Probably a bit late, now that it is fitted to the stock, but I'd be inclined to throw something like that the wood stove overnight. If that doesn't anneal it, I don't know what will...Not you are likely to be using a woodstove in July, of course.
Not to open a can of worms, but if the tang is that hard, I'd be a bit worried about what the plug itself was like. A hard, potentially brittle tang is just a pain in the neck, but a hard, brittle plug could be a potential safety issue. Might be worth checking.