cryogenics

[Nathan the Machinist]

Diffusion takes time, so operations like soaking for quench and tempering need a hold time to be effective. Martensite doesn't form this way at all, it forms via instantaneous shear driven mechanisms, hence all that matters to it is temperature.

Kevin,

That totally makes sense to me and is completely congruent with what other knowledgeable folks (Alvin Johnson) have to say about the subject. However, at least some of the literature disagrees, there is so much contradictory information out there. Perhaps some of it just old.

One of the links above by Hardheart, Air Products and Chemicals, 2005, show significant a difference between a 30 min soak in LIN and a 24 hr soak. I've seen another chart that show diminishing returns over several hours time rather than an instant change. However, I'm beginning to suspect these conclusions may be based upon old information.

In case anyone wants to see it, this is a chart that shows W1 with about 12% retained austenite at room temperature. It goes to about 4% at -100, and 1% at -300.

http://www.panix.com/~alvinj/graphCooling.jpg

This chart also shows why you need to hurry and get it cold.

 

[rashid11]

I am puzzled as to why no formal study was ever done, that we're aware of.
There was a pdf on the web, by some university professor, about steels used by
knifemakers or something to that effect. He might want to do Part II - about Cryo
treatment and effect of same

Also, let's face it - most knifes this day & age see very light use, to where one can
not discern any difference .

Imagine an article from some1 who works in some fish processing facility where he and
say 100 of other workers use knifes all day long to gut, fillet fish. He might say that
since beginning of time they used knives X and had to sharpen them every 2 hr and recently management got them cryo-treated knives Y and these last a whole shift. Now
even something like this wouldn't tell us much ... steel could be different, HT might have
been botched etc. We need a scientific study.

Interesting point about cryo-treated gun barrels . Lots of folx think what we have with these is the "placebo effect". Shooters feel they have superior guns and that _alone_ lets them have better results (tighter groups etc).

 

[Kevin R. Cashen]

The first thing anyone needs to do when approaching this issue is to seperate retained austenite conversion from any other phenomena that may be ocurring, then you will have two categories - fact and speculation.

As for other literature that one can encounter, here is where I abrubtly run afoul of those folks who want to sell you a process on "it just works, we don't need to know why". To a person with such a driving curiosity that they can work this way any explanation could be possible, any anomolous or contrary literature is legitimate. If my whole position is somewhere in the ether any anomolous finding can be conveniently plugged into it becuase it is quite fluid and adaptable. There are tons more literature than what has been written on this topic showing that Elvis is alive somwhere, call me boring and close minded but I prefer to stick with the scantly documented traditional wisdom, suported by Occam's Razor, that he is under the turf at Graceland.

The actual process of martensite formation is instantaneous, but the mechanisms triggering it could be acumulative i.e. strain, outside energy introduced to the lattice, or very sluggish diffusive actions. At room temperature the retained austenite is only a problem in that is can give lower hardness readings, the real reason it must be dealt with is its possible conversion or break down into something else over time. Often this is looked at exclusively as how much austenite is there in the end, and no attention is paid to what became of the missing austenite. Any austenite that went to martensite I assure you did so in a flash, however if it managed to break down into some other product of ferrite and carbide it could have taken time. That is exactly the issue with waiting or holding at higher temperatures before zapping that austenite, the possibility of it going to a more stable structure that is still not martensite.