Before welding insert plates to reinforcing bars, preheating is sometimes needed to help reduce cracking at the welded joint. But preheating is costly and complicates the welding process.
Required preheat depends on carbon equivalent of the steel (a measure of weldability) and size of the rebar. Steels with high carbon equivalent values are less weldable and require more preheat. Most mill certificates for reinforcing steel don't supply all the chemical composition data needed to calculate carbon equivalent. When the carbon equivalent is not known, welding codes dictate the highest required preheat be used. To avoid needless preheating expense, request a complete chemical breakdown from the rebar supplier so that the carbon equivalent can be calculated.
In most climates, insert plates less than 1 1/2 inches thick are weldable without preheating. However, don't weld when the ambient temperature is below freezing.