Preheating Carbon Steel Plate Before Welding: When It Is Really Needed
Ordinary structural grades such as SS400, S235JR and Q235B are low-carbon steels with good weldability, so thin sections, up to about 20 mm, normally weld without preheat in mild weather. The cooling rate after the arc is simply not fast enough to form hard, crack-sensitive microstructures.
Preheat becomes necessary as thickness grows. From around 25 to 30 mm and up, a preheat of roughly 50 to 100 °C before welding is a sensible default, because thick plate pulls heat out of the weld zone and cools it too quickly. Restrained joints, tack welds that are fully welded over, and thick-to-thick connections behave like even thicker plate.
Cold weather changes the arithmetic too. When the plate temperature is below about 5 °C, most fabricators preheat even thin material, if only to drive off condensation before the arc strikes. Welding over a damp surface is a reliable way to get porosity.
One more rule that costs nothing: interpass temperature matters as much as preheat. Keep the joint between the minimum preheat and about 250 °C while welding, and let it cool slowly. That does more for crack-free welds than any single magic temperature.
We supply SS400, S235JR and Q235B steel plate in all thicknesses, with mill certificates showing the chemistry you need for your welding procedure.