Mild steel will discolor in air well below it's critical temperature. In a 10cm weld, your cup may only be shielding 2-3cm of it. The weld just outside that shielding may still be 400*C, and will turn blue in exposure to air. Normal. Harmless. The choices are to wire-brush it, get a bigger cup, or ...

Otto Nobedder wrote:Blue on mild steel is normal.

Welcome to the neighborhood.

Steve S

thank you
What do you mean it is normal ? if i weld like 10 cm there is something like dark blue crust and only if I wirebrush it, it looks ok

P9200012.JPG Other than the learning curve, I have never really had much of a problem getting both structurally sound and reasonably aesthetically pleasing welds in the past. After some pretty major recent surgery, I headed out to the shop today, just to run a few strands of weld & help get my ...