Key Points for Pre treatment of S235JR Structural Steel

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.