Cryo questions

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Bill1170

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Dec 20, 2007
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I spent some time tonight sharpening an aftermarket plane iron made from cryo'd A2 steel. The blade maker claims that the cryo helps toughness, which in turn helps with edge retention. That makes sense to me.

My questions:

1) How does cryo treatment change the steel?

2) Does it help all or only some alloys? Which ones are most helped by cryo treatment?

3) Can a blade still benefit from cryo treatment that is done years after the blade was first heat-treated?

Thanks in advance for educating me.
 
Cryo converts austenite to martensite. Austenite is a soft and malleable phase of steel that is reached when it is heated during heat treat. Depending on the alloy content of steel, the temperature at which the austenite hardens, or converts to martensite, varies. Also, the temperature at which all the austenite finishes changing to martensite varies. Some steels have very low marteniste finish temperatures, which is why cryo helps complete the hardening process for them. For other steels, their simple composition means there is no low tempereature needed for full marteniste conversion. The new marteniste is untempered, so following cryo another tempering is needed to relax stresses in the fresh martensite. Austenite will stabilize if not converted to martensite, so if cryo is not done relatively quickly, then a whole heat treat sequence will need to be repeated.

Another result of cryo is the precipitation of very fine carbides, but this requires soaks of eight hours plus, while austenite conversion is generally done in two or less.
 
Thanks Hardheart, that helps a lot. So I gather from what you wrote that cryo has to be done soon after heat treatment. Is it best done just after quench, or after tempering? It appears that cryo acts to "complete" the quenching process. I am also curious about the time scale needed for the retained austenite to stabilize to the point where cryo treatment will no longer convert it into martensite. Are we talking minutes, days, or months?
 
Time is going to depend on alloy content. Lots of carbon with stabilize the austenite faster, so does nickel. Liquid nitrogen is cold enough to restart the transformation after long delays, but the austenite may degrade into bainite during the interim. But again, that depends on the steel and the prior heat treatment. Continuing from quench to room temperature straight to cryogenic temps seems preferable from just a perspective of most complete transformation, but snap tempers are done as a safety factor to reduce the likelihood of quench cracks in the brittle untempered martensite.

Wildcat, the reason steel is heated and then cooled very quickly is to first get the atoms energized enough to change alignment, and then to quickly freeze them in a highly strained orientation without giving them a chance to return to a low strain alignment during slow cooling. This is why steel is strong after hardening. Think of it like a tightrope, people can walk across it because it is under a high strain that doesn't allow it to deform much. Same thing between soft steel and hard steel. The switch in properties requires phase changes that occurs in heating in cooling - but if the atoms are varied, available in large amounts, and some very large, then they don't change phases as evenly and quickly during heating and cooling. The temperatures necessary can vary a lot, and sometimes well below what we call room temperature.
 
Rapid cooling can realign the atoms and trap atoms in a structure of other atoms. Like cryo aus8a the carbon atoms create a "cage" that "traps" the larger nitrogen atoms. Look at the Journal of Chemical Physics on the electron spin relaxation of N@c60 in CS2.
 
Bill1170 said:
Thanks Hardheart, that helps a lot. So I gather from what you wrote that cryo has to be done soon after heat treatment. Is it best done just after quench, or after tempering? It appears that cryo acts to "complete" the quenching process. I am also curious about the time scale needed for the retained austenite to stabilize to the point where cryo treatment will no longer convert it into martensite. Are we talking minutes, days, or months?
Click to expand...

From my reading on A2, cryo will add 1-4 RHc points to A2 and increase the wear resistance up to @80% (250 C temper). However the impact resistance is reduced by > 50%. The conversion of austenite to martensite (increasing hardness) is fast at liquid nitrogen temperatures (cool slowly, hold for a few minutes, warm slowly to room temperature, then temper). Increasing the wear resistance takes longer (@ 24 hours at liquid nitrogen temperatures) due to the slower formation of carbides in the grain boundries. Cryo after tempering decreases the impact resistance with very little gain in wear resisance. Reference: Zbigniew Zurecki, Air Products...

D2 seems to respond better than other steels, at least as far as wear resistance is concerned (@ 500% increase).

ac700wildcat - for A2, cryo gives harder steel at a given temper, the blade will cut longer but break easier. If you temper the cryo blade to the same hardness then you will probably get back some of the toughness while keeping the better edge retention.
 
I am fascinated the use of "cryo" in differential tempering and in layering like a san Mai setup.
 
Bo, Yes, thats the nature of cryo soaked steel. They get hard afterward (brittle) lacking toughness. Subsequent tempers add back toughness to the blade. Do atleast 2 some steels need more. DM
 
This is a great thread, I want to thank everyone who has posted so far.

If I understand correctly, the cryo converts austenite to martensite, and the subsequent temper gives some of that austenite back? Someone also mentioned bainite, which I do not know about yet. Got to use my Google-fu.
 
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