Welding Lecture 5
Transcript of Welding Lecture 5
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Reactions in liquid weld metal
Solution of gas, causing gas-metal reactions or reaction
with elements dissolved in the liquid metal;
Evolution of gas;
Reaction with slag or flux
Metallurgical effects in fusion welding
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438.6/6355)/(1 2/1! T p sn
p = 1 atm
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Solubility of nitrogen in binary iron alloys under equilibrium
conditions at 1600 oC and 1 atm. nitrogen pressure
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Surface-active agents (like oxygen) cause an increase in
nitrogen content of the weld.
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Effect of alloying elements in electrode wire on the nitrogen
content of weld metals in an air welding atmos phere at a pressure
of 1 atm. GMAW; electrode positive, 25 V, 150 A.
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Porosity
Desor ption of dissolved gases causes porosity. There are two
main ty pes of porosity:
Globular: s pherical and near s pherical (0.05 mm to 5 mm)
Wormhole, blowhole or tunnel porosity: elongated cavities
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Dissolved gasses cause porosity. In addition, nitrogen causes
strain-ageing embrittlement. Formation of FeN also has an
embrittling effect.
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Oxygen content of pure iron as a function of partial pressure of
oxygen in Ar-O2 mixture of total pressure 1 atm. GMAW,
electrode positive.
Oxygen can be dissolved in the liquid metal either directly
from the arc or by reaction with slag or flux
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Effect of alloying elements on the oxygen content of iron weld
metal.
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Oxygen affects the weld metal pro perties in three ways: non-metallic
inclusions, oxidation of alloying elements and CO porosity.
SAW, Flux basicity = 1.1
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Oxidation loss of Mn and Si from consumable electrode (carbon
steel) during GMAW. Mn is lost by va porisation as well.
If consumable electrodes used, alloying elements are oxidised as
the dro plets are formed at the electrode ti p and during their transfer
across the arc. Alloy content may be modified by slag-metalreactions also.
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% element transfer from electrode or filler wire to weld de posit
(fusion welding of steel)
Process C Si Mn Ni Cr Mo Cu Nb Al Ti
Coated
electrodes
30/
75
50/
75
60/
100
50/
95
45/
85
90/
100
40/
50
5/
20
5/
20
Submergedarc
70/100
100/200
100/300
10075/100
100 10075/100
Carbon
dioxide
50/
200
50/
60
60/
70100 100 100 100 60 20 20
Inert gas
tungsten
arc (filler
rod)
70/
100
90/
100
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Only a small pro portion of carbon is burnt out if adequate
deoxidisers are present in flux / electrode wire. Otherwise, a lowcarbon weld de posit is obtained which, due to CO evolution in the
weld pool, is porous. C can be picked u p during CO2 welding.
Most alloying elements, particularly, Ti and Al, are oxidised tosome extent, unless protected by inert gas. Si and Mn may also
suffer oxidation but their content in the weld de posit is largely
de pendent on slag com position.
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FeOSi FeSiO 222
F e M n F e M nO
2
2
)(
)]([
SiO
F eo Si
K Si !
)(
)]([
M nO
F eO M n
K M n!
Where [Si] and (MnO) etc re present activities of the com ponents in metal (square
bracket) and slag (small bracket) res pectively.
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RT
G
Si
Si
e K (
!
RT
G
M n
M n
e K (
!
Therefore, K si
and K M n
decreases with decreasing tem perature,
and corres pondingly the two equation move to the left. It canthus be ex pected that surface layers of the solidified weld metal
would be lower in silicon and manganese contents than the
central parts.
Si MnG G (( & are positive
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)(
)(
Acid ox id es
s Basi cox id e B
7
7!
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Weld pool solidification
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The structure of the weld a ppears to de pend mainly on: com position
(solute content), and solidification parameter .
Weld pool solidification
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G: tem p gradient in the direction of solidification
R: rate of advance of solidification front.
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Dendritic growth in TIG-welded monel.
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Post-weld heat treatment (say at 600 oC for a min of 1 h per inch thickness) to minimise the residual stresses.
Possibility of tem per embrittlement during the thermal
treatment in carbon and low alloy steels.
Residual Stress
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Weld Cracking
Cracking during solidification (hot or solidification cracking)
Cracking in HAZ (liquation cracking)
cold cracking
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S and P increase the brittle tem perature range.
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14.53.124575190230 ! Mn Si Nb P S C U CS
S
S
H
H H H
E
!
Cracking index for SAW
Cracking if Ucs > 20-25
Risk of solidification cracking can be minimised by:
??