To harden mild steel or iron we need to first carborize it in a few steps
Powder all ingredients
6 parts charcoal
4 parts table salt
3 parts wheat flower
Enough water to make a paste
Coat steel to be hardened with generous layer of char-paste and allow to fully dry. It should resemble a corn dog.
When dry apply a 1/4 inch jacket of clay and allow to set up and then fire red hot for 20-80 minutes. Depending on the depth of desired carborization you may need to expiriment on heat times.
Once complete quickly remove from forge and gently break away the clay and quench in your desired quenching solution.
Now you have a blade with a core of mild steel or A36 and an outside layer of hard crystalline martensite or tool steel.
This is an ancient and fool proof way to create high carbon steel with a variable depth of penetration. You can fully harden the steel all the way through or just the surface based on how long you hold a red hot temp.