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RE: Tack Welds

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The primary problem with tack welds as others have stated is the unreliable
strength.  Because the welding process puts so little energy into the weld,
the molten weld metal cools down quite rapidly.  This rapid cooling impacts
the crystal structure of the hardened weld metal.  In addition I believe
that the lack of heat makes it easier for hydrogen to be absorbed into the
hardened weld metal.  The net result is that the tack weld will likely be a
source of cracks in addition to acting as a stress riser.  These cracks can
occur even when the member is not subject to loads,  This is why you do not
want to leave tack welds even if they are carrying no load. 
Non-destructive testing will not identify this problem.

A metalurgist or welding engineer should be able to provide authoritive

Mark Gilligan

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