new at HT...Home Cryo?

James_Terrio

James_Terrio

Joined
Oct 6, 2008
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OK guys, I'm diving in head-first... got me some nice CPMD2 and CPM154 from our friend Tracy, and have a Paragon oven on the way. So get ready for lots of dumb questions! :D

I've heard cryptic references to home cryo using dry ice and acetone or kerosene. Does anyone have a tutorial up on this? Folks are telling me that these two steels will benefit from cryo, but I want to avoid having to send them out. (that's why I ordered the oven!)
 
If you can find a dry ice supplier in your area, there's a couple of ways to do it. If all you can buy is the chips of dry ice, you'll want to get several pounds of it and make a slurry using kerosene (safer than acetone). Keep adding dry ice until the slurry stops boiling and you have a nice icy mix. That will give you approaching -100F. Or, if you can buy blocks of dry ice (many suppliers offer blocks of varying sizes), get two good size blocks and put your knife between the block. Keep the slurry or blocks in a good cooler or even a styrofoam cooler. If your blocks are big enough, you can treat several knives, otherwise use the slurry for multiple blades. Keep the blades in the slurry/blocks for 8 hours or so. That's the problem with dry ice; it's pretty much gone after one treatment, but the setup is much cheaper than that for liquid nitrogen. If you produce in enough volume, LN is the way to go as it can be kept for a month or two in a quality dewar where as dry ice is gone in a day or two.

The -70 to -100 you get with a dry ice slurry isn't as good as the -300 you get with LN, but it IS better than nothing. You will see some benefit from a dry ice cryo. Heck, many people don't even cryo and you still get a pretty decent result with CPM 154. The cryo really lets you get the most out of the steel by forming as uniform and complete a martensitic composition as possible.

--nathan
 
Thanks Nathan. After a bit of research, I talked myself into spending extra on the CPM type steels. I'd like to make the most of the steel if I can.

I'm unclear on one thing: do I need the kerosene if using blocks? I'll call around on Monday about blocks of dry ice, I know we used to get them years ago to make "fog" onstage. We got them from a place that sells/services fire extinguishers IIRC.

I'm hoping to do batches of 4-5 blades or so to help me save energy and keep things consistent. So two blocks in a cooler would work great I think.

Also, I've read that D2 benefits from cryo before tempering; is this true of CPM154 as well, or does it matter?
 
Cryo will always be done prior to tempering as part of the heat treating process. After you heat and plate or air quench your steel, once it is room temperature, it goes straight into the cryo. No Kerosene needed for the blocks as the knife will melt a depression into the blocks like a mold. The cryo helps continue the conversion of austenite into martensite and decrease the amount of retained austenite in the finished blade. After cryo, once the blade is at room temperature, you need to temper. Some people do a quick, low temperature "snap" temper prior to cryo, but from what I've seen, many don't consider this necessary and will go from quench to room temp to cryo for 8 hours.

--nathan
 
Gotcha - air quench to room temp, cryo for 8 hours, back to room temp, temper per instructions. I'll wander around the yellow pages Monday and look for some aluminum plate for quenching as well, now that I think of it.

Thanks man, this is exactly the kind of thing I need to know. Don't assume I know diddly :D

In another thread, someone mentioned putting a soft-back draw on D2, what's up with that? I know how the carbon-steel guys do it and why, but I didn't think it worked with air-hardening steels? :confused:
 
If you're talking about drawing back the temper on the handle and spine, i would think it would work the same as carbon steels. In that process, you're not going up to critical temp, just over tempering it to above your original temper. Don't take the blade to any glow, just take it beyond a purple oxidation color. This will soften the steel, so protect the edge by keeping it in water. The air hardening part only refers to the actual hardening from austenite to martensite.

--nathan
 
i draw the temper on the spine by clamping the edge in aluminum plates to keep the edge cool... had some very contolled results even using a regular propane torch
 
addendum... esp. in light of nathan's statement on the other thread: i didn't check performance after soft back draw... i know the back is softer because sandblasting produced a clear line. performance-wise i'm not a fan of differential tempering.
 
May I ask why? I doubt it matters much on a hunter or EDC, but on a 5"+ blade that might be used for chopping, it seems like a good idea.
 
drawing the temper to reduce hardness also reduces stength. Basically I believe the edge is the most vulnerable part, being thinnest, so the temper at the edge should be adequate for the bulky part of the blade. I do a lot of hard cutting tests...
 
blade with a soft back draw will deform more easily, and depending how soft, might not break at all but permanently deform (ductile failure). Through hard blade will snap, but will take more force to snap. High hardness blades can be incredibly difficult to snap (they often shatter beautifully). If the hard blade is going to be subjected to high shock loads, eg. smashing with hardened steel hammer, by all means do a soft back draw, but the blade will not be as strong and will not withstand lateral or torsional forces as well. If batoning with wood or a soft steel hammer you should be fine using a through temper. Remember the edge is the thin part that takes the most stress! If the edge survives shock loads the spine will too.
 
I guess I have the ABS bend test in the back of my mind somewhere. I've read many folks I respect a lot, say not to think of that as the be-all/end-all of performance. I'm trying to keep my mind and ears open. Thank you for your time and input, gentlemen.

All I care about is learning to make a darn good blade; whether the materials and techniques are traditional or controversial doesn't matter to me. As long as I can prove it to potential customers.
 
James_Terrio said:
In another thread, someone mentioned putting a soft-back draw on D2, what's up with that? I know how the carbon-steel guys do it and why, but I didn't think it worked with air-hardening steels? :confused:
Click to expand...

James,

I got this from a "famous" (famous to me...) fellow who differentially tempered most of his blades.

Put edge in shallow water (water won't go above boiling... 212F, or there abouts... colors will form gradient between spine and edge-area out of water). Very soft, very small flame on oxy-acety. and play the flame on the spine. Do it three times and take the past the dark colors. Should be the near edge doesn't get darker than straw. Need to re-polish inbetween to view color developement each time.

The point is even heating... even heating is slow, soft heating in this instance. The number of times and the slowness of the heating helps approximate a true temper for time. You are looking for spring tempering colors of silver gray (650F approx.) and it is past the browns, purples and blues.

Mentally, this is a little messy... you got questions, just ask...

Mike

Edit: http://www.anvilfire.com/index.php?bodyName=/FAQs/temper_colors.htm&titleName=Temper%20Colors%20:%20anvilfire.com
http://www.anvilfire.com/index.php?bodyName=/FAQs/temper_colors.htm&titleName=Temper%20Colors%20:%20anvilfire.com
 
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I'm familiar with temper colors from baking files :D Thanks for the link to the chart.
I've diff-tempered carbon-steel knives in a similar way, it just didn't occur to me that it was possible with D2.

I think I understand what you're saying, wnease, which may make it a moot point. Do you feel this holds true even with a big camp knife with a 9 or 10" blade?

All else being equal, including proper HT for each steel, how would a through-hardened or diff-tempered CPMD2 blade stack up against one made of 5160 in chopping tests? Not chopping concrete blocks; realistic stuff like 2x4's and pine knots.

How does a spring temper differ from a hard edge temper in terms of carbide size, if at all? Fine carbides are good, that's one reason for going with CPM-type steels...


Hey, I warned you about dumb questions :D Thanks again!
 
Mike,

Thanks, that explanation was very clear:thumbup:
 
i make blades up to around 3' and absolutely wail with them! Full power chops at SUVs. As for cpm D2 (haven't used it)i'd be confident with that in normal cutting media. It's tougher than D2 and I trust that.
 
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