basic metallurgy - a walkthrough

8/21/2019 Basic Metallurgy - A Walkthrough

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Materials II.

Content

1. The phase rule

2. Binary alloys with non-mixable elements.

3. Binary alloys with a continuous series of mixing crystals

4. Binary alloys with partly mixable solid phases. Complicated diagrams.

5. Cast structure of metals.

. !echanical properties and heat treatment of alloys.

". The annealing of unalloyed steel.

#. $ardening of steel

%. Carbonating and nitrating.

&'. Cast iron.&&. (luminium and (luminium alloys.&2. !agnesium- and magnesium alloys.&3. Copper and Copper alloys.

)roblems

1. The phase rule

( metallic element combined with one or more other elements forms an alloy if the resultmacroscopically consist of one metal sort. *xamples are steel+ cast iron+ bron,e+ messing+solder+ aluminium-+ magnesium-+ titanium- lead-+ tin alloys+ etc.

(n alloy that consists of 2 atom sorts is called binary+ of 3 atom sorts tertiary+ of 4 atomsorts uaternary+ etc. The most important element is called the base metal+ the otherelements are called alloy elements.

Admixtures and irregularities.

n metals there are irregularities which are not accounted for as alloys. Copper producedelectrolytically that is used for electric conductions contains a little oxygen. Because
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oxygen hardly influences the conduction it is considered a undangerous admixture. nmany cases a desoxydation substance is added+ which added in abundance+ increases theadmixtures.

/arious alloys carry names which are used for centuries. 0teel is an alloy of iron and

carbon+ bron,e of copper and tin+ messing of copper and ,inc and so is tombac. ( patentfor a new in1ented substance expires+ but the name can be protected permanently+ so thateach factory used to use its own name for an alloy. n the nomenclature the base metalcomes behind and the alloy element before. 0o aluminium-copper is copper alloyed withcopper.

(n mixture of crystals can be distinguished microscopically+ but not macroscopically.The crystals themsel1es in an alloy dont need to be either of the base metal or the purealloy elements. They can be mixing-crystals themsel1es. The alloy elements can formchemical connections with the base metal or mutually. f the connections purely concernsmetals we spea of intermetallic connections. n mixing-crystals the atoms of the alloy

element can replace those of the base metal they are placed substitutional6+ or can beabsorbed by those of the base metal they are interstitial6 placed.

n substitiutional crystals often there is ordering. 7e consider a mixed-crystal consistingof eual numbers of ( and B atoms. n the unordered state they are placed arbitrary o1erthe a1ailable lattice positions. f there is ordering the (-atoms will try to surroundthemsel1es with as many as possible B-atoms and 1ice 1ersa. 0ee figure below

8ig. 9rdering in a body centered cubic lattice with 5': ( and 5': B

This case also appears in Cu-;n alloys with around 5': ;n


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Definition of Phase

( system of substances can be thought of mechanical separated in parts which inthemsel1es ha1e a homogenous composition and structure. The homogenous parts with

the same composition are called a phase. ( system with more than & phase is calledheterogeneous. There is only & gas phase. The bordering plane between gas and liuid orbetween gas and solid state can be thought of as a physical border-plane. There can bemore than & liuid phase. ( system consisting of water+ oil and uic sil1er consists of 3liuid phases. t is thinable to separate the 3 parts mechanically. The number of solidphases can be 1ery large. *ach sort of crystals forms an apart phase because this can beseparated mechanically. ( mixing-crystal is & solid phase because it can only be separatedchemically.

( pure chemical substance will usually appear in 3 phase+ corresponding to the 3aggregation states. ( 4thphase occurs when the material has more than & modification in

the solid state.


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?1ap @ ?li.

Both uantities are dependent on temperature and pressure. f we consider the system at acertain temperature+ then only the pressure-dependency remains. 8or more components in& phase then also the concentration in that phase is a uantity on which the chemical

potential depends.

Phaserule

7e consider 2 independent euations with 3 unnowns x+ y and x. This system normallyis undetermined. (t one of the 1ariables


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This is called the phaserule. t is deri1ed for the general case that in each phase thereappears each component. f & component is missing then the number of relations islessened with & one the phase-rule still applies.

!"stems #ith 1 component $unar" s"stems%.

n this system there are only 2 1ariables+ namely the temperature and the pressure.(ccording to the phase-rule 8 @ 3 - p. 8rom this there follows that there are maximal 3phases in euilibrium with each other+ because 8 cannot become negati1e. Because thereis at least & phase+ we obtain the following possibilities

p @ & 8 @ 2

p @ 2 8 @ &

p @ 3 8 @ '

f there is & phase there are 2 degrees of freedom. f there are 2 phase there is & degree offreedom. 9ne of the 1ariables+ e.g. T can be chosen freely+ but then the other


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8ig. 0tate diagram of copper.

The cross-point of the 3 two-phase-lines is the triple-point.


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There are more triple-points now


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8ig. $eating- and cooling cur1e of iron

(t ""'EC


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( binary alloy consists of 2 components. Onown examples are lead-tin


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7e consider a binary system. The pressure is fixed at & atm+ so that already ha1e one ofthe degrees of freedom. The remaining numbers are 8@ 3 P p. 7e then obtain thefollowing possibilities

a. p @ &+ so 8@2

The 1ariables then are the temperature T and the concentration of ( a one-phase area in the binary system is gi1en by a surface.

b. p @2+ so 8@&

There are now 3 1ariables the temperature T+ the concentration of ( in the first phaseC(&+ and the concentration of ( in the second phase C(2.

9ne of these 1ariables can be chosen freely+ the other lie fixed. f we choose e.g. a certain

temperature then there follow the concentrations C(&

and C(2

of both phases ineuilibrium. n the diagram the chosen temperature is gi1en by two corresponding pointslying at the same height

8ig. The two-phase euilibrium.

(t each temperature we chose we find a pair of points in the diagram. The pair of pointsforms 2 lines. The cross-points of the line pair with a hori,ontal line gi1e the phase ineuilibrium at that temperature belonging to the hori,ontal line.

c. p @3+ so 8@'

There are now 4 1ariables+ namely the temperature and the concentrations of ( in the 3phases C(&+ C(2and C(3.

The number of degrees of freedom is ,ero+ the system gi1es the solution. This means thata 3-phase-system is only found at certain temperatures determined by the system+ and thatthe composition of the participating phases lie fixed.


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The 3-phase-euilibrium thus is gi1en by a hori,ontal line+ on which 3 points gi1e thecomposition of the 3 phases.

8ig. The three-phase euilibrium.

(s an example we consider the binary system gold-nicel which lies beneath %''QC.

8ig. )art of the binary system (u-Ki


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0ystems with one liuid phase when solidifying+ can gi1e in the one extreme a mixture ofcrystals the most usual is part-mixability.

The state ) diagram in the solid state at nonmixabilit".

n the area abo1e the melting points there can exist only liuid+ there is only & phasethere. ( liuid in which there is much ( and little B is considered a sol1ent ( with sol1edmatter B. f there is more matter sol1ed the solidification point is lowered. /ant $offga1e

$ere x is the number of !oles sol1ed matter per !ole sol1ent+ S is the gas constant per

!ole+ T is the solidification point in O and is the melting heat of the sol1ent per !ole.The formula only holds for ideal+ thinned solutions.

*xample we calculate the free,ing point lowering of the sol1ent sil1er.

T @ %& C it tra1els downwards and in the beginning if the solution is thinnedis straight.


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8ig. 0olidification lowering of a dissol1ed substance.

( liuid with composition )'at cooling at )&will gi1e solid matter (


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( mixture of ( and B+ with composition and temperature such that the state goes througha point in the angle 020'2+ must exist of 2 phases in euilibrium the solid ( and a liuid.(t a certain temperature there belongs a certain composition of the liuid and 1ice 1ersa.

The mentioned area is a 2-phase area.

The line pair that determines this area+ consists of the 1ertical 0'02and the obliue line0'2. Through euality of ( and B at the B-side of the diagram the same deduction can bemade. The cross point * of both soluble lines gi1es the euilibrium of the liuid withboth solid states 0(and 0B.

8ig. Binary state diagram of non-mixable phases.

(t cooling both solid states will crystalli,e simultaneously at constant temperature.Beneath this temperature the liuid has disappeared and there are only the solid phases 0(and 0B. The hori,ontal line


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which exactly has the composition *+ solidifies fully+ has a solidifying point. The point *is called the eutectic we spea of eutectic temperature andcomposition. ( solid ally with this composition is called an eutecticum. The eutecticum isa mixture of pure ( and B crystals.

Cooling cur&es.

7ith help of the state diagram it is possible to predict the cooling cur1e of an arbitraryalloy. 7e suppose that per unit of time the same heat is extracted and that the solidifyingheat and specific heat are constant. + then the solidifying cur1es can be constructed.

8ig. Cooling cur1es of alloys with an eutecticum.

$ereby use is made of the property that ratio of the phases can be read from the diagram.The alloy )2at T&is solidified for half while at T2for V. 0uch a series of cooling cur1escan be deduced from the state-diagram.

*onsoluble components and bondings in solid state.


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( bonding will be represented by the general formula (xBy. The indices are integers. Thisbonding / consists of WxL


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This is called an incongruent melting bonding. n such a state diagram the liuid line 8C*will ha1e a in at the point C.

8ig. Binary alloy with dissecting bonding.

This in must lie such+ that the extended of both parts of the liuid lines comes in aheterogeneous area. This is based on the theorem that the stable phase has the smallestsolubility. 9n the dashed line D$O the solubility of B in the liuid is gi1en by D and thesolubility of the bonding will be $. The liuid $ will howe1er gi1e B because it iso1ersaturated with it and thus cant be stable. The thermodynamic potential of the liuid$ is greater than that of liuid D plus the detached uantity B.

Examples of binar" allo"s #ith nonmixable solid phases.

The system bismuth-cadmium is an example of a binary alloy with unmixable solidphases


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8ig. 0tate-diagram bismuth-cadmium.

(n alloy with 2': Cd+ melted and afterwards solidified + has a structure as gi1en in fig.a.below> the eutectic alloy with 4': Cd that of fig.b. and with "': Cd that of fig. c.

8ig. 0tructure of bismuth-cadmium alloys.

The eutectic structure gi1es as intimate mixture of both phases. 7ith help of the micro-photographs it is possible to construct the course of the solidification.


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The system sil1er-lead is nown because a method for concentrating small amounts ofsil1er in lead is based on this diagram.

8ig. 0tate-diagram of sil1er-lead.

The lead is first melted and afterwards cooled to abo1e the eutectic temperature. Kowonly pure lead can crystalli,e out. f at this temperature the remaining melt is fully castaway+ this theoretically contains all the sil1er. This process is called patinsonating.

n the case of a strong asymmetric eutecticum lie that o (g-)b it often occurs that + if thelarge component present in the eutecticum is also the primary detached phase+ that theeutecticum cannot be recognised as such. The mentioned primary crystals continue togrow during the solidification of the eutecticum and thereby enclose the crystals of theother component. The last lie as round spheres or continuous layer along the crystalborders of the first. 0uch a structure is called degenerated eutecticum.


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ntermetallic bondings which are completely unmixable with the components seldomoccur. The system (u-)b is an example that also shows the rarity of 2 incongruentmelting bondings.

8ig. 0tate diagram gold-lead.

Bondings of metals with metalloids on the other hand are usually unmixable and oftenoccur. 0uch bondings mostly are impurities or admixtures which occur in a technical pure

metal+ lie sulphides+ phosphides+ oxides+ nitrides+ carbides+ etc. n these cases the part ofthe state diagram between the bonding and the metalloid is not important+ so that the partbetween metal and bonding is sufficient.

(n example is the system Cu-Cu29


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8ig. 0tate diagram copper-copper oxidule.

(t small uantities of Cu29 there forms a degenerated eutecticum.


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8ig. Negenerated eutecticum at copper-copperoxidule.

(t larger percentages of Cu29 this percentage can be estimated by metric estimate of theamount eutecticum in the micro-picture.

+. (inar" allo"s #ith a continuous series of mixing cr"stals

!tate diagram at complete mixabilit"

There exist some systems in which the components form mix crystals in all ratios+ so thatthere is only solid state+ at least in the euilibrium state. ( necessary condition forcomplete mixability is that both components ha1e the same crystal lattice and the

parameters dont differ too much mutual.

The state diagram at high temperature shows one homogeneous liuid area and at lowtemperature one area of the homogeneous solid state. n both cases the number of degreesof freedom is two because the temperature and the concentration can be changedsimultaneously without a change of phases occurring. These both areas are separated by a2-phases area.

f an alloy in half melted state is in euilibrium the composition of the liuid and the

solid state will not be the same. n the euilibrium liuid-solid the liuid will be richer inthe low melting component and the solid state richer in the high melting one. Becausethis holds for all concentrations in such a system between the melting points of thecomponents there lie 2 lines.


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8ig. 0tate diagram at complete mixability and complete diffusion

The upper line is called the liuidus-line the lower one is called the solidus line. ( pointbetween these lines represents a 2-phase state. $ereby the both coexisting phases lie onthe crossing points of the hori,ontal line through this point.

Cooling cur&es.

( melted alloy with a certain composition ) at cooling after reaching the liuidus line


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8ig. Cooling cur1es at complete mixability.

Diffusion in the solid state.

The in the beginning originated crystals at further solidification will grow with a layer+which is richer in the layer melting component than the nucleus. To maintain theeuilibrium the atoms of this component in the crystal will diffuse inwards and the that ofthe other outwards. ( diffusion in the solid state is much slower than in a liuid+ wherethe unordered motion promotes a continuous mixing of the atom sorts.

f atoms B diffuse in the metal (+ they must be sol1ed in ( either substitutional orinterstitial.

n a substitional mixing crystal diffusion is possible because there are empty spaces+1acancies+ in the lattice. They are a direct conseuence of the heat motion of the atoms> athigh temperatures there are more 1acancies in thermal euilibrium than at lower ones.

n an intersitional mixing crystal the sol1ed atoms can Uump from the one to the otherlattice-position+ because there are wide canals6 which connects these positions.

Cr"stal segregation.


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To approximate the practice case+ we assume that in the solid state there is no diffusionpossible. The solidification then is according to the figure below.

8ig. 0tate diagram at complete mixability and without diffusion.

The liuid ) at begin of solidifying will gi1e a solid state 8 which forms the nucleus ofthe crystals. The nucleus during solidification will not change in composition so that itstays on the 1ertical line 87. The liuid becomes (-poorer and must be cooled for furthersolidification. The segregated solid state + which has the composition gi1en by the solidusline+ forms the outer layer of the crystals. The a1erage composition of the crystals therebycomes one a line between the 1ertical 87 and the solidus line. n this is the ratio O!.

f the point $ is reached


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8ig+ 0tate diagram at complete mixability and incomplete diffusion.

The crystal segregation of a dendrite can be seen sharply.

8ig. Nendrites with crystal segregation


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,omogenous glo#ing.

$omogenous glowing undoes the crystal segregation+ whereby the temperature is chosenhigh to eep the time short.


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The two-phase area that lies between the one-phase areas does not need the ha1e thepre1ious shown fish-from. Two-phase areas with maximum and minimum also appear.

8ig. Two-phase-diagram of mixing crystals with a maximum


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8ig. 0tate diagram of copper and nicel.

(n example of a two-phase area with a minimum can be found in the upper part of thestate-diagram titanium-1anadium.

8ig. The state diagram titanium-1anadium.

-. (inar" allo"s #ith partl" mixable solid phases. Complicated

diagrams.


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state diagram #ith a eutecticum of mixing cr"stals.

Binary systems in which both components are soluble in each other+ can gi1e aneutecticum+ whereby the two phases are not the components+ but the mixing crystals+represented by the points C and N below.

8ig. 0tate diagrams with an eutecticum of mixing crystals.


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8ig. Netermination of the position of the solubility line

Z one-phase alloy [ two-phase alloy

(nother method is to measure the physical properties all samples glowed at &temperature. These obser1ations are other glow temperatures to find more points of thesolubility line.

7e consider the alloy a of the following figure+ where part of a binary system is drawn.n an ideal euilibrium case the solidification will be complete at R+ an after cooling a &-

phase alloy will be found under the microscope.

8ig. Crystal segregation.


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f there is no or little diffusion then the a1erage composition of the solid state will go e.g.along the line 9S. n this case the solidification e1en at the eutectic temperature isntcomplete+ but there will remain a uantity liuid of the eutectic composition+ which

relates to the amount of solid matter as S0 0*.

!tate diagram #ith a peritecticum

(t diagrams with an eutecticum the solidus- and liuidus- line of both components lowerwhen adding the other. t is also possible that the one pair lowers and the other one rises.

f this is the case a diagram with a peritecticum originates.

8ig. 0tate diagram with a peritectum


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A 0( 0B

(n alloy lying between C and N at solidification will gi1e mixing crystals of ( till the

temperature of the three-phase-line is reached.

Allo"s #ith a demixing area in the solid state.

( special case occurs if in the area of the homogeneous solid phase there is demixing.

8ig. Nemixing area in the solid state without bonding


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8ig. Nemixing area in the solid state. The bonding forms no mixing crystals


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f the 2ndphase is a liuid the liuidus line at this temperature will ha1e a in. f one orboth modifications form mixing crystals with the other component then the transitionpoint will change by addition of this last.


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f metals in the liuid state are not or little mixable+ then there are 2 liuid phases. (tsolidification there originate 2 solid phases abo1e each other which dont form an alloy. tis nown that lead gi1es 2 liuid phases with different metals lie copper+ ,inc+ iron andaluminium. (t high temperature the solubility often increases and the demixing area candisappear. ( nown example is the system ,inc-lead.


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eading complicated binar" state

diagrams

(T each temperature and each concentration it can be gi1en from which phases thesystem in the euilibrium state consists.

(T a 3-phase-line there originate 3 two-phase areas+ which describe the euilibrium ofthe 3 pair-wise combinations of the 3 phases. f+ from these 3 two-phase areas there lie 2abo1e the 3-phase-line+ then the 3-phase-euilibrium is eutectic in type> the opposite caseis the peritectic type.

8ig. Three-phase-euilibria


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8or this a spatial diagram is needed.


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8ig. Ternary 0tate diagram

8ig. )roUection of a ternary diagram.

/. Cast structure of metals.

0ormation of nuclei and solidification in a simple metal.

n the solid state the atoms are ordered in a lattice+ where the 1ibrate around theireuilibrium state. (s a conseuence of this heat motion+ at high temperatures a number ofthese lattice positions will be unoccupied. These 1acancies mae that neighbouring atomscan Uump to another unoccupied place. n the neighbourhood of the melting point eachatom maes around &'Uumps per second. (t melting the regular structure is not lost.


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*ach atom will ha1e other neighbours. n solid metal with close paced structure eachatom is surrounded by &2 nearest neighbours. n the liuid state this continuously changesbut seldom smaller than &2. The liuid state has another structure than the close pacedstructure and thus has a lower specific weight. (ll close paced metals at melting ha1e a1olume increase> this is in most cases 4-5:. Cubic body centered metals at melting ha1e

a 1olume increase of 2-3:. 0ome metals at melting ha1e a 1olume decrease+ bismuth+germanium+ and gallium. n the )-T diagram the melt line will incline to the left.

The continuous change in the mutual position of the atoms maes that the specific heatfluctuates+ whereby it can reach a maximum. This is the first step in the solidificationprocess the formation of a crystal nucleus. The chance that a density fluctuation causes atthe melting point causes a nucleus is small. Therefore The liuid can be cooled far belowthe melting point before solidification begins. This phenomenon is called undercooling.This is the lowering of the temperature of a liuid beyond the free,ing temperature and

still maintaining a liuid form. Kormal free,ing occurs when the atoms of the containerwalls impose an ordering on the liuid atoms causing them to arrange themsel1es into acrystalline structure and begin to grow. 7ithout the container+ the onset of free,ing


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8ig. 0tructure of uicly cooled copper


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8ig. 0tructure of slowly cooled copper

!olidification and cast structure of allo"s.

n melted alloys the formed nuclei in general has a different composition than that of theliuid.

The cast structure at alloys which form mixing crystals at large is the same as those insimple metals.

The difference is that there is a solidification traUectory instead of a solidification point.

(s a conseuence the dendritic seleton propagates through the whole liuid and that theliuid between the braches only solidifies later.

(n alloy with the eutectic composition will solidify differently than a simple metalbecause both inds of crystals influence each other solidification.


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t often occurs that there originate a structure in which both phases appear as alternatinglayers


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8ig. *xamples of eutectica

a. lamellar+ b. 1ane-lie + c. dendritic+ d. globular.

f the liuid alloy has a structure different from that of the eutecticum +one of the phaseswill begin to solidify and the eutecticum will fill the space which remains aftersolidification of a primary formed crystals. $ere also the primary solidification 1an bedendritic or globular. The difference with the pre1ious is the last case the intermediatespaces are filled with eutecticum. f the amount of primary solidified phase is large andthere remains little eutecticum then at solidification of this the primary phase will settle

on the present crystals and the remaining space will be exclusi1ely filled with the secondphase. $ere there originate small holes between the branches of the dendrites or a thinnetwor around the spheroid crystals. The last case is called degenerated eutecticum.

The shrin hole

( metal expands a few percentages ate melting. (t solidification the 1olume will become

smaller. (t casting the difference in 1olume causes shrin holes. n a cast bloc the shrinhole lies in the middle of the cross section an penetrates deep into the metal. f the after-flow at casting by uic solidification of transition between cast sample and riser is notpossible+ there will originate a shrin hole in the cast sample.


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8ig. 0hrin holes in risers


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2as inclusions

n the liuid different gasses are sol1ed of which oxygen. Kitrogen and hydrogen are themost important. The solubility of these gasses in solid state is 1ery small+ so that theyfreed at solidification. By the low specific weight the gasses rise and mae the metalbubble.

(t a certain cast method of steel by the freeing of C9-gas the 1olume of the solidifiedmetal is much greater that the shrin hole can disappear. (t rolling and forging the flawsor gas bells in the solidified metal are welded off.

2ra&itation segregation.

t is possible that a solidifying metal has a specific weight that differs from that of meltfrom which it originates. (s a conseuence these crystals will float or sin. Thisphenomenon is called gra1itation segregation. The most nown example is an alloy of%': 0n and &': 0b. The primary 0b0n crystals separated off gather in the upper partwhere hardness is much higher than in the lower part.


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To pre1ent this we choose an alloy of #':0n+&':0b and &':Cu. Kow there firstcrystalli,e a networ of Cu0n5 which pre1ents the following 0b0n-crystals to rise.


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8ig. Critical tensions at a continuous series of mixing crystals.

n the case of polycrystalline alloys+ consisting of mixing crystals+ the course of rupturelimit+ pull strength and hardness with concentration is as described abo1e.

The mechanical beha&iour of a mixture of cr"stals.

Contrary to a mixing crystal at a mixture of crystals the properties as strength+ hardnessand toughness lie in between those of the composing crystals.

The geometric distribution of the both crystal sorts in the metal has a large influence onthe mechanical properties. fthe structure consists of a networ of brittle crystal sortwhich encloses the tough one then the metal will be brittle compared with a metal withthe same amount brittle imbedded in the matrix of the other crystal sort. f the base metalsolidifies in dendrite form then a brittle bonding forms between the branches of will notas large a influence as at a spheroid solidification of the base metal whereby the bondingforms a closed networ. (fter casting and solidification the impurities often such a

networ.

ntermetallic bondings often will be brittle > their complicated crystal structure does notallow plastic deformation. (lso the ordering phenomena gi1es a lowering of thetoughness.

The glo#ing of cast pieces.

f the casting occurs slowly then the euilibrium will ha1e settled and a glow treatmentwill ha1e little influence. !ostly the cooling will be so uic that the euilibrium has not


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The soft glo#ing.

f a multiple-phased metal contains & phase in the form of lamellas + needles or a networ

then at heating there can occur a structure change+ whereby this phase goes into thesphere form. The tendency to transition of the lamellar into globular form comes from thedifference in the surface-energy. This is smallest in the at the globe form because here thesurface is the smallest relati1e to the content. This energy per unit of surface decreaseswith increasing radius of cur1ature+ in other words at increasing si,e of grains. The moststable form thus is a single spheroid crystal. (t longtime glowing the lamellas slowly gointo spheres


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epair glo#ing and the glo#ing out of allo"s.

The difference between the glowing of alloys and simple metals is not large. n bothcases the internal tensions must be lessened. This can also be obtained by repair glowingand recrystalli,ing in alloys.

(n example of the repair glowing is the application of this heat treatment on colddeformed messing. !essing is susceptible to corrosion+ whereby the effect penetratesdeep into the metal along the grain boundaries. The internal tensions let the corrodedmessing burst along the crystal boundaries


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The inter-crystalline corrosion which at messing is called the season sicness + does notoccur if the metal is glowed> thereby it is sufficient to warm the messing during half a

hour at 2"5-3''C. This heat treatment has little influence on the hardness+ toughness+

and strength.

The precipitation hardening.

This hardening method is also called 1apour or mist hardening and does not occur in puremetals+ but only in alloys. 7e consider a simple binary system (-B


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solution of B in + which in general has the tendency to separate off -crystals. after some time there occur the firstsubmicroscopic separations

because the direct atomic coherence is maintained


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Example of precipitation hardening.

8ig. precipitation hardening of duraluminium

a. (fter-annealing temperature 2'C.

b. (fter-annealing temperature &5'C.

5. The annealing of unallo"ed steel.

Iron forms.

martensite

FFF!artensiteFFF+ named after the Derman metallurgist (dolf !artens+ is a class of hard minerals occurring as lathe- or plate-shapedcrystals. 7hen 1iewed in cross-section+ the crystals appear acicular


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during uenching. n the %'s+ !artens studied samples of different steels under a microscope+ and found that the hardest steels had aregular crystalline structure. $e was the first to explain the cause of the widely differing mechanical properties of steels

!artensite is a body-centered cubic form of iron in which some carbon is dissol1ed.!artensite forms during uenching+ when face centered cubic austenite changes to thebody centered cubic structure without the precipitation of cementite. nstead+ the carbon

is retained in the iron crystal structure+ which is stretched slightly so that it is no longercubic. !artensite is more or less ferrite supersaturated with carbon. Compare the grainsi,e with tempered martensite.

8ig. 0tructure martensite

Tempered Martensite7hen martensite is tempered+ it partiallydecomposes into ferrite and cementite.Tempered martensite is not as hard as Uust-uenched martensite+ but it is much tougher.

Kote also that it is much finer-grained thanUust-uenched martensite.


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Austenite

(ustenite is a face centered cubic form ofiron in which some carbon is dissol1ed.

(ustenite forms abo1e the criticaltemperature.

8ig. (ustenite


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Cementite

Cementite is ron Carbide+ with the formula

8e3C.

0errite

8errite is a body-centered cubic form of iron

in which some carbon is dissol1ed.

6nallo"ed steel

0teel is an iron alloy which is forgeable. 0ome steel sorts ha1e more than &.5:carbon+ so it hard to gi1e an exact definition. The best definition would be an + insolidified or sintered state+ forgeable iron alloy in which there is no free carbon.Besides iron and carbon there are 1arious other impurities in the steel.


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The differences in cast steel and rolled steel lie in the different production techniues.(T solidification of steel there originate a relati1ely large dendrite structure. The lastsolidifying matter between the dendrites due to segregation is less pure. n these there

can also originate gas P or shrin holes. (t rolling the steel the structure becomesfiner by recrystalli,ation> the pores are pressed closed> the segregation areas arestretched into large paths.

Ironcarbon diagram

This holds for alloys of iron and carbon+ other elements dont occur. t was drawnafter research of Bramley+ ord+ !ehl and 7e1er. The euilibrium state and the 3-phase reactions can be deduced from the diagram. The liuid-area


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8ig. ron-carbon diagram.

structure of glo#ed steel.

Kormal glowing consists of heating the


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8ig. amellar pearlite.

(t steel with a low carbon content at cooling in the autenite area there first will formferrite and afterwards at reaching '.# : C in the austenite + transition in pearlite willoccur. (t ':C all is light and at '.# :C all is dar.


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8ig.


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8rom the boundary of the pearlite it can be seen that the ferrite crystals are formedglobularly. (t a low C content this leads to triangular pearlite islands6.

(t steel with a higher carbon content larger than '.#:


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8ig. 0tructure of normal glowed abo1e-eutectic steel.

*ormal glo#ing $annealing%

This is defined as heating of steel in the austenite area followed by cooling in calm airor cooling such that the pearlite is obtained in lamellar state.

8ig. 0chematic representation in time of normal glowing.

&-2 states at steel fabrication

2 deli1ery state

3- normal glowing

The structure before and after normal glowing is seen in the figure below.


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Before normal glowing

(fter normal glowing

7. ,ardening of steel


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The simplest method of hardening consists of heating the steel till the austenite area+

till abo1e "23C+ followed by uenching in water. By the high cooling 1elocity the

carbon diffusion is suppressed so far that there can not form pearlite or troostite.Nuring uenching the austenite will transform into ferrite+ and such that the carbonatoms cannot escape in time and in unsaturated state stay behind in the ferrite. The so

formed structure is called martensite. their formation time is less than &'-"sec> in such short times thediffusion processes hardly play a role. The transition form austensite to martensite isconsidered a umlapp process> of which the principle is setched below.


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8ig. ( future martensite-cell in austensite.

$ere two cells of the cubic body plane austensite is setched. The bold lines indicate

that the structure can be described as a tetragonal body centered with an axis ratio cLa@ 2&L2.

(t the transformation of the austenite lattice in the -lattice the displacements of

neighbouring atom relati1e to each other is small. Because of this the -lattice can

umlapp and this process can propagate with large 1elocity through the -lattice.

There exist an orientation relation between the austenite and the formed martensitecrystals. (t the transformation the tetragonal cell goes into a cube.

(t hardening there is a 1olume increase of &.3 :. The martensite diss are pressedout of the austenite. By this there originate tensile stresses in the austenite+ whichhinder a further growth of the martensite diss and new diss. (s a conseuence inhardened steel there is an amount of non-transformed austenite+ which is called rest-austenite.

8ig. Sestaustenite.


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The T.T.T diagram.

7e1er and *ngel around &%3' did experiments about austenite transformation and the

hardening of steel. These were put in a diagram as a function of temperature + theT.T.T diagram


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t appears that the transformation begins after a waiting period


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8ig. The T.T.T diagram of unalloyed pearlitic steel.


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8ig. Bainite. a. $igh bainite b. ow bainite. !agnification 25''x.

The structures originated are

a. )earlite. 8irst there originates a 8e3C-nucleus> for this carbon is needed. This isextracted from the near surroundings+ which therefore unmlapps to a ferritelamella > in this there only sol1es little carbon.


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b. Bainite. $ereby the -lattice umlapps to ferrite after the incubation time has

passed. n this carbon can diffuse much uicer than in austenite+ through whichin the umlapped areas direct separation of 8e3C originates+ which is finer is thetransformation temperature is higher.

c. martensite. $ere the -lattice goes into the -lattice+ thereby catching the carbon

which cant escape.

.

,ardenings methods

8rom the TTT diagrams the hardening methods used in practice can be described.


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8ig. Cooling diagrams in the TTT diagram.

The beha1iour of steel can be described by CCT


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8ig. 0phere-shaped troostite.

(n insufficient cooled steel+ shows + due to impairment after annealing or tempering+coloured places


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8ig. The relation between the critical cooling 1elocity and the carbon content atunalloyed steel.

The hardening depth.

f a steel sample is uenched the outer ,one cools much uicer than the inward

directed parts. The thicness of the shell6 in which there is complete hardening iscalled hardening depth. To measure this the steel must be grinded+ prepared andetched. Because the troostite can be affected the border can me seen and measured.

( well-nown method is the method of \ominy+ whereby a cylinder heated athardening temperature+ is uenched by a water stream. (lso 2 thic diss with smoothsurfaces can be pressed against each other and uenched commonly. n both casesthe course of the hardness is measured perpendicularly to the hardened surface. Thiscourse+ measured at hardened steel with '.#:C is gi1en below.

8ig. Course of the hardness perpendicular to the surface of hardened pearlitic steel.


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The corresponding structure image is gi1en below.

8ig. 0tructure of hardened pearlitic steel from the border to the ernel.

t will be clear that a sample can only be hardened thoroughly if the maximalthicness is not bigger than ca. 2 x the hardening depth. 9f the unalloyed steel sortsthe eutectic ones ha1e the largest hardening depth> this is ca 3-&'mm + so samplesfrom unalloyed cannot be hardened thoroughly if they are thicer than -2' mm.

7e consider the beha1iour of unalloyed undereutectic steel. This steel after hardening

is less hard than eutectic steel+ because martensite with a lower carbon content hasless internal tension and thus is less hard.


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8ig. The hardness of hardened unalloyed steel as a function of the carbon content


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8ig. the hardening temperature in dependency of the carbon content.

The austenitation of abo1e-pearlitic steel doesnt occur in the austenitic area+ but 5'-"'C abo1e (&. (fter glowing at this temperature this temperature the steel consists of doesnt mae sense> the end product cannot become harder.

The diffusion 1elocity of the carbon atoms is 1ery large due to the interstitialpositioning. The heating in the austenitic area for small samples doesnt ha1e theoccur more than &'-&5 minutes. (t larges samples the center warms less uic thanthe borders so that austenitation remains there.

Tempering and refinement.

0teel hardened by uenching is too brittle for use+ and after some time can crac.Therefore another treatment is needed annealing or tempering. The higher thetempering temperature the softer the and also tougher the end product.


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T9 remo1e the largest brittleness without hardness loss it is sufficient to heat the steel

at ' C during ] to & hour. $ereby there originates fine hexagonal iron carbide


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!oft glo#ing in ' stages

The soft steel is first austenitated in such a way+ that all cementite is sol1ed inaustenite. 8or abo1e-pearlitic steel this means glowing abo1e (&but beneath (cm>for pearlitic or under-pearlitic steel this must be glowed during short time abo1e (&.whereby the steel is transformed isothermally. The remaining cementite particles inthe austenite function as nuclei+ from which the transformation begins. Thereoriginates a soft glow structure of globular cementite. f the austenitic temperature isso high that all cementite in the austenite is sol1ed then the bac-transformation ismuch slower+ while the resulting structure if finer and partly lamellar + so that thehardness is higher.

This soft glowing gi1es the globular structure much faster than other methods. (disad1antage is that the temperature area + in which the globular transformation mustoccur


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,eat treatment of allo"ed steel.

8or alloyed steel sorts we use TTT diagrams. Beneath an example of K-Cr-!o-steelis gi1en.

8ig. TTT diagram of a wealy alloyed Ki-Cr-!o-steel


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Carbonating resources

T9 obtain an uniform carbonation a liuid or gas resource is used. 0olid carbon in aninert surrounding will only gi1e local carbonation+ after the contact is broen.

( liuid carbonation substance consists of a bath of molten salt mixture+ mostlychlorides of natrium


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The theoretical end state is reached if there is euilibrium between oxygen+ gas andmetal. To increase the carbon content in the austenitic area the steel must be heated.The carbonation occurs at temperatures in the neighbourhood of %'' C. The steel

surface thereby obtains a carbon content of '. &.2 :+ depending on the

temperature and the C9content of the carbonating gas. f the carbon concentration at

the surface is much higher than in the nucleus+ the carbon atoms will diffuse inwards.

8ig. )enetration cur1es at carbonating + schematically. a. after t hours. b after 4 thours.

f the air consists of #': of nitrogen+ and this gas is inert relati1e to the carbonationreaction+ the process is accelerated by remo1ing nitrogen. This is done by addition ofbarium carbonate+ that during carbonation slowly dissects according to

BaC93

Ba9+

C92

The occurring C92dri1es away the air and participates and the carbonation process.


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8ig. 0tructure of the carbonated shell.

,ardening of carbonated steel.

t seems attracti1e to harden the carbonated samples directly and sa1e the cost heatcosts at hardening.

(n obUection against this method is + that the structure of the austenite duringcarbonating+ which is short+ becomes rough. (t unalloyed steel this method isapplicable in the simplest cases. To use this method in more general cases+ light

alloyed steel sorts must be used+ lie chromiummolybdenumsteel. (t such a steel at

normal carbonation e1en after hours gi1es no grain growth+ so direct hardening has no


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obUections. (n ad1antage of such a steel is that complete hardening is reached byuenching in oil+ whereby the danger of cracing is much less than uenching inwater. The whole heat treatment is gi1en in the scheme below.

8ig. Carbonating and direct hardening of an alloyed carbonating steel+ schematic

(nother ad1antage of the use of unalloyed steel is that at uenching there are alsostructure impro1ements in the nucleus+ so that this becomes tougher and stronger thanin the case of unalloyed steel. (t the direct hardening there is uenched at the

carbonation temperature (%''%5'C=. this is much higher than the hardening

temperature which due to the carbon content at the border (0.8:)would be ideal

(around ""0C). The process then is as setched below.


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8ig. Carbonating and nondirect hardening.

Nue to grain growth there will be used a light alloyed carbonating steel. The firstcooling a must be done at such a 1elocity that there originates a fine troostitestructure+ if possible also in the nucleus. (T the following austenitating at the borderthere originates 1ery fine grained cementite in the austenitic ground mass. Theuenching b is done in oil.

0uch a treatment can also be done with unalloyed steel. The nucleus will then ha1e a1ery rough structure. This can be fixed by adding an extra growth phase =normalflowing of the nucleus after carbonating.

*itrating.

(T gas nitrating the steel to be hardened at about 5''55'C is glowed fro a long time

in an ammoniac stream. Mnder these circumstances the ammoniac at the steel surfacewill dissect according to

2K$32K +3$2+

whereby the iron wors a catalyst. The nitrogen in atomic form+ which herebyoriginates+ can sol1e interstitial in the steel =at 5''C some promilles and diffusesinwards.


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8or gas nitration special steel sorts are used. ( nitration steel usually contains '.3

'.4: C+ about &: (l and about &: Cr+ and further a few promille of !o. Thementioned alloy elements ha1e a large affinity to Kitrogen+ but the inward diffusing

Katoms will bond with the (l+ Cr+ and !o atoms to nitrides. This 1ery fine

dispersion of nitrides increases the hardness (well abo1e hardened steel= to more than

&''' g-mm2according to /icers method. The difference with carbonating is thathere the hardening occurs during the diffusion process and need not be obtained byuenching.

The diffusion is slow and is slower is the carbon content is higher. n practice anitration depth of some mm is obtained after hours of nitrating. Thus it is expensi1e.Temperature rise doesnt speed the process because abo1e 5#'C there forms austenitewhich at cooling transforms into soft braunite+ a eutectoid of ferrite and 8e4Kcorresponding with pearlite. Kitrating is applied to impro1e wear resistance andfatigue properties.

19. Cast iron.

The bonding 8e3C or cementite doesnt appear to be stable. The dissection 1elocity isso small that is appeared the cemenitite was apparently stable. 7e spea of metastablebonding.

The dissection relation 8e3C 38e A C is ending and cannot be written as a

euilibrium reaction. 8or reaching a reasonable dissection 1elocity a high temperatureis needed.

There are certain elements which accelerate the reaction silicium+ nicel. 9therelements ha1e an opposite effect manganese and chromium.

!etastability of cementite implies that also the iron-carbon diagram is metastable.

The solubilit" of cementite and graphite.

ron+ at a suitable chosen temperature brought into contact with cementite + willabsorb this bonding till there is saturation. The same is the case with iron in contactwith graphite. The solubility of cementite+ expressed in percents of carbon+ will bedifferent than that of graphite. ( stable substance will ha1e less inclination to go intosolution than a metastable substance. The solubility of graphite at a certain


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temperature will be less that that of cementite. This holds for the solubility inaustenite as that in ferrite.

The stable ironcarbon diagram.

8rom the abo1e their follows that the 3 solubility lines NC+ *0 and )R forgraphite are to the left of the corresponding solubility lines for cementite. Thecementite line expires+ so that the 3 mentioned solubility lines form the border of 2-phase areas in which graphite occurs besides liuid+ austenite and ferrite. Thediagrams extends to &'':C.

The 3-phase line austenite-liuid-graphite becomes higher than that of the 3-phase-

line ferrite-austenite-cementite. The upper line is followed. The liuid solidifies as amixture of austenite and graphite.

IronIron Carbide Phase Diagram


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8ig

8igure shows the euilibrium diagram for combinations of carbon in a solid solution of

iron. The diagram shows iron and carbons combined to form 8e-8e3C at the .":C endof the diagram. The left side of the diagram is pure iron combined with carbon+ resultingin steel alloys. Three significant regions can be made relati1e to the steel portion of thediagram. They are the eutectoid*+ the h"poeutectoid(+ and the h"pereutectoidB. Theright side of the pure iron line is carbon in combination with 1arious forms of iron calledalpha iron


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(t 4.3: C and 2'E8+ the transformation is eutectic+ called ledeburite.


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8ig . Time-Temperature )aths on sothermal Transformation Niagram

a.


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d.


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The iron-carbon phase diagram showing the eutectic and eutectoid reactions.

Drey cast irons are softer with a microstructure of graphite in transformed-austenite andcementite matrix. The graphite flaes+ which are rosettes in three dimensions+ ha1e a lowdensity and hence compensate for the free,ing contraction+ thus gi1ing good castings freefrom porosity.

The flaes of graphite ha1e good damping characteristics and good machinability


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iron. The magnesium is freuently added as an alloy with iron and silicon


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2re" ferritic cast iron.

(t solidification of ferritic cast iron the stable diagram is completely followed


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Drey cast iron+ 8e-3.2C-2.50i wt:+ containing graphite flaes in a matrix which ispearlitic. The lamellar structure of the pearlite can be resol1ed+ appearing to consist ofalternating layers of cementite and ferrite. The specled white regions represent a

phosphide eutectic. *tchant Kital 2:

n decreasing order of carbon content we ha1e white+ grey pearlitic and grey ferritic castiron. There is no sharp transition between these sorts.

!hrining and gro#th of cast iron.

(n alloy in general is more castable is the melting point is lower an the 1olume changeduring solidification and further cooling is smaller. Drey cast iron satisfies theseconditions. ( cast temperature of &2''-&35'QC is easily obtained.

Because dissection of cementite into iron and graphite is paired with a 1olume increase+cast iron at solidification will shrin less than cast iron and the less if it is more ferritic. (ferritic cast iron with much carbon will expand e1en at solidification. The cooling after

solidification is paired with graphite separation+ so the shrin is relati1ely small.


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larger. n the iron-carbon diagram the eutectic carbon content is 4.3:. Becausemanganese and sulphur are to be neglected we following formula is used for the eutecticcarbon content

4.3 P


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8ig. 7eichelt diagram for thicness of 3' and mm.

The form of graphite.

(s can be seen in the following figures the graphite has the form of lamellas+ because thestructure of the graphite is hexagonal. The si,e of the graphite lamellas relates to theirnumber the more there are+ the finer they are.


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8ig. Drey ferritic cast iron. 0tructure composites graphite+ ferrite+ and steadite


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8ig. Drey pearlitic cast iron.

!pheroidal 2raphite Cast Iron

The chemical composition of the cast iron is similar to that of the grey cast iron but with'.'5 wt: of magnesium. (ll samples are etched using 2: nital.

0pheroidal graphite cast iron+ 8e-3.2C-2.50i-'.'5!g wt:+ containing graphite nodules ina matrix which is pearlitic. 9ne of the nodules is surrounded by ferrite+ simply becausethe region around the nodule is decarburised as carbon deposits on to the graphite.*tchant Kital 2:


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0pheroidal graphite cast iron usually has a pearlitic matrix. $owe1er+ annealing causesthe carbon in the pearlite to precipitate on to the existing graphite or to form further smallgraphite particles+ lea1ing behind a ferritic matrix. This gi1es the iron e1en greaterductility. (ll samples are etched using 2: nital.

Graphite nodules in a ferritic matrix.

Draphite nodules in a ferritic matrix. 0ome carbon deposited during tempering is also1isible. *tchant Kital 2:

(lacheart Cast IronBlacheart cast iron is produced by heating white cast iron at %''-%5'oC for many daysbefore cooling slowly. This results in a microstructure containing irregular thougheuiaxed nodules of graphite in a ferritic matrix. The term GblacheartG comes from thefact that the fracture surface has a grey or blac appearance due to the presence ofgraphite at the surface. The purpose of the heat treatment is to increase the ductility of thecast iron. $owe1er+ this process is now outdated since spheroidal graphite can be


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produced directly on casting by inoculating with magnesium or cerium. (ll samples areetched using 2: nital.

Blacheart cast iron. Blacheart cast iron. *tchant Kital 2:

Mechanical properties

7hite cast iron is due to its content of cementite 1ery hard and brittle and can only beprocessed by grinding. t is used for its hardness in e.g. bullets.

Drey cast iron contains graphite in the forms of lamellas which causes a change in

mechanical properties compared with cast steel. The round mass which consists ofpearlite and


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8ig. *xplanation of the mechanical properties of grey cast iron.

f the tension at the border of a graphite lamella surpasses the pull strength then thelamella cracs open. Because of this the tension in the remaining ground mass increasesfurther and the crac expands o1er the whole surface. (T the pull test a low pull strengthand low stretch is found.

(t the pressure test the lamellas are pressed closed and gi1e resistance. The thumb rulethat the hardness is trice the pull strength doesnt hold for cast iron. (nother conseuenceis that the elasticity modulus at pressure is larger than at pull+ so that the bending strengthis larger than the pull strength. The nodular cast iron is stronger+ tougher and stiffer thanthe corresponding lamellar because the spheroid graphite influences the structure muchless and causes tension concentration than the lamellar. 0ee fig.


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8ig. Kodular ferritic cast iron.

Internal tensions in cast pieces.

n a cast piece there are internal tensions. The character of these tensions is determinedby the way of casting+ solidifying and cooling and by the form

(t a cylindrical rod the solidification will start at the outside. The inner will shrin and atcooling shrin e1en further than the shell because the temperature is higher. (s aconseuence in the nucleus there will originate pull- and in the shell pressure tensions. fthe cylinder is closed off at the outer side it will become shorter.


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8ig. ( cast piece for pro1ing internal tensions. The side-rods solidify first+ the middlepiece later> the first will ha1e pressure the last load. This is shown by increase of thedistance between the mars.

2lo#ing of cast iron

The heat treatment of cast iron is mostly restricted to tension free glowing at about5''QC. f also the processability must be impro1ed there is glowed at a highertemperature. n both cases there is chance at growing which deteriorates the mechanicalproperties.

The processability becomes larger if there is more grain+ and by disappearance ofcementite. Tension free glowing of white cast iron doesnt mae sense because there theledeburite net doesnt change much. Because the internal tensions mostly appear in theshell + the glowing occurs after the preprocessing.

$ardening and tempering of cast iron is applied little. 0amples of white cast iron willcrac at hardening by 1olume increase. Drey pearlite cast iron is hardened in oil to a1oidcracs. The present alloy elements silicon+ manganese+ phosphor+ etc+ delay the critical


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cooling 1elocity by obtaining hardening in oil by uenching. The tempering temperaturedepends on the desired properties.

0orging and rolling of cast iron.

Drey cast iron in the austenite area is forgeable because there all cementite is taen in theaustenite> white cast iron difficult. The forgeability is not as good as that of steel becausethe cast lamellas gi1e rise to cracs and because the silicon maes the austenite stiffer andless tough. (t enduring forging the graphite lamellas are stretched in the direction of theforging or rolling. n this stadium the originated cracs can be welded closed.

Kodular cast iron is better forgeable than ordinary.

0orgeable cast iron.

ac of toughness is a disad1antage for cast pieces. This can be undone by maing castiron that contains no graphite lamellas. This is the case at nodular cast iron. There are 2methods of heat treatment to obtain the desired result.

The oldest process is researched by Seamur in &"&-&"25+ consist of heating of whitecast iron in a wea oxidi,ing surrounding. The carbon is oxidi,ed+ but not the iron

C92A C 2C9 This C9 gas can reduce iron ore according to

C9 A 38e293C92A 28e394

By diffusion there originates a carbon stream from the middle to the surface. f theprocess is long then the result is wholly ferritic iron without carbon. This method is called

glow freshening> it is only applicable on cast pieces with small wall thicness.

The other process+ the tempering white cat iron is heated in a neutral surroundings undersuch circumstances that the cementite is dissected. The carbon that hereby originates+forms no lamellas but nests of sphere-lie separations of temper carbon.


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Tempering

The white cast iron to be tempered will ha1e to contain more silicon than the sortdestined for anneal freshening. This content must be low enough to pre1ent separation ofgraphite during solidification. To obtain this the process must be split into 2 parts. The

first part occurs at high temperature


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8ig. 0tructure of completely tempered cast iron.

:earesistant ,ighChromium Cast

IronThis cast iron is used in circumstances where a 1ery high wear resistance is desirable. 8orexample+ during the 1iolent crushing of rocs and minerals. t contains a combination of1ery strong carbide-forming alloying elements. ts chemical composition is+ therefore+8e-2.C-&"Cr-2!o-2Ki wt:.

The white phase is a chromium-richcarbide nown as !"C3. The matrixconsists of dendrites of austenite+ someof which may ha1e transformed intomartensite. There may also berelati1ely small uantities of otheralloy carbides.

The white phase is a chromium-richcarbide nown as !"C3. The matrixconsists of dendrites of austenite+ someof which may ha1e transformed intomartensite. There may also berelati1ely small uantities of otheralloy carbides.
http://www.msm.cam.ac.uk/phase-trans/2001/adi/white1000.jpghttp://www.msm.cam.ac.uk/phase-trans/2001/adi/white500.jpg


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Austempered Ductile Cast Iron

The chemical composition of the cast iron is 8e-3.52C-2.5&0i-'.4%!n-'.&5!o-'.3&Cuwt:. (ll samples are etched using 2: nital. Colour micrographs are produced by first

etching with 2: nital+ followed by open air heat treatment of the metallographic sampleat 2"'oC for 3 h. This oxidises the sample and produces interference colours which arephase dependent.

Nuctile iron as-cast. Kodules of graphite+ pearlite


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Austenitised at 950C, austempered at 350C for 64 min.

11. Aluminium and Aluminium allo"s.

The base rough material is bauxite which in general consists of 5'-': (l293+ &'-2':8e393+ 2-5: 0i92+ &-3: Ti92+ some other impurities and $29. By a series of chemicaltreatments from this pure (l293is won+ which contains some hundredths of percents 0i92and 8e293. 8rom the (l293there can be won (l with electrolysis. The (l293then is sol1ed

in molten ryolite


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)roperty 8e Cu (l !g0pecific weight


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8ig. The binary state diagram (l-0i.

The alloy with about &3: 0i+ is called silumin+ is the classical aluminium-cast alloy. The

shrin is about half of that of pure aluminium.

(t slow cooling there originates rough+ plate-formed crystals+ which mae the alloybrittle. The eutecticum degenerates so that the 0-crystals become extra large. This can betacled by casting of '.&: Ka to the melt in the form of some Ka slat


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( well-nown alloy of this type is (lCu 4.5 + with a elongation limit of &' gLmm2. Thishas a good processability.

(n alloy which at high temperatures has good properties + originates by addition of 8e


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Allo"s of the t"pe Al;n.

)ure (l-;n alloys+ pre1iously used much+ are nowadays seldom used because of poor

mechanical and corrosion properties.

The alloy (l;n&' P &2Cu4- 2 has good cast properties and reasonable strength+ used fromotor blocs.

( nown alloy for sand casting is (l;n 5.5 !g '. Cr '.5 Ti '.2. (fter casting andcooling there occurs precipitaion hardening at room temperature> the full strength isobtained after 3 wees. The hardening time can be shortened by after annealing at higher

temperatures

b. By reduction of !g9+ usually with 0ilicon. The !g9 can for example beobtained by calcinating of magnesite

2!g9.Ca9 A0i


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The elasticity modulus is much lower than that of aluminium. !agnesium is 1eryignoble


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8ig. eft part of the state diagram !g-(l.

(s a conseuence of the large width of the this is degenerated> there are isolated -areas in the -ground metal. By

the deformation of the -phase the (l-content of the -matrix will be smaller than

corresponds with the nominal composition.

f the (l-content increase this effect becomes stronger. Because the strength of the

!g-(l-alloys is mainly determined by that of the -mixing crystal+ the !g-(l alloys

cast in sand will ha1e increasing strength with the nominal (l-percentage. This willha1e a maximum at around : to decrease afterwards.

8ig. )ull strength Band rupture stress


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1+. Copper and Copper allo"s.

Copper is mainly won from sulphidic minerals+ e.g. copper stone


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8ig. Change of electrical conduction of copper by different elements.

8rom the figure it shows that especially phosphor is damaging.

The crystal structure of copper is plane centered cubic


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8ig. Binary state diagram Cu-;n.

The -phase has + lie copper+ the cubic face centered structure. The -phase has thecubic body centered structure. $ereby the -phase is unordered. n the -phase there

is ordering> the -alloys can be considered intermetallic bondings Cu;n.

-mixing crystals ha1e a cubic structure with 52 atoms per unit cell. They ha1e the

character of an intermetallic bonding with formula Cu5;n#. The remaining structures


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!essing sorts with a ;n-content between 4 and 5': at high temperature consist of

homogeneous -crystals> these are well warm deformable. Below 45'C there is

ordering and the alloys consist of -crystals. These are hard and brittle + so that such

alloys arent usable technically.

( ;n-content between 3% and 4: gi1es this causes a good warm deformability.

(fter etching in

the alloy is relati1ely tough. (t percentages close to 4: the crystal are surrounded

completely by the . The breaing stress is small.

8ig. )ull strength of nead messing in different states as a function of the ;n-content.

(ron4e.


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8ig. The binary state diagram Cu-0n for 0n-contents `4':.

The - phase has the fcc or cubic plane centered structure.> the -phase is bcc lie the

-phase. The -phase loos lie the -phase in messing it has the structure Cu3&0n#

and is hard and brittle. The transition 1elocities below 5''C are so small that the

theoretical -solubility line is ne1er followed. n practice the -solubility below 5''

is temeparture independent. Because the reaction Aat 33'C is slow+ a bron,esort at 4''C containing the -phase in general at room temperature will also consists

aprtly of the .

The


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8ig. Brinell hardness


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8ig. Binary state diagram Cu-(l for (l-contents ` 2':.

(er"llium copper.

This is one of the most nown precipitation-hardening alloys. They ha1e highstrength and good conduction of electricity and heat. )art of the state diagram is gi1enbelow.


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8ig. )art of state diagram Cu-Be.

The most used composition is CuBe&.%Co '.2+ whereby cobalt is added to lesse thechance to discontinuous precipitation during after-annealing. The temperature for

solubility annealing lies at about #''C+ while the temperature for after annealing

usually lies between 3&' and 35'C. The mechanical properties can be impro1ed by

applying cold deformation


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The binary system is simple.

8ig. The binary state diagram Cu-Ki.

Copper and nicel form in the solid state a continuous series of fcc mixing crystals.

The strength of copper-nicel alloys rises with the Ki content> at about ': Ki themaximal 1alue is obtained at a still good toughness> the breaing stress there is more

than 4':. !uch used Cu-Ki alloys are

Cupronicel+ Cu with &'-3' Ki and about &:8e. The presence of 8e impro1es thecorrosion resistance against sea water.

Constantan + CuKi45. 9f all Cu-Ki alloys it has the largest electrical resistance.

!onel CuKi#8e3!n&+ with a little 0i and C. t is used fro its good corrosionagainst seawater.

!older

!ostly consist of a tin-lead-alloy. n the figure below the binary state diagram of tin-lead is gi1en.


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8ig. Binary system )b-0n.

Problems

IronIron Carbide Phase Diagram Example


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8ig & 8e-8e3C )hase Niagram

8igure & shows the euilibrium diagram for combinations of carbon in a solid solution of

iron. The diagram shows iron and carbons combined to form 8e-8e3C at the .":C endof the diagram. The left side of the diagram is pure iron combined with carbon+ resultingin steel alloys. Three significant regions can be made relati1e to the steel portion of thediagram. They are the eutectoid*+ the h"poeutectoid(+ and the h"pereutectoidB. Theright side of the pure iron line is carbon in combination with 1arious forms of iron calledalpha iron


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austenite @


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Damma that transforms to pearlite compostion of austenite @ '.#: C. (mount ofaustenite @ 2g.

(t "2"EC


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!olution

a.

basic metallurgy - a walkthrough

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