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Tempering a blade multiple times
#1
I have a few basic questions on tempering. 
 
What happens to hardness if a blade is tempered more than once? 
 
How is hardness is changed if a blade is tempered to 450° the first time and then, say, 300° the next time?
 
Or, conversely, 400° the first time and 600° the second?
 
And lastly, how long does a blade have to be at tempering temperature to make a difference?  What if the blade only reaches tempering temperature for a few seconds and then was allowed to air cool?
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#2
What happens to hardness if a blade is tempered more than once?

That depends on temperature and time tempering 2x 20min @ 180°C should result in the same hardness as tempering once for 40min @ 180°C

And lastly, how long does a blade have to be at tempering temperature to make a difference? What if the blade only reaches tempering temperature for a few seconds and then was allowed to air cool?

Hollomon–Jaffe parameter is propablywhat you should look at although I do not know where you would get the steel specific parameter at the moment.

https://en.wikipedia.org/wiki/Hollomon%E..._parameter
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#3
This all raises a lot of questions for me. Not earth shattering questions but curiosity questions. Say a blade is tempered at 450 degees and turns out to be Rockwell 58. So if it's tempered again at 350 degrees will it be Rockwell 60 or at least harder than it was before? How about if you go the other direction? I know that all these steels have different temperature reactions but hopefully you get my drift.
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#4
Once it is tempered at 450 you can’t temper at a lower temperature to make it harder. Tempering allows the steel to reach a more “relaxed” equilibrium state. It wouldn’t go backwards to a less relaxed state.
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#5
Thank you very much Mr. Larrin! That's what I was looking for and makes sense to boot. Its not like you can take your favorite kitchen knife and your kitchen oven and dial in whatever hardness level you want. So I take it then that when we talk about over heating edges and softening the steel that we are talking about more like much higher temperatures and not tempering temperatures.
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#6
Overheating can be from overtempeing like exceeding the temperature of the original temper all the way up to “burning” away steel.
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#7
(10-25-2018, 11:14 PM)Bud Wrote: So I take it then that when we talk about over heating edges and softening the steel that we are talking about more like much higher temperatures and not tempering temperatures.

Well Mr. Bud, I seem  to be hearing the same story of much higher temperature generation, so I'm glad I'm not alone. 

Tempering a fully hardened blade is usually done twice, for two hours each time. Temperature is the variable. Simple carbon steel is tempered at significantly lower temperature than simple stainless. The very high alloy particle steel commonly used these days could take tempering temperatures as high as 600°F or more.  

The point being most knives these days should be very heat tolerant compared to the simple carbon steels people have been using for hundreds of years. Surprisingly, virtually no one in the world seems to have noticed softened edges of simple carbon steel in all that time. If you consider how much sharpening has been done by people who literally lived or died by the quality of their edges, it's hard to imagine no one jotted down some notes for posterity. 

Anyway, the first temper has a greater affect than the second temper, so my first temper is always at a lower temperature. With 52100 full hardness after cryo is regularly 65-66. A two hour soak at 360 brings hardness down 2-3 points. The second temper at 375 usually leaves me with RHC 61-62. That's pretty hard, which means relatively brittle. A typical blade might only flex 15° before snapping in two. 

If I raise the tempering temperature to 400-425, the hardness would drop to RHC 57-58, but would increase the flexibility/toughness substantially. Perhaps the same blade would flex closer to 25° before breaking. The reason my blades are unbreakable is simple. I temper the spine a third time at a very high heat of around 600-700°F for maybe ten seconds before quickly quenching in oil. This produces spring tempered martensite with RHC 50 on the spine, while preserving HRC 62 on the edge. 

Near as I can tell, the overheated edge theory we are hearing so much about doesn't seem to be based on a normal source of heat. I don't understand anything about extra heat coming into the picture from molecular degeneration. It sounds interesting, kind of like how splitting an atom releases energy, but I don't think that grinding steel releases any energy at all.  

If there is anything to this theory of spontaneous high  heat energy generation due to  molecular separation or however you describe it, wouldn't someone be harnessing this energy resource?
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#8
Thanks Mr. Mark. Only a few months ago I didn't even know the difference between tempering and annealing. I've never done any of it, but thanks to educated folks like yourself I think I now understand the basics.
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#9
I'm happy to be mildly useful occasionally.
Smile
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#10
"I don't understand anything about extra heat coming into the picture from molecular degeneration. It sounds interesting...".

I agree Mr. Mark.  It is interesting and makes for great endless conversation on knife sharpening forums.  I think the endless verbiage probably comes from, among other reasons, folks wanting to do a good job.   What's missing in the conversation is that it most likely has imperceptible impact on basic knife sharpening.

And like you have been saying all along, all of our knives are power ground sharp when they are born.
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