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You could also wash a light clay coating over the blade to shield it from oxygen.

Hardening is the process of taking a piece of steel to the austenizing temperature, and then quenching it in a coolant.
At that point, there are still residual stresses left in the steel, and it can...

Oil or water depends on the steel.
In general if you have a piece of steel and you don't know how to treat it, first heat a sample to non magnetic and quench in oil.
If you can clamp it in a vise...

That is Waaaaay too hot.
A dull red is what you are shooting for. Reduce the airflow to decrease the heat. that will also give the blade a better chance to heat evenly.
And get a strong magnet. You...