Most welding procedures including AWS only require a minimum of 50 degrees F preheat for mild steel (A-36 plate and similar) and that is only to remove any residual moisture upon initial arc strike. Higher preheat on low carbon steel adds no value to the toughness. Chrome alloys require preheat to avoid hardening of the metal and thus lowering ductility. Some of the high manganese and high carbon steels also need preheating to around 200F to prevent root pass cracking. Most of the metals used in fabrication are A 36 grade and just dont require anything above 50 F to start welding on them. AWS welding procedures dont allow use of any 60xx series rods, only 70xx and higher.