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  #1  
Old 12-18-2016, 03:32 PM
Toni Toni is offline
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Why multiple tempers?

Yeah, title says it all. What's the reason why I see so many recommendations of tempering a knife twice, or sometimes even thrice? Why won't a single temper cut it?
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  #2  
Old 12-18-2016, 04:14 PM
samuraistuart samuraistuart is offline
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Excellent question. I'll cite some material from the Verhoeven PDF, as he is a metallurgist.

1. There are a few things going on during tempering. The first would be strain relief caused by the quench.
2. The first temper also will allow the formation of super small carbides, carbides so small you need an electron microscope to see them.
3. The 2nd temper decomposes retained austenite into ferrite and carbides.
4. The 3rd temper is really for the higher alloy steels, in the higher temperature range, where those super small carbides from round 1 are transformed into cementite.

Most low alloy steels like O1, 52100, 1095 etc are only tempered with 2 cycles, whereas higher alloy like D2 and stainless steels benefit from 3 temper cycles.
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  #3  
Old 12-19-2016, 11:46 AM
Toni Toni is offline
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Quote:
Originally Posted by samuraistuart View Post
3. The 2nd temper decomposes retained austenite into ferrite and carbides.
Now, I might be mistaken, but doesn't retained austenite increase the toughness of an item?
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  #4  
Old 12-30-2016, 03:20 AM
Toni Toni is offline
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Also, how should I figue out the proper tempering temperatures for these different tempers? Usually manufacturer's sheets only refer to the hardness of the temper.

Last edited by Toni; 12-30-2016 at 08:17 AM.
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  #5  
Old 12-30-2016, 08:47 AM
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Ray Rogers Ray Rogers is offline
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In most cases, the same temperature is used for all the temper cycles...


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Old 12-30-2016, 10:34 AM
Toni Toni is offline
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Quote:
Originally Posted by Ray Rogers View Post
In most cases, the same temperature is used for all the temper cycles...
Oh, When I looked at the Verhoeven pdf he said that the different tempering stages use different temperatures?

I'm getting even more confused about this whole tempering thing.

Last edited by Toni; 12-30-2016 at 10:45 AM.
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  #7  
Old 12-30-2016, 11:11 AM
Doug Lester Doug Lester is offline
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I don't have Verhoeven's pdf, I have his book. From what I recall most of the changes caused by tempering occur within two hours so you will do just fine with two one hour cycles. I remember no mention in the book that the cycles should be of different temperatures. This came up on one of the other boards not too long ago and Kevin Cashin chimed in and said that for most steels a single two hour cycle would do too. His opinion was that this three two hour cycles was part of the idea that the more the better.

Doug


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Old 12-30-2016, 11:27 AM
samuraistuart samuraistuart is offline
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Toni, retained austenite is not a desirable structure in a blade. Yes, it can facilitate more impact toughness, but this is my understanding....If you take a knife that has significant RA % and chop into "whatever", the damage may be a bit less with the RA present. However, on that spot that impacted the medium being struck, you now have a weak area that will continue to fail to a greater extent if the RA wasn't there to begin with. Sorta like, OK RA is fine to have for impact toughness...but it only works once (on that spot). That is WAY over simplified, and I have no data to support that claim. Just what I have heard.

No, Verhoeven does not say to use different tempering temperatures. If you have a target hardness of 61HRC, then you need to temper at a temperature that will give you 61HRC. The charts are a good place to find that data. Sometimes the charts aren't exact, tho, so you have to "dial in" the temperature to reach the desired HRC number. Example, if you are working with a new steel, and you don't want to overshoot and end up with lower HRC than planned, it is a good idea to "walk in" that temper. Start low. If goal is 61HRC and 61 comes at 400F, but you're not sure and haven't tried before and nailed it down, then it's a good idea to do a lower cycle first, say 375F or even 350F, and walk it up.

There are steels that need higher tempering temperatures, I say "need" but that's not entirely accurate. This is for the secondary hardening of particular alloys like D2 and stainless. However, the concept is still the same. Pick the temperature that gives you the desired HRC number, and stick with that temperature. Do 2 or 3 cycles AT that temperature....unless, again, you need to "dial it in".

Note the Japanese with their "low alloy" carbon steels. They temper once, often only a few minutes. But good to note most often their knives are kitchen knives, run very hard, and are strictly kitchen knives with the thin grinds. Not a knife you'd wack with.
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  #9  
Old 12-30-2016, 01:04 PM
Toni Toni is offline
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Thanks to both of you for clearing these things up! Now, I trust and believe what you say as you have a lot more experience about these things than I do, but I'll just copy this off of verhoeven pdf's page 101:

"During tempering the martensite begins to decompose into ferrite and carbides. At the
lowest tempering temperatures very fine metastable carbides (usually epsilon carbide, Fe2.4C)
form (called stage 1 tempering) and at higher temperatures they are replaced by the usual
carbide, cementite, Fe3C, (stage 3). At intermediate temperatures, if retained austenite is
present it will decompose (stage 2)."
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  #10  
Old 12-30-2016, 01:47 PM
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Ray Rogers Ray Rogers is offline
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That quote doesn't say to use three different tempering temperatures on a single knife as I read it. It merely says that you get different formations according to which temperature you choose to use for tempering out of the acceptable range of temperatures. The spec sheet will show you a range of temperatures for your steel and indicate the relative hardness/toughness characteristics that result from using that temperature. It doesn't say you have to use all of those temps on one job ...


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  #11  
Old 12-30-2016, 01:54 PM
Toni Toni is offline
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Ah, that clears it up. Thank you!
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