Tehnički materijali
1. Klasifikacija Klasifikacija tehničkih materijala Tehničkim Tehničkim materijalima, materijalima, u širem smislu, nazivaju se krute, tekuće i plinovite stvari, koje se koriste u izradi delova, za izgradnju, u pogonu i održavanju sredstava. Tehički materijali korišteni pri izradi delova i izgradnji mašina, po pravilu kruti, nazivaju se tehnički ili konstrukcijski materijali (građevinski, mašinski, elektrotehnički...). U lite litera ratu turi ri se čest čestoo kori korist stii i term termin in ma mate teri rija jali li u užem užem smis smislu lu,, pri pri čemu čemu se podrazum podrazumevaj evajuu samo tehnički tehnički materijali. materijali. Kruti, tekući tekući i plinoviti plinoviti materijali koji se koriste koriste u pogonu i održavanju delova mašina nazivaju se "pogonski materijali". Tehnički materijali se mogu podeliti u pet grupa: • Metali • Nemetali • Poluprovodnici • Kompozitni materijali Materijali u svakoj od ovih grupa imaju različite strukture i svojstva. Selek Selekcij cijaa materi materijal jalaa se bazir baziraa na njihov njihovim im osobin osobinam ama, a, koja koja prave prave razlik razlikuu između između materijala. Materijali imaju više različitih osobina. Najvažnije kategorije osobina materijala koja se moraju uzeti u obzir kod selekcije materijala su: su karakteri karakteristike stike materijala materijala koje se odnose odnose na njegovu njegovu strukturu strukturu i njegov njegovoo formir formiranj anjee iz osnovn osnovnih ih eleme elemenat nata. a. Ova svojs svojstva tva se ispitu ispituju ju u labora laborator torijim ijimaa i vizuelno se ne mogu odrediti. Često je nužno promeniti ili uništiti materijal da bi se odredile njegove hemijske osobine. Hemijske osobine ,
su karakt karakteri eristi stike ke materi materijal jalaa koje koje se odnose odnose na intera interakci kcije je ovih ovih materijala sa različitim formama energije i drugim formama materije. Fizičke osobine se obično mere bez uništavanja ili menjanja materijala. Na primer: boja je fizička osobina, koja se može odrediti jednostavno gledanjem. Gustoća materijala se određuje merenjem težine i volumena predmeta. Fizičke Fizičke osobine, osobine,
opisuju kako se materijal ponaša kad je izložen delovanju sila. Najvažnija su čvrstoća i tvrdoća. Čvrstoća prema načinu, kako sile deluju imamo čvrstoću na istezanje, pritisak, savijanje, uvijanje i smicanje. Ispitivanja mehaničkih osobina se izvode sa destrukcijom (uništavanjem) materijala. Na primer: tvrdoća se ispituje utiskivanjem tvrđeg predmeta (šiljka ili kugle) u materijal. Mehaničke osobine ,
su svoj svojst stva va koja koja se odno odnose se na obli oblikk ma mate teri rija jala la i njeg njegov ovee površ površins inske ke karakt karakteri eristi stike. ke. Moguć Mogućee dimenz dimenzije ije,, oblik, oblik, hrapa hrapavos vostt površi površine ne i tolera toleranc ncije ije materijala su često najvažniji faktori pri selekciji. Tehnološ Tehnološka ka svojstva svojstva,,
1
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Tehnički materijali
2. Struktura tehničkih materijala Struktura materijala se može promatrati na nekoliko nivoa. utiču na vezivanje atoma, ona nam pomaže pri katego kategoriz rizaci aciji ji materi materijal jalaa u metale metale,, polupr poluprovo ovodni dnike, ke, kerami keramičke čke materi materijal jalee i polime polimere. re. Iz atomske strukture mogu se izvući glavni zaključci o mehaničkim i fizičkim osobinama materijala. Raspored elektrona, koji okružuju jezgro atoma, utiču na električna, magnetska, toplotna i optička svojstva materijala. Raspored elektrona takođe utiče na veze između samih atoma. Atomska struktura i raspored elektrona,
na sled sledeć ećem em nivo nivou, u, se posm posmat atra ra rasp raspor ored ed atom atoma. a. Meta Metali li,, poluprovodnici, mnogi keramički materijali i neki polimeri imaju sređen raspored atoma. Ovi materijali materijali se nazivaju nazivaju kristalni kristalni materijali. materijali. Kristali Kristali se definišu kao delovi delovi krute materije materije u kojima su atomi pravilno raspoređeni. Neki keramički materijali i polimeri kao i rastvori nemaju nemaju sređen sređen raspored raspored atoma. Ovi materijali materijali se nazivaju nazivaju amorfni amorfni materijali materijali ili staklasti materijali (glassy) i ponašaju se potpuno različito od kristalnih materijala. Na primer: staklasti amorfni polietilen je proziran, a kristalni polietilen je neproziran materijal koji propušta svetlo. Raspo Raspore red d atoma, atoma,
Mikrostruktura, je struktura poliranih i nagriženih materijala, koju dobijemo uz pomoć
mikroskopa uz povećanja većim od 10x. Mikrostruk Mikrostruktura tura pokazuje pokazuje:: prisutne prisutne faze, faze, njihov njihov volumeni deo u materijalu, primese, veličinu zrna i toplotnu obradu materijala. Zrnastu strukturu imaju mnogi metali, poluprovodnici i keramički materijali. Veličina zrna i njihov oblik utiču na ponašanje materijala. U nekim specifičnim slučajevima, kao što je, na primer: u silikonskim čipovima želi se proizvesti materijal koji sadrži samo jedno zrno ili materijal koji ima samo jedan kristal. Međutim, kod mnogih metala je prisutno više faza. Faza materijala ima jedinstven raspored atoma i svojstva. Kontrolom vrste, veličine, rasporeda i količine faza u materijalu kontrolišu se osobine materijala.
a) Atomska struktura b) Kristalna struktura c) Mikrostruktura (100x)
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Tehnički materijali
2.1 Mikrostruktura tehničkog materijala Sadržaj Sadržaj ugljenika ugljenika (C), vrst vrstaa i koli količi čina na legi legira raju jući ćihh elem elemen enat ataa tako takođe đe utič utičuu na mikrostrukturu legirajućih čelika. Na slici 2, prikazana je ova zavisnost. Čelici se prama mikrostrukturi dele na: • • • • • •
podeutektoidne eutektoidne nadeutektoidne ledeburitne feritne (α , β ) austenitne (γ )
Slika 2 -Mikrostruktura legiranih čelika u ravnotežnom stanju- 2
Slika 3
-Količinski udeo faza u dijagramu stanja -3
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Tehnički materijali
sadrže u svojoj mikrostrukturi ferit i perlit. Ovi čelici su tzv. konstrukcioni čelici i mogu se svrstati u grupu nisko ili srednje legiranih čelika. Podeutektoidni Podeutektoidni čelici,
Slika 4 -Šematski prikaz mikrostruktura podeutektoidnog čelika sa 0.5% C- 4
Slika 5 -Mikrostruktura podeutektoidnog čelika-5
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Tehnički materijali
Eutekto Eut ektoidni idni čelici, čeli ci,imaju perlitnu strukturu, a nadeutektoidni sadrže pored perlita i
"
F 3C e . Ovi čelici se uglavnom koriste za izradu alata jer se lako obrađuju na strugovima, dobo kale u ulju, a nakon dobrog otpuštanja imaju izuzetno dobre osobine.
Slika 6 -Šematski prikaz mikrostruktura eutektoidnog čelika sa 0.77% C i izgled
mikrostrukture perlita koji se sastoji iz lamela ferita (bela faza) i cementita (tamna faza) (600×i 1000×)-6
Slika 7 -Mikrostruktura eutektoidnog čelika-7
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Tehnički materijali
Ledeburitni (a) čelici, spadaju u grupu
Mikrostruktura ovih čelika sastoji se od
visokolegiranih i visokougljeničnih čelika.
"
F 3C e , perlita i legirajućih elemenata ledeburitnih
karbida. Ovi čelici imaju visoku tvrdoću, otpornost na habanje i veoma dobre rezne osobine.
Slika 8
-Mikrostruktura ledeburitnog čelika-8
U ovu grupu ledeburitnih čelika spadaju: alatni čelici za rad na hladno i svi tzv. brzorezni čelici. b) Feritni ( b) čelici, takođe imaju visok sadržaj legirajućih elemenata koji
sužavaju γ područje i (Cr, Si, Mo, V) i sadrže oko 0,1 do 0,2% ugljenika (C). Ovi čelici su maki i jako plastični, ne mogu se kaliti niti poboljšavati jer izostje α ↔ γ transformacija. Osobine feritnih čalika se mogu poboljšati usitnjavanjem zrna plastičnim deformisanjem na hladno i naknadnom rekristalizacijom, obično imaju primenu kao čelici za magnete.
Slika 9 -Mikrostruktura feritnog čelika-9
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Tehnički materijali
mogi da imaju različit sadržaj ugljenika (C), ali takođe moraju da imaju visok sadržaj legirajućih elemenata, koji ostvaruju γ područje (Mn, Ni). Kao i feritni čelici čelici ovi čelic čelicii se ne mog moguu kaliti kaliti,, pobolj poboljša šavat vatii niti niti norma normaliz lizov ovati ati,, jer nemaj nemajuu α ↔ γ transformaciju. Međutim, kod austenitnih čelika u poređenju sa feritnim čelicima, pošto je temperatura rekristakizacije viša neko kod feritnih čelika, javlja se veća opasnost od stvaranja krupnog zrna. Plastičnim deformisanjem na hladno i naknadnom rekristalizacijom može se usitniti zrno. Austenitni čelici su otporni na koroziju i postojani su na oksidaciju. Tvrdoća austenita je od 160 do 200HB, jačina od 200 do 800Mpa i izduženje od 40 do 50%, sve u funkciji količine rastvorenog austenita u γ području. c) Austenitni ( c) čelici,
Slika 10
-Mikrostruktura austenitnog čelika- 10
Mikrostruktura ovih čelika sastoji se od legiranih austenitnih zrna sa dvojnicima žarenja. Na slici 2, možemo uočiti da postoji i poluferitni, odnosno poluaustenitni čelici. Kod ovih čelika, sadržaj legirajućih elemenata manji je nogo kod austenitnih, a veći nego kod perlitnih, pa je α ↔ γ transformacija ne potpuno izvršena.
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Tehnički materijali
2.2 Podela prema mikrostrukturi tehničkog materijala Mikrostruktura nekih manjih delova, preseka posle hlađenja na vazduhu može biti: • • •
perlitna martenzitna austenitna
Što prestavlja i podelu čelika, na slici 11, prikazan je termodinamički dijagram transformacije austenita (IR dijagram) sa ucrtanom brzinom hlađenja. Čelici koji u sebi sadrže legirajuće elemente, linija transformacije pomera se udesno.
Slika 11
-IR dijagram prelitnog (a), martenzitnog (b) i austenitnog (v) čelika- 11
Perlitni (d) čelici ,
kod ovih čelika pomeranje je najmanje, pa kriva brzine hlađenja sa hlađenjem na vazduhu seče linije u perlitnom području.
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Tehnički materijali
sadrže veću količinu legiranih elemenata. Linije transformacije su pomerene nešto više udesno, pa se mogu kaliti na vazduhu. Ovi čelici se standardnim metodama ne mogu obrađivati rezanjem, pa se zato retko koriste i primenjuju. Martenzitni čelici,
su visoko legirani sa hromom (Cr), niklom (Ni) ili manganim (Mn). Linija transformacije austenita su sasvim pomerene udesno. Na sobnoj temperaturi austenit je veoma stabilan, jer je linija početka martenzitne transformacije (Ms) pomerena ispod nule. Austenitni čelici,
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Tehnički materijali
3. Uticaj pojedinih legirajućih elemenata Od tehničkog interesa su samo oblasti podeutektoidnih i feritnih čelika. Silicijum (Si) sa manganom (Mn), najviše utiče na povećanje granica tečenja i zatezne čvrstoće čelika (svaki 1% Si povećava osobine za oko 100 MPa) . Silicijum takođe poboljšava otpornost na habanje, povećava prokaljivost, a pošto sprečava izlučijanje cementita, povećava i postojanost pri otpuštanju. Uglavnom se koriste za izradu opružnih čelika (Č2130, Č2135) , a čelici koji sadrže od 2,5 do 4% silicijuma (Si) i malim (C, ispod ispod 0,5% 0,5%)) imaju malim sadrža sadržajem jem ugljni ugljnika ka (C, imaju dobru dobru elektrootpornost i smanjuju magnetičnost, pa se kao takvi često koriste u elektrotehnici.
Slika 13 -Mikrostrukturni dijagram silicijumovih čelika-13
Slicijum je legirajući element ako je njegov sadržaj u čeliku veći od 0,6%.
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Tehnički materijali
3.1 Uticaj mangana (Mg) Mangan je legirajući element ako mu je sadržaj veći od 0,8%. Kod čelika se mangan nalazi u čvrstom rastvoru, a delom u cementitu, stvarajući mešoviti karbid ( Fe , Mn ) 3 C . Mangan u čelicima ne stvara posebne karbide.
Od tehničkog su značaja samo niskolegirani feritno-perlitni, odnosno perlitni čelici, kod kojih je sadržaj mangan do 2%, i visokolegirani austenitni čelici kod kojih je odnos C:Mn=1:10. Mangan Mangan u niskol niskolegi egiran ranim im čelici čelicima ma pobolj poboljšav šavaa proka prokalji ljivos vostt i značaj značajno no smanju smanjuje je kritičnu brzinu hlađenja. Ovi čelici su takođe osetljivi na pregrevanje, jer se mešoviti karbid ( Fe , Mn ) C , u odnosu na Fe C brže rastvara u austenitu. 3
3
Mikr Mikros ostr truk uktu tura ra za ma mang ngan ansk skee čeli čelike ke je tipi tipičn čnoo trak trakas asta ta,, čime čime je uslo uslovl vlje jena na raspoređivanjem nemetalnih uključaka MnS, MnO ili kompleksnih jedinjenja Mn ⋅ SiO u redove. 2
Slika 14 -Mikrostrukture koje se javljaju kod martenzitnih čelika-14
Ovakva mikrostruktura utiče na smanjenje žilavosti u poprečnom preseku i do 4x. Manganski čelici, se sadrže do 2%, koriste se kao čelici za poboljšanje (Č3130) i cementaciju (Č4320), čelici koji su otporni na habanje (Č3134), čelici za automate (Č3190, 3990 podešen je za cementaciju) , kao i alatni čelici. Jedan Jedan od najvaž najvažnij nijih ih mangan manganski skihh čelik čelikaa iz grupe grupe austen austenitn itnih ih čelik čelikaa je Hatfildov (Hatfield) čelik sa 1,2 do 1,4% C i 12 do 14% Mn (Č3160). Ovaj čelik ima visoku otpornost
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Tehnički materijali
3.2 Uticaj nikla (Ni) Nikl se potpuno rastvara u feritu i povećava njegovu čvrstoću i tvrdoću, pri čemu ne utiče na smanjenje žilavosti. Ovo predstavlja posebni značaj kod čelika koji se koriste za rad pri niskim temperaturama. Nikl povećava prokaljivost i snižava kritičnu brzinu hlađenja, niklu se takođe dodaje 2% konstrukcionog konstrukcionog čelika za cementaciju (Č5420) i poboljšanje (Č5430). Žilavost čelika sa 3% Ni i 0,3% C na − 60 0 C je oko 3x veća od žilavosti ugljeničnog čelika, sa istim sadržajem ugljenika na istoj temperaturi. Zbog svoje cene, nikl se često legira sa drugim elementima, najčešće sa hromom (Cr). U kličinu od 8 do 20% nikl se dodaje visokolegiranim hromnim čelicima za ventile. Čelicima otpornim na koroziju i kiseline, kao i čelicima otpornim na povšenim temperaturama.
Slika 15
-Mikrostruktura dijagram čelika legiranih niklom- 15
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3.3 Uticaj hroma (Cr) Hrom je element kojim se čelik najčešće legira, hromni čelici sadrže od 0,3 do 30 hroma (Cr). Hrom se rastvara u feritu, povećavajući mu čvrstoću i tvrdoću. Legure sa više od 13% hroma (Cr) potpuno su feritne u području od sobne temperature pa do tačke topljenja. Pošto postoji kontinuitet između δ i α faze, legirani ferit sa hromom se nazivaju δ ferit. Hrom se rastvara i u cementitu, a gradi i složene karbide: ( Fe , Cr ) C , ( Fe , Cr ) C , ( Fe , Cr ) C i krto jedunjenje FeCr (σ ) . Karbidi povećavanju tvrdoću i hromnim čelicima povećavaju otpornost na habanje. 3
7
3
4
Slika 16 -Mikrostruktura hromnih čelika-16
Za inženjers inženjersku ku praksu praksu značajni značajni su niskolegi niskolegirani, rani, martenzit martenzitni, ni, feritni feritni i ledeburit ledeburitni ni hromni čelici. Čelici za cementaciju mogu da budu jednostruku legirani (Č4120, Č4130) ili višestruko legirani, gde se pored hroma dodaju do 1% Mn, 1,5% Ni i 0,3% Mo (Č5420,
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Tehnički materijali
4. Podela hromnih čelika 4.1 Martenzitni hromni čelici Martenziti hromni čelici su visokolegirani hromni čelici koji sadrže od 12 do 17% hroma (Cr) i od 0,1 do 1% ugljenika (C). Najviše se koriste čelici koji sadrže od 11,5 do 13,5% Cr i do 0,15% C. Ovi Ovi čeli čelici ci se pri pri kalj kaljen enju ju hlad hladee na vazd vazduh uhu, u, posl poslee čega čega se vrši vrši otpu otpušt štan anje je.. U temperaturnom intervalu od 450 do 550ºC, otpuštanje se ne preporučuje jer se u tom intervalu izlučuju karbidi Cr C po granicama zrna, što utiče na pojavu otpusne krtosti i samim tim povećava sklonost prema koroziji. Ovi čelici spadaju u grupu nadeutekto nadeutektoidnih idnih čelika, čelika, pa se samim samim tim i mikrostruk mikrostruktura tura 23
sastoji iz perlita i
6
"
F 3C e . Ovi čelici karakterišu se visokom čvrstoćom, koriste se za delove
koji su u dodiru sa vodom i vodenom parom (lopatice turbina, parni kotlovi, itd.) , primer: (Č4171). Ako se ovim čelicima dodaje azot i ugljenik, povećava se čvrstoća, atehnička granica može se približno odrediti prema izrazu: R p 0.2
=
71 710 0 + 77 770 0 ⋅ ( %C + 2% N )
MPa ] [ MPa
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Tehnički materijali
4.2 Feritni hromni čelici Feritni hromni čelici su takođe visokolegirani čelici koji sadrže do 11 do 27% hroma (Cr) uz sadržej ugljenika od 0,1% (C). Najčešće se koriste čelici sa sadržajem od 17 do 26% Cr. Ovi čelici mogu da sadrže mangan (Mn) i silicijum (Si) (najviše do 2%), radi povećenja ravnomernog izduženja. Zbog povećanog sadržajem hroma stvaraju se karbidi tipa: M C , M C i M C , σ -faza) ili FeCrMo a uz dodatak malog sadržaja molbdena može doći do stvaranja FeCr ( σ ( χ -faza). Pri zagrevanju ovih čelika iznad temperature od 600ºC, feritna zrna su sklona rastu. Usitnjavanje zrna se može podstići dodavanjem titana (Ti) ili nobijuma (Nb). Tada i male količine TiC ili NbC sprečavaju rast feritnih zrna. Feritni hromni čelici su u odnosu na austenitne čelike hemijski manje postojani ali su zato otporniji na oksidaciju, a naručito na temperaturama višim od 900ºC. 3
7
3
23
6
4.3 Ledeburitni hromni čelici Ledeburitni hromni čelici su čelici koji sadrže 12% Cr i 2,1% C (Č4150) , spadaju u visokolegirane i visoko-ugljenične čelike. Ledeburitna mreža koja nastaje u procesu livenja, može se razbiti toplim kovanjem ili valjanjem. Mikrostruktura meko žarenog čelika sastoji se iz ferita, finih karbida tipa ( Fe , Cr ) C (uglav (uglavnom nom potiču potiču od razlože razloženih nih kristal kristalaa γ čvrstog rastvora) i posebno grubo izlučenih Fe ) C . ledeburitnih karbida ( Cr , Fe 3
7
3
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4.4 Visokolegirani nerđajući austenitni čelici Čelici legirani sa hromom (Cr) i niklom (Ni) čine posebnu grupu visokolegiranih nerđajućih austenitnih čelika. Ovi čelici sadrže od 18 do 25% Cr , od 8 do 20% Ni i od 0,03 do 0,15% C. Ovi čelici takođe mogu i da sadrže male količine molibdena, nobijuma i titana (stabilizacioni i karbidoobrazujući elementi). Mikrostruktura ovih čelika u žarenom stanju sastoji se iz poligonalnih zrna austenita sa dvojnicima žarenja, takođe u strukturi može biti i prisutnan δ ferit, ferit, zavisno od sadržaja hroma i nikla. Stabilnost austenita zavisi od povećanog sadržaja nikla, pa se mogu dobiti austenitni čelici u kojima je austenit stabilan i do -196ºC. Ako se čelik sa sadržajem od 17% Cr i 7% Ni plastično deformiše na hladno,onda dolazi do transformacije austenita u martenzit, čime se dobijaju čelici visoke tvrdoće. imaju visoku postojanots prema uticaju vazduha, vode kao i prema prema mno mnogim gim drugim drugim hemijs hemijskim kim sredin sredinam ama: a: imaju imaju visoku visoku žilavo žilavost, st, čvrsto čvrstoću ću i otporn otpornost ost na udare udare na vrlo vrlo niskim niskim tempe temperat ratura urama ma,, imaju imaju visoki visoki stepen stepen deform deformac acion ionog og ojačavanja, postojanost postojanost na visokim temperaturama, nemagnetični nemagnetični su i dobro se zavaruju. Hrom-nikl nerđajući čelici ,
Ovi čelici su osetljivi na izdvajanje karbida Cr C što je posebno opasno pri zavarivanju. Ovi karbidi se izdvajaju po granicama zrna, povaećavajući krutost i osetljivost ovih čelika na interkristalnu i naponsku koroziju. Zbog izlučenih karbida po granicama zrna, meta me taln lnaa osno osnova va u bliz blizin inii gran granic icaa zrna zrna gubi gubi svoj svojuu hemi hemijs jsku ku post postoj ojan anos ostt i dola dolazi zi do interkristalne korozije. 23
6
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Tehnički materijali
5. Osobine homnih čelika Hromni čelici otporni na kiseline koji se odupiru, pre svega elektrohemijskoj koroziji (organskim (organskim i neorganskim neorganskim kiselinama, kiselinama, jedinjenjima jedinjenjima azota, azota, rastvorima rastvorima soli-elektroli soli-elektrolitima, tima, vatrootporni ni čelici čelici koji se ne oksidišu na vazduhu i agresivnim životnim namirnicama) i vatrootpor sagorelim gasovima na temperaturi preko 600ºC . Prohromski čelici otporni na kiseline moraju sadržati više od 12% Cr , a vatrootporni čelici više od 6% Cr i dodatke silicijuma (Si) i aluminijuma (Al) radi sprečavanja dubinske korozije. Pri dugotrajnom držanju na temperaturi iznad 500ºC kod prohromskih čelika otpornih na kiseline dolazi do izdvajanja hroma (Cr) po granicama zrna i time osiromašenja čvrstog rastvora u hromu. Ako procenat hroma (Cr) u čvrstom rastvoru opadne ispod 12% nastaće selektivna sele ktivna međukristaln međukr istalnaa korozija korozij a.
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5.1 Mehaničke osobine prohromskih p rohromskih (nerđajućih) (nerđajućih) čelika Tablica 1 -Mehaničke osobine (nerđajućih) prohromskih čelika- 19
Prohromski (nerđajući) čelik
Rp, Mpa
Rm, Mpa
Izduženje, A, %
Feritni
275÷ 600
450÷ 700
20÷ 30
Austenitni
200÷ 380
500÷ 800
45÷ 60
Martenzitni
275÷ 1900
520÷ 2000
15÷ 30
Dupleks
315÷ 565
590÷ 740
20÷ 40
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6. Šta predstavlja nerđajući čelik? Nerđajući čelici poseduju niz izuzetnih osobina koje ih razlikuju od ostalih klasa čelika. Kao prvo, koroziono su otporni zahvaljujući prvenstveno uticaju legirajućih elemenata hroma, u širokom spektru koroziono agresivnih sredina. Ništa manje neobična osobina je nemagnetičnost nemagnetičnost austenitnih nerđajućih čelika koju duguju u prvom redu niklu kao legirajućem elementu elementu koji stabilizuj stabilizujee visokotem visokotemperat peraturnu urnu nemagneti nemagnetičnu, čnu, površins površinski ki centriran centriranuu kubnu kubnu rešetku željezo-austenit. Sve one vrste čelika koje se opisuju kao nerđajući čelik su topljene u posebnoj proceduri, poseduju visoki stepen čistoće i ravnomerno reaguju na tretmane toplotom. Sledeći ovu defin definici iciju ju nerđa nerđajuć jućii čelic čelici,i, ne moraju moraju isklju isključiv čivoo da budu budu legira legirani ni poseb posebno no visoki visokim m legiranim čelicima. Ipak, u daljem razmatranju ograničićemo se na visoko legirane nerđajuće čelike sa sadržajem hroma od najmanje 11%.
6.1 Klasifikacija visokolegiranog nerđajućeg čelika
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Tehnički materijali
Austenitski nerđajući čelik ,
koji se takođe navodi kao CrNi čelik, sa sadržajem Ni od preko 8%, je najbolja kombinacija u odnosu na mogućnost obrade, korozionu otpornost i mehaničke karakteristike. Zbog tog razloga, austenitski nerđajući čelik se upotrebljavaju u agresivnim sredinama, kao na primer: u slučaju kontakta sa morskom vodom koja sadrži hlor, hemijskoj ili prehrambenoj industriji.
Slika 20
-Kristalna rešetka austenita-21
koje se često navodi kao dupleks čelik zahvaljujući svojoj svojoj dvokompo dvokomponent nentnoj noj strukturi. strukturi. Visoka Visoka rastegljiv rastegljivost ost sa istovreme istovremenim nim poboljšan poboljšanjima jima korozione otpornosti omogućavaju ovim čelicima da se koriste posebno u oblasti tehnologije Austenitski feritni nerđajući čelik ,
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7. Dobijanje prohroma Nerđajući čelici su porodica materijala sa nizom jedinstvenih karakteristika. Zaštićeni su tankim slojem hrom-oksida po površini, koja se formira reakcijom hroma iz čelika i kiseonika iz vazduha, tako da prohrom ne zahteva nikakvu dodatnu površinsku zaštitu protiv korozije. Kaže se da prohrom ima jednu stvarno jedinstvenu karakteristiku: “da se sam zaleči”. U slučaju da površina pretrpi oštećenje sloj se momentalno sam obnavlja u prisustvu kiseonika. Ovaj transparentni pasivni pasivni sloj ima debljinu svega nekoliko nekoliko atoma i obnavlja se pod uticajem uticajem kiseonika i to bez obzira da li kiseonik potiče iz vazduha ili iz vode.
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Poluproizvod od prohroma prolazi kroz operaciju “formiranje”, počevši od vrućeg valjanja u kojoj prolazi kroz valjke. Poluge se formiraju u žice, a ploče se formiraju u ploče, trake, listove i dr.
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Kaljenje je neophodno da se u površinskim slojevima dobije martenzit i da se gruba zrna (površine i jezgra) nastala dugotrajnim žarenjem delova na 950ºC, transformišu u sitnija zrna. Prema Prema tražen traženim im osobin osobinama ama delova delova mog moguu se načini načini kalje kaljenja nja,, posle posle nauglj naugljen enisa isavan vanja, ja, podeliti na tri grupe: • • •
direktno kaljenje jednostruko kaljenje dvostruko kaljenje
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je veoma važna specifikacija za proizvode od prohroma, takođe je i od ključnog značaja u aplikacijama koja se često odnosi na sanitarne aplikacije. Glatka površina koja se najčešće daboija poliranjem, takođe pruža bolje otpornosti na koroziju. Grubo završna obrada se često koristi na metsima ge je potrebno dodatno podmazivanje elemenata Postoje razne vrste metoda za obradu, visoko reflektujuća završna obrada je proizvod hladno hladnogg valjan valjanja ja sa žaren žarenjem jem na kontro kontrolis lisan anoj oj tempe temperat raturi uri,, od brušen brušenja ja sa abrazi abrazivni vnim m sredstvima ili glačanja. Završna Zav ršna obrada obra da,
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8. Mašinska obrada prohroma (bušenje) Površinsko finalna obrada izvršena u fabrici, tj. valjanje, kaljenje i/ili još jednom toplotno tretirani materijal, potrebno je izabrati kako bi orginalna površina odgovarala kasnije željenoj površini, što je bliže moguće karjnjem gotovom proizvodu. Vezano za korozionu otpornost potrebno je pratiti u napred, da je materijal skladišten i transportovan na odgovarajući način. Ovo u osnovi znači:
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9. Obrada oblikovanjem prohroma (zavarivanje) Zavari Zavariva vanje nje prohro prohroma ma običn običnoo se vrši vrši TIG(g) postupko postupkom m zavarivan zavarivanja, ja, aparatima aparatima iz UltraSonic TIG Precision serije koja omogućava spajanje metala tankih i do 0.25mm. TIG TIG post postup upak ak se koris koristi ti za proi proizv zvod odoo zava zavari riva vanj nje, e, nava navari riva vanj njee i repa repara ratu turn rnoo zava zavariv rivan anje je nerđ nerđaj ajuć ućih ih i vatr vatros osta taln lnih ih čeli čelika ka,, kao kao i legu legura ra alum alumin inij ijum umaa (Al), bakra (Cu),Titanijuma (Ti), nikla (Ni), hroma (Cr) itd.
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