Cryogenics can benefit many steels mainly because the transformation temperatures from austenite to martensite is well below room temperature. Just think about it. Why would you expect the crystal structure of steel to be controlled by the temperature that we find comfortable.
Cryo treatment does many things for a blade steel including, but not limited to, greater austenite to martensite transformation, finer grain size, and improved carbide structure. This has been studied in great detail and you can find controlled testing done in various journals.
The problem with such testing is that it is often done by the people selling the equipment. It would be like going to a used car salesman and asking him his opinion of his products as compared to those competing against him. Everyone you ask always has the best cars.
In competition to cryo is the traditional multi-temper approach, that can also increase the martensite transformation percentage and refine the grain structure. No work has been done to my knowledge to show that it can generate the same level of carbide optomization and thus wear resistance.
Anyway, deep cryo is just a continuation of the quench part of the heat treatment, the blade *has* to be tempered afterwards otherwise you are working with untempered martensite which is extremely brittle (~66 or so RC). As well realize that on many blades, (the HI khukuris being one such type) not all of the blade is quenched in the first place.
For large chopping class blades, I would not think any significant advantage is going to be obtained from cryo added to the standard heat treatment because (a) you can already get the hardness you desire without cryo, and (b) wear resistance plays little role in chopping and other heavy work. It would be interesting to see if it has any effect on the karda.
If you are interested I could take the karda from one of my larger khukuris, do some edge retention work on it, do a deep dryo and temper (in a regular oven), and repeat the edge retention work to see if a difference was made. I would think little actually because if the austenite at room temperature tends to stabilize and resist transformation which is one of the reasons that you don't leave blades around untempered.
Finally, if you just jammed a khukuri, especially one of the larger ones into liquid nitrogen, it might experience enough thermal shock to cause micro-cracking. Phil Wilson has looked at this in detail and found no evidence for his blades, but they are 1/8" thick at max with full flat grinds and distal tapers, thus the surface area to volume ratio (and thus heat transfer) is high, on a 22" Ang Khola this is not going to be the case.
-Cliff
) but the process is a series of controlled temperature drops (using liquid nitrogen) which re-aligns the grain structure and creates a stronger, more uniform pattern throughout the piece. That is why I wonder about a forged blade - the structure might be so diverse due to differential hardening, packing, etc., that the stresses of the temp drops might be damaging. OTOH, if there is a way to do it gradually, and not exceed limits, it might enhance everything already present in the blade. IIRC, the blades mentioned in the article were finished, ground pieces.
