They are usually hardened steel.
I got baked before a timing setting session once. Forgot to remove the flywheel nut and went ahead and threaded the puller on. After muscling it with a breaker bar for oh, like 20 minutes, it finally stripped all of the threads out of my flywheel. I remember thinking, "This flywheel is on there really tight. WTF!" I even enlisted a few other guys in the shop to try to get this flywheel off. Haha.