Stress crack
A stress crack occurs when a component comes under tensile stress due to uneven heating or cooling and the material can no longer absorb this stress. With cast iron, a difference of often as little as 20 °C between the warm weld area and the cold surroundings is enough to trigger a crack.
How a Stress crack Forms
Metal expands when heated and contracts when cooled. If only one spot gets hot while the rest stays cold, the contracting hot spot pulls on the cold rest. If the material can't take that tensile stress, it cracks.
The Cast Iron Trap
Cast iron is brittle and can barely relieve stress. Often a difference of about 20 °C between the spot you just welded and the cool rest of the part is enough to trigger a Stress crack.
How to Avoid It
Preheating evens out the temperature, and slow cooling keeps the weld from contracting too fast. If you weld cold, keep the heat low (short beads, low interpass temperature) and peen the weld to relieve stress.