When I bought my sprocket from Surplus Center they sent me one with hardened teeth. I didn't realize at first so I dulled a few lathe bits trying to cut it. I learned that the discolored teeth are a sign of induction hardening of the metal. I couldn't scratch the metal with a file, which means it's too hard for normal machining. But, all was not lost. I was able to anneal the sprocket so that it was workable. Here's what I did. I heated the sprocket up with a oxy-acetylene rosebud tip. It was nice and hot. I then placed it into a shoebox of vermiculite for slow overnight cooling. The next morning, I was able to file and machine it normally. When I ordered, there was no way for me to know that the sprocket was hardened.

I learned a bit of metallurgy and got a nice looking sprocket in the meantime. Not too bad.