Welding Book Vietnamese

Xin li tc gi ca gio trnh nhng hy nn bit chia s kin thc! admin_nde

MC LC trang

Chng 1: khi nim v hn h quang tay, u v khuyt im 1.1 Thc cht c im ca hn h quang tay 2 1.2 H quang in .. 2 1.3 Tnh hn vt liu .. 3 Chng 2: Vt liu hn h quang tay 6 Chng 3: K thut hn 3.1 K hiu v cc loi lin kt hn. 10 3.2 V tr hn 11 3.3 Nhit trong ch hn 12 3.4 K nng c bn trong cc phng php hn 16 3.5 Ch hn 24 3.6 Cc phng php hn thng dng khc 27 Chng 4: ng sut v bin dng hn 4.1 Khi nim ng sut v bin dng hn ...... 30 4.2 Nguyn nhn gy ra ng sut v bin dng hn 31 4.3 Cc loi ng sut v bin dng hn .. 32 4.4 Cc bin php gim ng sut v bin dng hn 36 Chng 5: Khuyt tt mi hn v phng php khc phc 41 Ti liu tham kho 48

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CHNG 1 KHI NIM V HN H QUANG TAY 1.1 Thc cht c im ca phng php hn h quang tay: Hn h quang tay l mt trong nhng phng php hn nng chy, dng nhit ca h quang nung nng kim loi ch ni n trng thi chy, sau khi kt tinh n s to thnh mi hn ni lin cc chi tit vi nhau to thnh khi khng th tho di. V trong qu trnh hn, tt c cc thao tc nh: gy h quang, dch chuyn que hn duy tr h quang bo m chiu rng cng nh hn ht chiu di mi hn u do tay ngi th thc hin, nn phpng php hn ny c mt s c im c bn sau y:

- C th hn c tt c cc mi hn mi t th trong khng gian - Nng sut hn thp do cng dng in hn b hn ch - Kch thc v hnh dng ca mi hn khng u do tc hn (tc dch

chuyn ca h quang) gc nghing ca que hn, chiu di h quang thay i.

- iu kin lm vic ca th hn khng tt v nh hng ca nh sng v nhit h quang.

Mc d c kh nhiu nhc im, song i vi cc chi tit c chiu dy khng ln lm, i vi cc mi hn v tr ng v c bit hn trn, cho n nay hn h quang vn l phng php thch hp nht ng thi n cng l phng php hn ch yu ch to kt cu hn. V vy hu ht cc nc trn th gii hin nay, k c mt s nc c nn cng nghip pht trin nht, hn h quang tay vn chim t l ln trong tng s cc phng php hn. 1.2. H quang in: H quang in l hin tng phng in mnh v lin tc trong mi trng kh gia hai in cc, km theo to nhit v nh sng. gy h quang khi hn ngi th c th thc hin bng hai cch sau:

- Cho que hn tip xc vi vt hn theo phng thng ng khong 1/10s ri nhc ln khi vt hn mt don 2 5 mm th h quang s hnh thnh

- t nhing que hn so vi b mt vt hn mt gc no , cho mt u que hn chuyn ng tip xc vi b mt vt hn mt on ngn sau a que hn v v tr thng gc vt hn h quang cng hnh thnh.

Sau khi hnh thnh h quang s chy ca n ph thuc vo nhiu yu t: in p gia hai in cc, cng dng in hn, cht lng que hn, chiu di ct h

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quang (khong cch gia que hn v vt hn). Do cho h quang chy n nh trong sut qu trnh hn cn phi gi cho chiu di ct h quang lun lun khng i, iu ny quyt nh ch yu bi trnh tay ngh ca ngi th hn. Ct h quang c th xem nh dy dn bng kh c do cao, di tc dng ca cc lc khc n c th b ko di ra v dch chuyn i, tc l b thi lch ra khi v tr bnh thng ca n. nhng yu t nh hng n s thi lch h quang nh hng ca t trng dng in hn, nh hng ca gc nghing que hn, nh hng ca vt liu st t. Khi ct h quang b thi lch th cht lng mi hn s b nh hng. 1.3 Tnh hn ca vt liu 1.3.1 Khi nim tnh hn Dng khi nim tnh hn ch mc d hn hay kh hn i vi vt liu c bn no , ni cch khc tnh hn l t hp cc tnh cht ca kim loi hay hp kim cho php nhn c lin kt hn tho mn cc yu cu v cht lng cn thit. 1.3.2 Phn loi tnh hn 1. Vt liu c tnh hn tt: Bao gm cc loi vt liu cho php hn oc bng nhiu phng php khc nhau, ch hn c th iu chnh trong phm vi rng, khng cn s dng cc cng ngh phc tp m vn m bo c cht lng cao. 2. Vt liu c tnh hn trung bnh: Nhm ny ch thch hp vi mt s phng php hn nht nh, cc thng s ch hn ch c th dao ng trong phm vi hp, yu cu v vt liu hn cht ch hn. Mt s bin php cng ngh nh nung nng s b, gim tc ngui v s l nhit sau khi hn c th c p dng. 3. Vt liu c tnh hn hn ch: Gm nhng loi vt liu cho php nhn c mi hn mong mun trong iu kin phi p dng cc bin php cng ngh cao. Thng phi p dng bin php s l nhit trc khi hn hoc hn trong mi trng c kh bo v, ch hn nm trong phm vi hp. Tuy nhin lin kt hn c khuynh hng b nt. Phn ln cc loi thp cc bon cao, thp hp kim cao nm trong nhm ny. 4. Vt liu c tnh hn xu: Thng phi hn bng cng ngh c bit, phc tp v tn km. T chc kim loi mi hn ti, d b nt nng v nt ngui. C tnh v kh nng lm vic ca mi hn thp hn so vi vt liu c bn. V d gang v mt s hp kim c bit nm trong loi vt liu ny. 1.3.2 nh gi tnh hn ca kim loi v hp kim

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Tnh hn ca kim loi v hp kim c nh gi thng qua cc ch tiu nh l: - C tnh v thnh phn ho hc ca kim loi mi hn, ca vng nh hng

nhit ca mi hn. - Kh nng chng nt nng v nt ngui v n mn tinh gii ht.

1556CuNiVMoCrMn +++++

nh gi s b tnh hn ca thp theo thnh phn ca n theo cc cng thc sau: % CE = C + (%) Thng qua gi tr CE c th nh gi tnh hn ca thp thuc loi no. Theo kinh nghim sn xut ngi ta c th nh gi gn ng tnh hn ca thp

theo thnh phn ho hc bng cch so snh tng lng cc nguyn t hp kim vi hm lng ca cc bon c trong thp C(%)

Tnh hn ca thp theo % C Tng % HK (Mn, Si, Cr, Ni,

Tt Trung bnh Hn ch Xu

1.0 1.0 3.0 3.0

0.25 0.2 0.18

0.25 0.35 0.2 0.3 0.18 0.28

0.35 0.45 0.3 0.4 0.28 0.38

0.45 0.4 0.38

Nhit nung nng s b Tp : i vi thp cc bon trung bnh v cao, cng nh cc loi thp hp kim thng phi nung nng s b trc khi hn. Nhit nung nng trc khi hn c th tnh nh sau :

Tp = 350 25.0Ce (oC) 1.3.3 nh hng cc nguyn t n tnh hn ca thp - Cc bon (C) : thp c hm lng cc bon ti 0.25% c tnh hn tt. Hm lng cc bon cng cao tnh hn cng km v n lm tng kh nng thm ti ca kim loi mi hn v vng ln cn mi hn v nh vy tng kh nng nt. Khi chy cacbon d to ra lng ln cc r kh. - Mangan (Mn) : Vi hm lng nh (di 1%) mangan khng nh ti tnh hn ca thp. Khi hm lng Mn ln hn tnh thm ti s tng lm kim loi mi hn d b nt. - Silc (Si) : Hm lng Si tng s kh hn do tnh chy long ca kim loi ln v to thnh SiO2 di dng r x.

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- Niken (Ni) : Hm lng Ni tong thp c th thay i gii hn ln. Ni c tc dng lm nh ht tng do, bn ca thp. Ni c tc dng tt ti tnh hn ca thp tuy nhin n d b oxy ho bi vy cn bo v tt chng tc ng ca Oxy. - Molipen (Mo) : C trong thnh phn ca tt c cc thp hp kim chu bn nhit. N lm thp c ht nh, gi bn nhit cao, nhng lm gim tnh hn v l nguyn nhn gy nt mi hn v vng ln cn. - Pht pho (P) : l thnh phn c hi trong thp v lm tng kh nng nt ngui. - Lu hunh (S): l thnh phn c hi trong thp v lm tng kh nng nt nng

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CHNG 2: VT LIU HN H QUANG TAY 2.1 K hiu v c tnh cc loi que hn: Chng ny ch gii thiu k hiu que hn theo tiu chun M. Tiu chun c s dng ph bin trn th gii hin nay. Que hn in theo qui phm do hip hi hn M ( AWS American Welding Society ) ch r cc thng s c bn sau:

- Loi dng in m que hn c th c p dng ( dng AC hoc DC ). - Loi thuc bc ca que hn - V tr hn m que hn c th c p dng. - C tnh ca mi hn.

Chng loi que hn c th hin y trn bao b cng nh trn phn ui ca mi que hn, bao gm: E X 1X 2 Y 1Y2 E X 1X 2 Y 1Y2 Z Trong ch th ca cc ch v s nh sau: E ch th in cc hn h quang nng chy

X 1X 2: ch th gii hn bn ko (nh nht) n v ksi, ca mi hn khi mi hn c thc hin da trn qui trnh (WPS) c hp cch. Y 1Y2 ch th v tr hn m que hn c s dng, chng loi thuc bc trn que hn, dng dng in m que hn c th c p dng. Y1 ch th v tr que hn c s dng c th l 1, 2 hoc 4 1 - c hn mi v tr 2 - Ch hn c v tr ng v ngang 4 Hn c mi v tr, nhng ch hn c DOWN khi hn ngang. Z c th l A1, B1 biu th hm lng trung bnh ca cc nguyn t hp kim c trong kim loi p. Bng 2.1- 01 thnh phn cc nguyn t hp kim trong que hn hp kim

A1 B1 B2 B3 B4L B5 C1 C2 C3 D1 D2 D3 M % Mo

0.5 0.5 0.5 1 0.5 1 0.5 0.3 0.5 0.25 0.5

% Cr 0.5 1 2 2 0.5 0.35 - 0.65 % V 1 0.5 % Ni 2.5 3.5 1 1.25 2.5 %Mn 1.5 1.75 1.5 0.6 2.25

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Theo ASME section II part C, tiu chun do hip hi cc k s c kh M ban hnh (The American Scociety of Mechanical Engineers). Qui nh c tnh k thut ca cc loi dy hn, que hn v kim loi in y cho cng vic hn cc cu kin, thit b chu p lc ch r cc bng sau:

Bng 2.1-02 Ch th lp thuc bc v dng in s dng ca que hn K hiu Dng in s dng Loi thuc bc

E XX 10

E XX 11

E XX 12

Ch s dng DC+

AC or DC +

AC or DC -

Natri, xenlul cao

Kali, xenlul cao

Natri, titan

E XX 13

E XX 14

E XX 15

AC or DC (+, -)

AC or DC (+, -)

Ch s dng DC +

H titan

H titan, c bt st (30%)

H hydro thp

E XX 16

E XX 18

E XX 20

AC or DC +

AC or DC +

AC or DC +, -

H hydro thp

H hydro thp, c bt st (25%)

Oxit st cao

E XX 24

E XX 27

E XX 28

AC or DC +, -

AC or DC +, -

AC or DC +

H titan, c bt st (50%)

Oxit st cao, c bt st (50%)

Hydro thp, c bt st (50%)

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BNG 2.1-03 QUE HN THP CACBON V HP KIM THP

M hiu AWS

Loi thuc bc que hn V tr hn c p dng

Loi dng in c th c p dng

E6010 Natri, xenlul cao F,V,OH,H Dcep E6011 Kali, xenlul cao F,V,OH,H Ac hoc dcen E6012 Natri, titan F,V,OH,H Ac hoc dcen E6013 thuc h titan F,V,OH,H Ac hoc dcep hoc dcen

H-fillets Ac hoc dcen E6020

Oxit st cao F Ac hoc dcep hoc dcen

E6022 Oxit st cao F,H AC hoc DCEN H-fillets AC hoc DCEN

E6027 Oxit st cao, c thm bt st

F AC hoc DCEP hoc DCEN

E7014 H titan c thm bt st F,V,OH,H AC hoc DCEP hoc DCEN

E7015 Natri, thp hydro F,V,OH,H DCEP E7016 Kali, hm lng thp hydro F,V,OH,H AC hoc DCEP E7018 Kali, hm lng thp hydro

c thm bt st F,V,OH,H AC hoc DCEP

E7018 M Kali, hm lng thp hydro c thm bt st

F,V,OH,H DCEP

E7024 H titan c thm bt st b xung

F, H-fillets AC, DCEP hoc DCEN

F AC,DCEP hoc DCEN E7027

Oxit st cao c bt st b xung H-fillets AC hoc DCEN

E7028 Kali, lng thp hydro c thm bt st

F, H-fillets AC hoc DCEP

E7048 Kali, thp hydro c thm bt st

F,V- down,OH,H AC hoc DCEP

Ch thch: F: ch th v tr hn sp ( flat) H: ch th v tr hn ngang (horizontal) H fillet ch v tr nhng mi hn gc v tr hn ngang V- down: ch v tr hn ng nhng c hn theo chiu t trn xung V: ch th v tr hn ng (Vertical ) OH: ch th v tr hn nga

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AC: ch th chiu ca dng in hn l xoay chiu DC: ch th chiu ca dng in hn l mt chiu.

DCEP (DC+) ch th que hn ch s dng c my hn mt chiu v in cc hn c ni vi cc dng ca my hn DCEN (DC-) ch th que hn ch s dng c my hn mt chiu v in cc hn c ni vi cc m ca my hn.

BNG 2.1 04

Que hn thp cacbon thp theo AWS 5.1 Gii hn ko (Min) Gii hn chy (Min) M hiu

AWS Ksi MPa Ksi MPa gin di (%)

E6010 60 414 48 331 22

E6011 60 414 48 331 22

E6012 60 414 48 331 17

E6013 60 414 48 331 17

E6019 60 414 48 331 22

E6020 60 414 48 331 22

E6022 60 414 48 331 Khng ch th

E6027 60 414 48 331 22

E7014 70 482 58 339 17

E7015 70 482 58 339 22

E7016 70 482 58 339 22

E7018 70 482 58 339 22

E7024 70 482 58 339 17

E7027 70 482 58 339 22

E7028 70 482 58 339 22

E7048 70 482 58 339 22

E7018M 70 482 53-72* 365 496* 24

Ch : i vi cc loi que hn c thuc bc h baz trc khi s dng que hn phi c sy nhit t 3000 C n 4000 C trong thi gian t nht l 1 gi.

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CHNG 3 K THUT HN 3.1 K hiu v cc loi lin kt hn mi hn: 3.1.1 V tr k hiu

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3.1.2 Phn loi mi hn

Ghi ch: tng chng loi, cc k hiu ca mi hn c xem phn ph lc 1.

3.2 V tr hn V tr hn theo AWS c phn loi theo cc qui phm sau: i vi mi hn gip mi c - V tr 1G, 2G, 3G, 4G tng ng vi cc mi hn gip mi cc v tr

hn sp, hn ngang, hn ng v hn nga. - i vi cc kt cu hn gip mi l ng c cc v tr 1G (hn khi ng xoay),

2G, 5G, 6G v 6GR. Tng ng vi khi hn gip mi ng cc v tr ng c nh m trc ng vung gc vi mt phng ngang hoc nghing theo phng ng 15 , trc ng song song vi mt phng ngang, trc ng xin 45 theo phng ngang v trc trc ng xin 45 theo phng ngang nhng c vng gn ln cn v tr hn.

- i vi cc mi hn gc ta c cc v tr 1F, 2F, 3F v 4F tng ng vi cc mi hn gc v tr hn sp, ngang,ng v trn.

Theo qui phm ASME section IX cho vic hp cch th hn nh sau: - Cc th hn t chng ch cc v tr 2G hoc 3G hoc 4G ch oc php hn v tr v v tr 1G, Nu th hn ng thi t 03 chng ch cho 03 v

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tr th c php hn v tr 6G. Tuy nhin nu th hn t c chng ch 6G c th c php hn mi v tr loi tr v tr 6GR.

Min v tr ca cc loi v tr mi hn

Loi mi hn Vit tt ting Anh K hiu biu

nghing trc ()

Gc quay b mt ()

Bng (sp) F (flat) A 0 15 150 210

Ngang H(horizontal) B 0 15 125 150 210 235

Trn O.H (overhead) C 0 80

0 125

235 360

ng V (vertical) D E 15 80 80 90

125 235 0 360

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V tr ca cc mi hn gip mi cho ng , thp tm v cc mi hn gc

3.3 Nhit trong ch hn Khi hn cc mi hn chu p lc cao nhit trc, trong v sau qua trnh hn

c c bit ch . Cc thng s sau phi c a vo qui trnh hn v lun kim sot trong qu trnh hn. Nhit l yu t nh hng ti t chc ca kim loi hn, ng sut, bin dng sau khi hn v cc khuyt tt nh nt, r trong mi hn.

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3.3.1 Nhit trc khi hn v sau mi lp hn: Tng ng i vi mi loi vt liu hoc cc nhm vt liu tng ng s c ch nhit ph hp. Theo tiu chun ANSI/AWS D1.1 cho php cc min nhit tng ng vi cc nhm vt liu nh sau:

Bng 3.3 -01 Nhit trc khi hn v c php khi hn cc lp k tip

Vt liu Nhm

(Grade) Vt liu

Nhm

(Grade)

P.Php

hn

Chiu dy

ti im

hn hn

Nhit

nh nht

( C)

ASTM A36 ASTM A516

ASTM A53 B ASTM A524 n 19mm

Khng p

dng

ASTM A106 B ASTM A529

Grade I & II

ASTM A131 A,B, CS, D,

DS, E ASTM A570

tt c cc

nhm

t 19 n

38 mm 66

ASTM 139 B ASTM 573 Grade 65

ASTM A381 Y35 ASTM A709 Grade 65

t 38 n

63 mm 107

ASTM A500 A, B API 5L Grade B.

Grade E

ASTM A501 ABS

Grade A,

B,D,CS,DS

Grade E Hn

h

quan

g ta

y s

dun

g qu

e h

n kh

ng

phI

que

thp

hyd

ro

ln hn 63

mm 150

ASTM A36 ASTM A570 Tt c

ASTM A53 Grade B ASTM A572 Grade 42,50

ASTM A106 ASTM A573 Grade 65

ASTM A131

Grade A,B,

CS, D, DS, E,

AH 32 & 36

DH 32 & 36

EH 32 & 36

ASTM A588

n 19mm

Khng p

dng

ASTM 139 Grade B ASTM 595 Grade A,B,C

ASTM 242 ASTM 606

ASTM A381 Grade Y35

ASTM A607 Grade 45,50,

55 Hn

h

quan

g ta

y v

I que

hn

co

hydr

o thp

,

hn

h q

uang

ch

m, h

n b

n t

ng c

kh

t 19 n

38 mm

10

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ASTM A441 ASTM A618

ASTM 500 Grade A

Grade B ASTM 633

Grade A, B,

C, D

ASTM 501

ASTM 709 Grade 36,

50, 50W

ASTM A516 Grade 55 &

60; 65 & 70 ASTM A808

t 38 n

63 mm 66

ASTM A524 Grade I & II

API 5L Grade B

Grade X42

ASTM 529 API Spec. 2H Grade 42,50

ASTM A537

Class I & II ABS Grade A,B,

CS, D, DS,

E, AH 32 &

36

DH 32 & 36

EH 32 & 36

ln hn 63

mm 107

Ghi ch: 1. Khi nhit mi trng nhit -180 C, cng tc hn s khng c thc hin. Khi nhit trn b mt kim loi thp hn so vi yu cu c ghi trong bng iu kin bt buc phi gia nhit trc khi hn. Vic gia nhit mc ch ngn nga kh nng nt trong kt cu mi hn. i vi vt liu A514, A517, v A709 grades 100 v 100W nhit ban u v nhit trc khi hn cc lp k tip khng c vt qu 2050 C, i vi thp c chiu dy ln hn 38 mm th khng c php ln hn 2300 C. 2. Khi nhit ca kim loi thp hn 00 C, kim loi c bn c gia nhit ti

nhit ln nht l 210 C. 3. Gia nhit trc khi hn c th tnh theo cng thc : Tp = 350 25.0Ce (oC)

3.3.2 kh ng sut sau khi hn: - Cc mi hn c th phi yu cu kh ng sut sau khi hn bng phng php nhit, cng tc gia cng c kh trn b mt mi hn s c thc hin thch hp nht sau khi mi hn c s l nhit. - Nguyn cng kh ng sut s c thc hin theo cc yu cu sau:

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Bc 1: Nhit ca l khng c php vt qu 315 C ti thi im t mi ghp hn vo. Bc 2: Tng nhit ca l nm trong khong t 5900C 6500C, tuy nhin tc nung nng khng c php vt qu 2000C trong 01 gi. Bc 3: Vt hn s c trong l vi thi gian c ch trong bng sau:

Thi gian ti thiu m vt hn cn c gi trong l

Chiu dy ca vt liu ti 6.4 mm t 6.4 n 50.8mm ln hn 50.8mm

15 pht 1gi / 25.4 mm 2 gi + 15 pht i vi mi 25.4mm chiu dy trn 50.8mm

Bc 4: Lm ngui sau khi thi gian hon thnh. C th lm ngui trong l hoc ngoi khng kh tc ngui

3.4 K nng c bn trong cc phng php hn 3.4.1 Phng php hn nng chy h quang tay v tr 2G 1- V tr que hn: (xem hnh 3.4.1-1)

khi hn v tr hn ngang gc nghing ca que hn nm trong mt phng nghing vi mt phng ngang vi gc t 5 100 v to vi ng thng vung gc vi hng hn mt gc t 150 n 200 theo hng hn. 2- K nng xp lp v cch di chuyn (Xem hnh 3.4.1-2 & 3.4.1-3) : khi hn mi hn nhiu lp trnh t xp xp cc lp c m t nh hnh 3.4.1-2 & 3.4.1-3. nghing ca que hn c m t nh hnh 3.4.1-4, khi hn que hn c dao ng theo hnh 3.4.1-1

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Hnh s 3.4.1-1

V tr que hn v tr hn ngang v kiu di chuyn ca que hn.

Hnh s 3.4.1-2 Mi hn nhiu lp trong lin kt hn ch T

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Hnh s 3.4.1-3

Sp xp trnh t cc lp trong mi hn nhiu lp

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Hnh s 3.4.1-4 Gc nghing ca in cc hn 3.4.2 Phng php hn nng chy h quang tay v tr 3G

Hnh 3.4.2-4 nghing que hn khi hn 3G

1- V tr que hn: (xem hnh 3.4.2-1) Khi hn v tr hn 3G que hn nm trong mt phng vung gc vi mt phng hn v que hn to vi ng nm ngang mt gc l 15 30 (khi hn xung) v 10 n 15 khi hn ln. 2- K nng xp lp v cch di chuyn: khi hn mi hn nhiu lp trnh t xp xp cc lp c m t nh hnh 3.4.2-3 . nghing ca que hn c m t nh hnh 3.4.2.1, khi hn que hn c dao ng theo hnh 3.4.2-2. Chiu rng ca b hn ph thuc vo phng php dao ng que mc trung bnh th chiu rng ca b hn xp x bng 02 ln ng knh que.

Hnh 3.4.2-2 Cc phng php di chuyn que

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Hnh 3.4.2-3 xp lp hn cho mi hn nhiu lp

3.4.3 Phng php hn nng chy h quang tay v tr 4G 1- V tr que hn: (xem hnh 3.4.3-1)

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Khi hn v tr hn 4G que hn nm trong mt phng vung gc vi mt phng hn v que hn to vi trc ng mt gc l 10 15 .

Hnh 3.4.3-1 Gc que hn khi hn 4G 2- K nng xp lp v cch di chuyn: khi hn mi hn nhiu lp trnh t xp xp cc lp c m t nh hnh 3.4.3-2 cho mi hn 4F (que hn nghing gc 450), hnh 3.4.3-3 m t trnh t sp xp cc lp cho mi hn 4G. Dao ng que hn theo hnh 3.4.3-4.

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Hnh 3.4.3-2 Sp xp cc lp hn cho v tr hn 4F

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Hnh 3.4.3-3 Sp xp cc lp hn cho v tr hn 4G

Hnh 3.4.3-4 Dao ng que hn khi hn 4G

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3.5 Ch hn 3.5.1 ng knh ca que hn Que hn c dng trong thc t thng c ng knh t 2.0 5mm. La chn ng knh que hn da vo chiu dy, lin kt hn hoc cnh ca mi hn tiu chun ANSI/AWS D1.1 a ra phm vi s dng ng knh que hn nh sau ng knh ln nht cho php ca que hn s l:

- 8mm cho tt c cc mi hn v tr hn 1G loi tr mi hn lt. - 6.4 mm cho mi hn 2F. - 6.4 mm cho lp lt ca mi hn gc 1F v 1G khi lin kt hn c khe h l

6mm v c m. - 4.0 mm cho nhng mi hn s dng que EXX14 v que thp hydro v tr

3G & 4G. - 4.8 mm cho lp lt ca mi hn gip mi v cc v tr khc m khng c

lit k trn. Trong sn xut c th tnh ng knh que hn theo cng thc sau: D = S/2 + 1 mm cho mi hn gip mi D = K/2 +2 mm cho mi hn gc Trong : - S l chiu dy ca vt hn mm - K l cnh mi hn mm 3.5.2 Dng in hn Theo loi dng in chia ra:

- Hn h quang tay bng dng xoay chiu AC ( Alternating Curent). - Hn h quang tay bng dng mt chiu DC ( Direct Curent)

Hn dng xoay chiu c u im thit b n gin, d bo qun sa cha, gi thnh tng i thp nhng kh gy h quang, h quang chy khng n nh, do t cht lng mi hn khng cao. Hn dng mt chiu c u khuyt im ngc li. khi hn dng mt chiu c phn ra 02 cch ni dy:

- Ni thun: ni cc dng ca ngun in hn vi vt hn v m cc ca ngun in hn vi que hn. Trong cc ti liu bng ting Anh ni thun k

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hiu l DCSP ( Direct Current Strainght Polarity ) hoc DCEN ( D.C Electrode Negative ).

- Ni nghch: Cc dng ca ngun in hn ni vi in cc hn, cc m ca ngun in hn ni vi vt hn. Trong ting Anh ni nghch k hiu c k hiu l DCRP (DC Reverse Polarity ).

Khi hn bng in cc ni thun in cc c nhit lng ln hn so vi vt hn do vy in cc nng chy vi tc nhanh nhng chiu su ngu li thp v th s dng phng php ni thun hn cc chi tit c chiu dy mng vi vt liu c bn c nhit nng chy thp, dng hn p, hn gang

Trong trng hp hn h quang trong mi trng kh tr ( TIG ) bng in cc khng nng chy th s phn b nhit li ngc li so vi hn h quang tay, tc l nu ni thun th nhit lng tp trung in cc li thp hn so vi vt hn bi vy thng s dng ni thun tng tui th ca in cc. Chiu su ngu khi s dng ni thun trong phng php hn ny cng ln hn so vi ni nghch.

Cng dng in hn l mt trong yu t chnh quyt nh cht lng mi hn, ng vi mi ng knh ca que hn s c khong cng dng in ph hp. Trn mc ca mi loi que hn u c ghi khong cng dng in c p dng. Mt khc ta c th chn dng in theo cc cng thc gn ng sau y:

Ih = K d (1) K = 35 - 40 Ih = K1 d1.5 (2) K1 = 20 - 25 Ih = ( + d )d (3) = 20, = 6

Trong d l ng knh que hn (mm). K, K1, , v l h s thc nghim Kinh nghim cho thy khi hn bng que c ng knh D = 4 5 mm nn s dng cng thc (1). Vi ng knh nh hn nn dng cng thc (2) cn ln hn dng cng thc (3). Cn ch rng cc cng thc trn ch tnh cho khi hn v tr hn sp. Khi hn cc v tr khc cn phi hiu chnh nh sau:

- Khi hn ng v hn ngang dng in hn s gim t 10 15%. - Hn trn dng in gim 15 20%. - Khi hn cc chi tit c chiu dy S 1.5 d gim dng 10 15%. - Khi hn cc chi tit ln hn 3d hay hn cc lin kt ch T tng 10 15% Nu dng in hn ln qu s lm tng chiu su ngu nhng mi hn cng d

xy ra khuyt tt, dng in m nh th khng ngu .. nh hng ca dng in ti

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khuyt tt ca mi hn s c nu c th chng Khuyt tt v cc phng php kim tra mi hn. 3.5.3 Chiu di ca h quang hn Chiu di h quang l khong cch t u mt que hn ti b mt ca b hn phn bit:

- H quang bnh thng nu L = 1.1d (d ng knh que hn) - H quang ngn nu L nh hn 1.1d. - H quang di nu ln hn 1.1d

Ch : Nu chiu di h quang ln th qung ng dch chuyn ca cc git kim loi

lng t que hn vo vng hn di do d b tc ng xu ca mi trng khng kh, mt khc, hn vi h quang di in p h quang s tng, chiu su ngu gim, kim loi bn to, b mt mi hn g gh v d b khuyt tt chy chn. Nu chiu di h quang ngn th s chy ca h quang khng n nh, dng in c hin tng chp mch thng xuyn, in p h quang gim, chiu rng mi hn gim, b mt mi hn khng mn. 3.5.4 Tc hn Tc hn l tc dch chuyn que hn dc trc theo trc mi hn. Nu tc hn qu ln mi hn s hp, chiu su ngu gim, khng phng v mi hn c th b gin on. Ngc li, nu tc hn qu nh s d b hin tng chy chn, kim loi c bn b nung nng qu mc, vng nh hng nhit ln chiu rng v chiu su ngu ca mi hn tng.... Tc hn h quang ph thuc vo loi que hn, cng dng in hn v tit din ngang ca mi hn. Tc hn hp l c tnh theo cng thc sau: Vh = d * Ih / 3600 * *Fd. Trong : - Vh l tc hn (cm/s) - d l h s p ( d = 7 11 g/A.h) - Ih l cng dng in hn (A) - Khi lng ring ca kim loi p (g/cm3).

- Fd. l din tch thit din ngang ca kim loi tnh cho mt lp hn tng ng.

3.5.5 Sp xp cc lp hn trong mi hn nhiu lp:

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S lp hn ti thiu khi hn cc mi hn nhiu lp c tnh ton nh sau: N = (Fd Fl )/Fn + 1 Trong :

- N l s lp ti thiu. - F l din tch thit din kim loi p (mm) - Fl l din tch thit din lp lt. - Fn l din tch thit din cc lp k tip. Din tch thit din ngang ca kim loi p sau mt lp hn ph thuc ng knh que hn. Theo kinh nghim mi quan h c xc nh nh sau: Fl = (6 8)d (mm2) d l ng knh ca que hn Fn = (8 12)d (mm2)

Sp xp cc lp hn trong lin kt hn gip mi ph thuc vo chiu dy ca kim loi hn. Tiu chun ANSI/AWS D1.1 cho php chiu dy cc lp trong phng php hn h quang tay nh sau:

- Chiu dy ln nht ca lp hn lt trong lin kt hn gip mi cho php l: 6mm

- Cnh ca mi hn gc ln nht cho php khi hn lp lt ca mi hn gc cho php l:

o 10 mm i vi v tr hn sp ( 1G) . o 8 mm i vi v tr hn ngang v nga ( 2G & 4G ). o 13 mm v tr hn ng (3G)

- Chiu dy ca cc lp hn k tip vi lp lt ca mi hn gip mi v mi hn gc la:

o 3mm i vi v tr hn l 1G o 5mm i vi v tr hn l 2G, 3G v 4G.

3.6 Cc phng php hn thng dng khc 3.6.2 Hn MIG 3.6.2.1 Khi nim c bn v hn MIG

Hn MIG l phng php hn h quang c in cc nng chy, trong nhit c cung cp bi h quang c to thnh in cc v vng hn, vng h quang c nhit rt cao . Vng h quang c bo v bng kh tr (Ar hoc He) nu vng h quang c bo v bng kh hot tnh nh CO2 th gi l hn MAG, hn MIG c

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p c p dng rng di trong cng nghip hin i. N c th hn cc loi hp kim mu nh ng, nhm, hp kim mangan thp khng g v cc kt cu chu p lc cao. Phng php hn ny c th hn oc mi v tr trong khng gian vi chiu dy n 25 30 mm. Cc yu t chnh trong phng php hn ny cn quan tm bao gm:

Dy hn, khi hn MIG dy hn c ng knh t 0.8 2.4mm thng dy c m ng. Khi hn s dng ngun DC ni nghch,ngun AC s dng cho phng php ny l khng thch hp. c tnh ngoi ca ngun in hn thng thng l c tnh cng (in p khng i). Khi hn MIG la chn dy hn khi hn cc kt cu chu p lc da theo tiu chun ASME section II ( nu rt r ti chng 2 ca ti liu ny) tuy nhin nguyn tc c bn ca s la chn l kim loi in y c thnh phn c l tnh ging nh kim loi c bn.

Kh hn, kh hn c th l argon, helium c tinh khit 99.99% hoc hn hp 02 kh . Tuy nhin hin nay thng s dng kh argon , tc kh trong khi hn thng l t 15 200 lt/pht. La chn ch hn theo bng di y. 3.6.2.2 Ch hn MIG Dng in, ng knh dy in y, in cc, tc kh bao ph v tc hn c la chn theo bng 3.6.2-01

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Bng 3.6.2-01 Ch hn MIG

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CHNG 4 NG SUT V BIN DNG HN 4.1 Khi nim ng sut v bin dng hn

Hnh 4.1-01

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Khi c ngoi lc tc ng, trong vt th s xut hin mt trng thi ng sut, ng thi hnh dng kch thc ca vt th cng thay i so vi khi khng c ti. Nu nh trng thi ng sut v bin dng vt qua mt gii hn cho php th vt th c th b ph hu hoc mt kh nng lm vic tip theo. By gi xt trng hp hn p ln mt mp ca tm thp hnh ch nht v kho st s xut hin ng sut v bin dng tn ti sau khi hn xong.

Trc khi hn tm c dng hnh ch nht nh hnh 4.1-01a. Sau khi ngui hon ton tm b vng xung t pha c mi hn nh hnh 4.1-01b vi trng thi ng sut d nh trn biu hnh 4.1-01c. gii thch iu ny, ta gi s khi hn tm thp thnh 2 vng A v B. Vng A cha mi hn, khi hn nung nng ti nhit cao vng B c truyn nhit t A sang nhng nhit trung bnh thp. Khi Vng A b dn ra, cn vng B cng b dn ra nhng khng ng k v nhit tng t, tc l vng B s cn tr s gin ca vng A (H.4.1-01d). S cn tr ny khng phi l tuyt i, nn dn di vng A vn ln hn v lm cho tm thp b un v pha c mi hn. Tuy nhin trong vng A do khng thc hin c s gin n hon ton nn trong n s tn ti ng sut nn, cn vng B xut hin ng sut ko. Nu chiu rng ca tm ln, mp nhoi c th xut hin ng sut nn. Sau khi hn kt thc v chi tit hn ngui v trng thi ban u, trng thji vng A c nhit cao hn s co li nhiu hn so vi vng B. Nhng s co li cng khngc thc hin mt cch t do v s cn tr t pha vng B, nn s tn ti trng thi ng sut ko vng A v nn vng B. iu chng t rng trng thi ng sut v bin dng trong giai on nung nng v lm ngui hon ton tri ngc nhau. Sau khi hn biu ng sut dc c dng nh hnh 4.1-01g. ng sut d trong vng A c th t ti gii hn chy.

Qua v d trn nu ra ta thy rng: ng sut v bin dng hn l trng thi ng sut v bin dng do qu trnh hn gy ra v tn ti trong kt cu sau khi hn. Vic tm ra ngun nhn v phng php phng, hn ch kh nng tn ti ng sut v bin dng l iu cn thit khi ch to cc kt cu hn. 4.2 Nguyn nhn gy ra ng sut v bin dng hn C th k ra 3 nguyn nhn gy ra ng sut v bin dng hn:

1. Nung nng ng u kim loi vt hn. 2. ngt ca kim loi nng chy mi hn sau khi kt tinh. 3. S thay i t chc ca vng kim loi ln cn mi hn.

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Nung nng khng ng u kim loi vt hn lm cho vng xa ngun nhit khng hoc rt t b bin dng nhit, chng s cn tr s bin dng vng ln cn mi hn. Do vy s sut hin ng sut trong mi hn v vng ln cn mi hn, trng ng sut ny tn ti c khi kt thc mi hn v kim loi v trng thi ngui hon ton.

Kim loi lng mi hn b gim th tch do kt qu ng c tng t nh vt c. Do s co ngt c ca kim loi trong mi hn xut hin cc lc nn theo phng dc cng nh phng ngang so vi trc mi hn v to ra trng ng sut d trong .

Nhng thay i t chc kim loi trong vng ln cn mi hn l nhng thay i v kch thc v v tr sp xp ca tinh th kim loi, ng thi knm theo s thay i th tch ca kim loi trong vng nh hng nhit. S thay i cc b nh vy dn n vic to thnh ni ng sut. Khi hn cc thp hp kim v thp cc bon cao c khuynh hng ti th cc ng sut ny c th t n gi tr rt cao.

ng sut d trong vt hn kt hp vi ng sut sinh ra do ngoi lc tc dng khi lm vic c th lm gim kh nng lm vic ca kt cu v lm sut hin cc vt nt, gy. Bin dng hn lm sai lch hnh dng, kch thc ca cc kt cu hn. 4.3 Cc loi ng sut v bin dng hn 4.3.1 ng sut v bin dng dc ng sut dc l ng sut tc dng tc dng sng song vi dc trc ca mi hn v xut hin do s co dc trc ca mi hn (H.4.3-01). Nh ni trn, kimloi khu vc ln cn mi hn c ng sut dc l ng sut ko. Gi tr ng sut ko l rt ln, c th t ti gii hn chy ca vt liu. cc phn cn li ng sut dc l ng sut l ng sut nn. ng sut dc ging nhau trn mi mt ct ca mi hn (tr phn u v cui ca lin kt). Sau khi hn tm s b ngn mt on ld (co dc) v gim chiu rng mt lng ln (co ngang). Khi hn cc kt cu m trng tm mt ct ngang ca lin kt khng trng vi trc ca mi hn, th ngoi ng sut bin dng dc v ngang vt hn cn b un vi vng d f nht nh.

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Hnh 4.3-01 ng sut v bin dng gc a: Lin kt hn gip mi vi ng hn i xng, b: Lin kt hn gip mi vi

ng hn khng i xng, c: lin kt hn ch T

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4.3.2 ng sut v bin dng ngang

Hnh 4.3 -02

ng sut ngang xut hin do hin tng co ngang v co dc mi hn v ng

thi do s kp cht ca chi tit khi hn. Xt lin kt hn gip mi trn hnh 4.3 -02a. Gi s sau khi hn, tin hnh ct lin kt dc trc theo trc ca mi hn, khi ta s c hai tm ch nht c mt na mi hn cnh bin, tng t nh hn p ln mp tm gii thiu trn hnh 7-3, c ngha chng s b un nh hnh 4.3 -02b do hin tng co dc gy ra. Trong thc t hin tng ny khng th thc hin c c do chng cng nm trong mt lin kt lin khi. V vy trong lin kt hn s tn ti ng sut ngang c phng vung gc vi trc mi hn. ng sut do co ngang trong nhng iu kin bt li c th sut hin cc vt nt dc theo trc ca mi hn.

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ln v s phn b ng sut ngang ph thuc vo chiu dy kim loi c bn, tnh cht g kp cc chi tit trong khi hn, k thut v th t trnh by cc mi hn. Cng vi vic tng chiu dy kim loi v s lp mi hn, ng sut ngang cng tng ln. 4.3.3 Bin dng gc v s mt n nh cc b do hn

Hnh 4.3 -03 bin dng gc ca lin kt gip mi v lin kt ch T Cc bin dng gc do c co ngang khng ng u ca kim loi theo chiu dy ca lin kt. Gi s c lin kt gip mi, vt mp ch V nh trn hnh v 4.3 -03 a. Ta c th chia mi hn thnh nhiu lp c chiu di khc nhau ph thuc vo v tr

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ca chng so vi b mt ca mi hn. Khi ngui cc lp ny s co li vi nhng gi tr khc nhau tng ng vi chiu di ca chng. Kt qu phn trn ca mi hn co ngang ln hn so vi phn di v s lm quay chi tit hn mt gc no . chnh l bin dng gc ca lin kt do qu trnh hn gy ra. Khi hn cc lin kt gip mi vi kiu vt mp hnh ch V, cc bin dng gc c th t n = 3-7 vi chiu dy tm 6 -12mm v ln hn 7 vi chiu dy 13 20 mm. S dng kiu vt mp i xng s hn ch ti a bin dng ny. 4.4 Cc bin php gim ng sut v bin dng hn 4.4.1 Cc bin php trc khi hn Mt trong cc bin php u tin l la chn kim loi c bn, vt liu hn hp l khi tnh ton thit k. Kimloi c bn khng c khguynh hng d ti khi ngui trong mi trng khng kh, cn vt liu hn b xung phi c kh nng to mi hn c tnh do nh kim loi c bn. Ngoi ra cng phi thc hin cc bin php sau y: 1 trnh ng sut phng hay ng sut khi khng nn thit k cc mi hn tp trung hay giao nhau. 2. Khng nn thit k cc mi hn khp kn c kch thc nh v chng sinh ra ng sut phng ln. 3. C gng gim ti a ss lng mi hn trong kt cu, cc mi hn nn b tr i xng vi trng tm ca thit din to ra bin dng ngc chiu c tc dng trit tiu nhau. 4. Cc gn tng cng cn sp xp sao cho khi hn kim loi c bn s cng c nung nng hai pha ca mt khu vc gim s co ngang v ng sut khi ca ton kt cu 5. Khi hn gip mi nu chiu dy ca kt cu khng bng nhau th phi vt bt tm dy hn trnh s tp trung ng sut vng mi hn. 6. Khi thit k cc kt cu hn phc tp, cn tnh kh nng ch to tng b phn sau s t hp thnh kt cu ln. Nh vy s gim bt rng buc hn ch s co ngang ca mi hn v gim bt trng thi ng sut phng. 7. Trong cc kt cu dng hnh hp c nhng ng khp kn, hn ch bin dng cc b th cn t cc gn tng cng. 4.4.2 Cc bin php cng ngh trong khi hn

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1. Khi hn cc loi vt liu d b ti, vt liu c dy th cn xem xt vic nung nng s b trc khi hn, ng thi gim bt cng dng in hn trnh xut hin vt nt. Nung nng s b vt hn s lm gim ng sut v bin dng hn. 2. Khi mi hn b kp cht d sinh ra ng sut ln. Do trnh t thc hin cc mi hn trong kt cu phi lm sao cho vt hn lun trng thi t do, nht l i vi hn gip mi, l loi mi hn c co ngang ln. Khi hn phi hn theo mt chiu hoc hn t gia ra hai pha, khng nn hn t hai u vo v d ch ra trong hnh 4.4-01 v 4.4-02 3. Cc mi hn i xng, song song nn hn ng thi bng nhiu th hoc thc hin xen k i xng. 4. Ch hn chn sao cho vng nh hng nhit cng nh cng tt. 5. Hn theo phng php phn on nghch s gim c bin dng v ni lc sinh ra ch tng khu vc nh v hng v vng ln cn i din. 6. kh bin dng gc thng dng phng php to bin dng ngc khi hn

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Hnh 4.4-01

Hnh 4.4-02

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4.4.2 Cc bin php cng ngh sau khi hn Thng thng sau khi hn trong vt hn vn tn ti trng thi ng sut v bin dng d. khc phc tnh trng nhm nng cao cht lng ca kt cu hn, ngi ta thng dng mt s bin php sau.

1. , vt hn c th tr b c ng sut d sau khi hn. Nhit ph thuc vo vt liu, vi thp thng trong khong t 560 660 C. Sau khi gi nhit trong thi gian ph hp th cho ngui mt cch t nhin.

2. Nn ngui. Bin php ch yu l nh gin hay cn nhng phn b co, cong vnh t kch thc v hnh dng nh thit k. Song bin php ny thng sinh ra bin cng v tng ng sut d lm cho vt hn d b nt, thm ch c th b gy. nn nn ngui t dng.

3. Nn nng. Nn nng l phng php c dng rng ri v n gin v kinh t. Nung nng cc b ri cho ngui t do c mc ch l lm co li nhng khu vc b gin ra do tc ng ca qu trnh gy nn. cn ch vic chn khu vc nung. V nu ch nung khng hp l lm cho bin dng thm phc tp hnh 4.4-03 gii thiu mt s v d v v tr nung nng nhm gim bin dng hn.

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Hnh 4.4-03

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CHNG 5 KHUYT TT V PHNG PHP KHC PHC

5.1 Cc dng khuyt tt chnh v bin php khc phc 5.1.1 Nt

Nt c th xut hin trn b mt mi hn, trong mi hn v vng nh hng nhit ca mi hn. Nt c nt nng xut hin nhit trn 1000 C, nt ngui xut hin nhit thp hn. Hnh 5.1-01 m t cc loi nt xut hin trn mi hn v vng nh hng nhit.

Bng 5.1-01 Cc dng nt, nguyn nhn v cch khc phc Dng vt

nt Phng php kim tra Nguyn nhn Gii php cng ngh

Nt dc

1. Visual (bng mt) 2. . PT (thm thu 3. MT (bt t) 4. UT (siu m) 5. RT (chp phim)

1. s dng vt liu hn cha ng.

2. Tn ti ng sut d ln trong lin kt hn.

3. Tc ngui cao. 4. Lin kt hn khng hp

l. 5. B tr cc mi hn

khng hp l

1. S dng vt liu hn ph hp.

2. Gii phng cc lc kp cht cho lin kt hn. Tng kh nng in y ca vt liu hn.

3. Gia nhit cho vt hn, gi nhit cho lin kt hn gim tc ngui.

4. S dng cc lin kt hn hp l, vt mp , gim khe h gia cc vt hn.

5. B tr sole cc mi hn.

Nt vng gy

v kt thc h quang

- nt -

1. V tr kt thc h quang b lm, tn ti nhiu tp cht.

2. H quang khng c bo v tt.

1. S dng thit b hn ph hp, c ch ring cho lc gy v kt thc h quang.

2. S dnh cc bn ni cng ngh cc im u v kt thc h quang.

Nt ngang - nt - 1. S dng vt liu hn cha ng. 2. Tc ngui cao.

1. S dng vt liu cho ph hp. 2. Tng dng in v kch

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3. Mi hn qu nh so vi lin kt hn.

thc in cc hn. 3. Gia nhit trc khi hn

Hnh 5.1-01 Cc dng nt thng xut hin mi hn

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R kh

R kh c th sinh ra bn trong hoc b mt mi hn. R kh c th nm phn ranh gii gia kim loi c bn v kim loi p.

R kh c th phn b tp trung hoc nm di rc trong mi hn.

Bng 5.1-02 R kh, nguyn nhn v cch khc phc

Dng khuyt

tt

Phng php kim tra

Nguyn nhn Gii php cng ngh

R kh

1.Visual (bng mt)

2. . PT (thm thu

3. MT (bt t) 4. UT (siu m) 5. RT (chp

phim)

1. Hm lng cac bon trong kim loi c bn hoc trong vt liu qu cao.

2. Vt liu hn b m, b mt dnh sn, du, m, g, hi nc,

3. Chiu di ct h quang ln, tc hn qu cao.

1. Dng vt liu hn c % Cacbon thp.

2. Vt liu hn c sy kh, b mt c lm sch.

3. Gi chiu di h quang ngn, gim tc hn

4. i vi hn MIG, TIG, MAG s dng kh bo v ph hp, la chn khong cch gia chp kh vi vt hn. Kim tra lu lng kh.

5. i vi hn SAW m bo thuc khng b m (sy trc khi hn)

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Hnh 5.1-02 R kh Ln x (kt x). xem hnh 5.1-03

Ln x l loi khuyt tt d xut hin trong mi hn. X hn v tp cht c th tn ti trong mi hn, cng c th nm trn b mt mi hn, ch gip ranh gia kim loi mi hn v phn kim loi c bn. Ln x nh hng n bn, dai va p v tnh do ca kim loi mi hn, gim kh nng lm vic ca kt cu di tc dng ca ti trng ng.

Nguyn nhn - Dng in hn yu, khng nhit cung cp cho kim loi nng chy v

x kh thot ln b mt b hn. - Mp hn ch c lm sch hoc khi hn nh hay hn nhiu lp ch g

sch g hn. - Gc que hn cha hp l, tc hn qu ln. - Lm ngui mi hn qu nhanh lm cho x hn khng kp thot.

Bin php phng trnh: - Tng dng in hn cho thch hp. Hn bng h quang ngn v tng thi

gian dng li ca h quang. - Lm sch vt hn trc khi hn, g sch x hn trc khi hn lp k tip. - Thay i gc v phng php a in cc cho hp l. Gim tc hn,

trnh x hn chy trn ln vo trong vng hn hoc chy v pha trc vng nng chy.


Hnh 5.1-03 Ln x

Khng ngu (xem hnh 5.1-04) Hn khng ngu sinh ra gc mi hn, mp hn hoc gia cc mi hn (xem hnh

Nguyn nhn - Mp hn chun b khng hp l gc vt nh. - Dng in hn qu nh hoc tc hn nhanh. - Gc in cc hn v cch in cc khng hp l. - Chiu di h quang ln. - in cc chuyn ng khng ln theo trc mi hn.

Bin php phng trnh: - Lm sch lin kt trc khi hn, tng gc vt v khe h hn. - Tng dng in hn v khe h hn,

Chy chn v chy loang xem hnh 5.1-05 Chy chn

Chy chn l phn b lm (lm, khuyt) thnh rnh dc theo ranh gii gia kim loi c bn v kim loi p. Chy chn lm gim tit din lm vic ca mi hn, to s tp chung ng sut cao v c th dn n s ph hu ca kt cu trong qu trnh s dng.

Nguyn nhn: - Dng in hn qu ln - Chiu di h quang ln - Gc que hn v dao ng que cha hp l - S dng cha ng kch thc in cc hn.

Chy loang Chy loang l hin tng kim loi lng chy loang trn b mt ca lin kt hn (b mt kim loi c bn vng khng nng chy). Chy loang to ra s tp trung ng sut, lm sai lch hnh dng ca lin kt hn

Nguyn nhn

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- Gc nghing que hn khng hp l - Dng in hn qu cao - T th hn v cch t vt hn khng hp l.

Hnh 5.1-04 Khng ngu

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Hnh 5.1-05 Chy chn v chy loang

page 47 of 47

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