Preheat Temperature
The temperature to which the base metal is heated prior to welding, aimed at reducing cooling rates and preventing cold cracking.
Preheating is basically a simple principle: bring the material to welding temperature slowly and evenly, then let it cool down slowly too. This prevents the workpiece from contracting too much during rapid heating or cooling and building up stresses – which otherwise lead to cracks. If you preheat too little, especially with thick parts or cold weather, you get a hard, brittle weld area. But if you get too hot, the structure can coarsen and the material becomes brittle or prone to distortion. How much you need to preheat depends on the material and wall thickness. As a rule of thumb: the thicker the material and the higher the carbon content, the more preheat you need. It's important that the heat is distributed evenly over the entire weld zone, not just spot-wise.