Only if you "blue grinded" it.
If the steel turns blue, the hardening are ruined.
So cool it often, an dont overheat.
And yes, the can be re-hardened!

The easy way of hardenig is to heat the blade dull red. and throw it in a
can of water.

"EW" skrev i melding
...
I'm assuming that new lawn mower blades are sharpened, then hardened
before
sold. Question: after sharpening the blades on a bench grinder (after
much
use) is it necessary to harden them for longer life, and if so, how?
Thanks.

EW