View Full Version : Retained Austenite - does it hurt?


SteveS
12-28-2004, 12:03 PM
Let's say that I have 2 blades in S30V. Both as quenched at 61HRC. Then one is tempered to 59. The other gets a cryo treatment and comes out at 63, then tempered to 59.

Now I have two S30V blades at 59HRC. I presume the first still has retained austenite and is some how inferior. But what's the practical difference between the two? Edge retention? Toughness? What?

Thanks,

Steve

Quenchcrack
12-28-2004, 04:18 PM
Steve, Rockwell hardness is considered a Bulk Hardness, that is, it makes a HUGE impression compared to the size of the individual grains of metal. It is an average of the various constituents hardness. A few percentage points of retained austenite might not be measureable by Rockwell C. If you gain 1-2 points Rc after cyro treating, there must have been a considerable amount of retained austenite (20%?). More martensite will give better edge retention. The subtle danger in retained austenite is that it can transform to martensite by getting colder than the original quench temperature or by mechanical impact. If this happens after the blade is sold, this small amount of martensite will not get tempered. A crack can start in this material and possibly propagate to failure. The issue of retained austenite is of great concern when parts are being carburized. The high carbon case tends to retain austenite. If the part is in a wear application, as a large bearing pin, the mechanical impact and wear can transform the austenite, and possibly forming a crack. Now, this may be of very little practical concern to most knife makers. However, I notice that a lot of pro makers cryo treat as a matter of standard practice. Then again, a few always point the blade north when they quench it, too! :rolleyes:

mete
12-28-2004, 05:14 PM
In certain applications such as bearing some retained austenite is beneficial.Cryo should never be used to correct for poor heat treating . Heat treat properly because excessive austenitizing temperature increases retained austenite and also grain size [which can't be reduced by cryo]. Some gain in toughness and some loss in wear resistance would be seen without cryo, though it might not be significant.

SteveS
12-28-2004, 06:02 PM
Thanks guys, that helps quite a bit.

So the goal of cryo is not raising hardness for hardness sake. Rather it's for improving the performance of the blade in two ways:

* edge retention
* reduce the likelyhood of cracking/chipping

Cool!

Steve

shgeo
01-01-2005, 11:29 AM
.... Then again, a few always point the blade north when they quench it, too! :rolleyes:

Hopefully they will remember to have the magnetic declination set on their compasses or they may lose whatever effect they hope to gain... :rolleyes:

mete
01-04-2005, 11:41 PM
I should add here that in hte more complex alloys you will always have some retained austenite. I one test with O-1 cryo reduced retained austenite from 8.5 to 5.5% .I don't think that that's VERY significant . Poor heat treating can produce much higher amounts of retained austenite .Do the initial heat treating right and don't think you can correct a poor job with cryo.

Jerry Hossom
01-05-2005, 10:04 AM
It's been my experience that everything gets amplified in knife blades. Tiny cracks that might not affect a large part become a major source of failure on a fine edge if the blade is subjected to lateral stress. Anything that reduces crack formation, including finish, is a plus IMO. To be honest I'd never considered the thought that Quenchcrack introduced about subsequent transformation to untempered martensite, but it certainly makes sense and since the area of the blade most susceptible to carburization is the edge, that's the worst possible place. It would seem that it might also tend to amplify/promote any other source of failure, microchipping, etc.

I guess there's a good reason many of us use Paul Bos for heat treating. Exquisite control, zero decarburization, very fash quench to Ms, and cryo, on everything.

RJ Martin
01-05-2005, 05:14 PM
Steve: Yes, cryo treatment, followed by the proper tempering treatments, improves the edge holding and toughness of knife blades, particularly those made from the more highly alloyed steels used by professional makers. As you have seen in the recent posts on these forums, there is still considerable debate as to what cryo actually achieves and what it does not. Mete was quick to dispute my post on rifle barrels and eta carbide precipitation, and, he well may be correct. In a recent conversation I had with Scott devanna, a metallurgist with CRUCIBLE, he indicated that some of the stainless steels used for rifle barrels would in fact benefit from cryo. So, it just goes to show you that there are varying opinions, or that one answer doesn't fit all steels. And, there is always something new to learn.

Mete: In my earlier comment on the effects of cryo on oversoaked steel, I should have specified, as you did, that cryo should not be used to salvage badly treated parts. I was merely using that as an example of the tangeable changes that cryo can produce.

Jerry: Yes, Paul Bos is an excellent heat treater, used by many makers who choose to outsource this part of their knifemaking. If a maker lacks the equipment, expertise or interest to perform this critical operation in-house, I'd recommend Paul Bos hands down.

For me, one of the greatest rewards in knifemaking comes from taking that ground blade and turning it into a high performance cutting machine throught the precise application of time and temperature. I love to heat treat! I'd part with most of my knifemaking equipment before I would consider giving up my CRESS furnace, hardness tester and dewar.

tmickley
01-05-2005, 05:29 PM
Hey RJ, since you seem to hang out here the most, congratulations on getting the cover of March Blade. Very nice.

blgoode
01-27-2005, 01:16 PM
Speaking of March 2005 Cover. Way cool RJ and nice to read HT info. I need to visit this place more.

SteveS
01-27-2005, 02:13 PM
Now that I'm re-reading the thread, I hope the issue is settled:

We don't cryo to raise the hardness, we cryo to improve the performance.

Seems everyone agrees on that point (so to speak). SO the next time someone tells me the purpose of Cryo is raising hardness I can point them here.

(Purpose is a different thing than side-effect or attribute.)

Steve

Quenchcrack
01-27-2005, 07:53 PM
Don Fogg, Daryl Meier, myself and a cast of thousands just had a rambling 5-page discussion of retained austenite. If you have not had enough of this topic here, it is on Don's site under "metallurgy and other enigmas". His site is www.dfoggknives.com. :p

mete
01-27-2005, 09:17 PM
That thread is so long that Daryl had grandchildren during it !!.... The problem with "hardness " is that all too many look at hardness as the sole objective as a measure of performance .It's a lot more complex than that. ;)

Quenchcrack
01-28-2005, 07:21 PM
Mete brings up a good point; hardness is not necessarily the only factor in abrasion resistance. I mentioned it before somewhere: massive carbides, like you get with D2, make for extremely good abrasion resistance. High carbon makes massive carbides possible. 5160 is one of my favorite steels but it is NOT the most abrasion resistant steel! :o

tonn
01-29-2005, 09:13 AM
Here is some graphs:
http://www.internetbusinesslinks.net/album/tonn/graphs1.gif

a. shows starting temp and end temp related to C%

B. shows % of retained austenite related to C%


http://www.internetbusinesslinks.net/album/tonn/graphs2.gif

Here is hardness related to C%. Shaded area shows difference betveen cryo & no cryo.

Hope it helps & keep it to the north :D

BTW - softer blades cut more smoothly than harder ones!

mete
01-29-2005, 11:01 AM
I should have mentioned my favorite example of hardness vs wear resistance , that is Talonite. Hardness only in the low 50s but exceptional wear resistance because of soft matrix but lots of carbides.....Thanks for the graphs , Tonn. Now we know that it all holds true even in Estonia !

tonn
01-29-2005, 12:43 PM
I should have mentioned my favorite example of hardness vs wear resistance , that is Talonite. Hardness only in the low 50s but exceptional wear resistance because of soft matrix but lots of carbides.....Thanks for the graphs , Tonn. Now we know that it all holds true even in Estonia !

It's true only in big citys! Not sure about countryside :D

Quenchcrack
01-29-2005, 06:54 PM
Tonn, Interesting graphs but there is a danger in using such graphs without understanding what materials they tested and what conditions were used to create them. Questions I have are;
1. At what temperature were they austenitized?
2. What was the section thickness?
3. What was the quench medium?
4. What was the composition of the alloy used to test?
5. What method was used to determine the % retained austenite? Visual? X-ray diffraction?

Data posted without the details of the experiment can lead people to assume these curves apply to ALL steels and that may not be true.
:confused:

tonn
01-30-2005, 06:07 AM
Quenchcrack!
I never heard such kind of graphs used to heat treat any specific steel, for what steel manufacturer gives instructions. As discussion about retained austenite & sub-zero quenching raises time at time in many threads I find reasonable to post them just to make it more visual.

Graphs are from book for students. I met one author - he's a proffessor in department of material science in Technical University - and asked about sub-zero quenching. Belive or not he could'nt give any answer :D

Quenchcrack
01-30-2005, 10:30 AM
Tonn, I understand and appreciate your motivation for posting the graphs. They do illustrate the issues we have been discussing and I thank you for that. Perhaps I am being overly cautious that we do not lead the new bladesmiths down the wrong path by letting them think that all steels behave similarly and that the percentages of retained austenite are the same for every instance. Forgive me if I was too critical. :o

kababear
01-31-2005, 03:02 PM
the hardness-pic is for plain carbon steel as well as the diagram showing the martensite transformation temperatures. my proffessor (also material science) is using the same diagramms.