Welding Lecture 5

8/9/2019 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



438.6/6355)/(1 2/1! T p sn

p = 1 atm



Solubility of nitrogen in binary iron alloys under equilibrium

conditions at 1600 oC and 1 atm. nitrogen pressure



Surface-active agents (like oxygen) cause an increase in

nitrogen content of the weld.



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.



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



Dissolved gasses cause porosity. In addition, nitrogen causes

strain-ageing embrittlement. Formation of FeN also has an

embrittling effect.



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



Effect of alloying elements on the oxygen content of iron weld

metal.



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



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.



% 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



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.



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.



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



)(

)(

Acid ox id es

s Basi cox id e B

7

7!



Weld pool solidification



The structure of the weld a ppears to de pend mainly on: com position

(solute content), and solidification parameter .

Weld pool solidification



G: tem p gradient in the direction of solidification

R: rate of advance of solidification front.



Dendritic growth in TIG-welded monel.



Residual Stress



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



Weld Cracking

Cracking during solidification (hot or solidification cracking)

Cracking in HAZ (liquation cracking)

cold cracking



S and P increase the brittle tem perature range.



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:

??

Welding Lecture 5

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