(I apologize if I come across as a "young know-it-all". I prefer to think that I'm a "young know-enough-to-know-that-I-need-to-know-more." ;D )
@trikebuilder57 I agree that blades come more hardened at the edges than the middle, but I respectfully think that you might have the rest backwards: the centers of stock blades are left unhardened to maintain flexibility because they don't need to be hardened.
Applying heat to hardened steel to above "grey temper" over-tempers the steel, making it softer and less brittle. Since my MIG welder brought the steel to above critical temperature, any hardening in the area of the welds would have been completely wiped away ('normalized'). The post-heat would have slowed the cooling process, so that even if the steel was "air hardening" (which I sort of doubt), it would not have re-hardened.
This is partially good, in that the steel became and remained flexible enough not to crack during the rapid temperature shock of welding and dis-similar expansion/contraction rates (both in regards to the differing temperatures in close proximity; and between the harder unknown tool steel and mild steel in my welds.)
That's not so great for trying to maintain a hard cutting edge that won't immediately dull during use.