Chuong 4- hoa nhiệt luyện - vật liệu học

8/14/2019 Chuong 4- hoa nhit luyn - vt liu hc

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Chng 4 nhit luyn thp

- chng ny s nghin cu nhng bin i v t chc

v c tnh tng ng ca thp khi nung nng ri lm ngui

tip theo khi nhit luyn.

- Thp l vt liu rt thng dng v l hp kim nhy cm

nht vi nhit luyn v cng ngh ny rt ph bin trong

ch to c kh

4.1.Khi nim v nhit luyn thp

4.1.1.S lc v nhit luyn

a./n

- Nhit luyn l cng ngh nung nng kim loi, hp kim

n nhit xc nh, gi nhit ti mt thi gian

thch hp ri sau lm ngui vi tc nht nh lmthay i t chc bin i c tnh v cc tnh cht

khc theo phng hng chn trc

- Nhit luyn l phng php gia cng (treatment) c nhng

c im ring khc vi cc nguyn cng gia cng c kh

khc:+ c, hn nung nng n trng thi lng, nhit luyn

lun trng thi rn (nhit nung thp hn ng rn).

+ khc vi ct gt, bin dng do (rn, dp) khi nhit luyn

(tr c - nhit luyn) hnh dng v kch thc sn phm

khng thay i hay thay i khng ng k.


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+ kt qu ca nhit luyn c nh gi bng bin i

ca t chc t vi v c tnh, khng th kim tra bng v

ngoi bng mt thng.

b.Cc yu t c trng cho nhit luyn- t nht cng c c trng bng ba thng

+ Nhit nung nng 0nT : nhit cao nht m qu trnh

phi t n.

+ Thi gian gi nhit tgn: thi gian ngng nhit nung

nng.

+ Tc ngui Vngui sau khi gi nhit.

c trng tng ng vi ba giai on ni tip nhau ca

qu trnh nhit luyn: nung nng, gi nhit v lm ngui.

- Kt qu nhit luyn c nh gi bng cc ch tiu sau:

+ T chc t vi bao gm cu to pha, kch thc ht, chiu

su lp ha bn... y l ch tiu gc, c bn nht

+ cng l ch tiu c tnh d xc nh v cng c lin

quan n cc ch tiu khc nh bn, do, dai,

thng thng c kim tra theo t l (quan trng c th

phi kim tra c 100%), nht thit phi thc hin

+ cong vnh, bin dng: trng hp quan trng yu cu

ny rt kht khe, nu vt qu phm vi cho php cng

khng th s dng c.

c.Phn loi nhit luyn thp


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S b c th phn loi cc phng php nhit luyn thp

vi nhng c im ch yu nh sau:

Nhit luyn, thng gp nht: ch dng cch thay i

nhit (khng c bin i thnh phn v bin dngdo) bin i t chc trn ton tit dingm nhiu

phng php:

Nhit luyn s b :mc ch l lm mm thp d gia

cng ct v dp ngui, gm v thng ho

Nhit luyn kt thc: to c tnh ph hp vi yu cu lmvic c th, gm ti ( th tch hoc b mt) v raml

hai nguyn cng i km nhau

Ha - nhit luyn: dng cch thay i nhit v bin

i thnh phn ha hc b mt lm vng ny c bin

i t chc v c tnh mnh hn.thng tin hnh bng cch thm, khuch tn mt hay

nhiu nguyn t nht nh.

Thm n nguyn t c: thm cacbon, thm nit...

Thm a nguyn t c: thm cacbon - nit, thm cacbon

- nit - lu hunh...

C - nhit luyn: dng cch thay i nhit v bin

dng do bin i t chc v c tnh trn ton tit

din mnh hn khi nhit luyn n thun.

3.1.2.Tc dng ca nhit luyn i vi sn xut c

kh


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- Nhit luyn l khu quan trng v khng th thiu c

i vi ch to c kh v :

a.Tng cng, tnh chng mi mn v bn ca

thp- Mc tiu ca sn xut c kh l sn xut ra cc c cu v

my bn hn, nh hn, khe hn vi cc tnh nng tt

hn.

Sau ti + ram, ti b mt, thm cacbon, thm cacbon -

nit... bn v cng ca thp c th tng ln t 3n 6 ln my hay kt cu c th nh i tit kim

kim loi (h gi thnh), nng lng (nhin liu) khi vn hnh.

- Cht lng ca my, thit b cng nh ph tng thay th ph

thuc rt nhiu vo cch s dng vt liu ng v nhit

luyn bo m.

b.Ci thin tnh cng ngh

- cn mm d ct hoc do d bin dng ngui.

c.Nhit luyn trong nh my c kh

- cc nh my c kh vi quy m nh v trung bnh, bphn nhit luyn khng ln v thng t tp trung.

- cc nh my c kh c quy m ln v rt ln, cc chi tit

my c gia cng hon chnh t khu u n khu

cui trn dy chuyn c kh ha hoc t ng ha trong

bao gm c nguyn cng nhit luyn.


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4.2.Cc t chc t c khi nung nng v lm ngui

thp

- Bn cht ca nhit luyn l bin i t chc, chuyn

pha, vy phi xt xem nung nng thp, lm ngui....- C s l thp tng ng vi gin pha Fe -C, i t loi

thp c t chc n gin nht - P ( thp cng tch) vi

0,80%C ri m rng ra cho cc loi thp cn li (trc v sau

cng tch).

4.2.1.Cc chuyn bin xy ra khi nung nng thp -S to thnh austenit

a.C s xc nh chuyn bin khi nung

- L gin pha Fe C (gii hn khu vc thp v trng

thi rn )

- T chc ca thp cng tch: P ( hn hp c hc ca F vXe)

- Cc thp trc v sau cng tch c t chc phc tp hn:

P+F hoc P+XeII.

T chc c bn ca mi thp l F v Xe

Khi T< Ac1 trong mi thp vn cha c chuyn bin sau

lm ngui khng c bin i g.

Khi T= Ac1, P trong mi loi thp chuyn bin thnh Fe

(As) theo phn ng:

P = [Fe + Fe3C]0,80%C Fe(C)0,80%C


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trong khi F v XeII ca cc thp trc v sau ct cha

chuyn bin.

nung nng qu nhit Ac1 mt cht:

Thp ct chuyn bin hon ton: hon ton

Thp trc v sau ct c chuyn bin khng hon ton()

Nung n Ac3 (thp tct) v Accm (thp sct) s c s ha

tan F v XeII cn d vo , T0 cng cao lng hai pha ngy

mt t i

Khi T> Ac3 v Accm : s ho tan F v XeII d vo cc thp

trc v sau ct kt thc v ch cn mt pha hon ton .

khi nung nng trn GSE mi thp (d c cacbon cao,

thp) u c duy nht dung dch rn vi nng

cacbon khc nhau v bng chnh thnh phn cacbon cathp.


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Hnh 4.2. Gin pha Fe - C (phn thp).

- Trong cc chuyn bin khi nung nng chuyn bin P

thnh l c s v l mc tiu phi t ti i vi phn ln

qu trnh nhit luyn

b.c im ca chuyn bin peclit thnh austenit

Nhit chuyn bin

- chuyn bin xy ra A1 = 727oC, iu ny ch ng

khi nung nng v cng chm, thc t tc nung ln hn

nhiu T chuyn bin phi cao hn, v nung cng ln, T

chuyn bin cng cao.

- Dng cch nung nng ng nhit xc nh thi gian

xy ra chuyn bin cc nhit khc nhau Gin

chuyn

bin ng nhit P : nung nhiumu cc nhit khc nhau, gi khng i nhit , xc nh thi

gian bt u v kt thc chuyn bin.


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- Gin chuyn bin ng nhit P ca thp ct c

dng ca hai ng cong , ng 1- thi im bt u v

ng 2- kt thc chuyn bin

nhit nung (ng nhit) cng cao, thi gian btu v kt thc (tnh bng khong cch trn trc honh)

v thi gian hon thnh chuyn bin cng ngn.

VD: Nung vi V2 > V12 < 1

Trong thc t, khi nung chm (), t c chuyn bin

phi nung nng qu nhit ti hn tng ng t nht l 20

30oC v khi nung nhanh c th ti hng trm oC (nung cm

ng).

Kch th c ht austenit

- austenit khng tn ti T thng, nhng cc sn phm to

thnh t cc ht nh do, dai hn so vi t ht lntrong bt k trng hp no phi t c ht nh

- Chuyn bin P cng theo c ch: to v pht trin

mm nh kt tinh: Mm c to nn gia b/m F v Xe ca

P, b mt phn chia gia chng rt nhiu mm to

thnh rt nhiu ht austenit lc mi to thnh rt nh mn(c th nh hn cp 8, ti cp 9, 10


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- Hiu ng ny cn c tn dng trit trong nhit

luyn.

- Ht to thnh cng nh mn khi P ban u c phn

tn cng cao (tc Xe trong n cng nh mn) v tc nung nng cng ln.

- nhit Ac1, lc mi to thnh ht austenit kh nh

mn, tip tc tng nhit hoc gi nhit lu s lm

cho ht ln ln. y l qu trnh t nhin, lm gim nng l-

ng d tr Khi lm ngui, kch thc ht austenit khnggim i, vn gi nguyn cp ht ln nht t trc .

- c tnh pht trin ca ht austenit cn do hai loi thp:

thp bn cht(di truyn) ht ln v nh

Thp bn cht ht ln ht pht trin nhanh v u

n mi nhit khi nung nng d to thnh lnlm ngui to ra cc sn phm gin.

Thp bn cht ht nh ht austenit pht trin chm

lc ban u, ch khi > 930 950oC ht mi pht trin

nhanh v rt nhanh nhit luyn thng dng, nhit

thng


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- Khng xy ra cc chuyn bin mi song li l cn thit

:

+ Lm u nhit trn tit din, cho li cng c

chuyn bin nh b mt.

+ C thi gian hon thnh cc chuyn bin khi

nung nng. +Lm ng u thnh phn ha hc ca v

lc u ht c thnh phn khng ng nht: ni trc l F

ngho C, Xe s giu C. Trong giai on ny C (v nguyn t

hp kim) s san bng nng .

+ Thi gian gi nhit ch cn va nu ko di ht

ln

4.2.3. Cc chuyn bin ca austenit khi lm ngui

chm

- Khi lm ngui, austenit s chuyn bin thnh t chc no

vi c tnh ra sao quyt nh c tnh ca thp khi lm

vic hay gia cng tip theo

- Trc tin xt cho trng hp ngui chm: kho st lm ngui

ng nhit cho thp ct (0,80%C) (t chc ban u P).

a. Gin chuyn bin ng nhit ca austenit qu

ngui (gin T - T - T) ca thp cng tch

- T gin pha Fe - C, khi lm ngui, P 727oC (tc T

= 0) khi ngui rt chm khng xy ra trong thc t.

-Khi lm ngui ng nhit (t gp trong thc t): lmngui nhanh xung di Ar1gi nhit o thi gian


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bt u v kt thc chuyn bin t [F+ Xe]

*Gin T - T - T

- Tin hnh cho thp ct tp hp ca cc im bt u

v kt thc chuyn bin gin chuyn bin ng

nhit ca As (gin T - T - T) (transformation-

temperature-

time)

- Ch "C" bn tri biu th s bt u, cn ch "C" th

hai biu th s kt thc ca chuyn bin

- y l gin rt quan trng i vi nhit luyn thp v

c s dng rt nhiu xc nh t chc sau khi lmngui , cn nm vng.


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*Cc sn phm ca s phn ha ng nhit ca austenit

qu ngui

- Khi b ngui (tc thi) xung di 727oC cha chuyn

bin ngay sau mt thi gian nht nh, trc khi chuynbin, qu ngui - khng n nh ( trn 727oC l loi

n nh).

- Trn gin c nm khu vc:

+ trn 727oC n nh,

+ bn tri ch "C" u tin - qu ngui,

+ gia hai ch "C" - [F + Xe]

+ bn phi ch "C" th hai - cc sn phm phn ha ng

nhit qu ngui hn hp [F + Xe] c mc nh mn

khc nhau,+ di ng Ms (~ 220 240oC) M +

Lm ngui ng nhit austenit vi cc mc qu ngui

khc nhau

Qu ngui xung st A1 (trn di 700oC), sau thi gian di

(~ 100s) bt u phn ha kt thc chuyn bin

(sau ~ 2000s). To thnh hn hp F - Xe tm th to P

(tm), cng thp nht, HRC 10 15 (HB 180 220)

Qu ngui T thp hn (trn di 650oC), bt u v

kt thc phn ha sau thi gian ngn hn r rt (saugn 3 v 100s). Hn hp F-Xe tm s mn (nh) hn


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xoocbit, cng cao hn, c HRC 25 35.

Qu ngui nhit thp hn na (nh li ch

C) (km n nh nht, T= 500 600oC), chuyn

bin rt nhanh (sau khong 0,5 0,8 v 8s). Hn hp F-

Xe tm nh mn hn na. gi l trxtit, vi cng

cao hn, c HRC 40.

3 chuyn bin trn u l chuyn bin P, cn X, T c coi

l cc dng phn tn ca P.

Qu ngui nhit thp hn na (on di ca ch

"C"), khong 450 250oC, chuyn bin li ko di ra

c ch thay i cht t t chc gi l bainit (B), c

th coi bainit l hn hp F - Xe tm song cn mn hn

na (hn c T) vi cng cao hn, c HRC 50 55,song c im hi khc nhau (nhiu khi c th b qua)

nh sau:

+ F hi qu bo ha C (0,10%)

+ cacbit st c cng thc cha hn l Fe3C (gn l FexC vi

x=2,43,0)

+ ngoi ra cn mt lng nh (d).

(ging vi chuyn bin M chuyn bin B l chuyn bin

trung gian)

- Vy v c bn c th coi peclit (tm), xoocbit, trxtit v

c bainit c bn cht ging nhau l hn hp c hc cng


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tch ca F v Xe tm song trong theo th t tm cng

nh mn hn v cng cng cao hn.

- Sau khi lm ngui ng nhit t chc nhn c l ng

nht trn tit din.b.S phn ha ca austenit khi lm ngui lin tc

-Thc t thng lm ngui lin tc, c th s dng gin

chuyn bin ng nhit xc nh t chc to

thnh khi lm ngui vi tc khc nhau

Tc ngui V1 < V2 < V3 < V4


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+ Lm ngui chm cng l - vect V1, ct cc ng cong

ch "C" st A1: P tm vi cng thp nht.

+ Lm ngui trong khng kh tnh - vect V2, ct cc -

ng cong ch "C" phn gia ca nhnh trn xoocbit.

+ Lm ngui trong khng kh nn - vect V3, ct cc ng

cong ch "C" phn trxtit.

+ Lm ngui trong du - vect V4, ch ct phn li ca -

ng cong ch "C" th nht mt phn thnh T v phn

cn li s chuyn bin thnh M, (T+M),

- Lm ngui trong nc lnh - vect V5, khng ct ng cong

ch "C" no, chuyn bin M.

2) T chc t c thng l khng ng nht trn ton

tit din (ngoi ngui nhanh hn trong li nn thng c t

chc vi cng cao hn.

3) Khng t c t chc hon ton B

4) Nhng iu trn ch ng vi thp C, vi thp hp kim

v tr ca ng cong ch "C" dch sang phi (mc

khc nhau)

c.Gin T - T - T ca cc thp khc cng tch

- Cng c dng ca hai ch C nhng c thm nhnh ph

pha trn, bn tri ca ch "C" th nht tit ra F hoc Xe

II (thp trc hay sau ct) v mt ng ngang A3 hay Acm.

- C ba c im khc bit sau y.

1) V tr ca cc ng cong (ch "C" v nhnh ph) dchsang tri mt cht, cng xa cng tch s lch ny cng


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nhiu. (tnh n nh ca As qu ngui km hn thp

cng tch) khi nung kh t c As ng nht.

Gin T -T - T ca thp khc cng tch.

2) Khi lm ngui ng nhit vi qu ngui nh (ng

1) hay lm ngui chm lin tc (ng 2), As tit ra F (i

vi thp tct) v Xe II (i vi thp sct) gp nhnh ph

hn hp F - Xe.

Sn phm ca s phn ho: F + P (i khi F + X) (tct) v P

+ XeII (i khi X + XeII)(sct)


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S tit ra F hay Xe II khi lm ngui ng nhit vi qu ngui nh (1) v chm lin tc (2).

3) Khi lm ngui ng nhit vi qu ngui ln hay

ngui nhanh vect ngui khng gp nhnh ph

As phn ha ngay [F+Xe] vi %C 0,80 cng tch gi.

4.2.4.Chuyn bin ca austenit khi lm ngui nhanh

- Chuyn bin mactenxit (khi ti)

- Nu ngui nhanh As vi v thch hp sao vect ngui

khng ct ng cong ch "C" khng kp chuyn bin

thnh [F+Xe], m ch c chuyn bin th hnh (chuyn

kiu mng tinh th) t Fe sang Fe (khng c s tp trung

ca cacbon to nn xmentit Fe3C) xy ra nhit

thp (t 250 220oC tr xung) mactenxit( gi l ti)

Gin T -T - T v tc ti ti

hn Vth (m v

0

mT

thi gian v nhit ng vi As km n nhnht).

tm


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- v ngui nh nht gy ra chuyn bin ny Vth

(vect tip xc vi lng ca ng "C")

- khi lm ngui nhanh lin tc As (vvth) nhn c

mactenxita. Bn cht ca mactenxit

- /n: M l dd rn xen k qu bo ha ca cacbon trong Fevi nng cacbon nh ca As, c kiu mng chnh phng

tm khi v c cng cao.

- Gii thch cc c tnh ca M:

+ Do lm ngui nhanh, C trong Fe (As) khng kp tit ra

(Xe), n nhit tng i thp chuyn mng ca

Fe (tm mt) sang Fe (tm khi):

Fe (C) Fe (C) (M)

( nng C trong hai pha ny bng nhau)

+ Fe qu bo ha C ( ha tan C trong F rt thp :0,006

0,02%).

+ Nguyn t C ha tan vo Fe bng cch xen k vo cc l

hng tm mt ca mng ( gia cc mt bn hay gia

cc cnh ca c s) ko di mt cnh ra mnh hn so

vi hai cnh kia, lm cho mng t lptk (ca Fe) chnh ph-

ng tm khi. T s c/a c gi l chnh phng ca M


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c s ca

mng tinh th mactenxit.

b. Cc c im ca chuyn bin mactenxit

1) Ch xy ra khi lm ngui nhanh v lin tc As vi V Vth.

(khng xy ra khi lm ngui ng nhit)2) Chuyn bin khng khuch tn: C hu nh gi nguyn v

tr, cn Fe chuyn di v tr to kiu mng mi chnh ph-

ng tm khi

3) L qu trnh to ra cc tinh th mi vi tc pht trin

rt ln, ti hng nghn m/s. Tinh th M c dng hnh kim,u nhn lm vi nhau cc gc 60 hay 120o.

4) Ch xy ra trong khong gia hai nhit (im) bt

u Ms v kt thc Mf (start-s; finish -f). Ngoi khong

As qu ngui khng chuyn bin thnh mactenxit.

V tr ca hai im ny ph thuc vo thnh phn C vHK ca As


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5) Chuyn bin xy ra khng hon ton. khng bao gi t

c t l 100% M, m vn cn li mt lng nht nh pha

As (austenit d.)

- Nguyn nhn tn ti As d: do s khc nhau v th tch

ring ca hai pha: VM > V phn As cha chuyn bin b

sc p ngy mt tng n mc khng th chuyn bin

ht c.

Lng As d tn ti trong thp ti ph thuc vo:

+ Lng nguyn t hp kim trong As nhiu tng lng As d,

gim Ms v Mf

+ Lng C trong M cng nhiu th tch ring ca n cng ln

lm lng As d cng nhiu.

c.C tnh ca mactenxit

- M l quyt nh c tnh ca thp ti. C tnh ni bt

ca n l cng v gin.

cng cao

C ha tan xen k lm x lch mng tinh th Fe, C qu

bo ha cng cao, x lch mng cng mnh

VD: +M t C 0,25%, cng khng cao HRC 40,+ M vi C trung bnh, 0,40 0,50%, cng tng i cao,

HRC 50,

+ M cha C cao, 0,60%, cng cao, HRC 60.


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Ch : cn phn bit cng ca pha M v cng ca

thp ti v hai i lng ny khng phi lc no cng ng

nht vi nhau.

cng cao nng cao tnh chng mi mn v ch phthuy c nhng thp c 0,40%C.

Tnh gin

- L nhc im ca M hn ch s dng

- Do x lch mng v ng sut d (hay ng sut bn trong)

- cng cng cao tnh gin cng cng cao v c th bin

ng trong phm vi kh rng ph thuc vo:

+ Kim M cng nh tnh gin cng thp

+ ng sut bn trong cng nh tnh gin cng thp.

4.2.5. Chuyn bin khi nung nng thp ti (khiram)

- Sau khi ti: M +As d thp c cng cao, gin ln, tn

ti ng sut bn trongt chc khng n nh c

nguyn cng iu chnh : nung nng li ram.

a) Tnh khng n nh ca mactenxit v austenit

- < 727oC t chc n nh ca thp l [F+Xe] tc P

M+As d l cc pha khng n nh c khuynh hng

chuyn bin thnh hn hp ny.

- M khng n nh do qu bo ha C C tha s tit ra

dng Xe v phn cn li mt ht C tr thnh F:


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Fe(C) Fe3C + Fe

- As khng tn ti n nh di 727oC c khuynh hng

chuyn bin thnh:

Fe(C) Fe3C + Fe

- Ch : T thng xy ra rt chm (kh nhn thy) nung

nng thc y qu trnh chuyn bin ny.

- Hai pha ny khng chuyn bin ngay thnh [F+Xe] m

qua t chc trung gian l M ram theo s :

mactenxit

Mram [F + Xe].

austenit d

b. Cc chuyn bin xy ra khi ram

Theo s tng ca nhit nung nng, thp cng tch(0,80%C) ti vi t chc M+As d ln lt qua cc giai on

vi cc chuyn bin:

Giai on I (< 200oC)

As d cha chuyn bin

Ch c M chuyn bin:

(M ti) Fe(C)0,8 [Fe(C)0,25 0,40 + Fe2 2,4C] M ram

- cui giai on thp ti c t chc: M ram v As d.

Giai on II (200 260oC)

- C vn tip tc tit ra khi M C trong dd rn ch cnkhong 0,15 0,20%


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-As d M ram:

(As d) Fe(C)0,8 [Fe(C)0,15 0,20 + Fe2 2,4C] Mram

- Cui giai on, t chc M ram = + M ngho C (0,15

0,20%).

- M ram l t chc cng khng kmM tinhiu ( 1-2 HRC),

song li t gin hn do gim c ng sut bn trong (do C

tit bt ra khi dd rn gim cht t x lch mng).

Giai on III (260 400oC)

C hai pha u chuyn bin:

- Tt c C qu bo ha c tit ht ra khi M di dng cacbit,

chnh phng khng cn, M ngho C tr thnh F.

- Cacbit (Fe2 2,4C) dng tm mng bin thnh xmentit

Fe3C dng ht.- S chuyn bin nh sau:

Fe(C)0,15 0,20 Fe + Fe3Cht hn hp

Fe2 2,4C Fe3Cht F-Xe

- Cui giai on, t chc: hn hp F - Xe dng ht rt nhmn v phn tn, c gi l trxtit ram.

- Do C tit ht ra khi ddrn (M)

+ cng gim i r rt song vn cn tng i cao (HRC

45 vi thp cng tch).

+ Mt hon ton s bn trongtng mnh tnh n hih

max.


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Giai on IV (> 400oC)

Khi T > 400OC , khng c chuyn bin g mi, ch c qu

trnh kt t (st nhp, ln ln) ca cc phn t Xe dng

ht.- 500 650oC hn hp F+Xe ht nh mn v kh phn

tn c gi l X ramvi c tnh l c ch v ak max

- gn A1 (727oC) c hn hp F+Xe ht P ht.

Do Xe kt t cng ca thp ti gim i mnhd

gia cng ct.

Khi T > Ac1 s xut hin As n nh ( ging gin pha)

khi ram khng c nung nng qu A1.

4.3. v thng ha thp

-L nhit luyn s b, lm mm thp, chun b t chccho gia cng (ct, dp ngui, nhit luyn) tip theo

4.3.1. thp

a.nh ngha v mc ch

nh ngha

- l phng php nung nng thp n nhit nht

nh, gi nhit lu ri lm ngui chm cng l t -

c t chc n nh P (ng vi gin pha Fe - C) vi

cng thp nht v do cao.

- c trng ca : T khng c quy lut tng qut v lm

ngui vi tc chm t t chc cn bng.


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Mc ch

- C nhiu phng php m mi phng php ch t c

mt, 2 hay 3 trong s 5 mc ch sau:

1) Gim cng (lm mm) thp d gia cng ct.

2) Lm tng do d bin dng (dp, cn, ko) ngui.

3) Lm gim hay lm mt ng sut bn trong gy nn bi

gia cng ct, c, hn, bin dng do.

4) Lm ng u thnh phn ha hc trn vt c b

thin tch.

5) Lm nh ht thp.

Theo chuyn bin pha P As xy ra khi nung nngchia

ra hai nhm : c v khng c chuyn bin pha.

a)Cc phng php khng c chuyn bin pha-T


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VD: - Vt c gang quan trng nh thn my ct phi

kh b phn ln (70 80%) s bn trong khng lm

cong, bin dng sng trt lu trong kho hay ngoi tri

(T thng), s gim dn

+ 450 600oC trong 1 2h s kh b hon ton s.

- Sau khi gia cng c s bn trong s tng ln phi c

T = 200 n 450oC

kt tinh li

- kt tinh li cc thp qua bin dng ngui, b bin

cng cn khi phc tnh do, cng mc nh trc b

bin dng.

VD: i vi thp cacbon, T KTL= 600 700oC gim

cng v lm thay i kch thc ht.

c. Cc phng php c chuyn bin pha

-Thc t thng gp loi c chuyn bin pha nhit

cao hn Ac1 xy ra chuyn bin P As khi nung nng

vi hiu ng lm nh ht khi lm ngui chm As ht nh

P vi kch thc ht nh.C cc phng php

hon ton

- p dng cho thp tctvi (% C = 0,30 0,65%) nung ti

trng thi hon ton l As ( cao hn Ac3):

0u

T = Ac3 + (20 30oC)


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Mc ch:

- Lm nh ht: nung qu Ac3 khong 20 30oC th ht As

vn nh, nn khi lm ngui tip theo nhn c F + P ht

nh.

- Lm gim cng v tng do d ct gt v dp

ngui ( HB 160 200)

- Nung nng hon ton As ng nht lm ngui

nhn c F+ P ( P dng tm)

khng hon ton v cu ha

- p dng cho thp dng c c thnh phn cacbon cao

0,70% ( tt c cc thp ct, sct v thp tct vi 0,70%C )

nung nng thp ti trng thi khng hon ton As ( > Ac1 ,

< Accm ):

0

uT

= Ac1 + (20 30

o

C) = 750 760

o

Cvi mi thp

- T chc to P ht(khng phi l P tm):Nu hon ton

As ng nht P tm ( HB > 200 220) v Xe II dng

li thp gin.thp sct khng hon ton

- khng hon ton T > Ac1 mt cht As cha kp ng

u ha thnh phn (cn cc phn t Xe ca P cha chuyn

bin xong hoc cn XeII) to thnh P ht d dng ( HB Ac3 hay Accm), gi nhit lm ngui tip theo


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trong khng kh tnh As phn ha thnh t chc

gn n nh: P phn tn hay X vi cng tng i thp

(cao hn mt cht).

- Cc nt c trng ca thng ha so vi l:+ Nhit : ging nh hon ton ton nhng c p dng

cho c thp sct: 0thT = Ac3 + (30 50oC) cho thp

tct 0thT = Accm + (30 50oC) cho thp

sct.

+ Tc ngui: nhanh hn i chtkhng phi dng l

khi lm ngui nn kinh t hn .

+ T chc v c tnh: t chc t c l gn cn bng

vi cng cao hn i cht.

b.Mc ch v lnh vc p dng

Nhm vo ba mc ch sau.

1) t cng thch hp gia cng ct cho thp cacbon

thp ( 0,25%). (Loi thp ny nu hon ton s t

cng qu thp (HB < 140 160) qu do, phoi kh gy,kh ct gt)


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Ch : bo m tnh gia cng ct:

+ thp 0,25%C thng ha,

+ thp 0,30 0,65%C hon ton,

+ thp 0,70%C khng hon ton ( cu ha).

2) Lm nh xmentitchun b cho nhit luyn kt thc.

3) Lm mt li xmentit II ca thp sau cng tch.

4.4.Ti thp

4.4.1. nh ngha v mc ch

a.nh ngha

- Ti thp l phng php NL bao gm: nung thp ln cao

qu nhit ti hn Ac1 lm xut hinAs, gi nhit ri

lm ngui nhanh thch hp bin n thnh M hay cc t

chc khng n nh khc vi cng cao.

- c trng ca ti:

+ T ti > Ac1 c As (c th ging hoc thng ha).

+Tc lm ngui nhanh s nhit cng nh s t chc

u ln, d gy nt, bin dng, cong vnh.

+ T chc to thnh cng v khng n nh.

b. Mc ch

t cc yu cu sau y.


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1) Nng cao cng v tnh chng mi mn (kt hp vi

ram thp tng tui th ca cc chi tit my, dng c .

Cc thp C 0,35%C khi ti cng HRC 50chng mi

mn khng ng k.

2) Nng cao bn v sc chu ti ca chi tit my. Nu

cc thp C 0,10%hiu qu tng bn thp

- y l nguyn cng gn nh cui cng, thc hin trn chi

tit gn thnh phm ngui nhanh d bin dng, thm

ch c khi nt, v gy lng ph nghim trng

4.4.2.Chn nhit ti thp

a.i vi thp trc cng tch v cng tch ( 0,80%C)

-T ti > Ac3 (trng thi hon ton l As)

0

tT = Ac3 + (30 50oC)Sau ti M + As d.

b.i vi thp sau cng tch ( 0,90%C)

- T ti > Ac1 (trng thi khng hon ton As: (+ XeII)

0

tT = Ac1 + (30 50oC) 760 780oC

Sau ti M + XeII +As d.

c.Gii thch

- i vi thp tct, nu ti khng hon ton (tc < Ac3):

+ M + , l pha mm lm thp cng cathp ti n cn gy ra im mm xu ti c tnh


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- thp sct, nu ti hon ton (>Am) hon ton As cng

nhng ht As ln gin, ch cn >A1 As+XeIIti

M+XeIIcng, ht nh

Ch

- Cch chn T ti nh trn theo gin pha Fe - C ch p

dng cho thp C.

- i vi thp HK

+ i vi thp HK thp (v d 0,40%C + 1,00%Cr) gn

ging, thng tng ln (10 20oC).

+ i vi thp HK trung bnh v cao, T ti khc nhiu vi

thp C tng ng, phi tra trong cc sch tra cu

4.4.3.Tc ti ti hn v thm ti

a.Tc ti ti hn

- V ti hn tc ngui nh nht cn thit As

chuyn bin thnh M.

- V ti hn ca thp cng nh cng d ti, khng cn

ngui nhanh cng t c Mbin dng nh v khng b

nt quan trng vi cc chi tit c hnh dng phc tp.

b.Cc yu t nh hng n tc ti ti hn

+ Thnh phn HK ca Asquan trng nht. as cng giu

NTHK (tr Co) ng "C" cng dch sang phiVth cng nh

VD: thp vi 2 3% NTHK Vth 100oC/s,

vi 5 7% NTHK Vth 25oC/s.


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- Cc phn t rn cha tan htvo As thc y to thnh

hn hp F- Xe, lm tng Vth.

+ As cng ng nht cng d bin thnh M, As ng nht

gim Vth.

c. thm ti

- Khi lm ngui (trn chi tit c ng knh D), tc ngui

phn b trn ng knh c dng Parabol

- Ch c lp b mt c chiu dy c ti: Vng>Vth

- Phn li c Vli < Vth : Khng c ti

- Nu Vli > Vth chi tit c ti thuCh :

Nu lm ngui nhanh ng Parabol c nng cao

giao vi Vth cng gn li thm ti tng

d. ngha ca thm ti


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- Sau ti bn, cng ca thp tng ln nhiu ln lp

ti qu mng ( thm ti nh) th hiu qu ny l khng

ng k

- Khi lp ti dy ( thm ti su) sc chu ti ca chitit tng. c bit khi ti thu c tnh cao v ng

nht trn ton tit dinc bit quan trng i vi cc

chi tit ln chu ti nng.

- Chn cc mc thp ph hp vi kch thc ca sn phm

trnh lng ph vt liu.

e.Tnh thm ti v tnh ti cng

- Tnh ti (tnh ti cng) l kh nng t cng cao khi

ti, ph thuc ch yu vo lng C ca As ( t ph thuc

vo lng NTHK) Thp c C cng cao tnh ti cng cng

ln.

- Tnh thm ti l kh nng tng chiu dy ca lp ti cng,

ph thuc ch yu vo mc hp kim ha ca As (t

ph thuc vo lng C) Thp HK cng cao tnh thm ti

cng ln.

4.4.4.Cc phng php ti th tch v cng dng. Cc

mi trng ti (ti th tch)

- Khng th lm ngui thp vi tc ty a thp vo

trong mi trng no (thng l cht lng) s phng

thc lm ngui khi ti cng l c hn.


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- Theo phng thc lm ngui (cch s dng mi trng

ngui- mi trng ti) c mt s phng php sau

Hnh 4.18. Ph-ng thc lm

ngui trong cc

phng php ti:

a. trong mt mi trng,

b. trong hai mi trng,

c. phn cp,

d. ng nhit.

a.Ti trong mt mi trng v cc mi trng ti th-

ng dng

- L phng php ti c dng rng ri nht.

- Lm ngui khi ti nh biu din bi ng a , nhng chi

tit vo trong mi trng lm ngui nhanh thch hp chon khi thp c ngui hn.

- Chn mi trng lm ngui - mi trng ti c ngha

quyt nh.

Yu cu i vi mi tr ng ti

* Mi trng ti phi t c cc yu cu:


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+ phi lm ngui nhanh thp t c t chc M

+ khng lm thp b nt hay bin dng (bin dng

nh trong phm vi cho php),

+ tnh n cc yu t kinh t, an ton v bo v mi tr-ng

ngcong l tng lm

ngui khi ti.

- Mi trng ti em dng phi c kh nng lm ngui khc

nhau cc khong nhit biu din bng vect ngui

tng quan vi gin T - T - T ca thp

1) Lm ngui nhanh thp trong khong As km n nhnht 500 600oC ( khng kp phn ha thnh hn hp F -

Xe) mi trng ti phi c kh nng lm ngui thp vi

>Vth

2) Lm ngui chm thp ngoi khong nhit trn -

khong nhit bt u xy ra chuyn bin M (300

nhit


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200oC). gim ng sut t chc khi xy ra chuyn bin

nybo m thp ti khng b nt v t cong vnh.

- Trong thc t cha tm c mi tng ti no tha mn tt

c hai yu cu chnh k trn.

Cc mi tr ng ti th ng dng

N c

- l mi trng ti mnh, an ton, r, d kim v thng dng.

- Nc lnh (10 30oC) lm ngui thp kh nhanh c hai

khong nhit cng cao khi ti nhng d gy ra

nt, bin dng.

- Nc trong b ti b nng ln (> 40oC) lm gim mnh tc

ngui nhit cao (t 500 600 xung cn c

100oC/s khi nc b nng ln 50oC, n cn lm ngui chm

hn c du) gim mnh kh nng ti cnggi cho nc

b ti c lnh bng cch tun hon nc

-Nc (lnh) l mi trng ti cho thp cacbon, khng thch

hp cho chi tit c hnh dng phc tp.

Du- L mi trng ti thng dng c cc c tnh hu nh

i lp vi nc.

- Du lm ngui chm thp c hai khong nhit t

gy bin dng, nt nhng kh nng ti cng li km.


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- Du nng v du ngui c kh nng ti ging nhau th-

ng ti trong du nng, 60 80oC c long (linh

ng) tt khng bm nhiu vo b mt thp sau khi ti.

- Nhc im : thng khi qu nng (> 150oC) s bc chy

lm ngui du. Ngoi ra du bc mi gy nhim mi tr-

ng, hi cho sc khe ngi.

- Du l mi trng ti cho thp hp kim (loi c Vth nh, b nt hng khng sa cha cs >c cong vnh

Ngn nga:......

2. xy ho v thot cc bon: to vy v mt C lp b

mt

- Nguyn nhn:nung trong mi trng c oxy, CO2, hi

nc gy OXH vi Fe v C


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OXH lm xu b mt chi tit thp

Thot C gim cng

Ngn nga: Nung trong khng kh khng c tc dng

OXH v thot C Kh bo v: T hi t thin nhin c cc thnh

phn i lp: OXH v hon nguyn

CO/CO2 ; H2/H2O ......

Kh quyn trung tnh:

Ni t tinh khit

Argon (Ar) ch dng trong phng th nghim

(t)

Nung trong chn khng: 10-4- 10-5 mmHg

Phng php th cng: Nung trong hp ng than ci

hoc rc than trn y l....

3. cng khng t:

- cng cao

- cng thp

4. Gin cao :

- Ht ln

4.7. Ti b mt:Nguyn l chung: b mt c nung nhanh n T0 ti, li

vn ngui lm ngui nhanh, b mt c ti , li mm

4.7.1. Ti cm ng:

p dng rng ri ch tiu kinh t k thut cao

1. Nguyn l: Hin tng cm ng in t


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Vt dn c dng in i qua t trng bin

thin chi tit t vo t trng c s dng

cm ng (cng tn s) nung nhanh b mt

Tn s cng cao chiu su lp nung cng mng( hng vn Hz)

2. Vng cm ng :

l b phn gy dng cm ng

bng ng, rng, hnh dng bao ly chi tit

C th ti : - Mt ngoi

- Mt trong

- Mt phng

3. T chc v tnh cht thp ti cm ng :

%C : 0,4-0,6

Nhit : Do nung nhanh chuyn bin xy ra To cao

v ngn ( hn thng thng 100-2000C)

T chc: nh mn M kim nh mn

thng trc ti tn s ti + ram cao ( Xram )

B mt cng : 56-58HRC/ li do dai: 10-15 HRC


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( bnh rng , trc khuu)

4. u im: Nng sut cao

Cht lng tt

D t ng hoNhc im: khng p dng cho chi tit phc tp

Ho nhit luyn Hiu qu ho bn b/m hn ti cm ng & c ngdng

rng ri

Nguyn l chung:1. /n: lm bo ho vo b/m thp mt hay nhiu nguynt thay i thnh phn ho hc lm bin i tchc v tnh cht theo hng nh

2. Mc ch: Nng cao cng, tnh chng mi mn v bnmi (thm C. N, C-N, B)

Nng cao tnh chng n mn in ho v hohc(thm Cr,Al, Si)

3. Cc giai on ca ho NL: Phn ho: to nguyn t hot ca nguyn t cnthm Hp ph: nguyn t hot c hp th vo b/m chitit cn thm to ra gradien nng cht thm

Khuych tn ( giai on quan trng nht)

4. nh h ng ca nhit v thi gian: Nhit :T0 cng cao Vkt ca cc nguyn t cng ln lpthm chng t c chiu su qui nh

(T0 khngc qu cao) Thi gian:

T=const, thi gian cng di chiu su lp thmtng (chm)


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Thm cc bon:

1. /n: lm bo ho cc bn vo b/m thp C thp ( 0,1-0,25%) ti v ram tip theob/m c cng cao cn

li th do dai

2. Mc ch: B/m thp cng ( 60-62 HRC), chng mi mn cao,chu mi, li do dai p dng cho chi tit chu ti v va p

3. Yu cu ( sau thm):

%C b mt 0,8-1,0 % sau ti + ram thp , tchc : M kim nh mn + ccbit nh mn, phn tn

Li: ht nh, M kim nh mn cng 30-40 HRC( khng c F t do)

4. Nhit :

T0 thp trng thi hon ton (trn A3): 900-9500C

5. Thi gian: ph thuc vo 2 yu t

Chiu dy lp thm: thng t 0,5-1,8 mm ( chim0,1-0,15 Dct)

Tc thm: Vthm th lng> Vthm th kh> Vthm th rn

VD: th rn : 0,1mm chiu su/1h nung gith kh: 0,2mm chiu su/1h nung gi

6. Cht thm :

Th rn: 80-95% than g + Na2CO3, hoc Ba CO3(

xc tc)

Phn ng: 2C + O2 2CO ( khi thiu xy)


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2CO CO2+ Cngt

Cngt khuych tn vo b/m thp ( nng tng

dn)

Tc dng ca xc tc: Ba CO3 BaO + CO2

CO2 + Cthan 2CO

2CO CO2 + Cnguyn t b/m thp

- Thm cacbon th rn song qu trnh xy ra li

thng qua pha kh

Nhc im :

+Bi

+ %C : 1,2-1,3

Th kh:

- C th dng trc tip cc kh c kh nng sinh ra

cacbon nguyn t nh CO, CH4 thm ch to kh

thm cacbon t kh t thin nhin, thnh phn ch

yu l mtan CH4 c tc dng thm rt mnh.

- Thnh phn: CO 95-97%; CH4 3-5 %

CH4 2H2+ Cnguyn t

u im:

+ %C : 0,8- 1,0 (c th pha long v pha ch kh thinnhin sao cho c nng thch hp to ra lng

cacbon nguyn u im ni bt )

+ d c kh ho v iu chnh

- Khi khng c kh thin nhin ngi ta c th dng duha hay cn


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Th lng ( t dng)

7. Nhit luyn sau thm: nht thit phi Ti + Ram

Ti trc tip : Sau thm h nhit n 850-860

0C tiu im : BD nh

Ti 2 ln :

Ln 1: cho li T> A3 : 880-9000C

Ln 2: T> A1 : 760-780 0C

Nhc im : BD ln ( do nung nhiu ln) t dngp dng cho thp C v HK c yu cu cao v c tnh

Ti 1 ln :

Nhit ti trung gian cho c b mt v li:

yu cu cng b/m: T= 820-8500 C

yu cu cng li : T= 860-8800 C

Ram: 180-2000C = 1-1,5 h

So snh: ti b mt thm C

b/m 56-60HRC 60-62

HRC

li: 15-20 HRC 30-40HRC

Thm Ni t:

1. /n: bo ho Nit vo b mt thp nng cao

cng, tnh chng mi mn ( mnh hn C), to ra ng sut

d nn, chng r tt.2. c u to lp thm N:


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- C s xc nh t chc lp thm nit vo thp l

gin pha Fe - N : Lp thm giu nit, nu tnh t ngoi

vo s c cc pha sau: - pha xen k Fe2N,

- pha xen k Fe4N, - ferit nit (hay dung dch rn ca nit trong Fe).

Nitrit vi cng rt cao, nh mn (phn tn): cngcao 65-70 HRC sau thm N khng phi nhit luyn

3. c im ca thm N:

T= 480-6500

C, kh NH3Phn ng : 2NH3 3H2+2Nngt

Do T0 thp khuych tn kh thi gian di lp thm

mng

VD: T=5200C, = 24 h = 0,25-0,3 mm

Sau thm khng phi ti : do bn cht ca lp

thm l Nitrit c cng cao

Lp thm c th gi cng khi T>5000C

(


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Tthm 5600C ch yu thm N

4. Thm C-N th lng: c, t dng

Thnh phn: NaCN, KCN cht thm

NaCl, Na2CO3pha longT0C thp: 50% NaCN + Na2CO3, NaCl

Hoc NaCN+ KCN t l 1:1

T0C cao: < 30% hoc 10% NaCN hoc KCN

5. Thm C-N th kh:

So vi thm C th kh c u im hn hn:

- dng chung thit b vi thm C th kh ( thm vo 5-

10% NH3)

- t chc lp thm Cc bit-Nitrit ( 60-65HRC ), tnh chng

mi mn tng ( 50-100%)

- - Do chng mi mn tng ln mnh, lp thm c th

mng hn lp thm cacbon khong 20 30%.

- V d bnh rng khi thm cacbon su 0,90 1,20mm,

khi thm cacbon - nit ch cn 0,50 0,80mm

ngn hn

gim nn bin dng chi tit , tng c tui th ca

l.- lp thm mng hn thm C khng to mui (NH3) tng

tc thm

Ch cn thm 840-860 0C 9300C thm

- Sau thm : Ti (trc tip )+ ram


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Chuong 4- hoa nhiệt luyện - vật liệu học

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Transcript of Chuong 4- hoa nhiệt luyện - vật liệu học