[goforthjp]

What causes a cast iron weld to crack right down the center?

[79jasper]

Incorrect preheat and cool down.

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[dsmabe]

Ive seen most people beat the crap outta the weld immediately after welding to help reduce internal stresses.

[TRACKRANGER]

There's a bit of technique involved. Normally the ball end of a hammer is used to apply stress relief between weld passes. The method is to provide light hammering of the weld area as it cools. "Beating the crap" out of cast iron it is not likely to make it want to be your best friend.

Also, excessive hammering can work-harden and actually increase stresses in the weld region.

Proper pre-heat and very slow cooling of the job (over several hours) can be additionally beneficial in relieving these stresses.

Ranger

[TRACKRANGER]

Also, what weld technique is being used (Bronzing, SMAW, etc)?

[weldin mike 27]

If you do not apply correct techniques, the weld can become extremely hard and will not cope with shrinkage stresses on cooling .
These are pre heat, peening, and slow cooling in warm sand. Or if you are using the "cold method" keeping the runs very short, peening and waiting between beads so you can rest your hand on the job.

Mick