More Information on Cryogenic Treatment of Steel

Me2, thanks for the article, need to print it out to really take it all in. What is interesting to me off the top is the relationship between cryo treatment and compression and tensile strength. First time I have seen this.
 
We've never mentioned dislocations in the knife forums so if your confused just ask. If we look at an annealed structure the atoms within a crystal lattice should be all lined up -the atoms nicely in orderly columns and rows. Heat treating [or cold work] produces imperfections in the lattice .There are various types of these dislocations . They can move through the lattice and anything that impeeds the movement strengthens the metal.They can be pinned by precipitates like carbides or interact with other dislocations .Now that we have electron microscopes we can see these dislocations and with a video see them move!! If you look at the structure of martensite you'll see that it's packed with dislocations .

This link and discussion should be moved to the Maker's forum.
 
I thought about putting it there, but for the sake of wider exposure, figured general would be better. I read this article a few days ago and the dislocation related discussion made me hesistant to post it here. Not for the lack of ability to understand, but the absence of the term on pretty much any knife forum. But there are more than enough people around here that know and can explain what they are and what they do that it shouldn't be a hindrance.

I must say, I'm a little confused about the tensile samples. They are shown in the diagram, but no mention is made of the results. The yield strengths are incredibly high, but are from compression, from what I remember. The article also shows that the correlation between hardness and strength is only a general one. The highest yield strength samples have the lowest hardness, though all the hardnesses are well above the norm for knives. I've read it twice and will probably read it again, and once more along side the article from Ireland that was done in the mid 90's. The focus of the earlier article was determining the difference between freeze treating and cryogenic treating. It is also a good article, but left me with questions that this one seems to answer pretty well.
 
I couldnt open it from my phone. It sounds a bit too technical for me anyhow. Could sombody explain the benefits of cryogenic treatment in laymans terms? Is it part of the heat treat process or can it be done anytime? Does it increase edge retention, chipping resistance, toughness, or what? I am asking because I may want to get my sebenza treated just to make it "better" :D
 
The general opinion is that it is best done as part of heat treatment. You can do it any time if you want, but the improvements are questionable and little studied in the research I've read. It can improve toughness, wear resistance, edge life, and increase hardness, but all these things depend on the other parts of the heat treatment prior to and following the cryogenic treatment.

What we know for sure it does is reduce retained austenite, if not elmininate it entirely, thereby increasing hardness. Austenite is a softer atomic arrangement of steel and cryogenic treatment removes most or all of what is left after quenching, if done properly. We also know that cryogenic treatment before tempering causes/encourages the formation of very small, very hard carbide particles. This increases wear resistance, but doesn't necessarily increase hardness. Both higher hardness and higher wear resistance can lead to longer edge holding, as long as your uses for your knife don't cause it to dull by chipping/fracturing at the edge.

Claims have been made for years that cryogenic treatment improves toughness, and this article is the first I've read that lays out a mechanism for this. Toughness would prevent dulling by chipping/cracking in chopping uses, such as machetes and large bush blades.

One of the basic topics dealt with in the article from Ireland mentioned above was the wide claims for all kinds of property improvements using cryogenic and just low refrigeration (dry ice temperature ranges). That article basically says there are many different ways to perform low temperature treatments, and they have different effects, so decide the properties you want and choose your cryogenic and heat treatment procedures accordingly. I'd really like to give you a simple answer, but this topic has been debated and researched at least since the 80's, so I'm afraid there isn't one.
 
According to one of the latest papers cryo makes small changes in the lattice which , on tempering , permits small carbides to form [ eta carbides] on tempering. At this point in time we can demonstrate that both cryo [-300 F] and sub-zero [-100 F ] reduce retained austenite but only cryo forms eta carbides .Other things so far have not been demonstrated and some are just hype. But these treatments are definitely part of the HT.
The complication is that each steel is different and to get the optimum properties each steel has to be studied thoroughly as to the entire HT schedule !!!
For blades cryo means increased hardness ,wear resistance and toughness.For makers on this forum cryo is often standard treatment for hypereutectoid steels especially 1.00% C or more.This is especially true of stainless steels .
 
this would explain some of the benifits of cryo on non-ferrous metals wouldn't it? Although if you think the research on steel is muddied and confused you should look for Cryo research on non-ferrous metals it is even worse.
 
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