We don't really know what the carbon content is though, do we? "Weldable steel" can be anything, and the chinese factories that make "weldable steel" most likely melt down anything that stuck to the magnet and sell it back to us.
Don't get me wrong, I'm not saying it will harden. I'm saying we don't know if it will. Chances certainly are not in our favor, but I'm willing to let him at least try. We learn more from experience than by accepting someone else's word.