View Full Version : After heat treat in what order do you ?????
08-11-2011, 09:32 PM
Im working on a knife for my brother in law, 8" skinner CPM 154. My first time working with CPM 154, i dont know if i was just imagining it but it seemed to grind alot easier than 440c, and get hot quicker but cool faster.
anyway tonight i heat treated by heating to 1400 and letting set for 30min then 1950 for 45 min. Then plate quench until fully cooled. Here is where my question is, I put the blades in my dry ice/acetone slurry for the night and i plan on double tempering at 500 for two hours tomorrow.
should i have tempered once, then cryo, then double temper, or ihave i dont it right this time?? i just dont know what the best recipe is here.
08-12-2011, 05:27 AM
You can flash temper at 300 then go to dry ice and do the double temper after but most of the time I go straight to cryo then come back with tempering.
08-12-2011, 06:46 AM
This might draw some backlash, but I personally think your wasting your time/money with dry ice/acetone....that mixture will get you about a max of -85F....not enough to cause changes in the steels matrix. At one time I thought it was the way to go too.....then I started having samples of steel/blades spectrographed. There is almost no measurable change in steel that has been "cryo treated" with dry ice/acetone, and one that came straight out of the hardening. OK, now that I've said that, let's go to you actual question.
There are always tremendous stresses built up when a blade is hardened. More stress is induced when a blade goes into cryo. As a precautionary measure, most folks will do a "snap temper" right after the hardening operation, to relieve some of those stresses. (temper for a short period of time, before going into a cyro treatment) In the vast majority of cases, IF a blade is hardened to it's max potential, and then put straight into cryo without a stress relieving cycle, tiny cracks will develop. Most of the time these cracks are so small that you will not notice/find them until finial finishing.
Personally, I will do a single tempering cycle, cryo in liquid nitrogen, then do two more tempering cycles, at 25F higher than my pre-cryo tempers.
08-12-2011, 07:04 AM
Crucible's spec sheet for cpm154 actually suggests the dry ice cryo for converting RA and goes into a little detail about tempering before vs tempering after.
08-12-2011, 07:28 AM
Just in the interest of getting lots of opinions, We go straight from hand warm to cryo, because cryo has it's greatest effect as a continuous (more or less) part of the quench. No cracking problems (yet :3: ). Part of my logic is that the steel went from 1950F to about 120F in less than a minute without much problem. That's probably more stress than the last 440 degrees or so in the LN.
I'm interested in Ed's post because he has followed it with testing. I've also read the 85F thing about the slurry, but I don't understand it. Dry Ice is about 109F - enough for cryo according to crucible - and if you keep adding dry ice to acetone, I don't see why they wouldn't eventually equalize - ie why would the heat transfer stop at -85? Maybe it's a time thing.
We stopped using Dry Ice because it disappears too fast - but maybe there's more :31:...
Ed, how do spectrographs differ after a liquid nitro cryo??
08-12-2011, 07:48 AM
Thanks for the info, i have a dewar on order right now to start using liquid nitrogen but it wont be here for a week or so and his birthday is coming up so i needed to get these done for him.
08-12-2011, 07:51 AM
Hello ED , I have cryo treated with liquid nitrogen a few times, never tried the dry ice . I think dry ice is -109.3 f. instead of
-85f. I dont know if that would make much difference ? Where as liquid nitrogen is around -346 f. A lot of difference .
08-12-2011, 04:42 PM
Ed, how do spectrographs differ after a liquid nitro cryo??
According to the individual that does my spectrographing, the results can actually vary greatly, depending on steel type, how well the initial heat treatment (hardening) was accomplished, and several other factors.
Assuming that the heat treatment was optimal (quenching) and again, depending on the steel type, cryo treating in liquid nitrogen can convert as much as 95+% of the retained austinite. This, in turn, can add 2-3 more Rc points of hardness (this is why I temper twice at 25F higher than my initial "snap temper". From what I've found through testing, the higher the alloy content, the greater the need for cryo treatment.
Dry Ice is about 109F - enough for cryo according to crucible -
That just might be true for a specific steel type(s). I don't have the equipment or facilities to test it for myself....I have to trust what the individual who does my testing tells me (and I have no reason to doubt his word). I brought up the same point with him about the -109F thing with dry ice, but he insisted to me that when mixed with acetone, under a "moderate ambient" temperature, -85F would be about the best one could hope for.
He is also the source for my not believing that -85F or even -109F has any effect in converting retained austinite, because according to him, that change does not begin until -180F, and ends at approx. -300.
I'm not saying all of that is absolute, but it's what I've learned, and through testing is what I believe.
I highly encourage folks to get their own testing done, and conduct experiments to see just how well your methods are working.
08-13-2011, 07:09 AM
All information welcome and absorbed. I think you're right that it needs more people questioning and testing their methods. In our case, it would be just for interest, because we already use liquid nitrogen, but we have suggested the use of dry ice to some based on Datasheets - and we at least try not be wrong.:3:
Maybe someday, dry ice cryo will go the way of edge packing.
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