quench question again

bartblade,

The only real way is metallographic techniques, to find retained austenite. But when you work with steel long enough, you can "get the idea" when it is present and when it has (or mostly has) be converted by the results you obtain from such cyro processes and the subsequent change in hardness.

itrade,

Is the steel really unchanged from it's original forging? Or did time improve its knife characteristics? "the secrets of forging 300 years ago are lost. Modern technology cannot reproduce them." etc. Is it because there was a secret or because of 300 years of aging?

I do not feel there was a "secret". Also, I am doubtful that 300 years would necessarily provide any realistic benefit to a steel. Especially since, in terms of retained austenite, there would have had to been large alloying element percentages present in that steel when they made that old knife. A factor usually necessary to create the possibility of retained austenite. Fact is, alloying in steel (on purpose, not including silicate slag inclusions) is a relatively modern invention. So, back then, you're dealing with steels that today, more closely resemble the carbon steels we all know. Thus, carbon steels and those with low alloys typically do not have a tendency to retain austenite.

I have heard references of Scandinavian blades buried in snow to help them hold a better edge and Japanese wood chisels that were stored away and left to "age" over the year(s) to improve their edge holding, but mostly I think such things are erroneous and simply passed down traditions without further examinations. However, and with a resounding, BUT :cool:, anything is possible and the right situation could be present to make those and other effective bladesmithing techniques, facts. As mete would say, isn't metallurgy fun?! :cool:
 
Yes Jason, no disagreement on any of that. It wants to transform and enough time will cause that. However, I would not call that any type of tempering. Do you not agree that cryogenic tempering is just that - tempering?? And that left alone is 'aging' and that cryogenic tempering is also commonly refered to as 'aging'??

Are you suggesting that a -20 degree treatment for a reasonabe amount of time (let's say three or four days) will transform a large percentage of retained austenite?? If yes I'll test it out with the type steel you recommend.

RL
 
Bart,

Leave the steel in dry ice cryo for as long as the dry ice will last. If you can get two days out of it leave it in there. I used to put mine in the refrigerator to try to extend its life. I used a cooler and it was to large for my small freezer or I would have put it in there. I never did mix my dry ice with acetone but did use kerosene to make the ice bath with. Kerosene will, I found, eat through a styrofoam cooler so its best to line it with something or you could have a mess. Acetone can be more dangerous.

RL
 
Yes Jason, no disagreement on any of that. It wants to transform and enough time will cause that. However, I would not call that any type of temepring. Do you not agree that cryogenic tempering is just that - tempering?? And that left alone is 'aging' and that cryogenic tempering is also commonly refered to as 'aging'??

No. Cryogenics is not tempering. Tempering requires a re-heating. Cryogenic treatment or cold treatment are more accurate terms.

Are you suggesting that a -20 degree treatment for a reasonabe amount of time will transform a large percentage of retained autenite??

I am suggesting that it is possible, not gauranteed.
 
I suggest it is tempering in that it transforms austenite to martensite and therefore effects hardness. In addition to that it has been refered to as 'cryogenic tempering' in publication. Here is but only one source that uses that phrase: http://www.cryogenic-tempering.net/
RL
 
rlinger,

I can't just say three or four days or three or four months for that matter, will work. You have to examine it case by case. That was the point I was trying to get across.

If you want to test than: If you can get a knife steel that has some retained austenite in it, say ATS-34, prove it does first or the percentage better yet, then run a series of tests with different cold treatments. Try room temp, then refridgeration, then freezer, then dry ice/acetone, then liquid nitrogen. Longer times for warmer temps, shorter times for colder temps. If you could get ahold of liquid helium, I'd say "what the hell" and add that too for shi#'s and giggles.

Whether the freezer method works or not, its not a formula you can just plug into everything. If your want the easiest gauranteed results, get a dewar and dunk some blades. If you're feelin' adventurous and have time on your side, there are other ways.
 
But you are not giving him anything definitive to work with with any degree of assurance. I gave him a economical means of transforming his retained austenite in a reasonable period of time without having to guess whether it would work well or not.

I have a scrap piece of ATS-34 that is about 1 X 1 1/4. Just to satisfy myself I did not give Bart bad advise when I suggested the household freezer really wouldn't help him I will quench and temper once and Rockwell it, immediately after I will put it beside the ice cream for a week (7 days) and test again. I did a similar test using ATS-34 and deep cryo (LN) a few weeks back. The HRc value increased by 3 1/2 points after deep cryo of somewhere between 12 and 16 hours (I do not remember the exact amount of time). I could use the same HT temperatures and times as I did for that one. I am not being sarcastic; I am curious to know. If it shows a marked improvement I will make a thread of it. I wonder if it be better I test after day two, day four and day seven to get a better picture?? Have you tried this??

RL
 
Last Saturday at 3:00 PM I placed a test piece of ATS-34 in the house freezer; no kidding - right behind the ice cream. It is very early Friday next and will wait til 3:00 PM tomorrow, Saturday, to pull it. The test piece showed 58.5 HRc before putting in the freezer. I have not taken it out or intend to until the 7 days are up.

I view this seriously and am doing this in an effort to learn for myself whether I be right or wrong about what I had said about it. After all I had never tried this before and will be happier than not if the house hold freezer does indeed appear to age this steel sufficiently enough to consider it beneficial to our purposes. If it does I will post it here. If it does not I will not.

RL
 
I should have continued to follow the thread it's become interesting. Jason has been studying metallurgy, that's good. Whether we call it cryogenics or something else is only a matter of semantics. I was amused that the cryogenic-tempering website, only as a side note , talks about tempering (with heat ) after chilling. I must strongly remind you to always temper after chilling because the chilling forms UNTEMPERED martensite which is extremely brittle so must be tempered.This is why I don't like the term cryogenic tempering too easy to cause confusion. Each steel has its own characteristics so we can't generalize about what works and what doesn't as far as temp and time . In addition austenitizing temp can have a great effect on the amount of retained austenite and apparently some use cryo to correct sloppy hardening. That reminds me of the time I saw 4340 with 100% retained austenite !! There are applications where some retained austenite is beneficial such as bearings. What is important is that it doesn't transform during use since you would then have untempered martensite. In this regard the multiple tempering can stabilize the retained austenite.... Bartblade we'll have to wait a while for Roger to determine which ice cream flavor does the best job of getting rid of austenite.
 
Here is how I do it.
1) I use only low alloy carbon steel. Stainless requires very exact heat treatment and leaves no room to improvisation: if you want a good quality blade you have to respect the heat treat parameters of the steel to the degree, give or take 5-10 degrees.
With carbon steel you just have to heat to non-mag, soak for some more seconds to a minute and quench.

2) I use two burners like yours, but don't use them in open air. I took a soft refractoory brick and hollowed it inside creating a caviti with an oval section, and practised two holes at 1/3 and 2/3 respectively of the brick's length so that they are angled 45° upwards.
I put the burners in there so that the hot gases swirl in the chamber and even out the temperature as well as possible.

3) I put the blade inside, heat to non-mag, let air cool, and repeat (normalize. This refines the steel's grain and allows you to see if there is any residue tension in the steel (the piece will warp). if the piece warps I straighten it while still red hot with a few raps on the anvil. You don't actually need an anvil for this. A bench vise is sufficient. If there was any warp, I normalize again.

4) I heat to non-mag, soak and then immediately quench.

5) I grind away scale to expose bright steel, then bring back to the forge and use the hot gases coming out of the forge mouth to heat slowly up the steel to the appropriate color (dark straw to brown for a knife).

I then finish the knife.

Keep in mind that some steels are water-quenching, others are oil quenching, but in the sections most knives are, water quenching is a big NO. Even if the steel is marked as water quenchable you'll probably crack it. I'd say not to use water unless you are under 50 points of carbon.
Under 40 water is safe and under 25 superquench is in order.

FREEZER: I don't think is any good. Cryo treatment transforms remaining austenite in hi-alloy SS steel into martensite but for what I know liquid nitrogen is required, or at the very least an acetone/dry ice bath. Don't ever think of quenching a hot blade in such a bath unless you wanna see serious hollywood special effects and get seriously crispy! the blade is put in there cold
 
Alarion, tempering by color - the color is the color of oxide and only that .It depends on temperature and alloy. The tempering process is to stabilize the very unstable untempered martensite. To properly temper, the steel should be held at least one hour at temperature . That can be much better done in a kitchen oven since only 400F is typically needed. How many times have you been to the arms and armour museum in Milano ?
 
I thought I was going to be gone today and not get back in time to pull the test piece from the house hold freezer so I pulled it last evening after 6 days 7 hours in the freezer. I do not believe my freezer to be close to -20 F. but rather closer to 0 F.. So this may not have been a completely fair test. However, since there may have been a seemingly small change I thought I should post my Rockwell results.

The test piece had been tempered before putting in freezer and it had tested 58.5 HRc (rounded to the nearest 1/2 point) before placing in freezer for 6 days 7 hours. After pulling from freezer and allowing to still air warm to room temperature I Rockwell tested it across its surface with 6 tests. The results were 58 HRc with each of the six testings (rounded to the nearest 1/2 point).

RL
 
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