Why would anyone want to harden lawnmower blades? They are meant to
cut very soft material (grass) by hitting it at high speed and
splitting the stems. Hardnedd of the blade is irrelevant to grass
cutting performance.

But the blades still dull and need to be sharpened.
If the edge was harder, they would not dull as fast.
I have a relatively small yard, and I still need to
sharpen blades every 3-4 cuttings.

cs
If you are serious about "fixing" the wear problem, I would look into
a rod that Lincoln (and probably others, too) have on the market,
which I believe is called "Abrasoweld" or something in that order.
It's used to "build up" plowshares, etc., that have contact with the
earth. It resists wear from the dirt. You could lay a bead of that
on each cutting edge, then sharpen and it probably would last a while
longer. HTH
Ken.