Question about heat treating and annealing

[johnnyKnives]

If I take a piece of metal and anneal it I make it softer. When I'm done I want to make it harder again. I put it back in the fire.

What's the difference since they're both in the the fire heating to a critical temperature? Is it because of the quenching in the heat treating versus the slow cool down in the annealing? Thanks.

 

[Jason Fry]

Short answer is "yes." The rate of cooling is one of the key components in your question. For the long answer, read the stickies.

 

[Stacy E. Apelt - Bladesmith]

There are many posts on the subject of metallurgy, including some in the stickys. This information should be studied and understood before starting a knife.

Short and non-technical explanation:
When you anneal steel, you heat it until the structure becomes austenite, around 1400F, and then slowly cool it so the structure converts into pearlite, starting around 1000F.

When you harden steel you heat it until it becomes austenite and super-cool it ( the quench) avoiding the pearlite range and taking it to the martensitic start point. At that temperature ,around 450F, the steel converts into martensite.....which is what knife blades should be.

The martensite will be very brittle until it is tempered by raising it to near the martensitic start point again and allowing the structure to slightly convert into pearlite. This is usually done between 350F and 500F for knife blades.