Every details of Preheat/PWHT is given in WPS or you can find in design codes.

When hydrogen related cracking may be there due to high thickness due to dissolved hydrogen presence there is a chance of delayed cracking which takes up to some hours,when gaseous hydrogen try to escape through grain boundary. It all depends on preheat temperature, the electrode filler you had used, how you cool your metal after preheat or your PWHT just follows Preheat.