Tempering is described as improving the hardness and elasticity of (steel or other metal) by reheating and then cooling it.
For the 108X and 1095 metals, I do 2 cycles at 425 degrees for 2 hours each. When I started, I did 3 cycles, but after some testing, I think the twice is fine. At the end of two hours (I set the timer on my phone, so I don’t forget) I just turn the oven off and let everything cool back to room temperature after each cycle. What is most important is that the blade get below 100F after any temper cycle.
The Time and number of cycles does not affect the hardness as much as temperature. The important factor is temperature. The number of cycles does not affect it nearly as much as temperature as well. If you temper two times at 400 and put it in a 3rd time, the third temper will drop it again, but the amount will be small enough that for most practical purposes it can be ignored.
If you forget and leave it in temper for 6 hours instead of one, that will also drop it, but again for most practical purposes it can be ignored.
Two – one-hour tempers or three two hours tempers will yield different hardness, but the difference will most likely only be one point (which is in of itself is a normal variation from piece to piece and hardness tester to hardness tester)
To affect tempering, the piece must be above approximately 300 degrees, so light heating will not affect it.
More on tempering in the heat treating post
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