Breaking of Tack Weld During Preheating for Welding

<...It was observed...> So, what did the observer actually see and under what circumstances? Surely this is the best avenue of enquiry?

What material do you try to weld?

As the learned PWSlack has already explained the cause, no more need be said.

If you were looking for a solution, which apparently you are not, I'd look into increasing the length, or frequency, of the tack welds.

^{_{My time has expired.}}

As PWSLack stated, the thermal expansion stress was too much in regards to the length and placement of tack welds, and the cracked tack welds must be removed.

Preheat the piece at a much slower rate. That should help to minimize thermal stresses.

Preheating before tacking is as critical as the welding.

Even if the tack weld is fully removed, the damage to the HAZ from tacking without the prescribed preheat will remain.

Tacking without preheat can also increase the risk of Hydrogen inclusions

Solution is as above - preheat prior to tacking, ensure tacking is of sufficient size to allow for the thermal and structural stresses applied during the welding of the root run. Ensure tack is visually examined and then tapered to allow for the successful welding of the root run.