Elmax, tempering low or high

V

Van Zanten

Joined
Sep 20, 2011
Messages
104
Hi everybody,

I have ordered some 2,5mm Elmax to make a couple of kitchen knives. The question is about the heat treat. This is my recipe:

  • Preheat the oven at 1080 degrees Celsius, put the blade (wrapped in foil) in, austenitize for 30 minutes
  • Quench in a special straightening tool, in the foil, in peanut oil @ 50 degrees Celsius
  • Cleaning
  • Submerging in liquid nitrogen for 3 hours
  • High tempering at 520 degrees for two hours to precipitate the secondary carbides
  • Aircool
  • Second temper at 350 degrees Celsius for 2 hours

What would you guys prefer for a kitchen knife, a low temper at 200 degrees or the high temper with the benefit of secondary carbides precipitation?

p36z.png

Look at the hardness increasing at 450 degrees Celsius


Kind regards,

Johan van Zanten
 
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Peanut oil would be way too fast....use aluminum quench plates.

Dunking in liquid nitrogen is not "cryo". Cryo done properly involves reducing temp very slowly till
the temp of liquid nitrogen is reached...holding ....then increasing the steel slowly till room temp
is reached.

Use the lower tempering temps. Tempering in the secondary hardening range will reduce
corrosion resistance.
 
Thank you Russ,

I worry about the quench plates not cooling concave sections fast enough. I don't do flat grinds... Would that be a problem? Also, the foil will slow the oil quench down...

Can you give me practical advice on how to do the cryo? I have a canister style dewar:

jspf.jpg


Ok, so the corrosion resistance is better at the lower tempering temp. But what about the increased toughness at the higher temps (regarding a kitchen knife). I think the hardness will be very similar, low vs high temper.

I have found this thread about how to perform the cryo in the meanwhile: http://knifenetwork.com/forum/showthread.php?t=21121
 
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In my opinion, the best course would be to send them to Peters Heat Treating.
I've used some Elmax, and that's where I send it. It comes back clean, straight, and
60-62Rc.

Aluminum quench plates will be faster than air quenching, more controlled than oil, and provide
some assurance against warpage...if the blades are of uniform thickness over most of their length.
When I use quench plates, I leave most of the grinding for after HT.

A high temper may result in better toughness....but how tough does a chef's knife need to be.
Edge retention and corrosion resistance would be a higher priority, I think.
 
The higher tempering temperature in the secondary hardening hump does not produce higher toughness. Both toughness and corrosion resistance decrease in this tempering range.
 
I won't e helpful with any information, but that knife is beautiful. Any other photos?
 
Thanks for the help guys. I'll be talking to my teacher in metallurgy soon about the problem, would like to know more precise why to choose a low temper.

I want to do the cryo myself. I hope to get some information about the right way to do it.

@ PT Doc, thank you! [removed] here you can see some more about the knife! You can also click januari, mei, and augustus.
 
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Me2 is correct - when tempering @ the approximate peak of the secondary hardening curve both corrosion resistance and toughness are sacrificed.
 
Let me take a stab at it, until you can ask a real metallurgy teacher. The precipitation of the chromium carbides in the high temper range is a common issue in hardenable (martensitic) stainless steels. When the CrC carbides precipitate, they pull Cr out of the matrix of the steel and bind it up with carbon. Cr so bound cannot aid corrosion resistance. So you have a carbide with an area around it depleted of chromium, which leads to corrosion issues. Unfortunately, one of the places the carbides precipitate is at the grain boundaries, between each grain of steel. What you have now is a very small amount of chromium carbide, but it just happens to go around each grain. Now you have a very nice path in a very brittle carbide for any crack to follow once it starts. Thus, you have a decrease in corrosion resistance and in toughness both. Choosing a low temper range avoids all this mess.

The problem is you also have increased retained austenite at lower tempering temperatures, so you either deal with increased toughness with a very soft phase sprinkled into your steel (retained austenite is very soft), or find a way to get rid of the retained austenite other than high tempering temperatures. On cue, cryogenic or cold treatment enters from stage left. This reduces and might be able to eliminate all the retained austenite. Generally speaking, if you can get %RA (retained austenite) down to 5% or less, you're doing really well for these high alloy stainless steels.

There are other reasons for a low temper, but they are a bit more complex, so I'll have to leave that alone.
 
Thank you Tsdevanna and me2 for this explanation.

Me2: I understand what you mean. So people who do a high temper do this because they want to get rid of RA, and they might not be able to do a cryo? The thing is, I already had a feeling a low temper would be better, but somebody at Uddeholm told me to do a high temper. I will call him about it and tell him about your advise.

Something else, do you think the oil quench in foil would be too fast for elmax, (forget about warping, I have a fixture for that).
 
Just talked to the Uddeholm guy. He said the advice you gave me is correct. He mentioned wear resistance and dimensional stability increases at a high temp. I don't need dimensional stability... About edge retention, wear resistance and toughness are both great... what to choose ;-)

So far, low tempering seems better. What temperature? 2 times 2 hours at 250? 220? (degrees Celsius)
 
Thats one reason to do high tempers. Years (decades) ago, knifemakers generally were not aware of the difference and just followed the manufacturer specs. The trouble was steels like ATS 34 were popular. Its intended for high temperature use, so knifemakers were using the high temper. The high temper does increase wear resistance, so you need to know what properites you want.

I think peanut oil would be too fast in general. The plate quench is surprisingly fast, or you could get the slowest quenching oil you can find. In theory, a faster quench will leave you with less RA to deal with, but practically it may not make much difference. All I can say there is to try both and see which you like better. It would not surprise me to learn plate quenching is faster than a really slow oil.
 
For a steel like this, what I'd recommend will seem far out there and is based on work by Roman Landes and other research, so take it for what you paid for it. I'd say use a middle or lower austenizing temperature, slow oil or chilled plate quench, then low temp treatment, either dry ice or LN2, -100 F or below, followed by a 350 F temper, water quench, and repeat at 390 F or lower based on desired hardness which I'd want 60 HRc or better, again followed by water quenching. That hardness may require an austenizing temperature on the higher side, now that I think of it.
 
The waterquench after the first temper is a good idea, i'm not so sure about the second water quench. It might convert some RA to untempered martensite.

I'd want 60 HRc as well, I prefer my blades on the harder side, besides that, 59/58 for a high end kitchen knife seems pretty low. 350 F, 390F seems real nice, it's what I do for O2. Ain't it a little low for stainless?

1080 austenizing temp?
 
The water quenches aren't for converting RA. They avoid some precipitation reactions after tempering that may lower toughness. If the low temperatures and first temper and water quench didn't do it, the last one won't either IME. The austenizing temperature is tricky. A lower temperature means less RA to deal with in the first place, but that's because there is less carbon in solution. The chart shows only 4% when using 1050 C, which is pretty low to start with. You'd probably be able to get that down to 1% and pick up a couple points in hardness with a low temperature treatment and a relatively fast quench (slow oil or chilled plates or faster), but it still might not make it to 60, which might still be ok. 58-59 that is all tempered martensite is better than 58-60 that is a mixture of tempered martensite and softer phases (pearlite, RA, bainite, ferrite, etc). The more I think about this, the more I don't think I'd like Elmax in a knife.
 
Oh, upon further review, there is a second low temp/cryo cycle after the first temper. I forgot that part. Like I said, take it for what you paid for it.
 
Can you make this more clear? :

They avoid some precipitation reactions after tempering that may lower toughness.
Click to expand...

You would be the first to not like Elmax in a knife I think ;-)

If I may ask, what is your background in heat treatment, metallurgy
 
That is generic information I got from Roman (referenced above). I'll dig into it to see what else I can find. He applies it to HT of most knife steels and says it also helps with issues when using knives in cold weather, though he doesn't mention the mechanism in specifics. It also likely applies more to the high temperature tempers, as I know there are temperature ranges that cause problems if the steel is slow cooled through or held in those ranges, which are naturally different for each steel. I'll see what I can dig up.

As for not liking Elmax, I have particular tastes. What can I say.
 
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