Sub-zero treatment

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MichealD84

Joined
May 22, 2005
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Hi, Does sub-zero treatment really improve performance in A2 and D2. How does this process should be done properly and should cryogenic treatment be done with niquid nitrogen or dry ice ?
 
OK,this comes up every so often.To avoid the"I always do..." I am going to quote from the reference manual on the subject "Metallurgy Theory and Practice" (Everyone should read this book once a month)
SUB-ZERO TREATMENTS
It has been discovered that any austentite retained at room temperature may be transformed into martensite by cold treating,i.e.cooling the steel to sub-normal temperatures in the -70F to -110F range.Cold treating is carried out by cooling the workpieces in dry ice or in special refrigerating units......(ed.Liquid nitrogen)...Cold treating to transform retained austentite will not result in any loss of hardness of martensite and will result in maximum hardness being obtained.Conventional tempering operations always result in some hardness being lost.......the tool should be given a low temperature (300-350F) temper operation before the cold treatment and this should be followed (ed.- after the cryo) by a second low temperature temper treatment......
Stacy
 
http://www.bucorp.com/coldworksteels/aza_bohler_item.asp?iItem_Id=40

This site by Bohler/Uddeholm North America has data data sheets for tool steels, including A2 and D2 and also has a very good article on heat treatment of tool steels.

They recommend sub-zero treatment as soon as the quench gets down to room temperature, followed by tempering. I have been using this as my main tool steel reference for 3 or 4 years now with good success.
 
An initial low temperature temper [300-350F] called a "snap temper" may reduce the risk of cracking . Always temper or freeze immediately after quenching. Freezing is done at from -100F to -300F .Dry ice and acetone is sometimes used for -100F and liquid nitrogen for -300F.Any of the complex steels such as D2, A2 can benefit from the reduction of retained austenite.Sub-zero quench must always be followed by tempering.
 
Sorry for the "Highjack" but is -300 more beneficial than -100 per say ? In other words is the liquid nitrogen quench "better" than the dry ice-acetone quench in that it transforms "more" of the retained austenite ? Incidently this is the first time I have heard of using the last combo.Interesting !! I have heard of alcohol-dry ice quench.
 
Not really.-70 to -100 is the bottom of the transformation.The big advantage to liquid nitrogen is it can be stored and reused.Dry ice and acetone are good for one treatment and gone.The dry ice mix is dangerous,too.LN is inert,basicly.
Stacy
 
The -100F does the job of reducing retained austenite. Added benefits of -300F are still questionable ,not by any means demonstrated.
 
bladsmth said:
The dry ice mix is dangerous,too.
Click to expand...

How is the dry ice mixture dangerous? I was going to try an ethyl alcohol and dry ice mixture for cold treatments at home, but I would like to be aware of any precautions that I have to take.
 
Khukuri Monster said:
How is the dry ice mixture dangerous? I was going to try an ethyl alcohol and dry ice mixture for cold treatments at home, but I would like to be aware of any precautions that I have to take.
Click to expand...
The normal dry ice method is dry ice and acetone.
Acetone is nasty stuff...
Liquid Nitrogen is basically inert, and the only hazards are displacing Oxygen in a room, and freezing your fingers off...
 
Since steel is a very conductive metal, I fail to see the need to use acetone. If you lay the blade on a slab of dry ice and cover it with another, it will melt a depression that just fits it into both and equilibrate temperature in a very short time. The dry ice will last for several hours longer than the recommended soak time at low temp (3-4 hours per Uddeholm's specs).

I have been doing it this way for 4 years now and have not had any blades crack.
 
shgeo - That will work.You still have to go get the dry ice,bring it home ,cryo the blade,and then your dry ice is gone.If you had 30 blades to do,it would be nearly impossible to do with dry ice slabs.In industrial setups,hundreds to thousands of pieces are run through the same LN.One thing to remember is that at -100 degrees air and steel do not transfer heat the same as at higher temps (actually,at -100 air is a fairly good insulator)
 
Many thanks to shgeo for pointing me in the direction of the Uddeholm data info. :D According to them, you get a hardness increase of 1-3 HRC in both A2 and D2 when you sub zero quench.
Scott
 
FWIW Bark River cryos all of their A-2 blades and is very concerned about the heat treat process in general. Mike Stewart, the company's owner, says the cryo makes a huge difference in the performance of their knives.
 
bladsmth said:
shgeo - That will work.You still have to go get the dry ice,bring it home ,cryo the blade,and then your dry ice is gone.If you had 30 blades to do,it would be nearly impossible to do with dry ice slabs.In industrial setups,hundreds to thousands of pieces are run through the same LN.One thing to remember is that at -100 degrees air and steel do not transfer heat the same as at higher temps (actually,at -100 air is a fairly good insulator)
Click to expand...
Good point about the number of blades, I rarely do more than 3 or 4 at once.

The steel transfers heat conductively very well on contact with the dry ice and equilibrates in temperature very fast, forming a cast of itself that seems to stay in contact with the blocks. I agree that convective heat transfers, which would involve steel/air flow are slowed greatly at these temps. The thermal conductivity should only get better at lower temperatures however, if there is any change at all, due to the slowing of random electronic motions at the crystyalline level in the steel.

I get my dry ice from the grocery store 3 miles away from the house.
 
S.Shepherd said:
Since the closest place to me that has dry ice is over an hour away, I make my own :D
Click to expand...
now just how can you go about doing that i will need to cryo some blades soon and not ready for Ln
 
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