The business about heating in a pile of charcoal and slow cooldown is not always necessary. Cracking occurs as a result of thermal stresses, which happen if the shape has a loop (or loops) such that the hot part and cool part are pulling in different directions. The simplest example would be a cast iron ring. After the repair, the heated part cools and shrinks. The rest of the ring is fighting the shrinkage, generally resultig in a crack near the repair. On the other hand, a simple butt-weld can shrink without developing stress, and all the fancy precautions are unnecessary.
If the grinder safety shield does not form any kind of loop around the cracked area, chances are you can just fix it and let it cool.
One thing that has not been mentioned is peening. If you weld in small increments, and peen the repair as it cools, you will relieve much of the stress and reduce the likelihood of cracking.