Chuong3 - Vật liệu học

8/14/2019 Chuong3 - Vt liu hc

1/35

Chng 3 Hp kim v gin pha

- Trong thc t khng dng thun kim loi nguyn cht,

m thng l t hp cc cht c bn trn.- Khi ha trn cc nguyn t, hp cht ha hc tng tc vi

nhau, to nn cu trc mi v c tnh cht khc ivt liu

tr nn a dng hn, thch ng hn trong s dng

- Cc nguyn l cho hp kim cng hon ton thch hp v

ng dng c cho h vt liu v c - ceramic v c th ccho h vt liu hu c - polyme.

3.1.Cu trc tinh th ca hp kim

3.1.1.Khi nim v hp kim

a.nh ngha

- /n: Hp kim l vt th ca nhiu nguyn t v mang tnh

kim loi

- Hp kim c to thnh gia hai kim loi vi nhau (latng: Cu

v Zn); c th l gia mt kim loi vi mt kim (nh thp,

gang: Fe v C); gia nguyn t chnh l kim loi vi hai hay

nhiu nguyn t khc (hp kim phc tp)

- Nguyn t kim loi chnh, cha nhiu nht (>50%) c gi

l nn. Thnh phn ca cc nguyn t trong hp kim (v

trong ceramic) phn trm khi lng, trong polyme c

biu th bng phn trm th tch.

b.u vit ca hp kim

67


2/35

- cc vt liu em dng thng l hp kim v so vi cc

kim loi nguyn cht v l do sau:

1) Hp kim c bn ( b, ch, h ) cao hn hn kim loi

nhng vn gi c do , dai cao

2) Tnh cng ngh a dng v thch hp (kh nng ch

bin VL). ( GCAL, c, gia cng ct gt, nhit luyn)

ch to sn phm c kch thc, hnh dng chnh xc,

b mt nhn bng... vi nng sut cao.

3) Trong nhiu trng hp, luyn hp kim n gin v r hn

so vi luyn kim loi nguyn cht, do khng phi chi ph

kh nhiu nguyn t ln vo.

VD: So vi luyn st nguyn cht, luyn hp kim Fe-C (thp

v gang) n gin hn do nhit chy thp hn, khng

phi hay t phi kh b cacbon trong sn phm ca l cao.

c.Mt s khi nim

Cu t l cc nguyn t (hay hp cht ha hc bn vng)

cu to nn hp kim.

VD: latng (hp kim Cu-Zn) c hai cu t l Cu v Zn.

C th phn bit cu t ha tan vi cu t dung mi

H l t dng ch mt tp hp vt th ring bit ca

hp kim trong iu kin xc nh hoc l mt lot hp kim

khc nhau vi cc cu t ging nhau.

68


3/35

VD: h Cu- Zn

Pha l nhng phn ng nht ca h (hp kim) c cu

trc v cc tnh cht c - l- ha xc nh, gia cc pha c

b mt phn cch.

VD: Nc 00C l h mt cu t (hp cht ha hc bn vng

H2O) v c hai pha (pha rn: nc , pha lng: nc).

Trng thi cn bng (n nh)

- H trng thi cn bng (n nh) khi cc pha ca n

u c nng lng t do nh nht trong cc iu kin v

nhit , p sut v thnh phn xc nh v khng bin

i theo thi gian.

- H vi cc pha trng thi cn bng c bn,

cng thp nht, khng c ng sut bn trong, x lch mng

tinh th t nht v c hnh thnh khi lm ngui vi tc

chm.

69


4/35

Hnh 3.1. S biu th cc v tr n nh (1), khng n

nh (2) v gi n nh (3).

Trng thi khng cn bng (khng n nh)

- L trng thi (t chc) lun lun c xu hng t bin i

sang trng thi cn bng, n nh, nht l khi b nung

nng nng lng t do nh hn.

- nhit thng qu trnh bin i chm, kh nhn

thy trong thc t trng thi khng cn bng ny vn

tn ti lu di (v mt l thuyt khng th tn ti vnh

vin).

- Trng thi khng cn bng c ngha quan trng trong

thc t v thng p ng c cc yu cu c tnh (bn,

cng) cao hn.

-Trng thi khng cn bng c hnh thnh vi tc

ngui nhanh v rt nhiu hp kim, c bit l thp c s

dng (lm vic) trng thi ny.

Trng thi gi n nh

- Gi n nh thc cht l khng n nh nhng thc t

li tn ti mt cch n nh ngay c khi b nung nng

trong mt phm vi no .

d.Phn loi cc tng tc

- Phng php ch to hp kim thng dng nht l ha trn

(nu chy ri lm ngui) cc cu t.- Hai trng hp xy ra: khng v c tng tc vi nhau.

70


5/35

* Khi hai cu t A v B khng tng tc vi nhau

cc nguyn t, ion ca tng cu t khng an xen vo

nhau, chng gi li c hai kiu mng ca cc cu t thnh

phn, di dng cc htring r ca hai pha nm cnh nhau, t chc t vi biu th

ca hn hp A + B (hnh a)

Hnh 3.2. T chc t vi ca hp kim ca hai kim loi:

a.khng tng tc vi nhau A+B, b. ha tan vi nhau thnh dung dch

rn A(B).

* Khi hai cu t A v B c tng tc vi nhau nguyn t ca

cc cu t an xen vo nhau to nn mt pha duy nht,

c th c hai trng hp xy ra:

71


6/35

+ ha tan thnh dung dch rn: hp kim gi li mt

trong hai kiu mng ban u lm nn, c t chc mt pha

nh kim loi nguyn cht (hnh 3.2b),

+ phn ng ha hc vi nhau thnh hp cht ha hc, vikiu mng mi khc hn cc cu t thnh phn.

Ln lt kho st hai kiu tng tc ny.

3.1.2.Dung dch rn

a. Khi nim - phn loi

- Cu t gi li c kiu mng dung mi (chim a s),

cn cc nguyn t cht ha tan (t l t) sp xp trong

mng dung mi mt cch u n v ngu nhin to

thnh pha ng nht

- Thnh phn (hay cn gi l nng ) c th thay i

trong mt phm vi m khng lm mt i s ng nht .

- K hiu: A(B) c kiu mng ca A l cu t dung mi, B lcu t ha tan ; hoc l bng cc ch ci Latinh: , , ....

- Cc nguyn t ha tan c sp xp trong mng tinh th

dung mi theo hai kiu, tng ng vi hai loi dung dch

rn: thay th v xen k(hnh 3.3) R rng y yu

t hnh hc c ngha quan trng.

72


7/35

Hnh 3.3. S sp xp nguyn t ha tan thay th v xen k

vo dung mi c mng lp phng tm mt, mt (100)

b. Dung dch rn thay th

- Cc nguyn t ha tan chim ch hay thay th vo ng

cc v tr nt mng ca kim loi dung mi c kiu mng

v s nguyn t trong c s ng nh ca cu t dung

mi.

- iu kin to thnh dung dch rn thay th :

Ch xy ra vi cc nguyn t c ng knh nguyn t gn

ging nhau ( sai khc khng qu 15%)

- V khng c hai nguyn t no c ng knh nguyn t

hon ton ging nhau thay th s gy ra x lch mng

- Vt qu gii hn no thay th ln nhau rt kh

mng x lch qu mnh, tr nn mt n nh cho n

nng bo ha m vt qu gii hn ny s to nn pha mi

gii hn ha tan.- Trong thc t c mt s cp kim loi c th ha tan v

hn vo nhau to nn mt dy cc dung dch rn c nng

thay i mt cch lin tc : 100%A + 0%B qua 50%A +

50 %B cho n 0%A + 100%B nh biu th hnh 3.4.

73


8/35

- iu kin ho tan v hn: (c 4 yu t)

Hnh 3.4. S thay th to nn dy dung dch rn lintc (ha tan v hn) gia hai kim loi A v B khi lng B tng dn Tng quan v kiu mng: nu cng kiu mng mi c

th ha tan v hn, khc kiu mng ch c th ha tan

c hn.

Tng quan v kch thc: ng knh nguyn t sai khc

nhau t (< 8%) mi c th ha tan v hn, sai khc nhau

nhiu

(8 15%) ch c th ha tan c hn, sai khc nhau rt

nhiu (>15%) c kh nng khng ha tan ln nhau.

Tng quan v nng in t (s lng in t ha tr

tnh cho mt nguyn t) c cng in t ho tr (cc

nguyn t khc nhau v ha tr ch c th ha tan c hn)

Tng quan v tnh m in. (kh nng tng tc ha

hc to thnh phn t) Cng tnh m in, d ho tan

vo nhau. Nu khc bit nhau rt nhiu d to nn hp

cht ha hc, pha trung gian

Hai tng quan sau l s gn nhau trong bng tun hon

74


9/35

Ch cn khng t mt trong bn yu t to

thnh dd rn ho tan c hn.

V d: Ag - Au (mng A1, r = 0,20%, cng nhm IB), Cu -

Ni (mng A1, r = 2,70%, IB v VIII), Fe - Cr (mng A2, r= 0,70%, VIB v VIII).

- Thng s phn b nguyn t ha tan trong mng dung

mi l u n, ngu nhin v c gi l dung dch rn

khng trt t.

- Trong mt s iu kin (nhit , nng ) cc nguyn

t ha tan phn b u n song li c quy lutdung

dch rn trt t c tnh cht gin hn.

c.Dung dch rn xen k

- Cc nguyn t ha tan phi c kch thc b hn hn

c th lt vo l hng trong mng ca kim dung mi

vn c kiu mng nh kim loi ch nhng s nguyn t trong

c s tng ln.

- Cc l hng u rt nh mt s kim vi bn knh

nguyn t b nh: Hyr (0,046nm), Nit (0,071nm), C

(0,077nm) v i khi c B (0,091nm) vc kh nng ha

tan xen k vo cc kim loi chuyn tip c bn knh nguyn

t ln nh Fe (0,1241nm),Cr (0,1249nm), W(0,1371nm), Mo

(0,136nm), V (0,132nm), Ti (0,145nm)...

- L hng ln nht trong cc mng tinh th kim loi l loi

tm mt ca A1 d= 0,414dngt tr H ra khng c kim no

nh n mc va kch thc ny thc t, vn c th

75


10/35

chen vo mt s l hng, gy ra x lch mnh mng

(hnh 3.5a) dung dch rn xen k ch c th l loi ho

tan c hn ( ha tan thng l nh v rt nh).

Hnh 3.5. S x lch mng trong dung dch rn:

a. ha tan xen k,

b. ha tan thay th khi rht > rch ,

c. ha tan thay th khi rht


11/35

3) Dung dch rn tuy cng c tnh dn nhit, dn in

song km hn kim loi nguyn cht, c th lm thay i

t ngt in th in cc c nh hng tt n tnh

chng n mn in ha- Trong tt c cc hp kim kt cu, dung dch rn bao gi

cng l pha c bn chim trn di 90% thm ch ch c

pha ny (100%) .

3.1.3.Pha trung gian

- Cc hp cht ha hc c trong hp kim thng c gi l phatrung gian bi v trn gin pha n c v tr gia,

trung gian gia cc dung dch rn c hn hai u mt.

- l cc pha c tnh cht gn ging cc hp cht ho hc

thng gp, nhng c mt s nt ca dung dch rn

a.Bn cht v phn loi

- c im

1) C mng tinh th phc tp v khc hn vi nguyn t

thnh phn.

2) Lun lun c t l kh chnh xc gia cc nguyn t v

biu din bng cng thc ha hc AmBn

3) Tnh cht khc hn cc nguyn t thnh phn vi c

im l gin (khc hn vi kim loi).

4) C nhit nng chy c nh v khi to thnh ta ra

mt lng nhit ng k.

77


12/35

- Phn ln cc hp cht ha hc trong hp kim c nhng

c im khc vi cc hp cht ha hc theo ha tr:

+ khng tun theo quy lut ha tr,

+ khng c thnh phn ha hc chnh xc (dao ng trong

mt khong quanh thnh phn chnh xc theo cng thc),

+ c lin kt kim loi.

Cc pha trung gian trong hp kim thng gp gm

c:pha xen k, pha in t v pha Laves.

Thng gp pha xen k, VD: cacbit, nitrit, hyrit (v borit).

cng thc M4X, M2X, MX hoc M3X, M7X3, M23X6.

(Cc nitrit Fe4N, Fe2N, Mo2N, Cr2N... c to thnh khi thm

Fe3C, Mn3C, Cr7C3, Cr23C6 )

3.2.Gin pha ca h hai cu t

- Gin pha chnh l gin biu th s bin i t

chc pha vo nhit v thnh phn ca h trng thi

cn bng.

Nh vy ni chung gin pha ch hon ton ng v

ph hp vi hp kim trng thi cn bng (lm ngui rt

chm hay trng thi ), cn trong trng hp lm ngui

thng thng (trong khng kh) gy ra t nhiu sai khc,

tuy nhin gin pha vn l c s xc nh cu trc ca

hp kim cho ( sai lch xut hin hon ton c th l gii

c).

78


13/35

V vy gin pha l cng c quan trng kho

st hp kim (v ceramic), qua c th bit iu chnh

yu nht l cu trc. 3.2.1.Gin pha v cng dng

- Gin pha c xy dng ch bngthc nghim.

- H mt cu t khng c s bin i thnh phn gin

pha ch c mt trc ( trc tung) trn nh du

nhit chy (kt tinh) v cc nhit chuyn bin th

hnh (nu c)

VD: Fe

- H hai cu t c hai trc: trc tung nhit , trc

honh thnh phn (% khi lng) vi nhng ng phn

chia cc khu vc pha theo theo cc nguyn tc sau:

* Xen gia hai khu vc mt pha l khu vc hai pha tng

ng.

* Mi im trn trc honh biu th mt thnh phn xc

nh ca h. ( Tri sang phi : t l cu t B tng ln, A

gimn, hai u mt tng ng vi hai cu t nguyn cht: A

(tri), B (phi).

VD: im C ng vi thnh phn c 30%B , 70%A),

im D: 80%B + 20%A.

79


14/35

Hnh 3.6. Gin

pha ca st.

Hnh 3.7. Cc trc ca gin pha h

hai cu t.

- ng thng ng bt k biu th mt thnh phn xc

nh nhng cc nhit khc nhau.

- Gin pha ca h hai cu t d dng xc nh c cc

thng s sau y cho mt thnh phn xc nh nhit

no :

Cc pha tn ti. Cn c vo im nhit - thnh phn

cho (ta ) nm trong vng no s c t chc pha t-

ng ng vi vng : nm vng mt pha, hp kim c t

chc mt pha; nm trong vng hai pha - c t chc hai pha.

Thnh phn pha.

Nu ta nm trong vng mt pha th thnh phn ca

pha cu to nn hp kim bng chnh thnh phn ca hp kim

chn.

80


15/35

Khi ta nm trong vng hai pha vic xc nh nh

sau: k ng nm ngang (ng nhit) qua ta ny, hai

giao im ca n vi hai ng bin gii vi hai vng mt pha

gn nht s ch r thnh phn ca tng pha tng ng. T l (v s lng) gia cc pha hoc t chc c xc

nh bng quy tc cnh tay n:

lng pha tri di on thng phi (n bn phi)

------------------ = --------------------------------------------------

lng pha phi di on thng tri (n bn tri)

ging nh s cn bng ca n by

Hnh 3.8. S cn bng ca n by.

Suy on tnh cht ca hp kim.

Theo quy tc kt hp th tnh cht ca hp kim l tng hp

tnh cht ca tng pha cu to nn HK theo t l bc nht

Nhit chy (kt tinh): thng hp kim nng chy (kttinh) trong mt khong nhit (bt u v kt thc) t-

ng ng vi hai ng chy ngang sut gin , ng chy

ngang trn c gi l ng lng ( cao hn ng ny hp

kim hon ton trng thi lng), ng ngang st di c

gi l ng rn (hay ng c) ( thp hn ng ny

hp kim hon ton trng thi rn).

81


16/35

Cc chuyn bin pha. S xut hin hoc bin mt

ca cc pha (khi nung v khi ngui chm) cng nh theo

nhit xy ra, tng ng vi cc ng di ng c.

D on cc t chc to thnh trng thi khng cn

bng (khi ngui nhanh).

3.2.2.Gin loi I

Hnh 3.9. Dng tng qut ca gin pha loi I (a) v gin pha

Pb - Sb (b).

- H hai cu t khng c bt k tng tc no chng to

nn hn hp ring r ca hai cu t

VD: H Pb - Sb

3.2.3.Gin loi II

82


17/35

Hnh 3.10. Dng tng qut ca gin pha loi II (a) v cc

gin pha h Cu - Ni (b)

- H hai cu t vi tng tc ha tan v hn vo nhau

VD: H Cu - Ni3.2.4.Gin loi III

- H hai cu t vi t-

ng tc ha tan c

hn vo nhau,

VD: H Pb - Sn

Hnh

3.12. gin pha h Pb - Sn- t chc khi kt tinh trng thi

cn bng ca hp kim 40%Sn (b).

+ AEB l ng lng

+ ACEDB - ng c

+ E im cng tinh: L(+ ) ( hp kim cng tinh)

Trc E : HK trc cng tinh

Sau E: HK sau cng tinh

83


18/35

+ CF; DG l ng gii hn ho tan ca dung dch rn:

ln nht nhit cng tinh, gim mnh nhit thp

+ C th chia HK ca h thnh 3 nhm:

Nhm cha rt t cu t th hai (bn tri F, bn phi G),

sau khi kt tinh xong ch c mt dung dch rn hoc ,

c c tnh nh gin loi II.

Nhm cha mt lng hn ch cu t th hai (t F n C

v D n G), ban u kt tinh ra dung dch rn, song

khi nhit h xung thp hn CF v DG chng tr nn

qu bo ha tit ra lng cu t ha tan tha di dng dung

dch rn th cp ( tha B tit ra pha II giu B, tha A

tit ra pha II giu A).

Nhm cha lng ln cu t th hai [t C (C) n D (D)],

ban u kt tinh ra dung dch rn (C hay D), pha lng

cn li bin i thnh phn theo ng lng n im E

kt tinh ca cng tinh

V d xt HK trc cng tinh 40%Sn ca h Pb - Sn

- T>245o

C hp kim hon ton trng thi lng.

- T= 245oC hp kim bt u kt tinh ra 2 vi 13,3%Sn

khi lm ngui tip tc c to thnh v pha lng t dn

VD: 200oC pha cha 18,5%Sn (a) v L cha 57%Sn

(a)

t l gia chng l : a / La = (57 - 40) / (40 - 18,5) =

17 / 21,5 vy pha a chim t l 44,2%

84


19/35

La - 55,8%.

- n nhit cng tinh 183oC, trc khi kt tinh cng tinh t

l gia hai pha ny l

C / LE = (61,9 - 40) / (40 - 19,2) = 21,9 / 20,8.

Cng ti nhit ny sau phn ng cng tinh LE (C +

D), hp kim c t chc C + (C + D) vi t l

C / (C + D) cng bng 21,9 /

20,8.

Nh vy trong t chc cui cng ca hp kim c hai loi dung

dch rn : loi kt tinh c lp trong vng + L ( cao

hn 183oC) v loi cng kt tinh vi nhit khng i

(183oC) v c gi l cng tinh.

Nu tnh t l gia hai pha (ch c trong cng tinh) v

(gm c loi c lp ln c loi cng tinh) th 183oC c

/ = (40 - 19,2) / (97,5 - 40) = 20,8 / 57,5,

chim t l 26,6%, chim t l 73,4%.

3.3.Gin pha Fe - C (Fe - Fe3C)

- Gin pha Fe - C c tng tc kh phc tp, rt in

hnh vic nghin cu c ngha va thc t v khoa

hc

- Thc t ch kho st mt phn (phn giu Fe) vi lng

cacbon n 6,67 % (h Fe - Fe3C) c coi l cn bng

gi n nh 3.3.1.Tng tc gia Fe v C

85


20/35

St: St nguyn cht k thut c c tnh nh sau:

HB 80; b 250MPa; 0,2 120MPa;

50%;

85%; aK 2500kJ/m2

c th hnh vi 2 kiu mng tinh th : lptk v lptm

a. Tng tc gia Fe v C:

- C nhiu tng tc vi nhau v rt in hnh: To ra c

dung dch rn, pha trung gian v hn hp c hc

1. Dung dch rn xen k ca C trong Fe

- Do kch thc nguyn t nh (rC = 0,077nm, rFe = 0,1241nm)

C ha tan c hn vo Fe dng dd rn xen k.

- Vi mng Fe v Fe ( mng lptk- A2) mt xp thp, c

nhiu l hng, mi l hng li c kch thc qu nh (l tmmt c r = 0,154rFe , l bn mt ln hn c r = 0,291rFe),

ch bng gn mt na kch thc nguyn t C Do vy v

nguyn l Fe v Fe khng c kh nng ha tan C hay

ha tan cacbon trong chng l khng ng k (c th coi

bng khng).- Thc t c th ho tan mt lng nh C ( 0,02% 727 0c v

vi Fe v 0,1% 14990C vi Fe do C nm cc vng sai lch

mng, ch yu trn bin gii ht

- Vi Fe - mng lptm (A1) c kch thc l hng ln hn (l

tm mt r = 0,414rFe), vn nh hn C mt cht nn ckh cha nguyn t C v gy x lch mng (ti a c th

86


21/35

n 2,14%C), vt qu gii hn trrne s lm mng qu x

lch v tr nn km n nh.

2. Cc bt Fe (xementit)

- Khi lng C a vo Fe vt qu gii hn ha tan ni trn, C

tha ra s kt hp vi Fe thnh Fe3C gi l xmentit.

- Fe3C l pha xen k vi kiu mng phc tp, c thnh phn

6,67%C + 93,33%Fe.

- L pha khng n nh, trong mt s iu kin li b

phan ho thnh Fe v C ( graphit)

3. Hn hp c hc:

- Hp kim Fe-C thng c cu to tinh th l hn hp c hc

ca dd rn ca Fe v Xe, nht l nhit thng

- Hai dng hn hp im hnh l cng tinh v cng tch

3.3.2.Gin pha Fe - Fe3C

a.Gin pha Fe - Fe3C

Gin pha Fe - C (Fe - Fe3C) c trnh by hnh

3.18 vi cc k hiu cc ta (nhit , oC - thnh phn

cacbon, %) c quc t ha nh sau:

A (1539 - 0); B (1499 - 0,5); C (1147 - 4,3);

D (~1250 - 6,67); E (1147 - 2,14); F (1147 - 6,67);

G (911 - 0); H (1499 - 0,10); J (1499 - 0,16);

K (727 - 6,67); L (0 - 6,67) ; N (1392 - 0);

P (727 - 0,02); Q (0 - 0,006); S (727 - 0,80).

Mt s ng c ngha thc t rt quan trng nh sau:

87


22/35

- ABCD l ng lng xc nh nhit chy lng

hon ton hay bt u kt tinh.

- AHJECF l ng rn xc nh nhit bt uchy hay kt thc kt tinh.

- ECF (1147oC) l ng cng tinh, xy ra phn ng cng tinh

(eutectic).

- PSK (727oC) l ng cng tch, xy ra phn ng cng tch

(eutectoid).

- ES - gii hn ha tan cacbon trong Fe.

- PQ - gii hn ha tan cacbon trong Fe.

88


23/35

Hnh 3.18. Gin pha Fe - C (Fe - Fe3C).

b. Cc chuyn bin khi lm ngui chm

- Chuyn bin bao tinh xy ra 1499oC trong cc hp kim

c 0,10 0,50%C (ng HJB)

H + LB H hay 0,10 + L0,50

0,16 (3.1) song t n phn ng ny v xy ra

89


24/35

nhit qu cao v khng c nh hng g n t

chc ca thp khi gia cng v s dng.

- Chuyn bin cng tinh xy ra 1147oC trong cc hp

kim c>2,14%C (ng ECF)LC (E + Fe3CF) hay L4,3 (2,14 + Fe3C6,67) (3.2)

- Chuyn bin cng tch xy ra 727oC hu nh vi mi hp

kim (ng PSK)

S [P + Fe3CK] hay 0,8 [0,02 + Fe3C6,67]

(3.3)

- S tit pha Fe3C d ra khi dung dch rn ca cacbon

trong cc dung dch rn: trong Fe theo ng ES v trong

Fe theo ng PQ.

c. Cc t chc mt pha

trng thi rn c th gp bn pha sau.

*Ferit (c th k hiu bng hay F hay Fe)

- l dung dch rn xen k ca cacbon trong Fe vi mng lptk

(a = 0,286 0,291nm)

- lng ha tan C qu nh (ln nht l 0,02%C 727oC - im

P, nhit thng thp nht ch cn 0,006%C - im Q)

coi

Fe nh Fe nguyn cht , c tnh st t nhng ch n

768oC.

- Trn gin n tn ti trong vng GPQ , c tnh ca ferit

chnh l ca st nguyn cht: do, dai, mm v km bn.

- Ferit c th ha tan Si, Mn, P, Cr... nn s cng v bn hnsong cng km do dai i.

90


25/35

- Ferit l mt trong hai pha tn ti nhit thng v

khi s dng (< 727oC), song vi t l cao nht (trn di 90%)

ng gp mt t l quan trng trong c tnh ca hp kim

Fe - C.- T chc t vi ca ferit trnh by hnh 3.19a c dng

cc ht sng, a cnh.

Hnh 3.19. T chc t vi ca ferit (a) v austenit (b) (x500).

*Austenit [ c th k hiu bng , A, Fe(C) ]- L dung dch rn xen k ca C trong Fe vi mng lptm (a

0,364nm)

- Lng ha tan ng k cacbon (cao nht ti 2,14% hay

khong 8,5% v s nguyn t 1147oC - im E, 727oC

ch cn 0,80%C - im S).

- Khng c tnh st t m c tnh thun t, n ch tn ti

nhit cao (> 727oC) trong vng NJESG (tip gip vi Fe

trn trc st)

- Rt t khi quan st thy nhit thng

91


26/35

- Khng tham gia quyt nh tnh cht ca hp kim

khi lm vic nhng li c vai tr quyt nh trong bin

dng nng v nhit luyn v n l trung gian t

nhn c cc t chc theo yu cu- T chc t vi ca austenit trnh by hnh 3.19b c cc

ht sng, c th vi mu m nht khc nhau i cht (do

nh hng khi tm thc) v cc ng song tinh (song song)

ct ngang ht (th hin tnh do cao).

*Xmentit (c th k hiu bng Xe, Fe3C)-L pha xen k vi kiu mng phc tp, cng thc Fe3C,

6,67%C, (ng thng ng DFKL trn gin )

- Cng ( 800HB) v gin, cng vi F n to nn cc t chc

khc nhau ca hp kim Fe - C.

- C 4 loi xmenntit:

Xmentit th nht(XeI):

- c to thnh t pha lng do gim nng C trong hp

kim lng theo DC khi h nhit

- ch c hp kim c > 4,3%C.

- do to thnh nhit cao (> 1147oC) XeI c dng

thng, th to (hnh 3.24b) i khi c th thy c bng

mt thng.

-Xmentit th hai (XeII) :

- c to thnh do gim nng C trong theo ng ESkhi h nhit

92


27/35

- thng thy rt r hp kim c 0,80 - 2,14%C.

- do to thnh nhit tng i cao (> 727oC) to

iu kin cho s tp trung bin gii ht khi XeII vi lng

ln to thnh li lin tc bao quanh cc ht austenit

(peclit) (hnh 3.23) to ra khung gin, lm gim mnh

tnh do v dai ca hp kim.

*Xmentit th ba (XeIII) :

- c to thnh do gim nng C trong F theo ng PQ

khi h nhit

- s lng (t l) rt nh (nhiu nht cng ch l 2o/oo) nn

rt kh pht hin trn t chc t vi v thng c b qua.

- *Xmentit cng tch c to thnh do chuyn

bin cng tch austenit peclit. Loi xmentit ny c vai

tr rt quan trng, c trnh by mc tip theo.

d. Cc t chc hai pha

*Peclit (c th k hiu bng P, [Fe + Fe3C]).

- L hn hp cng tch ca F v Xe c to thnh t

austenit (0,80%C) v 727o

C- Trong P c 88% F v 12% Xe phn b u trong nhau

peclit l t chc kh bn, cng, do, dai p ng rt

tt cc yu cu ca vt liu kt cu v cng c.

- Peclit v cc bin th ca n (xoocbit, trxtit, bainit) c

mt trong hu ht cc hp kim Fe - C.Ngi ta phn bit hai loi peclit tm v peclit ht.

93


28/35

+

Hnh 3.20. T chc t vi ca peclit tm (a) v peclit ht (b) (x500).

Peclit tm (hnh 3.20a) thng gp hn c, c cu trc tm(lp phin), hai pha F v Xe u dng tm nm an xen

nhau, (cc vch ti mng -vi lng t hn) l Xe, vch sng

dy (vi lng nhiu hn, gi l nn) l F

Peclit ht(hnh 3.20b) t gp hn, c cu trc ht : Xe

dng thu gn nht (b mt t nht), phn b u trnnn ferit.

- Gia hai loi ny c s khc bit nh v c tnh: so vi P

ht, P tm c bn, cng thap hn, do, dai

thp hn i cht. ng nht d to thnh P tm,

km ng nht d to thnh P ht.

94


29/35

P ht n nh hn Ptm nn khi nung lu nhit

tng i cao (v d 600 700oC) P tm c xu hng chuyn

thnh Pht.

*Lburit [ c th k hiu bng Le, hay ( + Xe) hay (P+ Xe) ].

Hnh 3.21. T chc t vi

ca lburit - (P+Xe)

(x500).

- L l hn hp cng tinh

ca v XeI ( vi 4,3%C 1147oC) nh phn ng cng tinh;

-Khi lm ngui tip tc li c phn ng cng tch (727 0C)

chuyn bin thnh P nn t chc t vi cui cng quan

st c l hn hp ca P tm (cc ht ti nh) trn nnXe sng.

- L cng v gin (ti 2/3, l xmentit - 600HB) v ch c

trong hp kim Fe - C dng gang trng, t gp.

3.3.3.Phn loi

a.Khi nim chung v thp, gang

95


30/35

- Thp v gang u l hp kim Fe - C (ngoi ra c th

c t nhiu cc nguyn t khc), trong phn bit 2,14%C l gang( bn tr i E l thp, bn

phi E l gang)

gii thch c cc c im c bn khc nhau gia

chng.

-Tt c cc thp d vi lng C, t chc v c tnh rt khc

nhau nhit thng, khi nung nng cao hn ng GSE

ch c mt pha l vi do rt cao nn rt d bindng thp c coi l vt liu do

- c cung cp di dng bn thnh phm cn nng (dy,

thanh, ng, tm, hnh...) rt tin li cho s dng.

-Tnh c ca thp ni chung l thp (do nhit chy

cao, khng c t chc cng tinh) v t c s dng ch

to vt c.

- Tt c cc gang khi nung nng, gang khng th t c

t chc mt pha m bao gi cng cn Xe (hay grafit)

khng th em bin dng ngui ln nng

- C nhit chy thp hn, trong t chc c cng tinh vi

kh nng in y khun caogang l vt liu c, ch

c cung cp di dng vt c (cc sn phm nh hnh

nh chi tit my phc tp: thn b, hp, v hay n gin

nh ng ...).

96


31/35

- Gang c thnh phn cng gn cng tinh tnh c cng

cao.

- Tnh do ca gang rt thp (khng bao gi c em bin

dng do).

d. Cc loi thp, gang theo gin pha Fe - C

- Cn c vo t chc khc nhau trn gin pha ta c ba

loi thp v ba loi gang khc nhau.

* Thp (tng ng vi gin pha Fe - C)

- l loi hp kim ngoi Fe vi C < 2,14% c cha lng khng

ng k cc nguyn t khc, c gi l thp cacbon hay

thp thng, gm ba loi nh sau y.

+ Thp trc cng tch ( tct) vi lng C t 0,10 - 0,70% (bn

tri im S )

c t chc F(sng) + P (ti) (hnh 3.22)

Thng dng l cc loi thp 0,20%C; 0,30 0,40%C.

Khi lng cacbon tng ln th trn t chc t vi t l phn

peclit (mu ti) cng tng ln, cn phn ferit (mu sng)

gim i. VD:

Vi 0,10%C (hnh 3.22a) phn ti (peclit tm) chim

khong 1/8,

97


32/35

Vi 0,40%C (hnh 3.22b) l 1/2

- Thp cng tch vi thnh phn 0,80%C (c th x

dch mt cht) tc ng vi im S c t chc ch gm

peclit.

Hnh 3.22. T chc t vi ca cc thp trc cng tch

(x500): a. 0,10%C, b. 0,40%C, c. 0,60%C

Hnh 3.23. T chc t vi ca thp sau cng tch

(1,20%C) (x500).

98


33/35

- Thp sau cng tch ( 0,90%C )(thng ch ti

1,50%, c bit c th ti 2.0 2,2%) - bn phi im S

c t chc P + XeII thng dng li sng bao bc ly P tm

nh hnh 3.23.* Gang tng ng vi gin pha Fe - C (Fe - Fe3C) l gang

trng, rt t c s dng do qu cng, gin, khng th gia

cng ct c . Theo s khc nhau v t chc ta gp ba

loi gang trng sau.

- Gang trng trc cng tinh (< 4,3% - bn tri im C)

c t chc P + XeII + L (hnh 3.24a).

- Gang trng cng tinh (4,3%C - im C hay ln cn)

t chc ch l L (hnh 3.21).

- Gang trng sau cng tinh ( > 4,3%C - bn phi im C)

c t chc L + XeI (hnh 3.24b).

Hnh 3.24. T chc t vi ca gang trng (x500):

99


34/35

trc cng tinh (a) v sau cng tinh (b)

c.Cc im ti hn ca thp

- Nh thy r t gin pha Fe - C, trong khu vc ca thp

c rt nhiu ng ng vi cc chuyn bin pha khc nhau.

- n gin ha vic gi tn cc chuyn bin pha k

hiu cc ng tng ng bng ch A

- Khi c chuyn bin pha nhit bin i chm li i

khi dng hn) gi l cc im (hay nhit ) ti hn.

- Cc im ti hn thng dng nht gm :

+ A1 - ng PSK (727oC) ng vi chuyn bin P, c trong

mi loi thp.

+A3 - ng GS (911 727o

C) ng vi bt u tit ra F khikhi lm ngui hay kt thc ha tan F vo khi nung nng,

ch c trong thp tct.

Acm - ng ES (1147 727oC) ng vi bt u tit

ra Xe khi khi lm ngui hay kt thc ha tan Xe vo khi

nung nng, ch c trong thp sct.

+ C th d dng xc nh (gn ng) gi tr ca A3 v

Acm ca theo gin pha Fe - C

+ phn bit cng mt im ti hn cho hai trng hp:

nung nng v lm ngui, ngi ta thm ch c (chauffage) khi

nung nng, thm ch r(refroidissement) khi lm ngui.

100


35/35

Vi cng mt thp bao gi cng c: Ac1 > A1 > Ar1; Ac3

> A3 > Ar3..., (trong A tnh theo gin pha cn Ac

cao hn v ph thuc vo tc nung, Ar thp hn v

ph thuc vo tc ngui, tc cng cao s sai khccng mnh)

101

Chuong3 - Vật liệu học

Documents

Transcript of Chuong3 - Vật liệu học