8welding Metallurgy

8/20/2019 8welding Metallurgy

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Materials Behavior


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Basic Regions of a Weld

Fusion Zone: area thatis completely melted

Heat-Affected Zone:portion of the base metalnot melted but whose

mechanical properties

and microstructure were

affected by the heat of

the joining process

Base Metal


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Fusion Weld Zone

Fig : Characteristicsof a typicalfusion weld zonein oxyfuel gasand arc welding.


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Composite Zone Concerns


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Grain Structure

Fig : Grain structure in (a) a deep weld (b) a shallow weld. Notethat the grains in the solidified weld metal are perpendicular tothe surface of the base metal. n a good weld! the solidificationline at the center in the deep weld shown in (a) has grainmigration! which de"elops uniform strength in the weld bead.

(a) (b)


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Solidification ofWeld metal

• #olidification begins with formation of columnar

grains which is similar to casting

• Grains relati"ely long and form parallel to the

heat flow

• Grain structure and size depend on the specific

alloy

• $eld metal has a cast structure because it has

cooled slowly! it has grain structure

• %esults depends on alloys !composition and

thermal cycling to which the &oint is sub&ected.

• 'reheating is important for metals ha"ing high

thermal conducti"ity


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Weld Beads

Fig : (a) $eld bead (on a coldrolled nicel strip) produced by alaser beam. (b) *icrohardness profile across the weld bead.Note the lower hardness of the weld bead compared to thebase metal.

(a) (b)


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Heat affected Zone

• +eat effected zone is within the metal itself

• 'roperties depend on:

• %ate of heat input and cooling

• ,emperature to which the zone was raised

•

-riginal grain size !Grain orientation ! egree of priorcold wor

• ,he strength and hardness depend on:

• how original strength and hardness of the base metalwas de"eloped prior to the welding.

• +eat applied during welding which %ecrystallises

elongated grains of cold wored base metal.


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Weld Quality

• $elding discontinuities can be caused by inade/uate

or careless application

• ,he ma&or discontinuities that affect weld /uality are

• 'orosity

• #lag nclusions

• ncomplete fusion and penetration

• $eld profile

• Cracs

• 0amellar tears

• #urface damage

• %esidual stresses


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Cracs

• Cracs occur in "arious directions and "arious

locations

Factors causing cracs:

•

,emperature gradients that cause thermal stresses inthe weld zone

• 1ariations in the composition of the weld zone.

• 2mbrittlement of grain boundaries

• nability -f the weld metal to contract during cooling


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Cracks

Fig : Types of cracks (in welded joints) caused by thermal stresses thatdevelop during solidification and contraction of the weld bead andthe surrounding structure. (a) Crater cracks (b) arious types of

cracks in butt and T joints.


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Cracks

• Cracs are classified as +ot or Cold.

• Hot cracks 3 -ccur at ele"ated temperatures

• Cold cracks 3 -ccur after solidification

• 4asic crac pre"ention measures :

5.Change the &oint design !to minimize stresses

from the shrinage during cooling

6.Change the parameters! procedures! these/uence of welding process

7.'reheat the components to be welded

8.9"oid rapid cooling of the welded components


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Cracks in Weld Beads

Fig : Crac in a weld bead!due to the fact that thetwo components were

not allowed to contractafter the weld wascompleted.


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Perils of Welding Free-Machining Steels

Solidification cracing

due to impurity

elements

Sulfur! phosphorus!boron

"mpurity segregation at

weld centerline creates

low ductility area #ombines with shrinage

stress to cause cracing


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Manganese Can Prevent Solidification

Cracking

Manganese combines with sulfur to form MnS

particles

$se a filler metal with higher manganese to absorbsulfur

%&'(S-)#omposition%&'(-*0.06-0.15carbon0.07-0.10.9-1.4mananese1.4-1.8- -


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Residual Stresses:

• Caused because of localized heating and cooling

during welding! expansion and contraction of the

weld area causes residual stresses in the wor

piece.

• 2ffects:

• 5.istortion!$arping and bucling of welded parts

•6.#tress corrosion cracing

• 7.%educed fatigue life


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Distortion after Welding

Fig : istortion of parts after welding : (a) butt &oints (b) filletwelds. istortion is caused by differential thermal expansionand contraction of different parts of the welded assembly.


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Stress relieving of elds :

• 'reheating reduces problems caused byheating the base metal or the parts to be

welded

• +eating can be done electrically! in furnace or by -9$

torch and for thin surfaces by radiant lamp or hot air

blast.

•

#ome other methods of stress relie"ing :'eening! hammering or surface rolling

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8welding Metallurgy