Cracking in tack weld joint

Cause of cracking in tack weld joint?
How to avoid this cracking?
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Caused by:-
lacked of fusion of parent metal/not enough filler metal deposited into the weld, parent metal to cold, the tack weld to small/ not long enough, wrong filler metal, welding machine settings wrong/not as per WPS etc.

Avoid by ensuring:-
Correct WPS settings is followed/WPS is implemented properly with regard to filler metal, preheat of weld area, weld sequence etc., ensure proper heat input during welding, qualified welder to execute job function.

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Min. lenght of tack weld? and standards reference?

3 times the thickness of parent metal.
ASME B31.1

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AWS D1.1 2.17.2.3: Tack welds located outside the joint groove shall terminate not closer than
1/2 in [12 mm] from the edge of the connected part.

ASME IX table 469.1 note 2: Tack weld are not limited by pipe or tube diameters when their aggregate length does not exceed 25% of the weld circumference.

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