Small knives harden to much in air

[Bluemountain]

I tried searching for my question, but I can't seem to find anything.

I'm a beginner, this is my first knife and I decided to use a leaf spring to make a tiny 3 inch long by 1 inch wide knife to begin with. The shape is good, I heated it up and left it to air cool and after a few minutes I tried to file it down with a bastard file, but the file skated. I have no idea why it hardened, so I heated it up again and tried putting it in the dirt to cool and still the file skated. I tested the leaf spring and it doesn't skate a file, anything I'm missing? 

[ThomasPowers]

Yes; the "official" heat treating specification documents; like the ASM handbooks, are based on a cross sectional area of 1" .  With something as thin as a blade they may chill down fast enough to harden with a more gentle quench (so water hardening steels *might* quench in oil and oil hardening steels *might* quench in air!)

Also to anneal small things you need enough mass to keep the heat hot a while, so if you want to soften the blade, heat a large piece of steel to go into the DRY Sifted Wood Ashes with the blade as a "heat donor".  I get around this by annealing blades in my propane forge by allowing them to come up to temp at the end of the day and then turning the forge off and closing the front door to hold the heat in.

Some alloys are more sensitive than others. (Some alloys getting a full anneal is pretty much impossible!)