Chng 3: CNG NGH HN GIP MI

PGS. TS. Nguyn Vn Ho, Hn- Cng ngh kim loi---------------------------------------------------------------------------------------------------------------------

Chng 3: CNG NGH HN GIP MI

1-Cc loi chi tit in hnh v chun b chi tit hn

Nh chng ta bit hn gip mi c th tin hnh bng in tr hoc nng chy. D l phng php no i na nhn c mi hn c cht lng cn phi m bo cc iu kin sau:

a) Cc b mt hn ca mi pha phi ging nhau v bng nhau.

b) Cc mt u chi tit hn phi c bo v khng b oxy ho bi oxy ca khng kh trong qu trnh hn.

c) m bo kh nng bin dng ca hai chi tit hn bng nhau khi p chn.

thc hin nhng iu kin trn cn m bo hnh dng hnh hc chi tit hn gn ch ni ging nhau v din tch bng nhau, v vy chun b cc mt u ni ca chng v chn cng ngh hn thch hp ht sc quan trng .

Nhng yu cu i vi hnh dng hnh hc v phi khi hn gip mi in tr v nng chy l khc nhau . Hn gip mi in tr

Hn gip mi in tr phi m bo tip xc hon ton trc khi nung. Nu khng s to khe h gia chng to iu kin oxy ho lm gim cht lng mi hn.

Tip xc hon ton c m bo bng lm sch b mt v lc p ca my hn.

Vic tng tit din, chu vi cng nh gim cng vt hn s gy kh khn cho iu chnh trng kht cc mt u . Bi vy hn gip mi in tr nhng ng mng, tm v nhng chi tit hn khc vi tit din c chu vi ln s kh nhn c cht lng mong mun v do khng s dng trong cng nghip. Hn ch ca hn gip mi in tr khi hn vt c tit din c khng ln ( thanh c c ng knh n 15-20 mm), nhng ng mng vi ng knh qu b. Lin quan n bn mi hn loi ny khng cao v i hi phi nung nng chi tit trc khi hn nn trong cng nghip t s dng.

Ch yu ca hn gip mi l hn nng chy .

Hn GM nng chy (GMNC)

C hai chi tit hn cn phi m bo kch thc v hnh dng ch ni bng nhau. Ci m bo nung nng v bin dng do ng u khi hn. Kt cu hp l c ch ra trn hnh 3.1.

Chiu di l on c tit din bng nhau :

Hn tm v ng mng :

a: Lng d tng cng khi nng chy v chn mm (tng co ca chi tit khi hn nng chy),

: Chiu dy tm hoc thnh ng mm.

a) b)

Hnh 3.1- Kt cu chi tit hn gip mi: a- Khng hp l; b- Hp l

Khi hn nng chy yu cu tit din phi hn ch ni gn bng nhau. Cc ng knh phi trn v chiu dy khng uc khc nhau qu 15% (hnh 3,2).

a) b)

Hnh 3.2- S khc nhau cho php v ng knh v chiu dy khi hn gip mi. a) ng knh: D1 =< 1,15 D2; b) Chiu dy ng:

Tu phng php hn c th chun b phi bng cc phng php khc nhau: C th bng c kh hay ct bng kh.

Hn gip mi nng chy c th hn tt c cc chi tit c tit din c ( trn, vung, tm) cng nh hn cc chi tit c tit din khc nhau (ng mng, tm mng v rng). Hin nay trn nhng my hn c bit chng ta c th hn c nhng tm rng 500-2000 mm vi chiu dy khng nh hn 1,5 3 mm. Vi vic gim chiu dy tm s lm tng nguy him do dch chuyn gia cc mp mi hn. Chiu rng vt hn cng ln th chiu dy ti thiu ca vt hn cng ln v khi vn c th hn c mi hn gip mi m bo cht lng. Khi hn ng, thi im chn c kh nng lm phnh ng (lm gim bn ng). nguy him tng cng vi vic tng ng knh ng. Trong bng 1 gii thiu chiu dy ti thiu ca ng theo ng knh ca n m bo bn ca thnh.

Bng 1

Hn vng xuyn (hnh3.3):

a) b)

Hnh 3.3- Hn gip mi vnh xuyn

a- Vnh xuyn c ng knh D ln; b- Vnh xuyn c ng knh D nh

Kt cu chi tit hn phi m bo mt u vung gc vi ng tm ca chi tit hn. Trong trng hp c bit c th phn b nghing mt gc so vi tm chi tit (VD Khung ca s) (hnh 3.4).

Khi hn tit din ln kt cu hp l ch ra trn hnh 3.5.

Gc nghing nh nht cho php ca mt u phi hn khi hn nng chy ph thuc vo phng php hn nh ch ra trn hnh 3.4.

Hnh 3.4: nghing nh nht cho php ca cc mt u ca phi khi hn nng chaHn t ng: 0,2-0,3 mm

Hn khng t ng: 0,15a;

a: Lng d p mm

Hnh 3.5. Chun b mt u phi hn khi hn nng chy.

Khi vt hn c tit din ln trong thc t ngi ta to cn mt pha nh hnh 3.5. iu to iu kin nung nng ban u v duy tr qu trnh n nh ca hn nng chy.

Hnh 3.6: Mt s v d v ng dng ca hn gip mi nng chy

2- Hn gip mi in tr thp

Hn gip mi in tr hin nay ch c dng :

- Hn dy : d=6-8 mm

- Thanh : d =< 20 mm

Phng php ny d hn ph hp vi thp cc bon thp nhng bn thng gim, c bit do vng ni so vi vt liu c bn ( dai va chm mi ni 2-5 KG/cm2). bn gim lin quan n s oxy ho mnh lit ch hn. Vng ht ln rng. Vng ny b m rng ln s gy hin tng nung nng mnh lit chi tit trn mt chiu di ng k (xem trng nhit khi hn gip mi in tr). u im ca phng php ny l mp ni trn tru. iu c bit c ngha khi hn cc ng m phi m bo ng knh trong khng thay i. Khi hn gip mi in tr thng ng knh trong thng khng thay i, trong khi hn gip mi nng chy s to thnh ba via mi ni c trong vng ngoi khi hn ng, lm gim ng knh trong. Mc d kh khn kh ba via nhng ngi ta vn dng hn gip mi nng chy thay hn in tr do cht lng mi ni khi hn gip mi in tr khng cao.

Ch hn c bn ca hn gip mi in tr bao gm nhng thng s :

- Cng dng in hay cng sut hn Ih (A)

- Thi gian hn t (s);

- p lc chn P (N, KG) hay p (N, KG/mm2);

- Chiu di phn nh cp ca chi tit khi hn.

Dng in v thi gian nung nng ph thuc vo vt liu hn, kch thc tit din vt hn, thi gian hn nung nng.

p lc chn thng xc nh gin tip qua lng d chn, ngha l co ca chi tit khi p.

Hn GM T thp cc bon thp c th chn:

J = 10-100 A/mm2, thng dng 20 60 A/ mm2Mt ln dng khi hn tit din ln.

Mt dng in ph thuc vo thi gian nung nng cho (nng sut hn). Ch hn cng mm i hi cng sut hn cng ln.

Mt dng hn c th xc nh theo cng thc:

H s k ph thuc tnh cht vt liu hn, tit din v chiu di cp phi khi hn v tra bng 2. Bng 2: H s kVt liu hnChiu di cp l ( d- ng knh phi trn)ng knh phi hn (mm)H s k

Thp cc bon thpd4 1010

( 0,7 1,0 )d10 408

ng2d-27

ng thau1,5d-20

Nhm (Al)1,5d-12 - 15

Cng sut my ph thuc dng hn,thi gian hn t (s), kt cu my hnC th tra th hnh xc nh cng sut hn (hnh 3.7).

Hnh 3.7 : S ph thuc ca tn hao cng sut khi hn vo thi gian

nung nng thanh thp cc bon thp v ng knh thanh.

V d :

CT38, 16; tra bng W = 10-60 KW

Hn phi c, cng sut ring : 0,12-0,15KW/mm2, khi hn ng 0,2 KW/mm2

Nhit khi hn thp t C : 900-12500C.

Thc t hn tin hnh 1200-12500C v nhit ny d bin dng do v xy ra kt tinh li nhanh

Thi gian nung nng:

Thi gian nung nng rt quan trng v nm trong khong: t = 0,5- 15 s

Hn dy d =< 8 mm : t = 1-2 s

Lc p khi hn nh hng thc s n cht lng mi ni, thng dng l p lc ring p (KG/mm2 hoc N/mm2). Hn tt nhit 12000C v p lc ring thng dng : p=1,5-3 KG/mm2.

Khi hn trn my khng t ng, p tay, giai on u : p=1-1,5 KG/mm2.

Cui qu trnh lc p tng ln

Khi hn thp C thp c d =4-10 mm co khi chn n =1,5-2,5 mm v c b nh lng d chn. .

3- c im khi hn nng chy

Hn nng chy hin nay c dng ph bin v ngha kinh t ln.

Cc phng php hn nng chy :

a- Hn nng chy khng nung nng s b

b- Nung nng s b bng nng chy

c- Nung nng s b bng in tr

n gin nht l hn nng chy lin tc v gm 2 giai on:

Lm nng chy v p ( p c dng in v p khng c in).

Giai on lm nng chy cn ch 2 c im sau:

- Gim nhit vng gn mi ni.

giai on hai (Chn) bao gm chn c dng in v khng c dng in.Chn c dng in c ngha rt ln lin quan n qu trnh chuyn t nng chy sang trng thi chn. Khi chn tc dch chuyn tng ln ng k (so vi tc nng chy) lm cho khe h gia cc mt u nng chy bt u gim nhanh. in p gim v dng in tng. iu gy kh khn cho vic ph hu cc cu ni on mch kim loi lng - Tit din cc cu ni tng n ni cc cu ni khng ph hu c do vy nng chy s dng li.

iu quan trng l chuyn t nng chy sang chn cn thc hin vi tc cao.

S chuyn i lun c kh nng xy ra khi m s ph hu cc cu on mch c lin quan n mi trng kh bo v khe h gia cc mt u nng chy b bin mt nhng khe h gia cc mt u vn cha ng kn.

Giai on p khng c dng in thc hin qu trnh hn vi p lc ring ng k.

p lc ny ln y kim loi lng ra khi mi ni v bin dng do vng gn mi ni. p lc ph thuc vo ba yu t c bn sau:

- Vt liu hn

- Mc chn v c xc nh bi kch thc vt hn v cng ngh hn.

- Nhit vng ni

Khi hn chi tit c tit din ln trn my hn t ng c cng sut ln khng nung nng s b . Lc chiu rng vng b nung nng mnh lit s nh nhng bin dng cng s kh khn hn.

Khi hn tm mng v rng do nung nng khng ng u trn mp hn nn dng p lc ln hn khi hn chi tit c.

Bin dng do khi chn lin quan n lng d chn. Tr s ca n lin quan n kch thc vt hn v iu kin nung nng khi hn. Thng thng di khi p : = 2-8 mm, trong thc t khng ph thuc loi thp hn.

Mc chn c iu khin theo lc hoc theo tr s chn. Phng n hai :i hi lc p my hn phi ln nhng dm bo cht lng mi ni n nh hn.

m bo cht lng MH(mi hn), c 2 chi tit trc khi hn phi c nung n nhit nh nhau v kt cu ch MH phi thch hp.

Nhit ng u lin quan n on di ra:

Cc vt hn cng vt liu th di ca chng bng nhau.

Vt liu khc nhau:

- Chi tit c dn in km, dn nhit thp, on nh nh.

Khi hn tm : : l 1,5 - 2 mm, : chiu dy chi tit.

a) b)

Hnh 3.8: Mi hn nng chy. a- Phi c ; b- Phi ng

Khi hn nng chy mi ni c ba via. gn mi ni th kim loi b un. Khi hn ng to ba via c hai pha lm gim ng knh trong (hnh 3.8).

Trong thc t hn nng chy phc tp nung nng s b.

Nung nng s b c thc hin bng on mch xen k hoc nng chy ngt on (Cc b mt tip xc chu k).

S hn nng chy lin tc c nung nng s b bng in tr v ngt on c ch ra trn hnh 3.9

a-Hn nng chy lin tc:

1- co tng cng khi nng chy; 2- co khi chn; 3- co khi chn c in

4- co khi chn khng in; 5- co tng cng khi hn; 6- Thi gian tng cng ca mt chu k hn; 7- Thi gian ng in; 8- Thi gian nng chy. Tc dch chuyn t t v tng cng sut ng k thi im chn.

b- Hn c nung nng bng in tr

1- co tng cng khi nng chy; 2- co khi chn; 3- co khi chn c in

4- co khi chn khng in; 5- co tng cng khi hn; 6- Thi gian tng cng ca mt chu k hn; 7- Thi gian ng in; 8- Thi gian nng chy;9- co chi tit khi nung nng s b.

co chi tit nung nng rt nh, cng sut i hi ng k v gim khi nng cao nhit chi tit.

c- Hn c nung nng s b bng nng chy

1- co tng cng khi nng chy; 2- co khi chn; 3- co khi chn c in

4- co khi chn khng in; 5- co tng cng khi hn; 6- Thi gian tng cng ca mt chu k hn; 7- Thi gian ng in; 8- Thi gian nng chy;9- co chi tit khi nung nng s b.

Hnh 3.9 : S kt hp gia in v c khi hn nng chy

Cng sut my i hi khng ln ph hp vi co chi tit.

Thi gian mi ln on mch thng 0,5-1,3 gy(s).

S ln on mch ph thuc kch thc vt hn, cng sut my, vt liu v nm trong khong 3-20 ( trong trng hp ngoi l c th ln hn, VD: hn ong . mng c tit din n 20.000 mm2 c th t ti 100 hoc ln hn).

So snh hn c nung nng s b vi hn nng chy lin tc:

u im:

- Cng sut my nh

- Rt ngn thi gian nng chy

- Gim ba via trong khi hn ng

Nhc im:

- Ko di thi gian

- Phc tp qu trnh t ng

- Kh kim tra nung nng s b

- Vng qu nhit tng i ln mi ni gim do

Nung nng khng ng u trn tit din hn.

Hn nng chy lin tc c dng trong sn xut ln hn nhng tit din ln 100-1500 mm2, hn tm mng, chiu rng ln.

Hn c nung nng s b c ng dng rng ri sn xut dng c, ng ray, ni ng.

Hn nng chy khng t ng c tin hnh nung nng s b.

Ni chung ch hn nng chy c xc nh bng cc thng s sau y:

- c tnh v thi gian nung

- Chiu di cp ca chi tit

- Lng d v tc nng chy

- Cng hn

- Lng d chn

- p lc chn

- Tc chn

Trn hnh 3.10 ch ra s lng d hn nng chy

Hnh 3.10: Chiu di cp v lng d khi hn nng chy

a-Chiu di cp; b-Lng d tng ca mt chi tit; o- Lng d nng chy ca mt chi tit; oc- Lng d chn ca mt chi tit. b + b- Lng d tng cng khi hn .

.

4-Hn gip mi nng chy thp cc bon thp

-Nhn c MH cht lng cao, khng phi nung nng s b tr khi hn tit din ln.

- Lc p ln: Thng 10-12 KG/mm2, ht nh, gn MH c th nh hn vt liu c bn. Nhng P ln lm th b un nhiu lm gim do MH.

- Sau khi hn nng chy khng gia cng nhit tip nhn c bn cao khi chu ti trng tnh, va chm v chu k.

- dai va chm ca MH khng cao do ht to ( 6-12 KGM/ cm2) so vi nhng ch tiu yu cu. nng cao

dai va chm ta tin hnh gia cng nhit sau khi hn. Khi thng ho nhit 9300C hay thp (630-6500C) t c bn mi gn vt liu c bn.

- Chiu di cp mt pha chi tit trn hay vung:l = 0,75 d - Cng sut ring khi hn nng chy thp cc bon thp v thp hp kim thp:

Cng sut ring : Tra bng 3.

Bng 3

Cng sut ring, KW/mm2

Loi chi titHn nng chy c nung nng s bHn nng chy lin tc

v dintchtit din mm2SX n chic v hng lot nhSX hng lot ln v hng khiDng SX trung bnhDng SX hng lot

TD vung 2000

TD vung 2000

DT kn bt k 1500

DT kn bt k 1500 0,05-0,08

0,03-0,05

0,1-0,12

0,08-0,10,12-0,16

0,05-0,12

0,18-0,22

0,1-0,180,15- 0,25

-

0,25-0,35

-0,25-0,4

-

s0,4-0,6

-

- Mt dng in hn J (A/mm2):

Thng J =10 50 A/ mm2.

Khi p chn, J tng 1,5-3,5 ln so vi khi nng chy n nh.

J hn nng chy = (1,5 2,5)J hn in tr.

- Thi gian hn: Tra bng 4

Bng 4

ng knh phi Din tch tit din Thi gian hn khi hn khng t ng c nung nng s b, giyThi gian hn t ng nng chy

trn d mmvung mm2Nung nng s bNng chylin tc , giy

5

10

15

20

30

40

50

75

9020

80

180

320

700

1200

2000

4200

6400-

2

3

5

8

25

30

75

1202

3

4

6

6

8

8

15

20-

-

6

10

20

40

-

-

-

- Nhit nung nng s b:

m bo nhit u ni ng u 700-9000C.

Nhit cao, thi gian ln dn n ht ln lm gim dai va chm ak ( hnh 3.11).

Hnh 3.11: nh hng ca thi gian nung nng s b n dai va cham ak (KGm/cm2)

( ng knh ng 38 mm, chiu dy thnh 4,5 mm , thp 0,2%C, khi hn ng nng chy 5mm vi tc 0,2 mm/gy) .

- Tc nng chy:

Lc u gn bng khng, sau tng ph thuc mt dng in, mc nung nng chi tit hn, vt liu v vo tc nng chy.

Tc nng chy khi hn NC tit din ln (200-250 mm2) cui giai on c th t: vnc = 4-5 mm/s.

Khi F = 2000-3000 mm2, vnc khng vt qu 0,8-1,5 mm/s.

Hn thp C thp vnc khng c thp hn 2mm/s.

- Lng d nng chy

Lng d nng chy phi m bo:

- Kim loi chy ng u trn mt u

-To mng kim loi nng chy mt u

-m bo d dng chn. Chi tit v chi dy vt hn cng ln th nhit tng trn tit dincng chm v do thi gian cng di, lng d nng chy cng ln.

Khi hn c nung nng s b lng d nng chy v nung nng thp hn khi hn nng chy lin tc khong 30-50% ( c bit hn t ng).

Bng 5: Lng d nng chy v p chn

ngLng d hn khng t ng c nung nng s bLng d

knhLng dLng d Lng d khi chnKhi hn t

Mmtng cngnng chyC inKhng inng bng nng chy

5

10

15

20

25

30

40

50

75

905

8

9

10

13

16

20

22

26

323,5

6

6,5

7

9,5

12

14,5

15,5

19

240,5

0,5

0,5

0,75

1,0

1,0

1,5

1,6

1,8

2,01,5

2,0

2,5

3

3,5

4,0

5,5

6,5

7,0

8,0-

-

13

17

21

25

40

-

-

-

- p lc chn: nh hng n bn v c bit dai va chm ak (hnh 3.12).

Hnh 3. 13: S ph thuc ca bn, gii hn chy vo p lc chn

Khi hn thp cc bon thp : P = 3-5 KG/mm2 Khi hn t ng: P = 6-8 KG/mm2 - Lng d chn (Tra bng)

- Tc chn (hnh3.14).

Hinh 3.14 nh hng ca tc chn n dai va chm

ca mi hn khi hn thp cc bon thp.

Vi thp cc bon thp khng c thp hn 10-15 mm/gy.

S ph thuc ca dai va chm ca mi hn vo tc chn c ch ra trn hnh 3.

5- c im hn GM nng chy thp Cc bon v thp hp kim.

Tt nht l Hn nng chy. Khc thp C thp ch: p lc p cao hn.

Nhng vt liu ny c chia thnh 4 nhm:

a) Thp cc bon trung bnh (C0,5% v thp HK vi hm lng C cao,

VD: OLCr15 (X15)c) Thp kt cu cc bon cao loi Pc lt (16Cr6M)

d) Thp khng g : Loi 18% Cr V; 9% Ni.

c im cng ngh:

Nhm a: Khi hn nng chy c th hn nh thp t C. p lc ring khi chn c th ly : p=1-1,5 KG/mm2,

Khi nung nng s b c th chn p n 4-6 KG/mm2.

Nhng MH i hi cht lng cao cn GC(gia cng) nhit sau hn.

Nhm b: Kh hn hn loi a v %C cao, khong kt tinh ng k, nhy ti.

Khc phc:

- Gim nhit nung nng gim chiu rng phn nng chy.

- Tng p khi p n 6-8 KG/ mm2 khi hn khng t ng c nung nng s b,

n 7-10 KG/ mm2 khi hn nng chy lin tc

- trnh nt c th lm ngui chm sau hn

Nhm c:

C xu hng to nt sau hn v kh nng chng li bin dng do nhit cao.

Tng i d hn hn thp cc bon cao nhng i khi b nt nn thng GC nhit sau hn.

Ni chung ging thp cc bon cao.

Nhm d: (loi Ostenit)

Hon ton c kh nng hn

in tr tng i cao, cng sut ring thp hn mt t so vi hn thp P.

C th tng p lc p do bn tng i cao ca n nhit cao.

Khi hn tit din c hoc ng mng p =< 12-14 KG/mm2

Khi hn tm cn nng p n 20-25 KG/mm2.

Tm hn cng rng, p lc cng cao.

Khi hn thp Ostenit hm lng trong mi hn ng k so vi hn thp C thng nn tc nng chy v p c bit cn ln.

VD:

-Tng Vnc tng t 1- 3 mm/s dn n ak = 6-15 KGm/cm2.

-Tng Vnc tng t 12- 30 mm/s dn n ak = 10-18 KGm/cm2..

Lng d nng chy v p ly nh thp c.

Chiu di kp gim 25% so vi thp C thp cng loi.

Chn ch hn gip mi : Gm cc bc sau:

1-Ch s cng ngh ( GM in tr, nng chy, khng nung nng, c nung nng s b )

2-Xc nh cng sut my hn theo bng trn.

3-Xc nh ch cng ngh

4-Chiu di cp l

5- Lng d

6- Lc p

7-Tc p

8-Thi gian ca qu trnh

5- Hn gip mi Kim loi mu v HK ca chng

Hn ng kh khn v dn in v nhit rt cao nn khng to c lp KL lng mt u.

Nhng thanh c d = 8-10 mm c th hn bng in tr vi mt dng in J cao gp 3-3,5 ln so vi hn thp c thp, p= 1- 1,5 KG/mm2 , phi p nhanh v bt buc phi p t ng.

Nhng thanh Latin c th hn GM in tr v khi mt dng in thp hn mt t so vi hn ng v bng 2,5 ln mt hn thp C thp.. ng v ng thanh c th hn tt vi thp bng hn nng chy.

Nhm v hp kim nhm c th hn GM in tr v nng chy.

Mt dng in cao hn 2 ln so vi J hn thp.

Ni chung khi hn nhm v HK ca chng i hi my phi c cng sut ln v iu khin t ng.

Hnh3.12: S ph thuc ca dai va chm vo tc chn (thp cc bon thp)

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