Manhole covers are hard ... but as noted, they are brittle. I believe they are castings, not hot rolled steel. In fact, I don't think they are steel at all, I believe they are cast iron, which has excellent compressive strength but is VERY brittle and has little tensile or bending strength.dasmi said:I know that 7.62x54R will penetrate a steel man hole cover at about 50 yards. Granted, it was old and brittle, but it went clean through, and blew a nice big chunk away as it exited. You'll want something thick, and hard.
Lucky said:Hey hey to harden steel, can you go over it with a torch? What heat, roughly, and what fuel, O2+mapp or acety?