Most people think S235JR is so mild that preheating never matters. Not quite. While this low‑carbon steel rarely cracks under normal conditions, there are three situations where a little warmth goes a long way.
First, plate thickness over 20–25 mm. Thicker sections cool faster, trapping hydrogen and causing cold cracks. Second, shop temperature below 5 °C – chilly steel makes the weld chill too quickly. Third, highly restrained joints like corner welds or repair work.
What temperature? Aim for 50–100 °C. No need to blast it. Use a contact thermometer or Tempilstick. Heat evenly about 75 mm on each side of the joint. A propane torch or induction coil works fine. Keep interpass temperature in the same range.
One mistake: overheating. Above 200 °C does nothing good – it can even coarsen the grain. Another: preheating only the surface. Make sure the heat soaks through.
For most shop welding on thin S235JR (under 20 mm) at room temperature, just weld. Preheating is a tool, not a rule. Use it only when needed, and your weld will stay crack‑free.
