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