I have a small, cute C-clamp that I bought at a garage sale. The owner must have made a custom fixture or something, since there was a hole drilled in the frame. When I clamped something, the frame cracked at the hole, and the clamp bent. I bent it back, but it was was flimsy due to the fatigue. Cast iron cannot take much bending. I had heard about cast iron welding rod, and wanted to repair it, but it seems to be hard to find. Somebody was selling a few sticks on Ebay for $80. Too much. So the lousy clamp went into the to-do box until I found a shard of cast iron from a shattered drain cover in the street. I used an oxy-propane torch, and it flowed in like "buttah". It was a good strong repair, but the decarburized zone became susceptible to rust. Another one of those "stupid metal tricks".
Anyway, cast iron cannot take much bending, heat or no heat. A forged steel clamp is much more forgiving.