Reply To: Prototype Plane
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George Paul said in his blog that after been in oven he cools blade down in warm water.
Copy of a bit of his blog below.
To harden the iron I used charcoal in a charcoal barbecue pit, but you could use a coal fire or a blow torch of some kind. A little extra air from a hair dryer increases the temperature quickly (No, not the heat from the dryer, the forced air) and soon the steel reaches the cherry red I need to quench it. Not all steels are created equal and depending on the steel you are using, it may take a hardness straight away or not. I have never had a problem but it can happen. I heat until the steel reaches cherry red and then plunge into used engine oil. To test for hardness, try a file on the steel. If it glides of it is now fully hardened. Now I temper the steel in the oven by leaving it in there at 300-degrees for an hour. Remove it and cool in warmish water and the steel should be hard but sharpenable.