Is anyone still using S90V ( CPM 420V ) ?

I think myself foolish for putting such pressures on untempered steel but I Rockwelled test piece #1 as quenched and it was 63 HRc. Before allowing test piece #2 to fall below what I thought was about 120 F. I put it in the 400 F. tempering oven. Test piece #1 is in deep cryo. Each piece will be tempered twice @ 400 F..

After the first temper of test piece #2 I will post the HRc value of it, this evening. Test piece #2 goes in cryo after Rockwell. Each test piece will be tempered twice for 2 hours 15 min. per.

I was concerned about the foil I used rated at 2000 F. but it held through quench just fine and all I got, noticably, was discoloration of the test pieces. The foil failed though when I put the pliers to it to unwrap the packet, so at temperatures such as S90V requires 2000 F. foil should be handled with the care you would normally afford your warp subject blades - deliberate care.

RL
 
Test piece #2 has gone through 1st temper and is now in cryo. After 1st temper it tested 60 HRc.

RL
 
That's fantastic, Roger. Looks like this steel can be HT to 60. the only question that comes to mind is that these are small pieces, and they would cool ( Quench ) very quickly compared to a blade. It sure looks good, though. Jason, thank you for your report on the use of your knife. I realize that this is not a tough steel, but carving wood is a real life test, because it is very easy to twist while cutting, thereby placeing lateral stresses on the blade.
I wonder, at HRC 60,CPM S90V might be the best edge holding stainless steel. I know what you are thinking, edge holding on what material. Thanks again Roger.
 
The heat treat for test piece #2 is complete (soak, temper, cryo, temper), It is 61 HRc.

Test piece #1 is now in final (2nd) temper. This evening I will have its results, which I expect to be harder. I will also report the HT receipe I used for both test pieces.

RL
 
Okay, test piece #1 is out of final temper and Rockwell tested. IT IS 63 HRc.

Essentially, both pieces were heat treated identically except that one piece was placed in cryo before 1st temper and the other between 1st and final temper.

The test pieces were heat treated as follows:
---------------------------------------------

Two approximately 1 inch square test pieces of CPM S90V, each 1/8th inch thick, were foil wrapped end-to-end, not stacked, in the same packet.

Ramp to 2100 F., soak 20 minutes

Rapid air quench

Deep cryo test piece #1, only, after cooling to room temp.

Temper test piece #2 before falling below approx. 120 F.

Deep cryo test piece #2 after 1st temper and cooling to room temp.

Temper both test pieces so that both are double tempered.
-----------------------------------------------

All tempers: 400 F.

Time for each temper: 2 hours 15 min.

Test piece #1 was in deep cryo 16 1/2 hours

Test piece #2 was in deep cryo 13 1/2 hours.
------------------------------------------------

Results
-------
Test Piece #1: 63 HRc
Test Piece #2: 61 HRc

In summary: test piece #1 was 63 HRc as quenched; #2 was not tested as quenched. Number 2 was 60 HRc after first temper. Cryo between tempers seems to have enhanced #2 hardness by 1 point, which is significant. Cryo before temper of #1 seems to have greatly enhanced its hardness. I have noticed the same type results by cryo tempering ATS-34. I was doubtful of these values because of what the data sheet told me to expect. I double checked my instrument with a standard to be sure I was getting good results.

Just for grins I am tempering test piece #1 again @ 700 F. to see if I can get it down to 60 - 61 HRc. In using this steel for a blade I would cryo before temper since it is indicated strongly by my testing and advise given me by others that a far greater percentage of austenite is transformed by doing so. I will report the 700 degree temper results.

RL
 
Tempering again at a higher temperature, in this case 700 F., did not change the hardness of the test piece. It remains at 63 HRc. This may mean I have just learned another basic lesson in tempering. It seems to have been set by the original temper sessions.

One thing that is remarkable I think is the relative ease of grinding after hardening. I ground at slow speed after the 700 degree temper to see if I could get rid of the previous dimples caused by the Rockwell tester. I was impressed by the relative ease in doing this. This steel may be both a great edge holder and offer ease in resharpening. Its probably just because I nailed the HT :D. It does grind as if the grain is well refined.

That is all except that I will be happy to send the test pieces to anyone good at judging grain if you want to test them. One note: I expect to leave town for a couple weeks beginning this coming Sunday, so sooner the better but I will keep the pieces so later is still okay.

RL
 
Kevin, well now - slow speed yes but I ground it with a 120 grit ceramic. I really wasn't attempting to get a fine finish but rather a flat surface and not too rough for Rockwell testing. Actually, it stood out to me how easily it ground at 63 HRc. I remain impressed. There is after all the opportunity that I did nail it and the grain is fine enough for that. I have offerd to send them to an unbiased expert, so I am not trying to BS anyone.

RL
 
Good grief! These results are astounding. For starters, I am amazed that you achieved this high Rockwell. I would have bet big money that subsequent tempering at 700F would have lowered the Rc. The fact that it didn't, reduces the concern that heat from subsequent grinding could ruin the temper. How do you get HRC 60? Temper at 450-500F initially. Now we know how Phil Wilson got the HRC of 62-63 that Cliff reported. How much more brittle is this stuff at Rc 63 as compared to Rc 60? Roger, I do indeed understand your fasination with the heat treating process and results. I know we all appreciate your efforts, not only here, but on the other threads as well. My hat is off to you, and thanks is just not enough. Great work!
 
Don, very happy to be able to do these type things. I would have to guess that 700 - 750F. might get us about 60 HRc, which is about were I would want it if mine (59 - 61 max.). I am of course considering deep cryo before temper included in that.

Let me know if you want more tested.

As for the results of re-temper at 700 F. I would like for Mete to help explain.

Roger
 
Roger, I to would like Mete to comment about the Rc not getting lower with a subsequent 700F temper. This goes against all my prior understanding of heat treating. It is still strange to me that the HRC values are much higher than Crucible lead us to believe. Is it possibe that these small pieces cooled much quicker than a blade, thereby becoming harder as quenched. Remember that Maria had reccomended 2100, oil quench, and temper at 500F for a 59-60 Rc. It just might be, however, that cryo right after quench has made all the difference.
On a slightly different note, think about all those people heat treating without a Rockwell tester. It is tough enough, to try to understand what is going on, even using a Rc tester.
 
(speaking of air quench steels)

Don, it is written and I believe I have found that small pieces, such as we make, can match oil quenching with a rapid air quench. For thin parts I have confidence in the foil wrapped air quench. We often read here of some of us using aluminium plates to quench their foil wrapped blades. The only reasons I have not tryed that myself is because I find it difficult for me to understand how I can quickly get two flat pieces of metal perfectly flat against a double beveled blade, and too because I have found a rapid air quench for these relatively small parts sufficient enough. (I should explain: in double beveled what I mean to say is beveled on both sides of the blade.)

Because I do not remember you mentioning that Maria specified cryo to obtain the 59-60 I would suggest my results agree with what she told you. Remember on test piece #2 I got 61 HRc by cryo between tempers and that I tempered at 400 F.. By that, I would expect about 59 without cryo and 100 degrees higher on the temper. By keeping my temper at 400 F. and without cryo I am guessing, with good reason, I would have gotten 60. Sixty HRc is what I got after first temper and before cryo on test piece #2.

Far more austenite will be transformed to martensite by cryo before temper.

As for the small test pieces; they are 1/8th in cross section. The blades will be at the very most 1/8th in cross section and only at the ricasso. The vast majority of your blades of the same stock will be thinner and therefore quench more quickly where it really matters most.

RL
 
In another post where rlinger asked about this 700F not resulting in any decrease in hardness, Jason Arnold replied:
Originally posted by Jason Arnold
Perhaps secondary hardening? A drop off occurs in HRC after a particular tempering temp and then begins to raise again as alloy carbides precipitate. Hard to say for sure without watching the whole process. Hope that helps.
Click to expand...

Here is a Q&A I harvested from a couple Paul Bos Posts:
Q. Is there an advantage to tempering [ATS-34] in the 400-450 deg F range as apposed to the 900 deg F range. Seems to me there was better corrosion resistance or something. The lower temp is a lot more user friendly as well. I have done the heat treat and cryo and now want to temper back to the 59-60 Rc range.

A. About 90% of my customers ask me to do the 950 temper, the others ask for the 400 temper. Paul.

Q. Can you tell me what the outcome is for the two different temps. Do they test to +/- 60Rc at both temps? Is the temper time the same ie: 2 x 1 hour. They seem like such drastic differences in temp that there should be a different outcome.

A. Your 14-4 steels have a secondary tempering range like D-2 and other tool steels. At 400 degF you get a 60 R/C, then at 950 degF it comes back up to 60 R/c. All tempering should be 2 tempers at 2 hrs. per temp. Paul.
Click to expand...

Ok, but S90V is not a 14-4 steel, it's 14% chrome but only 1% moly. But with 9% vanadium, that could also be a factor in secondary hardening, if that's what's going on.

So digging a bit more...last paragraph on this page:
http://www.key-to-steel.com/Articles/Art91.htm
Chromium, molybdenum and vanadium, on the other hand, which migrate to the carbide phase when diffusion is possible, bring about a retardation of softening, particularly at the higher tempering temperatures. These elements do not merely raise the tempering temperature; when they are present in higher percentages, the rate of softening is no longer a continuous function of the tempering temperature. That is, the softening curves for these steels will show a range of tempering temperature in which the softening is retarded or, with relatively high alloy content, in which the hardness may actually increase with increasing tempering temperature. This characteristic behavior of the alloy steels containing the carbide-forming elements is known as "secondary hardening" and results presumably from a delayed precipitation of fine alloy carbides.
Click to expand...

Or it could be a simple flat curve in temper for S90V as mete suggested could be the case (boy, a hardness vs. tempering temp family of curves (based on as-quenched) would answer a lot of questions... listening Crucible?):
Originally posted by mete
Crucible doesn't give the tempering hardnesses for S90V so I assume there is little change. The S60V doesn't change very much. More simple steels would change.
Click to expand...
 
Rob, That was nice of you to post that. Thank you. I wanted to email Don so to call his attention to this but have been real buisy today.

On ATS-34 I have rather recently done three knives, all deep cryo before temper, at different temper levels. The one I tempered in the mid 900 F. range is 59.5 HRc. The one I tempered at 475 F. is 61 HRc. The one I tempered at 600 F. is 60 HRc. I always double temper these stainless steels for 2 hr. 15 min. per. I have tested test pieces of ATS-34, cryo before temper, and achieved 62 HRc with 400 F. tempers.

I am inclined to have to agree with the secondary hardening theory. They are after all classified, I believe, as such steels. What agrivates me is I knew they are going in and it was under my nose the whole time.

Don has decided to trust me to HT a couple S90V blades for him and by my request has kindly agreed to send me two more test pieces with those blades. I want to HT one test piece with cryo and 700 F. tempers to see what I can get. I hope its closer to 60 HRc.

Thanks again Rob for this info. I hope Don sees it to.

BTW, yes - I plan to no longer temper martensitic stainless above about 750 F. because of loss of corrosion resistance and because of the chance of embrittlement (although I believe the embrittlement problem occurs closer to 1000 F.) .... and I want again to thank Kit for calling my attention to that some few months back.

RL
 
Aw, shucks... cross posting info and a quick research blurb was the least I could do. And I'm always asking myself... "Self... what is the least I can do...".

Computer stuff is easy...
Originally posted by Don Powell
...These results are astounding. ... How much more brittle is this stuff at Rc 63 as compared to Rc 60? ... I know we all appreciate your efforts, not only here, but on the other threads as well. My hat is off to you, and thanks is just not enough. Great work!
Click to expand...
... but the heat treating stuff takes equipment and real work. Very interesting results by the way! Thanks for posting the heat treat steps in detail so they can be understood. That doesn't always happen when people post, as you know.

* Blade #1...
rlinger
Far more austenite will be transformed to martensite by cryo before temper.
Click to expand...
The risk is of course that not doing a snap temper might produce some stresses that could show up as small microcracks, latent brittleness. The subsequent tempering should have helped mitigate a part of that risk, maybe not all.

* Blade #2... seems more typical that a temper (at least snap) is done before cryo, probably safer microstructure-wise.

Would be interesting to know what Phil had found out... is a snap temper really required before cryo with the CPM's, or with any of the CPM's in particular.

Don's question about residual toughness/brittleness at Rc63 vs. Rc60 is a key question. The heat treat methodology taken to arrive at the two hardnesses is a big driver, but I sure wish Crucible would post tempering temp vs. toughness curves for ALL of their steels, not just those typically used where toughness is required (O1, L6, S5, S7, A2, etc). They do post some HRC vs. Charpy for D2.

440V/S60V is known to be quite a bit more brittle than 420V/S90V.

Here is a blurb combining what little data I could find on 440V/S60V and on 420V/S90V from Crucible:
Code: 
               HRC    Charpy C	
		       Ft-lbs	
CPM S60V	56 	16	
CPM S60V	59 	12	
CPM S60V	60 	11	
CPM S90V	58	19
440C     	56	26	
440C	        58	16	
AISI D2	        59	21
S60V's toughness drops a lot (~1/3) from Rc56 to Rc60. S90V may be tougher at a given hardness, but it's not a "tough" steel. But S90V looks like it can be run 3 HRC's harder than S60V, in very general terms, while keeping toughness in the area generally expected by the general public.

Hard to draw generalizations from other steels typically, but some of the other steels have a "cliff"... the toughness drops precipitously above some hardness threshold, i.e. it's not a nice smooth "bell curve" of hardness vs. toughness, tends to be steep on the "right side". Unfortunately, Crucible usually just stops the data presentation at some point and doesn't show the user where the cliff might be.

I'd be concerned that Rc63 blades must be made for the person who understands this quite clearly, values resistance to edge roll highly, and confines the scope of work to thoughtful slicing with no twisting (the whittling example is a good one).

I'll regurgitate a couple of selected blurbs from RJ Martin that I stashed away. He's pretty experienced with the CPM's also, seems to be thoughtful about his approach and his heat treating (in-house):
Gentlemen: Here it is, plain and simple:
The more highly alloyed the steel, the more benefit from cryo. THis is because the higher the alloy content, the lower the Mf (Martensite Finish)temp. The colder you go, the more transformation you get. LN2 is capable of producing over 99% transformation in any knife steel that was properly heat treated to begin with. As to cooling rate, the "slow down", soak and "slow-up" cycles are to prevent thermal shock from uneven cooling in thicker parts, like cylinder blocks, etc.
Being thin, knife blades can generally be dunked right in with no ill effects, since they cool rapidly through their thickness.
A snap temper, or a full temper prior to cryo produces smaller grain size and reduces the thermal stress caused by the LN2 treatment.
Double tempering after cryo is the best. A single temper is a necessity.

from another RJ post:

Actually, cryo performed after a single temper, or "snap temper" (a temper lower than the actual temnper temperature but, high enough to relieve stress-generally, 300F) appears to offer the best benefit. In the early days of cryo, steel manufacturers stressed the need to cryo right after quench. This tends to produce larger grain size, and, more internal stress. If using LN2, the effect of the cooling is so strong that you can cryo at any point after the first temper-days, weeks, months and you will still convert retained Austenite to Martensite.

What is important is getting that just quenched steel right into the tempering oven while it is still about 150F so that the martensite responds well to the initial temper. By tempering the martensite formed during quench before performing the cryo treatment, you are helping the steel to not be overstressed during the cryo treatment.

Remember, the desired microstructure in a finished blade is tempered martensite, not martensite. You only need to cryo one time, and follow it with one or more tempers.

and from yet another post:

Snap tempering is done in the range of 300-350F for about 30 minutes, and relieves stress and sets up the microstructure for the deep freeze. An alternative is to perform one, full temper cycle before cryo. Research has been done that shows that going right into cryo without a temper of some sort can cause large grains-a slow deep freeze may help this-I'm not sure. Doing the temper (snap or full) IMMEDIATELY when the blades are still quite warm is key

Stay Sharp,
RJ Martin
http://www.martinsite.com
Click to expand...
 
Originally posted by rlinger
BTW, yes - I plan to no longer temper martensitic stainless above about 750 F. because of loss of corrosion resistance and because of the chance of embrittlement (although I believe the embrittlement problem occurs closer to 1000 F.) .... and I want again to thank Kit for calling my attention to that some few months back.
RL
Click to expand...
To rlinger's point, and from Crucible again, from S90V's data sheet:
Temper: Double temper at 400-750° F (200-400°C). Hold for
a miniumum of 2 hrs. each temper. For optimum stress relieving
and dimensional stability, S90V may be double tempered at
1000-1025° F (540-550°C), but tempering above 800°F (425°C)
may result in some loss of corrosion resistance. A freezing
treatment may be employed between the first and second tempers,
if desired. Freezing treatments should always be followed
by at least one temper.

PLEASE NOTE: Tempering between about 800 and 1000°F
(425 and 540°C) is not recommended. All martensitic stainless
steels suffer from embrittlement when tempered in this range.

[S90V]: Tempering Temperatures
500°- 800°F (200°-425°C) Best Corrosion Resistance & Wear Resistance
800°-1000°F (425°-540°C) AVOID THIS RANGE (Embrittlement)
1000°-1025°F (540°-550°C) Stress Relieving and Dimensional Stability
Click to expand...
 
Rob, all sounds good to me. Thanks again for going to the effort of posting such good stuff for all us.

Roger
 
It just gets better and better; this is what these forums are for. The very smart and knowlegeable people, that participate in these exchanges, continue to expand the knowledge and understaning of our knife steels. Thank you, Roger, Rob, Jason and others. OF COURSE it must be the secondary hardening that caused the 700F tempering to not lower the Rc. The point about the increased grain size and possible micro fractures, when cryo is done right after quench, is interesting, but Paul Bos does this routinely on high alloy stainless, seemingly without problems. So much of the information from the steel companies refers to much thicker parts, not our relatively thin blades. There may not be good correlation between those thick parts and our thin blades. It would certainly be nice to have hardening and tempering curves available from Crucible on all their CPM alloys.
 
Here is a blurb I harvested from a Paul Bos post:
Cryogenic Quenching
The freezer I use has a programable controle that steps down slowly to -280 degF using Liquid Nitrogen. All St/Stls are taken down as soon as they hit room temp from quench, I hold them for a minimum of 10 hrs. then bring them back to room temp the same way I took them down.

Then I do a double temper, 2 hrs. per temper. Tool steels are given a snap temper after quench before I do the Cryo then they go down the same as the ST.\ Stl. then double temper after. The reason I dont stick them right in the L-N is they could crack from thermal shock, It has happend to me before.

Dry ice and alcohol will get you down to -120 to get rid of your retained austenite, but the -250 to-300 will give you better edge retention and toughness. Liquid Nitrogen L-N dewers can be found at your local welding store.
Click to expand...
So Paul does say he does a snap temper after quench before cryo for "tool" steels.

So, left to wonder about what is a "Tool" vs. a "Stainless".

Guesswork:

Tool steels:
include A2 and D2. Maybe 3V?
M2 would be a HSS.

Stainless would include:
ATS-34 and BG-42 (14-4's for bearings)
440V/S60V
420V/S90V
440A/B/C
 
Sometimes the difference between stainless and tool steels is a bit vague and often the original use is one thing but other uses are different , don't worry about it......Secondary hardening - the addition of V, Mo, or W can cause formation of certain carbides durin g tempering at higher temperatures .Amounts of about 2%V, 8%Mo, and 15% W are needed. To get the secondary hardening effect the carbides and alloying elements must be thoroughly dissolved. Therefore low austenitizing temps or short times will not result in this secondary hardening and higher temps will promote more though there is obviously a limit because of the detrimental effects of too high an austenitizing temp......The labeling of steels as air, oil, or water haredening is only a general thing . Some of the typical knife steels called oil hardening are certainly air hardening in thin sections. That is all part of the subject of Hardenability......Roger that's a nice rifle on your postings but certainly you've made more . How about changing the picture or even sticking a knife in there.
 
You must log in or register to reply here.
Back
Top