5 ferrous metals & alloys 2

8/15/2019 5 Ferrous Metals & Alloys 2

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CHAPTER 5

Metallurgy, Properties, and Uses of Ferrous Metals and Alloys-II


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Impurities in teel and T!eir E"e#ts• Elements #ease to $e impurities and $e#ome alloying

elements %!en t!eir amounts %ill $e as !ig! as &' to () *+

• Impurities present in plain #ar$on steels

ulfur )+). to )+)5 */

Red s!ortnesssteel $e#omes $rittle and un%or0a$le at !ig!

temperatures, !appens %!en sulfur #om$ines %it! iron to

form iron sul1de+

P!osp!orus )+). to )+)5 */

P!osp!orus- t!e most undesira$le of all elements found in

steels2 produ#es #old s!ortnesssteel is $rittle at room

temperature and #annot $e #old-%or0ed/+

ili#on )+)5 to )+. */

ili#on3s e"e#t is 4ery slig!t+ In#reasing sili#on #ontent raisest!e elasti# limit and ultimate strengt! of steel %it!out

redu#ing du#tility 4ery greatly+ E#essi4e amount of t!ese

#auses steel to $e#ome %ea0 and $rittle+

Manganese )+6 to &+5 */

Presen#e of manganese in#reases strengt! of steel, $ut if limit


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E7uili$rium 8iagram of Iron and IronCar$ide

• T!e me#!ani#al properties of alloy depend upon t!eproperties of t!ese p!ase or p!ases of %!i#! t!e alloy is#omposed and upon t!e manner in %!i#! t!e se4eralp!ases are asso#iated+

•

Referen#e is usually made to t!e iron and iron #ar$idesystem rat!er t!an t!e iron and #ar$on system, $e#auseunder e7uili$rium #onditions, #ar$on is al%ays in t!e formof iron #ar$ide (+(9* C/, #ommonly #alled #ementite, andnot of free #ar$on+


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Allotropi# Forms of Iron

• Alp!a iron : room temperature to &(9) F;


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olid olutions of Car$on in Iron

• Maimum solu$ility of #ar$on in

Alp!a iron- attained : &666 F9.6 C/, %it! )+)6 * #ar$on,

#alled ferrite+

=amma iron- o##urs : .)(5 F&&6) C/, %it! &+9 * #ar$on,

#alled austenite+ 8elta iron- attained : .9&5 F&'


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Eute#ti# and Eute#toid of Iron and IronCar$ide

• Eute#toid- an intimate me#!ani#al miture of t%o or morep!ases !a4ing a de1nite #omposition and a de1nitetemperature of transformation %it!in t!e solid state+

• T!e formation of a eute#toid in4ol4es t!e formation of a

me#!ani#al miture of t%o p!ases from a solid solution+• T!e eute#toid of t!e iron-iron #ar$ide system #ontain )+>6

* #ar$on and forms on #ooling, under e7uili$rium#onditions, : &666 F9.6 C/+ t!is eute#toid is #omposed ofan intimate me#!ani#al miture of ferrite and iron #ar$idein lamellar #om$inations+ T!is eute#toid !as $een gi4en t!ename pearlite+


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Transformation in t!e Range ) to &+9Per Cent Car$on

• teel is an alloy of iron and #ar$on in %!i#! t!e #ar$on #ontents does note#eed &+9 *+

• Alloys of t!e iron-iron #ar$ide system #ontaining less t!an &+9 * #ar$on%ill ne4er !a4e any p!ases t!at are deri4ed from t!e eute#ti#+

• Proeute#toid- any p!ase t!at pre#ipitates prior to t!e formation of t!e

eute#toid+• Alloys #ontaining less t!an )+>6 * #ar$on %ill #ontain proeute#toid ferrite+

• Alloys #ontaining more t!an )+>6 * #ar$on %ill #ontain proeute#toid

#ementite+

• Hypoeute#toid steels- steels #ontaining less t!an )+>6 * #ar$on+

• Eute#toid steel- steels #ontaining )+>6 * #ar$on+

• Hypereute#toid steels- steels #ontaining more t!an )+>6 * #ar$on+• Under #onditions of slo% #ooling, maimum strengt! o##urs at

approimately eute#toid #omposition+ Furt!ermore, t!e du#tility as

re?e#ted $y redu#tion of area and per#ent elongation de#reases %it! anin#reasing amount of #ementite+

f l


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Heat Treatment of teels• Heat treatment is t!e alteration of t!e me#!ani#al

properties of an alloy t!roug! t!e #ontrol of t!ermal e"e#ts+

• T!e temperatures on t!e e7uili$rium diagram at %!i#! a

transformation ta0es pla#e are #alled critical points orcritical temperatures+

• Criti#al temperatures are found to $e a"e#ted $y t!e rate of!eating or #ooling+ T!e greater t!e rate of !eating or#ooling, t!e greater %ill t!e temperature de4iate from t!e

e7uili$rium #riti#al temperature+• @it! 4ery rapid #ooling, t!e transformation o##urs at lo%

temperature and t!e resulting stru#ture is 0no%n asmartensite+ Can $e #ompared %it! t!e stru#ture ofpearlite only t!at martensite possesses an a#i#ular

stru#ture %!ereas pearlite possesses a lamellar stru#ture+• It s!ould $e remem$ered t!at as t!e #ar$on #ontent

in#reases, t!e du#tility of t!e steel de#reases and t!ereforein ma#!ine parts %!ere relati4ely good du#tility is re7uired,t!e #ar$on #ontent s!ould not $e too !ig!+

•

For proper !ardening, steel must $e in t!e austeniti##ondition prior to 7uen#!ing+ To e"e#t t!is #ondition, t!e


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Hardena$ility

• Hardena$ility- t!e a$ility of a steel to $e !ardened t!roug!to its #enter in large se#tions+

• T!e !ardena$ility of steels may $e #ompared $y 7uen#!ing$ars of di"erent diameters and determining, for parti#ular

7uen#!, t!at diameter $ar %ill Bust #ompletely !ardent!roug! to t!e #enter+


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Tempering of teel

• Tempering of steel is t!e pro#ess of !eating a !ardenedsteel to any temperature $elo t!e loer #riti#al temperature,follo%ed $y any desired rate of #ooling+

• p!eroidiing- t!e pro#ess %!ere a steel is tempered for along time at a temperature Bust under t!e A#

& point, t!e iron

#ar$ide %ill #ompletely agglomerate to form sp!eres+

• 8istempering- a treatment %!ere a steel %it! suD#ient!ardena$ility may $e treated isot!ermally to produ#e t!edesired properties+ T!is eliminates t!e need of tempering

after t!e 7uen#!ing operation+ T!is is appli#a$le only tosteels of suD#ient !ardena$ility or for pie#es of steel ofrelati4ely small t!i#0ness+

• Martempering- t!e steel is 7uen#!ed from a temperaturea$o4e t!e #riti#al to a temperature $elo% t!e 0nee of t!e TTT diagram at %!i#! t!ere is an appre#ia$le time $eforetransformation %ill $egin+


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Annealing and ormaliing of teel

• Annealing of steel is t!e pro#ess of !eating to Bust a$o4et!e upper #riti#al temperature, follo%ed $y 4ery slo%#ooling+

• In some #ases, grain re1nement %it!out maimum softness

is desired, or one may %is! to redu#e t!e etent ofsegregation in a #ast material+ In t!ese #ases, t!e steelmay $e !eated to any temperature a$o4e t!e upper #riti#altemperature, follo%ed $y #ooling in still air+ T!is treatmentis referred to as normaliing+


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Case!ardening of teel

• Case!ardening is t!e appli#ation of any pro#ess %!ere$y t!e

surfa#e of t!e steel is altered so t!at it %ill $e#ome !ard+

• T!e met!ods of #ase!ardening steel is di4ided into t%o general

#lasses

T!e 1rst #lass in#ludes t!ose pro#esses $y %!i#! t!e#omposition of t!e steel is #!anged to a small dept! $elo% t!e

surfa#e $y t!e di"usion of some element or elements into t!e

steel from t!e outside as $y #ar$uriing, nitriding,

#ar$onitriding, and #yaniding+

T!e se#ond #lass in#ludes t!ose pro#esses $y %!i#! t!esurfa#e of t!e steel is lo#ally !eated $y !ig!-fre7uen#y #urrents

or $y a ?ame to a temperature a$o4e t!e transformation

temperature and t!en 7uen#!ed+


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Car$uriing

• Ca$uriing is a #ase!ardening pro#ess $y %!i#! t!e #ar$on

#ontent of t!e steel near t!e surfa#e of a part is in#reased+

• T!e !ardening is o$tained $y !eat treatment after

#ar$uriing+

• T!e in#rease of #ar$on #ontent of t!e steel #an $ea##omplis!ed $y t!is met!ods

olid #ar$uriing

i7uid #a$uriing

=as #ar$uriing


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itriding

•

itriding is anot!er met!od of #ase!ardening in4ol4ingdi"usion, in %!i#! t!e steel to $e #ase!ardened is

ma#!ined, !eat-treated, pla#ed in an air-tig!t $o, and

!eated to a$out &))) F+

• e4eral limitations of nitriding

An epensi4e spe#ial alloy su#! as nitralloy is used+ T!e manipulation of t!e pro#ess must $e done %it! great

#are and re7uires a s0illed operator+

T!e support in t!e $oes are diD#ult $e#ause t!e gases

must rea#! all parts to $e nitride+

It is ne#essary to anneal t!e steel $efore nitriding to

remo4e all strains+

o ma#!ining #an $e done after t!e steel !as $een nitride,

e#ept grinding+

• Car$onitriding- a pro#ess %!ere steels are #ar$uried andnitride simultaneousl + T!is ro#ess is used for #ases


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Cyaniding of teel

• Cyaniding is anot!er #ase!ardening treatment in4ol4ing t!epro#ess of di"usion+

• Cyaniding I, to a #ertain degree, a #ar$onitriding pro#ess$ut in #ar$onitriding, t!e dept! of #ase o$tained is deepert!an t!at o$tained %it! t!e #yaniding pro#ess+


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Flame Hardening

• Flame !ardening is a pro#ess of produ#ing a !ard surfa#e ina steel !a4ing a suD#iently !ig! #ar$on #ontent to respondto !ardening $y rapid #ooling of t!e surfa#e+

• T!is pro#ess does not in4ol4e a #!ange in #!emi#al#omposition of t!e surfa#e of t!e steel+


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Indu#tion Hardening

• Indu#tion !ardening- in tis pro#ess, t!e steel surfa#e to $e!ardened is surrounded $y a #oilatta#!ed to a !ig!-fre7uen#y AC sour#e+

• Indu#tion !ardening !as found 4ery great use as aprodu#tion met!od of surfa#e !ardening+ It !as $eenutilied in t!e automoti4e 1eld in t!e !ardening of #ran0$earing surfa#es, t!e !ardening of $us!ings and so on+


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Cast Iron

• Cast iron- alloys #ontaining more t!an &+9 * #ar$on+

• @!ite #ast iron- alloys of #ar$on and iron %!i#! #ontainmore t!an &+9 * #ar$on+


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Mallea$le Cast Iron

• A #ast iron t!at possesses t!e toug! #!ara#teristi#s typi#alof pure iron+

• T!e mallea$liing treatment remo4es all tra#es of iron#ar$ide lea4ing tiny grains of ferrite mied %it! temper#ar$onnodules of grap!ite/+

• T!e stru#ture gi4es t!e material a tensile strengt! of a$out56,))) l$;in., a yield point of a$out 6(,))) l$;in.+ And anelongation of a$out & * in .in+


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=ray Cast Iron

• =rap!iti# #ar$on- grap!ite in t!e form of ?a0es+

• =ray #ast iron- an iron-#ar$on alloy #onsisting of a stru#ture

of ?a0es of grap!ite em$edded in a matri of steel+

• Car$on #ontent 4aries from &+9* to 6+5*

• Used #!ie?y for parts t!at are not su$Be#ted to se4eretensile stress+

• Tensile strengt! 4aries from .),))) l$;in. to 5),))) l$;in.+


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8u#tile Cast iron

• T!is is produ#ed $y ladle addition of magnesium in t!e form

of ni#0el-magnesium alloy, or in some #ases $y t!e use of

ot!er elements+

• Also #alled nodular #ast iron+

• Has many properties similar to steel $ut is less #ostly toprodu#e t!an steel+

• 8u#tile #ast iron 1nds #onsidera$le use for #astings,

re7uiring lo%er porosity, for e7uipment used to #ondu#t

?uids, and for ma#!inery parts %!i#! re7uire a fair degree

of toug!ness %it! !ig! yield strengt!+ its ma#!ina$ility

seems to $e a$out similar to t!at of gray #ast iron and

produ#es a $etter ma#!ined surfa#e+

5 ferrous metals & alloys 2

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