iron dissolves carbon quite readily. Smelting results in an alloy () containing too much carbon to be called steel.[4] 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 and manganese in steel add to its tensile strength and make austenite more chemically stable, chromium increases hardness and melting temperature, and vanadium also increases hardness while reducing the effects of metal fatigue. To prevent corrosion, at least 11% chromium is added to steel so that a hard oxide forms on the metal surface; this is known as stainless steel. 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.

The density of steel varies based on the alloying constituents but usually ranges between 7,750 and 8,050 kg/m3 (484 and 503 lb/cu ft), or 7.75 and 8.05 g/cm3 (4.48 and 4.65 oz/cu in).

Even in the narrow range of concentrations which make up steel, mixtures of carbon and iron can form a number of different structures, with very different properties. Understanding such properties is essential to making quality steel. At room temperature, the most stable form of iron is the body- centered cubic (BCC) structure α-ferrite. It is a fairly soft metallic material that can dissolve only a small concentration of carbon, no more than 0.021 wt% at 723 °C (1,333 °F), and only 0.005% at 0 °C (32 °F). If steel contains more than 0.021% carbon at steelmaking temperatures then it transforms into a face-centered cubic (FCC) structure, called austenite or γ-iron. It is also soft and metallic but can dissolve considerably more carbon, as much as 2.1%[7] carbon at 1,148 °C (2,098 °F), which reflects the upper carbon content of steel.

When with less than 0.8% carbon, known as a hypoeutectoid steel, are cooled from an austenitic phase 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, resulting in a cementite-ferrite mixture. Cementite is a hard and brittle intermetallic compound with the chemical formula of Fe3C. At the eutectoid, 0.8% carbon, the cooled structure takes the form of pearlite, named after its resemblance to mother of pearl. For steels that have more than 0.8% carbon the cooled structure takes the form of pearlite and cementite.

Perhaps the most important polymorphic form is martensite, a metastable phase which is significantly stronger than other steel phases. When the steel is in an austenitic phase and then quenched it forms into martensite, because the atoms "freeze" in place when the cell structure changes from FCC to BCC. Depending on the carbon content the martensitic phase takes different forms. Below approximately 0.2% carbon it takes an α ferrite BCC crystal form, but higher carbon contents take a body-centered tetragonal (BCT) structure. There is no thermal activation energy for the transformation from austenite to martensite. Moreover, there is no compositional change so the atoms generally retain their same neighbors.

Martensite has a lower density than austenite does, so that 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 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 water quenched, although they may not always be visible.

1.1 Heat treatment

There are many types of processes available to steel. The most common are and quenching and . Annealing is the process of heating the steel to a sufficiently high temperature to soften it. This process occurs through three phases: recovery, recrystallization, and grain growth. The temperature required to anneal steel depends on the type of annealing and the constituents of the alloy.

Quenching and tempering first involves heating the steel to the austenite phase, then quenching it in water or oil. This rapid cooling results in a hard and brittle martensitic structure.The steel is then tempered, which is just a specialized type of annealing. In this application the annealing (tempering) process transforms some of the martensite into cementite, or spheroidite to reduce internal stresses and defects, which ultimately results in a more ductile and fracture-resistant metal.

1.2 Steel production

Iron ore pellets for the production of steel Main article: Steelmaking See also: Steel production by country

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 reduce the carbon to the correct amount, at which point other elements can be added. This liquid is then continuously cast into long slabs or cast into ingots. Approximately 96% of steel is continuously cast, while only 4% is produced as cast steel ingots.The ingots are then heated in a soaking pit and hot rolled into slabs, blooms, or billets. 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 foundries these processes often occur in one assembly line, with ore coming in and finished steel coming out.Sometimes after a steel's final rolling it is heat treated for strength, however this is relatively rare.

1.3 History of steelmaking

Bloomery smelting during the Middle Ages

1.3.1 Ancient steel

Steel was known in antiquity, and may have been produced by managing , iron- smelting facilities, where the bloom contained carbon.

The earliest known production of steel is a piece of ironware excavated from an archaeological site in Anatolia (Kaman-Kalehoyuk) and is about 4,000 years old.Other ancient steel comes from East Africa, dating back to 1400 BC.In the 4th century BC steel weapons like the Falcata were produced in the Iberian Peninsula, while Noric steel was used by the Roman military.The Chinese of the Warring States (403–221 BC) had quench-hardened steel, while Chinese of the (202 BC – 220 AD) created steel by melting together wrought iron with cast iron, gaining an ultimate product of a carbon-intermediate steel by the 1st century AD.The Haya people of East Africa discovered a type of high-heat which allowed them to carbon steel at 1,802 °C (3,276 °F) nearly 2,000 years ago.This ability was not duplicated until centuries later in Europe during the .

1.3.2 Wootz steel and

Evidence of the earliest production of high carbon steel in the Indian Subcontinent was found in Samanalawewa area in Sri Lanka.Wootz steel was produced in India by about 300 BC. Along with their original methods of forging steel, the Chinese had also adopted the production methods of creating Wootz steel, an idea imported into China from India by the 5th century AD.In Sri Lanka, this early steel-making method employed the unique use of a wind furnace, blown by the monsoon winds, that was capable of producing high-carbon steel. Also known as Damascus steel, wootz is famous for its durability and ability to hold an edge. It was originally created from a number of different materials including various trace elements. It was essentially a complicated alloy with iron as its main component. Recent studies have suggested that carbon nanotubes were included in its structure, which might explain some of its legendary qualities, though given the technology available at that time, they were produced by chance rather than by design.Natural wind was used where the soil containing iron was heated up with the use of wood. The ancient Sinhalese managed to extract a ton of steel for every 2 tons of soil remarkable feat at the time. One such furnace was found in Samanalawewa and archaeologists were able to produce steel as the ancients did long ago.

Crucible steel, formed by slowly heating and cooling pure iron and carbon (typically in the form of ) in a crucible, was produced in Merv by the 9th to 10th century AD.In the 11th century, there is evidence of the production of steel in Song China using two techniques: a "berganesque" method that produced inferior, inhomogeneous steel and a precursor to the modern Bessemer process that utilized partial decarbonization via repeated forging under a .

1.3.3 Modern steelmaking

A Bessemer converter in Sheffield, England

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.Originally using charcoal, modern methods use , which has proven to be a great deal cheaper.

1.3.3.1Processes starting from bar iron

In these processes pig iron was "fined" in a finery forge to produce bar iron (wrought iron), which was then used in steel-making.

The production of steel by the was described in a treatise published in Prague in 1574 and was in use in Nuremberg from 1601. A similar process for case hardening armour and files was described in a book published in Naples in 1589. The process was introduced to England in about 1614.It was produced by Sir Basil Brooke at during the 1610s. The raw material for this were bars of wrought iron. During the 17th century it was realised that the best steel came from oregrounds iron from a region of Sweden, north of Stockholm. This was still the usual raw material in the 19th century, almost as long as the process was used.

Crucible steel is steel that has been melted in a crucible rather than being forged, with the result that it is more homogeneous. Most previous furnaces could not reach high enough temperatures to melt the steel. The early modern crucible steel industry resulted from the invention of Benjamin Huntsman in the 1740s. Blister steel (made as above) was melted in a crucible or in a furnace, and cast (usually) into ingots.

1.3.3.2Processes starting from pig iron

A Siemens-Martin steel oven from the Brandenburg Museum of Industry

White-hot steel pouring out of an

The modern era in steelmaking began with the introduction of Henry Bessemer's Bessemer process in 1858. His raw material was pig iron.This enabled steel to be produced in large quantities cheaply, thus mild steel is now used for most purposes for which wrought iron was formerly used.The Gilchrist-Thomas process (or basic Bessemer process) was an improvement to the Bessemer process, lining the converter with a basic material to remove phosphorus. Another improvement in steelmaking was the Siemens-Martin process, which complemented the Bessemer process.

These were rendered obsolete by the Linz-Donawitz process of (BOS), developed in the 1950s, and other oxygen steelmaking processes. Basic oxygen steelmaking is superior to previous steelmaking methods because the oxygen pumped into the furnace limits impurities.Now, electric arc furnaces (EAF) are a common method of reprocessing scrap metal to create new steel. They can also be used for converting pig iron to steel, but they use a lot of electricity (about 440 kWh per metric ton), and are thus generally only economical when there is a plentiful supply of cheap electricity.

1.4 Steel industry

A steel plant in the United Kingdom

Steel production by country in 2007

See also: History of the modern steel industry, Global steel industry trends, Steel production by country, and

It is common today to talk about "the iron and steel industry" as if it were a single entity, but historically they were separate products. The steel industry is often considered to be an indicator of economic progress, because of the critical role played by steel in infrastructural and overall economic development.

In 1980, there were more than 500,000 U.S. steelworkers. By 2000, the number of steelworkers fell to 224,000.

The economic boom in China and India has caused a massive increase in the demand for steel in recent years. Between 2000 and 2005, world steel demand increased by 6%. Since 2000, several Indian and Chinese steel firms have risen to prominence like Tata Steel (which bought Corus Group in 2007), Shanghai Baosteel Group Corporation and Shagang Group. ArcelorMittal is however the world's largest steel producer.

In 2005, the British Geological Survey stated China was the top steel producer with about one- third of the world share; Japan, Russia, and the US followed respectively.

In 2008, steel began trading as a commodity on the London Metal Exchange. At the end of 2008, the steel industry faced a sharp downturn that led to many cut-backs.

1.5 Contemporary steel

Bethlehem Steel in Bethlehem, Pennsylvania was one of the world's largest manufacturers of steel before its 2003 closure. See also: Steel grades

Modern steels are made with varying combinations of alloy metals to fulfill many purposes. Carbon steel, composed simply of iron and carbon, accounts for 90% of steel production.[1] High strength low alloy steel has small additions (usually < 2% by weight) of other elements, typically 1.5% manganese, to provide additional strength for a modest price increase.Low alloy steel is alloyed with other elements, usually molybdenum, manganese, chromium, or nickel, in amounts of up to 10% by weight to improve the hardenability of thick sections.[1] Stainless steels and surgical stainless steels contain a minimum of 11% chromium, often combined with nickel, to resist corrosion (rust). Some stainless steels are magnetic, while others are nonmagnetic.

Some more modern steels include tool steels, which are alloyed with large amounts of tungsten and cobalt or other elements to maximize solution hardening. This also allows the use of and improves the alloy's temperature resistance.[1] Tool steel is generally used in axes, drills, and other devices that need a sharp, long-lasting cutting edge. Other special-purpose alloys include weathering steels such as Cor-ten, which weather by acquiring a stable, rusted surface, and so can be used un-painted.

Many other high-strength alloys exist, such as dual-phase steel, which is heat treated to contain both a ferritic and martensitic microstructure for extra strength.Transformation Induced Plasticity (TRIP) steel involves special alloying and heat treatments to stabilize amounts of austentite at room temperature in normally austentite-free low-alloy ferritic steels. By applying strain to the metal, the austentite undergoes a phase transition to martensite without the addition of heat. Maraging steel is alloyed with nickel and other elements, but unlike most steel contains almost no carbon at all. This creates a very strong but still malleable metal.Twinning Induced Plasticity (TWIP) steel uses a specific type of strain to increase the effectiveness of work hardening on the alloy.Eglin Steel uses a combination of over a dozen different elements in varying amounts to create a relatively low-cost metal for use in bunker buster weapons. Hadfield steel (after Sir Robert Hadfield) or manganese steel contains 12–14% manganese which when abraded forms an incredibly hard skin which resists wearing. Examples include tank tracks, bulldozer blade edges and cutting blades on the jaws of life.

Most of the more commonly used steel alloys are categorized into various grades by standards organizations. For example, the Society of Automotive Engineers has a series of grades defining many types of steel.The American Society for Testing and Materials has a separate set of standards, which define alloys such as A36 steel, the most commonly used structural steel in the United States.

Though not an alloy, galvanized steel is a commonly used variety of steel which has been hot- dipped or electroplated in zinc for protection against rust.

1.6 Uses

A roll of steel wool

Iron and steel are used widely in the construction of roads, railways, other infrastructure, appliances, and buildings. Most large modern structures, such as stadiums and skyscrapers, bridges, and airports, are supported by a steel skeleton. Even those with a concrete structure will employ steel for reinforcing. In addition, it sees widespread use in major appliances and cars. Despite growth in usage of aluminium, it is still the main material for car bodies. Steel is used in a variety of other construction materials, such as bolts, nails, and screws.Other common applications include shipbuilding, pipeline transport, mining, offshore construction, aerospace, white goods (e.g. washing machines), heavy equipment such as bulldozers, office furniture, steel wool, tools, and armour in the form of personal vests or vehicle armour (better known as rolled homogeneous armour in this role).

1.7 Historical

A carbon steel knife

Before the introduction of the Bessemer process and other modern production techniques, steel was expensive and was only used where no cheaper alternative existed, particularly for the cutting edge of knives, razors, swords, and other items where a hard, sharp edge was needed. It was also used for springs, including those used in clocks and watches. With the advent of speedier and thriftier production methods, steel has been easier to obtain and much cheaper. It has replaced wrought iron for a multitude of purposes. However, the availability of plastics in the latter part of the 20th century allowed these materials to replace steel in some applications due to their lower cost and weight.

2.Mechanical properties of materials

These properties are related to behavior under load or stress in tension, compression or shear. Properties are determined by engineering tests under appropriate conditions, commonly determined mechanical properties are tensile strength, yield point, elastic limit, creep strength, stress rupture, fatigue , elongation (ductility), impact strength (toughness and brittleness), harness, and modulus of elasticity(ratio of stress to elastc strain-rididity). Strain may be elastic (present only during stressing) or plastic (permanent) deformation. These properties are helpful in determining whether or not a part can be produced in the desired shape and also resist the mechanical forces anticipated.

The words mechanical and physical are often erroneously used interchangeably. The above are mechanical properties. Sometimes modulus of elasticity is considered to be a physical property of a material because it is an inherent property that cannot be changed substantially by practical means such as heat treatment or cold-working. The mechanical properties of materials are explained as follows

2.1 Strength

It is the ability of a material to resist the externally applied forces without breaking or yielding. The internal resistance offered by a part to an externally applied force is called stress.

2.2 Stiffness

It is the ability of a material to resist deformation under stress. The modulus of elasticity is the measure of stiffness.

2.3 Elasticity(Elastic Limit)

It is the property of a material to regain its original shape after deformation when the external forces are removed. This property is desirable for materials used in tools and machines. It may be noted that steel is more elastic than rubber.

2.4 Plasticity

It is property of a material which retains the deformation produced under load permanently. This property of the material is necessary for forgings, in stamping images on coins and in ornamental work.

2.5 Ductility

It is the property of a material enabling it to be drawn into wire with the application of a tensile force. A ductile material must be both strong and plastic. The ductility is usually measured by the terms, percentage elongation and percentage reduction in area. The ductile material commonly used in engineering practice (in order of diminishing ductility) are mild steel, copper, aluminium, nickel, zinc, tin and lead.

2.6 Brittleness

It is the property of a material opposite to ductility. It is the property of breaking of a material with little permanent distortion. Brittle materials when subjected to tensile loads snap off without giving any sensible elongation. Cast iron is a brittle material. 2.7 Malleability

It is a special case of ductility which permits materials to be rolled or hammered into thin sheets. A malleable material should be plastic but it is not essential to be so strong. The malleable materials commonly used in engineering practice (in order of diminishing malleability) are lead, soft steel, wrought iron, copper and aluminium.

2.8 Toughness

It is the property of a material to resist fracture due to high impact loads like hammer blows. The toughness of the material decreases when it is heated. It is measured by the amount of energy that a unit volume of the material has absorbed after being stressed up to the point of fracture. This property is desirable in parts subjected to shock and impact loads.

2.9 Machinability

It is the property of a material which refers to a relative case with which a material can be cut. The machinability of a material can be measured in a number of ways such as comparing the tool life for cutting different materials or thrust required to remove the material at some given rate or the energy required to remove a unit volume of the material. It may be noted that brass can be easily machined than steel.

2.10 Resilience

It is the property of a material to absorb energy and to resist shock and impact loads. It is measured by the amount of energy absorbed per unit volume within elastic limit. This property is essential for spring materials.

2.11 Creep

When a part is subjected to a constant stress at high temperature for a long period of time, it will undergo a slow and permanent deformation called creep. This property is considered in designing internal combustion engines, boilers and turbines.

2.12 Fatigue

When a material is subjected to repeated stresses, it fails at stresses below the yield point stresses. Such type of failure of a material is known as fatigue. The failure is caused by means of a progressive crack formation which are usually fine and of microscopic size. This property is considered in designing shafts, connecting rods, springs, gears, etc.

2.13 Hardness

It is a very important property of the metals and has a wide variety of meanings. It embraces many different properties such as resistance to wear, scratching, deformation and machinability etc. It also means the ability of a metal to cut another metal. The hardness is usually expressed in numbers which are dependent on the method of making the test. The hardness of a metal may be determined by the following tests

 Brinell hardness test  Rockwell hardness test  Vickers hardness (also called Diamond Pyramid) test, and  Shore scleroscope.

3.Metal forming processes 3.1Introduction Practically all metals, which are not used in cast form are reduced to some standard shapes for subsequent processing.

Manufacturing companies producing metals supply metals in form of ingots which are obtained by casting liquid metal into a square cross section. . Slab (500-1800 mm wide and 50-300 mm thick) . Billets (40 to 150 sq mm) . Blooms (150 to 400 sq mm)

Sometimes continuous casting methods are also used to cast the liquid metal into slabs, billets or blooms.

These shapes are further processed through hot rolling-forging or extrusion, to produce materials in standard form such as plates, sheets, rods, tubes and structural sections.

3.2Sequence of operations for obtaining different shapes

3.3Primary Metal Forming Processes

. Rollins . Forging

. Extrusion

Tube and wire drawing and Deep drawing

Although Punching and Blanking operations are not metal forming processes however these will be covered due to similarity with deep drawing process.

3.3.1ROLLİNG

3.3.1.1 CHANGE İS GRAİNS STRUCTURE İN ROLLİNG

3.3.1.2 Salient points about rolling

. Rolling is the most extensively used metal forming process and its share is roughly 90% . The material to be rolled is drawn by means of friction into the two revolving roll gap . The compressive forces applied by the rolls reduce the thickness of the material or changes its cross sectional area . The geometry of the product depend on the contour of the roll gap . Roll materials are cast iron, cast steel and forged steel because of high strength and wear resistance requirements . Hot rolls are generally rough so that they can bite the work, and cold rolls are ground and polished for good finish . In rolling the crystals get elongated in the rolling direction. In cok rolling crystal more or less retain the elongated shape but in hot rolling they start reforming after coming out from the deformation zone . The peripheral velocity of rolls at entry exceeds that of the strip, which is dragged in if the interface friction is high enough.

. In the deformation zone the thickness of the stiip gets reduce* and it elongates. This increases the linear speed of the at the exit. . Thus there exist a neutral point where roll speed and strip speeds are equal. At this point the direction of the friction reverses. . When the angle of contact a exceeds the friction angle A the rolls cannot draw fresh strip . Roll torque, power etc. increase with increase in roll work contact length or roll radius

3.3.1.3Pressure during rolling

Typical pressure variation along the contact length in flat rolling. The peak pressure is located at the neutral point. The area beneath the curve, represents roll force. Friction in rolling: It depends on lubrication, work material and also on the temperature. In cold rolling the value of coefficient of friction is around 0.1 and in warm working it is around 0.2. In hot rolling it is around 0.4. In hot rolling sticking friction condition is also seen and then friction coefficient is observed up to 0.7. In sticking the hot wok surface adheres to roll and thus the central part of the strip undergoes with a severe deformation.

ROLL PASSES TO GET A 12mm ROD FORM 100*100mm BİLLET

3.3.1.4 Roll configurations in rolling mills . Two-high and three-high mills are generally used for initial and intermediate passes during hot rolling, while four-high and cluster mills are used for final passes.

. Last two arrangements are preferred for cold rolling because roll in these configurations are

supported by back-up rolls which minimize the deflections and produce better tolerances. Various Roll Configurations(a)Two-high(b)Three-high(c)Four-high(d)Cluster mill(e)Tandem mill

Other deformation processes related to rolling

3.3.2 Forging

. Forging is perhaps oldest metal working process and was known even during prehistoric days when metallic tools were made by heating and hammering. . Forging is basically involves plastic deformation of material between two dies to achieve desired configuration. Depending upon complexity of the part forging is carried out as open die forging and closed die forging. . In open die forging, the metal is compressed by repeated blows by a mechanical hammer and shape is manipulated manually. . In closed die forging- the desired configuration is obtained by squeezing the workpiece between two shaped and closed dies.

. On squeezing the die cavity gets completely filled and excess material comes out around the periphery of the die as flash which is later trimmed. . Press forging and drop forging are two popular methods in closed die forging. . In press forging the metal is squeezed slowly by a hydraulic or mechanical press and component is produced in a single closing of die, hence the dimensional accuracy is much better than drop forging. . Both open and closed die forging processes are earned out in hot as well as in cold state. . In forging favorable grain orientation of metal is obtained

Open and closed die forging

Grain orientation in forging

Forging machining

Barreling in forging

Flash less forging or precision forging

3.3.2.1Self reading in forging

. Edging . Cogging . Upsetting . Heading . Swaging . Fullering . Radial forging etc.

3.3.3 Extrusion

. It is a relatively new process and its commercial exploitation started early in the nineteenth century with the extrusion of lead pipes. Extrusion of steels became possible only after 1930 when extrusion chambers could be designed to withstand high temperature and pressure. . In extrusion, the material is compressed in a chamber and the deformed material is forced to flow through the die. The die opening corresponds to the cross section of the required product. . It is basically a hot working process, however, for softer materials cold extrusion is also performed.

3.3.3.1 Direct and Indirect Extrusion

. In direct extrusion metal flows in the same direction as that of the ram. Because of the relative motion between the heated billet and the chamber walls, friction is severe and is reduced by using molten glass as a lubricant in case of steels at higher temperatures. At lower temperatures, oils with graphite powder is used for lubrication. . In indirect extrusion process metal flows in the opposite direction of the ram. It is more efficient since it reduces friction losses considerably. The process, however, is not used extensively because it restricts the length of the extruded component.

3.3.3.2 Impact Extrusion

It is similar to indirect extrusion. Here the punch descends rapidly on to the blank which gets indirectly extruded on to the punch and to give a tubular section. The length of the tube formed is controlled by the amount of metal in the slug or by the blank thickness. Collapsible tubes for pastes are extruded by this method.

Hydrostatic Extrusion In this process the friction between container wall and billet is eliminated, however, this process has got limited applications in industry due to specialized equipment & tooling and low production rate due to high set up time.

3.3.4 Draw Large quantities of wires, rods, tubes and other sections are produced by drawing process which is basically a cold working process. In this process the material is pulled throufih a die in order to reduce it to the desired shape and size. In a typical wire drawing operation, once end of the wire is reduced and passed through the opening of the die, gripped and pulled to reduce its diameter.

By successive drawing operation through dies of reducing diameter the wire can be reduced to a very small diameter. Annealing before each drawing operation permits large area reduction. Tungsten Carbide dies are used to for drawing hard wires, and diamond dies is the choice for fine wires.

Tube Drawing

 Tube drawing is also similar to wire drawing, except that a mandrel of appropriate diameter is required to form the internal hole.

 Here two arrangements are shown in figure (a) with a floating plug and (b) with a moving mandrel

 The process reduces the diameter and thickness of the tube.

3.3.4.1 Deep Drawin This operation is extensively used to for making cylindrical shaped parts such as cups, shells, etc form sheet metal. As the blank is drawn into the die cavity compressive s t r e s s is set up around the flange and it tends to wrinkle or buckle the flange.

Deformation of workpiece during punch travel

Defects in drawing

(a)wrinkling in the flange or(b)in the wall (c)tearing(d)earing(e)surface scrathes

The effect of wrinkling and buckling can be seen from the way a trapezoid on the outer surface of the blank is stretched in one direction and compressed in another direction to become a rectangle on the cup drawn. . Wrinkling and buckling is avoided by applying a blank holder force through a blank holder. . Blank holder force increases friction and hence the required punch load. Therefore, blank holder force should be just enough to prevent wrinkling of the flanae. . The edges of the punch and die are rounded for the easy and smooth flow of metal. . Sufficient clearance is also provided so that sheet metal could be easily accommodated. In sufficient or large clearance may result into shearing and tearing of sheet. . A drawn cup can be redrawn into a smaller cup but it must be annealed to prevent failure. 3.3.4.2 Punching and Blanking

. Punching and blanking operations are not metal forming operations but are discussed together with metal forming because of their similarity with deep drawing operation.

. Objective of punching and blanking is to remove material from the sheet metal by causing ntpture. the punch and die comers are not provided with the any radius.

. Tool steel is the most common material for tool and die. Carbides are also used when high production is needed.

3.3.4.3Defects İn Rolling Schematic illustration of typical defects in flat rolling:(a)wavy edges;(b)zipper cracks in the center of the strip;(c)edge craks;and(d)alligatoring.

3.3.4.4 Defects in forging

Examples of defects in forged parts.(a)laps formed by web buckling during forging;web thickness should be increases to avoid this problem.(b)Internal defects caused by oversized billet;die cavities are filled prematurely,and the material at the center flows past the filled regions as the dies close.

The shaping of thin sheets of metal (usually less than ¼ in. or 6 mm) by applying pressure through male or female dies or both. Parts formed of sheet metal have such diverse geometries that it is difficult to classify them. Sheet forming is accomplished basically by processes such as stretching, bending, deep drawing, embossing, bulging, flanging, roll forming, and spinning. In most of these operations there are no intentional major changes in the thickness of the sheet metal. See also Metal forming.

Stretch forming is a process in which the sheet metal is clamped between jaws and stretched over a form block. The process is used primarily in the aerospace industry to form large panels with varying curvatures.

Bending is one of the most common processes in sheet forming. The part may be bent not only along a straight line, but also along a curved path (stretching, flanging). In addition to male and female dies used in most bending operations, the female die can be replaced by a rubber pad (Fig. 1). The roll- forming process replaces the vertical motion of the dies by the rotary motion of rolls with various profiles. Each successive roll bends the strip a little further than the preceding roll.

Bending process with a rubber pad. (a) Before forming. (b) After forming.

While many sheet-forming processes are carried out in a press with male and female dies usually made of metal, there are some processes which utilize rubber to replace one of the dies. The simplest of these processes is the Guerin process (Fig. 2).

Guerin process, the simplest rubber forming process. (a) Before forming. (b) After forming.

A great variety of parts are formed by the deep-drawing process (Fig. 3), the successful operation of which requires a careful control of factors such as blank-holder pressure, lubrication, clearance, material properties, and die geometry.

Deep-drawing process.

Many parts require one or more additional processes. Embossing consists of forming a pattern on the sheet by shallow drawing. Coining consists of putting impressions on the surface by a process that is essentially forging, the best example being the two faces of a coin. Shearing is separation of the material by the cutting action of a pair of sharp tools, similar to a pair of scissors. See also Coining.

The spinning process forms parts with rotational symmetry over a mandrel with the use of a tool or roller. There are two basic types of spinning: conventional or manual spinning, and shear spinning. The conventional spinning process forms the material over a rotating mandrel with little or no change in the thickness of the original blank. In shear spinning (hydrospin-ning, floturning) the deformation is carried out with a roller in such a manner that the diameter of the original blank does not change but the thickness of the part decreases by an amount dependent on the mandrel angle. Shear spinning produces parts with various shapes (conical, curvilinear, and also tubular by tube spinning on a cylindrical mandrel) with good surface finish, close tolerances, and improved mechanical properties. See also Metal coatings; Spinning (metals).

4.METALLIC STRUCTURES

This page decribes the structure of metals, and relates that structure to the physical properties of the metal.

. The structure of metals . The arrangement of the atoms

Metals are giant structures of atoms held together by metallic bonds. "Giant" implies that large but variable numbers of atoms are involved - depending on the size of the bit of metal.

Note: Before you go on, it might be a good idea to read the page on bonding in metals unless you are reasonably happy about the idea of the delocalised electrons ("sea of electrons") in metals.

4.1 12-co-ordination

Most metals are close packed - that is, they fit as many atoms as possible into the available volume. Each atom in the structure has 12 touching neighbours. Such a metal is described as 12-co-ordinated.

Each atom has 6 other atoms touching it in each layer.

There are also 3 atoms touching any particular atom in the layer above and another 3 in the layer underneath.

This second diagram shows the layer immediately above the first layer. There will be a corresponding layer underneath. (There are actually two different ways of placing the third layer in a close packed structure, but that goes beyond the requirements of current A'level syllabuses.)

4.2 8-co-ordination Some metals (notably those in Group 1 of the Periodic Table) are packed less efficiently, having only 8 touching neighbours. These are 8-co-ordinated.

The left hand diagram shows that no atoms are touching each other within a particular layer . They are only touched by the atoms in the layers above and below. The right hand diagram shows the 8 atoms (4 above and 4 below) touching the darker coloured one.

4.3 Crystal grains

It would be misleading to suppose that all the atoms in a piece of metal are arranged in a regular way. Any piece of metal is made up of a large number of "crystal grains", which are regions of regularity. At the grain boundaries atoms have become misaligned.

Note: Within a crystal grain you get rather subtle irregularities known as dislocations. It isn't important to know about these for UK A level Chemistry (or equivalent) purposes, although they turn out to be essential in discussing the workability of metals at a higher level. I haven't included a description of them here because it is quite difficult to visualise how they work, and I don't want to add unnecessary complications.

4.4 The physical properties of metals

4.4.1 Melting points and boiling points

Metals tend to have high melting and boiling points because of the strength of the metallic bond. The strength of the bond varies from metal to metal and depends on the number of electrons which each atom delocalises into the sea of electrons, and on the packing.

Group 1 metals like sodium and potassium have relatively low melting and boiling points mainly because each atom only has one electron to contribute to the bond - but there are other problems as well:

 Group 1 elements are also inefficiently packed (8-co-ordinated), so that they aren't forming as many bonds as most metals.  They have relatively large atoms (meaning that the nuclei are some distance from the delocalised electrons) which also weakens the bond.

4.4.1.1 Electrical conductivity Metals conduct electricity. The delocalised electrons are free to move throughout the structure in 3- dimensions. They can cross grain boundaries. Even though the pattern may be disrupted at the boundary, as long as atoms are touching each other, the metallic bond is still present.

Liquid metals also conduct electricity, showing that although the metal atoms may be free to move, the delocalisation remains in force until the metal boils.

4.4.1.2Thermal conductivity

Metals are good conductors of heat. Heat energy is picked up by the electrons as additional kinetic energy (it makes them move faster). The energy is transferred throughout the rest of the metal by the moving electrons.

4.5 Strength and workability

4.5.1Malleability and ductility

Metals are described as malleable (can be beaten into sheets) and ductile (can be pulled out into wires). This is because of the ability of the atoms to roll over each other into new positions without breaking the metallic bond.

If a small stress is put onto the metal, the layers of atoms will start to roll over each other. If the stress is released again, they will fall back to their original positions. Under these circumstances, the metal is said to be elastic.

If a larger stress is put on, the atoms roll over each other into a new position, and the metal is permanently changed.

4.5.2 The hardness of metals

This rolling of layers of atoms over each other is hindered by grain boundaries because the rows of atoms don't line up properly. It follows that the more grain boundaries there are (the smaller the individual crystal grains), the harder the metal becomes.

Offsetting this, because the grain boundaries are areas where the atoms aren't in such good contact with each other, metals tend to fracture at grain boundaries. Increasing the number of grain boundaries not only makes the metal harder, but also makes it more brittle. 4.5.3 Controlling the size of the crystal grains

If you have a pure piece of metal, you can control the size of the grains by heat treatment or by working the metal.

Heating a metal tends to shake the atoms into a more regular arrangement - decreasing the number of grain boundaries, and so making the metal softer. Banging the metal around when it is cold tends to produce lots of small grains. Cold working therefore makes a metal harder. To restore its workability, you would need to reheat it.

You can also break up the regular arrangement of the atoms by inserting atoms of a slightly different size into the structure. Alloys such as brass (a mixture of copper and zinc) are harder than the original metals because the irregularity in the structure helps to stop rows of atoms from slipping over each other.