Strange Cryo on AEB-L Anomaly

Sando

Sando

Knife Maker
Joined
Jul 4, 2002
Messages
1,148
It just happened again and it's happened on all my AEB-L tests. I'm sure there's an explanation, but heck if I know what.

After I remove the AEB-L from the liquid nitrogen the hardness reads 60/61. That's just too low. But if I put it in an oven for just a few minutes at 300-400, the hardness will jump up to 62/63!?

That's where it's supposed to be, but why do I have to warm it to get the cryo to 'stick'?

So far I've repeated this with 4 test coupons and 2 blades. It's always the same thing when it comes out of the LN and into the oven. Here's from today's log:

  1. Put in 1725 degree oven - Wait 20 minutes
  2. Plate quench
  3. Wait 30 minutes
  4. Put in 1950 degree oven
  5. Let oven ramp to 1990 degrees (takes 5 minutes)
  6. Wait 20 minutes
  7. Plate quench
  8. 59hrc
  9. Liquid Nitrogen - Wait 2 hrs
  10. 61hrc
  11. 350 oven - wait 2 minutes
  12. 63hrc
The increase from steps 10 to 12 is just bizarre to me.

I've tested with and without the pre-quench and leaving it in LN for 10 hours and austenitizing from 1975 to 1990. I have experience with 154CM, ATS34, S30V, and Elmax. None of those steels do this and AEB-L always does for me.

Any explanations?
 
I’m just taking a guess, as I’m no expert. I suspect you have some retained austenite (in spite of the cryo) due to the 20min soak. You only need 5 min at that temp. The RA converts to martensite during temper. Now I’ll let the real experts DevinT and Larrin chime in.
 
I'm so GLAD you posted this. It mirrors my results to the T! IDK what the heck is happening. I've actually seen this with "440C" stock that makes me think it was AEB-L! I thought my heat treat oven was mussed up to the point I retested it with temp sticks. Ran tests with my RC hardness tester with certified test blocks. Which all came back perfect!

I've seen a post from JT who posted similar results once I think. Hope he weighs in!
 
Last edited:
At the -300F temp, the steel is just super cold martensite. There are some changes that slowly occur. It is upon returning to room temp and often to tempering temp, that the eta carbides precipitate ... which gains a couple points of hardness.
 
Last edited:
jdm61 said:
Don't argue with success? :D
Click to expand...

Hahahaha: Sound advice. I probably shouldn't even ask the question. It'll infuriate the god of Steel Transformations.

Stacy E. Apelt - Bladesmith said:
At the -300F temp, the steel is just super cold martensite. There are some changes that slowly occur. It is upon returning to room temp and often to tempering temp, that the eta carbides precipitate ... which gains a couple points of hardness.
Click to expand...

Thanks Stacy. That does leave me with 3 more questions:

  1. Why do other stainless steels harden up at room temperature after cryo, but AEB-L needs to get hot?
  2. If the cryo-driven transformation happens on reheat, is a long LN soak ever necessary?
  3. Would it be helpful to always pull a cryo-ed blade out of the tempering oven after a few minutes and cool, before the full tempering?
RE Question 1: Actually, I don't have proof other steels don't get even harder when entering tempering temp. I've never pulled one out quick enough to see.
 
Last edited by a moderator:
Read Larin's articles on cryo for a better explanation. My short answer was very simplified. What I was saying was that the steel will harden more as it returns to ambient. Some steels may need to get above 200F to fully have the extra hardness.

Things happen slowly at cryo temps, so you need several hours for the changes in the carbides to occur. The precipitation of the eta carbides is faster, and happens as the steel gets warmer.
 
I have posted about this in the past. I don’t even test hardness befor the first tempering. AEBL is very sensitive befor the first temper. Funny thing is no matter what the austenite temp is the hardness will settle down to about the same number as you temper hotter. Let me see if I can find test chart I made. Cryo will bump the hardness a point above what I have in the chart.

Photo%20Jun%2016%2C%201%2012%2013%20AM.jpg
 
JTknives JTknives Thanks! That's exactly what I needed to see. I don't feel crazy. Actually, I was afraid I was doing something drastically wrong. Given this new information I can see why you wait until after the first temper for a real hardness test. Sorry if this was an old question, but there's just so much data here.

Larrin Larrin I'm gonna reread your article, thanks. Tempering has always been a bigger mystery to me than hardening. It'll be good to understand it better.
 
JT I'd love to know if you let soak longer at 1900 then say 1975? Example 20 minutes at 1900 and 10 minutes at 1975.
 
All thoes tests where done with the same soak time. If you research the documents from the manufacture of this steel you will find that most people way over soak it time wise. I have a very strict schedule for heat treating AEBL that a fallow to a T. It is the product of a lot of testing and trial and error. It’s strict enough that it dictates how it’s processed from the moment we receive the blade. Some people might call it over kill or nit picking but it’s works and I don’t get warped blades anymore. And all my customers have nothing but praise to say.

So it takes time but it’s possible to really nail down a process for AEBL. Now AEBL is one of my favorite steels to heat treat.
 
JTknives said:
All thoes tests where done with the same soak time. If you research the documents from the manufacture of this steel you will find that most people way over soak it time wise. I have a very strict schedule for heat treating AEBL that a fallow to a T. It is the product of a lot of testing and trial and error. It’s strict enough that it dictates how it’s processed from the moment we receive the blade. Some people might call it over kill or nit picking but it’s works and I don’t get warped blades anymore. And all my customers have nothing but praise to say.

So it takes time but it’s possible to really nail down a process for AEBL. Now AEBL is one of my favorite steels to heat treat.
Click to expand...
How do you recommend heat treating like 6 knives at a time? If I soak for 5min buy the time I get to the 6th knifes is been soaking for 20min.
 
I don’t i detstand the obsession with packing the oven full of knives. Espicaly on steels like AEBL with such a short soak time.
 
JTknives said:
I have posted about this in the past. I don’t even test hardness befor the first tempering. AEBL is very sensitive befor the first temper. Funny thing is no matter what the austenite temp is the hardness will settle down to about the same number as you temper hotter. Let me see if I can find test chart I made. Cryo will bump the hardness a point above what I have in the chart.

Photo%20Jun%2016%2C%201%2012%2013%20AM.jpg
Click to expand...

JTknives JTknives , so one should always temper first at 300, in order to find out where you’re starting from in terms of hardness, then adjust the second temper to the desired target temp for the final hardness. Is this correct?

Thanks in advance,
Constantin
 
I was having a hard time getting past 62HRC with AEB-L until JT made his post above and Larrin followed it up with his info. I was tempering too high so I dropped my tempers to 300 and got the numbers I was trying for.
 
You must log in or register to reply here.
Back
Top