CPM154 cryo, temper, heat treat questions

G

gshamr

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Jan 18, 2002
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I have multiple questions that will apply to CPM154 that I will heat treat and home cryo soon, but these are general questions that could apply to A-2, D-2 or CPM154. I have bar stock of all of these steels. I hope to get an answer or two from one of the many metal gurus on this forum, such as Mete, Nathan the Machinist, or Stacey Apelt. I say that because they answer with specific supporting info that I can understand. Every time I think there is a general consensus on something, a successful knifemaker posts something to the contrary that I think is overkill.

1) Is it really necessary to cryo over 4 hours? I suspect the answers will be "probably not, but it's hard to confirm one way or the other".

2) Is it necessary to hold a blade at a pre-heat temp if you put it in a cold oven and it takes at least 40 min to bring to austenizing temp?

3) Is it ever necessary to soak at austenizing temp for more than 45 min?

4) If you don't cryo D-2, but want to temper it 3 times to convert as much austenite as possible, should they all be a minimum of 2 hours?

5) Does acetone in a slurry at -100 degrees and in a cooler really pose any risk of combustion...short of dropping a match or spark into it?

Thanks for any replies.
Gerry Hamrick
 
I don't know about CPM 154, but, I do know some about D2

I have been placing the knives in a cold oven and having it go to 1850 f holding for 45 min +. I have ordered a programmable PID and will be adding the hold temp to my cycle. But, the main reason I am getting a programmable is for annealing, not hardening.

I use 2+to 4 hours in acetone/dry ice. I am never in a hurry here and just find something else to do. I have not been able to detect any difference in performance between the short and the longer, but, whats the hurry. I use a bucket that is wrapped in fiberglass insulation, cover it with another piece of insulation and it stays cold a long time. I always try to do small batches of knives when doing HT and cyro to save on cost of dry ice.

With the dry ice in the acetone I doubt it gives off enough vapor for combustion, next time I do it I will make a very small batch outside and try it. But, I do treat it with caution and keep it outside. I also store it outside in a cabinet that I keep my gas cans and other highly flammable stuff in. Everyone should have a storage area for these items which is outside and detached from their main structures. Why have a something like this in or attached to your shop. In the event of a fire it makes a bad situation impossible. I use a large metal job box like one of these
http://www.badgerladder.com/products.aspx?CatID=9

I then temper twice at 425 f for 2 hr each.

I send one of my blades to Nathan the Machinist and he used in in cutting test against several others and he said it preformed with the best. He sent it back without resharpening after the testing and it would still shave hair.

I did do a large recurved chopper from D2 which I did NOT cyro. I did temper 3 times at 425 for 2 hours each and then draw the spine to 850 f. It preformed very well until I sold it and I have not heard any complaints from the new owner and it has been well over 6 months.
 
Thanks for the info. I have no problem with 4 hours in cryo but 8 hours would require me to do my heating, cryo and tempering over a two day period. Not a big deal but I see no reason to do it that way if I'm just wasting time.

The issue with acetone combustion is because I live in a condo and don't have a garage. I don't want to leave my cooler outside when I'm not watching it because we have problems with local thieves and goofballs here.

Do you have any idea who you would take your JS test under?
 
Can anyone tell me why Paul Bos and others tempers CPM154 at the higher range when Crucible advises against it? Is there any problem with tempering in the lower range of 450-500 to get 61 rc? It would be easier for me to temper in my kitchen oven than in my kiln. Thanks.
 
Acetone's flash point is about -5*F so I'm guessing if you play it safe and keep it in a ventilated area you should be safe. JUST BE CAREFUL.
 
Thanks, Rusty. I will put it outside if I'm not going anywhere or doing cryo overnight.
 
gshamr said:
Can anyone tell me why Paul Bos and others tempers CPM154 at the higher range when Crucible advises against it? .
Click to expand...

I talked to Paul about that once. The gist of it is, he will HT it however you ask him to. Some folks want the high temp temper so they don't have to worry about overheating it in post-HT grinding. The down side is it lowers edge stability and reduces corrosion resistance. It creates a larger carbide volume at the expense of a more carbon lean martensite and less free chrome. Good stuff for an extruder screw pushing GR PVC - bad stuff for a knife blade (IMO).

I tested Jim's HT against my own D2 and Bob Dozier's and found them all to be very similar in tests on cardboard, leather, and hardwood. And they are all quite a bit better than other benchmarks such as Chris Reeve's S30V and Spyderco VG10, and D2 that I heat treated to HRC 61 that had retained austenite. In that test, Jim's was actually probably the best of the three, but they were all very similar. There is no magic voo-doo. D2, when it is processed to reach it's full potential, is an excellent steel for a knife. I talked to Dozier in Atlanta this summer about his D2 (which does not include cryo) and he was very tight lipped about it. I can not achieve the results that he achieves without cryo. But my HT with cryo (and Jim's, which is the same as mine) equal it in every test. And it isn't rocket science.

To answer your questions

1) Is it really necessary to cryo over 4 hours? I suspect the answers will be "probably not, but it's hard to confirm one way or the other".

I can't tell much difference. In theory, a trip to Mf is not time dependent. But some people theorize there is more to cryo than just Mf and most of us leave it there for a few hours. If you want to leave it there for 30 min - I expect you will achieve the vast majority of the benefit.

2) Is it necessary to hold a blade at a pre-heat temp if you put it in a cold oven and it takes at least 40 min to bring to ausentizing temp?

There are temperature ranges where carbides are forming big clumps in the wrong places that you don't want to dwell at. Ideally you could preheat it then quickly run it to austenitizing temperatures. That is great if you're using two ovens, or one fast oven. In your situation I see little value to preheating it if it is going to be in there on a slow trip to high temp.

3) Is it ever necessary to soak at ausentizing temp for more than 45 min?

No, not for CPM 154. But it shouldn't hurt anything. I like D2 to be 30-45 min.

4) If you don't cryo D-2, but want to temper it 3 times to convert as much austenite as possible, should they all be a minimum of 2 hours?

I don't know how Dozier does it without cryo. But multiple tempers won't convert enough RA in D2 to achieve the edge stability that comes from cryo as part of the quench. I expect 154 will be the same. If you're going to take that approach, go to the freezer after the quench, I'd temper an hour, then cool it in the freezer, then temper an hour, cool in the freezer, then temper two hours. The idea being to convert as much RA as quickly as you can. That is just a swag though...

5) Does acetone in a slurry at -100 degrees and in a cooler really pose any risk of combustion...short of dropping a match or spark into it?

I don't know. I have never pressed my luck on that one. I don't do this near my home or main shop. Be carful with acetone (and gasoline and propane and etc. etc.)
 
Thanks Nathan. That's the kind of info I was looking for.

I wish I had waited to heat treat 2 D2 knives until I read this. I didn't cryo them.

I read a lot of old posts and look at a lot of knifemaking links. Some quote tempering times of 1 hour and some 2.5 hours. I wondered if there was a minimum time to be effective.

I did find something on cryo from a knifemaker on these forums. I believe it was Phil Wilson. He said that a guy from Crucible commented that the majority of cryo benefits appeared to occur in the first hour. It does appear to be a process that is not fully understood.

This is just a vent but, what is the deal with this mutant mill scale on CPM154? It's like ceramic.

Gerry Hamrick
 
gshamr said:
I have multiple questions that will apply to CPM154 that I will heat treat and home cryo soon, but these are general questions that could apply to A-2, D-2 or CPM154. I have bar stock of all of these steels. I hope to get an answer or two from one of the many metal gurus on this forum, such as Mete, Nathan the Machinist, or Stacey Apelt. I say that because they answer with specific supporting info that I can understand. Every time I think there is a general consensus on something, a successful knifemaker posts something to the contrary that I think is overkill.

1) Is it really necessary to cryo over 4 hours? I suspect the answers will be "probably not, but it's hard to confirm one way or the other".

2) Is it necessary to hold a blade at a pre-heat temp if you put it in a cold oven and it takes at least 40 min to bring to austenizing temp?

3) Is it ever necessary to soak at austenizing temp for more than 45 min?

4) If you don't cryo D-2, but want to temper it 3 times to convert as much austenite as possible, should they all be a minimum of 2 hours?

5) Does acetone in a slurry at -100 degrees and in a cooler really pose any risk of combustion...short of dropping a match or spark into it?

Thanks for any replies.
Gerry Hamrick
Click to expand...
1) The change to martensite is nearly instantaneous. 30 minutes is fine. Longer periods are used on the theory that carbides precipitate with long periods.

2)If it's a simple shape and not too thick you can just put it straight in at full austenitizing temperature.

3)Depends on the thickness, material, and autenitizing temperature. Just use the recommended soak time.

4) Not nearly as much retained austenite is converted by tempering as by using cryo.

5) I have no idea.
 
For D2 without cryo, you can try using the high temper to get rid of the retained austenite. 875F temper would give you 58-59 rc with a 1850F austenization, but you can try 1900F and plate quench. I never tried it but maybe it will give you 60-61 rc, worth a try.
 
Thanks for all the valuable info. I feel better since I don't plan to buy a hardness tester anytime soon.
 
Here it is, I found the chart for D2:

AISID2.jpg


1870F Austenization with 875F temper, gives 60 rc with no retained austenite w/o resorting to cryo. I was close!!!

If you have precise temperature control you can use the 1920F austenization and get 62 rc w/o cryo.

I think I'm getting closer to Bob Dozier's secret heat-treatment of D2.
 
gshamr said:
I read a lot of old posts and look at a lot of knifemaking links. Some quote tempering times of 1 hour and some 2.5 hours. I wondered if there was a minimum time to be effective.
Click to expand...

If tempering in an oven, I advocate the use of some sort of muffle or other such device that evens out the ups and downs experienced from the oven elements cycling on and off. I timed tonight how long it takes blades to come up to temp in my muffle and it is about an hour, so an hour at temp makes 2 hours and I go 2.5 just for good measure.

My oven, when I use the muffle, set at 265, gives a maximum temperature of 325 inside the muffle. When the oven is set at 375, I get 400 inside the muffle. Also, When set at 375, the oven temperature can reach nearly 500F and being that knife blades have considerable mass, they will come up to that temp and hold it long enough to make them useless if you really wanted a 375F temper.
 
cotdt said:
Here it is, I found the chart for D2:

AISID2.jpg


1870F Austenization with 875F temper, gives 60 rc with no retained austenite w/o resorting to cryo. I was close!!!

If you have precise temperature control you can use the 1920F austenization and get 62 rc w/o cryo.

I think I'm getting closer to Bob Dozier's secret heat-treatment of D2.
Click to expand...


I haven't had my coffee yet this morning, but looking at that chart I wonder if you and I are reading it differently.

The higher the austenitizing temperature, the more carbon is put into solution, the harder it is going to be to prevent RA. That chart seems to show a lot of RA with D2 heated to 1920 and tempered to HRC62. Am I reading it wrong?

I have proven, in repeated testing, that two pieces of D2, both at HRC 61-62, behave very differently, depending on their RA content. RA is a bad thing in a knife edge. Also, it is my opinion that D2 is better without using the secondary hardening hump - same as 154. This is why I recommend cryo with D2 before any temper at all. Perhaps I need my head examined.

...better go get some coffee...


Edit,

Okay, I had my coffee, I came back and looked at the chart again, I was reading it wrong. Which is kind of sad really, because I've read that chart before... Sigh...

Yes, you can probably austenitize at 1920 and temper at around 1000 F (that will probably remove most of the RA) and achieve almost HRC 62. However, there are disadvantaged to using the secondary hardening hump in D2.
 
Last edited:
WoW!

This started out as kind of a basic newbie question almost and has really turned interesting. FWIW, I don't presoak D2 (in the oven from cold) Austenize at 1850 for 45 min - Plate quench - Overnight in Liquid Nitrogen - and double temper at 400F for a very consistent 60 - 60.5.

I'm watching this thread with great interest and would like to thank all who are contributiing. It's nice to see an intellectual discussion without the pi**ing match. :-)

cotdt, could you reference that chart for us please? I can't see anything on the chart about toughness or grain growth or...??? It sounds intriguing but its rare for a pebble to enter a pond without ripples. I'm curious where those ripples might lead.

Rob!
 
KnifeMaker.ca said:
WoW!


I can't see anything on the chart about toughness or grain growth or...???
Rob!
Click to expand...

I have a chart somewhere that shows the grain size stays fairly flat through any reasonable austenitizing temperature range and I believe that grain size was around 10. The carbides both prevent it from getting much finer than that, but also prevent it from growing much larger than that. I think you'd have to get it over 2000F to start getting much grain growth in the first HT.

There are toughness charts tied to tempering temps and hardness, but you have to take these with a grain of salt because they don't say from what temp the steel was quenched, nor if it is with cryo or the timing of cryo.

RA improves toughness and impact strength on big massive stuff, but reduces strength at the scale of a knife edge.

Crucible has a chart that shows a toughness peek at HRC 60, tempered at 500. Higher than that you get carbide growth and RA converts. But wait, we want RA to convert - so what is the toughness if tempered to 450 and cryo quenched? I don't know, but I can tell you that at HRC 61, a hollow grind with a .020" edge can cut a nail in half without cracking. How important is impact toughness? I think the durability of the edge is more important in most applications (from a scalpel to a chopper) and the more homogenous structure you get with cryo and a low temper improve that.

Just my observations, I'm still learning this stuff too.
 
When I originally started researching cryo treatment there was a reference to time for retained austenite to martensite transformation being some where around 8 hours. Not a number that was written in stone just a point of reference to get the most out of your cryo.
 
bikermikearchery said:
When I originally started researching cryo treatment there was a reference to time for retained austenite to martensite transformation being some where around 8 hours. Not a number that was written in stone just a point of reference to get the most out of your cryo.
Click to expand...
I guess I should clarify my statement on cryo. When you get your knife in cryo immediately after quench (so it is essentially a continuation of your quench) then very short is good enough. However with more time wasted after quenching, a longer hold in cryo can convert some of the remaining retained austenite. This is why it's important to get the knife into cryo very quickly after quenching.
 
Nathan the Machinist said:
However, there are disadvantaged to using the secondary hardening hump in D2.
Click to expand...

What are the disadvantages? I'm very interested in hearing your thoughts on this.

KnifeMaker.ca said:
cotdt, could you reference that chart for us please? I can't see anything on the chart about toughness or grain growth or...??? It sounds intriguing but its rare for a pebble to enter a pond without ripples. I'm curious where those ripples might lead.

Rob!
Click to expand...

That was yesterday, I forget where I got that from as my memory is very bad for a 24-year old. When people send you blades for ht, is this the process that you use? I still need to use up that S30V you've sent me, then I will try the various different heat treats for D2.
 
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