View Full Version : S30V - gets hard, stays hard


SteveS
07-20-2004, 11:24 AM
Looking at the Crucible data sheet you can see that S30V at higher austentizing temps, this stuff doesn't temper to lower points - and now I know first hand - and it seems weird.

I've been using 1950 and 1975 (setting on my oven, but have no way to test its accuracy). But wasn't getting the full hardness. Well this time I set my oven at 1985 for this latest blade. Quench with aluminum plates and compressed air. Quick temper, LN soak. Then 2 tempers at 450 - didn't dent the hardness. Then another temper at 550 no real change.

Last night I gave it 2 tempers at 600 for a full two hours each. The blade is still at ~61HRC.

Point is once this steel is fully hardned it stays there! Not like carbon steels.

I'm not complaining, I was just astonished that tempering will not lower the hardness below a certain point.

That whole secondary hardening effect is kinda puzzling to me.

Steve

Quenchcrack
07-20-2004, 11:59 AM
I checked out the Crucible site for this stuff. Wow! No wonder it does not get soft! The high carbon combined with the chromium, moly and vanadium will form massive carbides. You probably are tempering the martensite formed during hardening but with this much alloy carbide, the bulk hardness is not going to change much. Tempering will not change the hardness of the carbides. This stuff should hold an edge but I sure would not want to re-sharpen it by hand!

SteveS
07-20-2004, 12:14 PM
Thanks! That makes sense and helps more than you know.

See if I've got this right: At higher austentizing temps it is forming more alloy carbides than at lower temperatures. These carbides are not effected by tempering.

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Given that, then the characteristics of the blade are going to be substantially different than a blade austentized at 1900, even if the tempering is such that the HRC's match.

Using the crucible tables. I could use 1900/400 and get 59HRC. Or 2000/600 and get 59.5. However the second blade is going to have far more carbides.

Now I wonder what blade is tougher?

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Also, I'm not seeing the hardness drop below 60 like their chart. This happened before with a blade treated at Michael Vagnino's (he has a better hardness tester). It came out of the LN at 64.5 (as I recall). It also never got below 60.

Steve

RJ Martin
07-20-2004, 05:04 PM
Steve: I use 1990F for S30V here in my shop, so we are very close. I also like to soak the steel about 20-25 min at temp. What soak time did you use? The hardness numbers you are getting sound about right-give or take a bit of variance for your Rockwell tester, which is common unless you have a real good unit and it is fully calibrated.

As far as the tempering, note that the tempering range runs up to 750F. You can definitely soften that steel more, you just need to temper it at a higher temp.

One other point-Have you verified that 600F on your furnace controller is really 600F in the furnace? Reason I ask is that some furnaces aren't quite as accurate at lower temps as they are at the higher end. And, the same goes for high temp. Many furnaces advertize +-1%, which is still 20F @2000F. And, my experience has shown that they change over time. Since you've gotten good results setting at 1985F, I'd use that from now on.

S30V is a wonderful steel-You're getting great results-lots of very small, really hard carbides and lots of free chromium for stain resistance. I like to say it has a "crisp" or "snappy" heat treat response-It does what it's supposed to!!

JGardner
07-20-2004, 06:09 PM
I heat treated a couple of S30V blades using 1975 F for 25-30 min with quench plates and compressed air. I don't have a Rockwell tester so this is kind of crude, but straight out of a quench plate quench I could very slightly pull metal off of the cutting edge with a file. The same knife after 24 hours in LN was hard enough that the file would skate across the cutting edge. No surprise there. The last two knives I have heat treated were tightly packed in foil held at 1975 F for 25-30 min then oil quenched, foil and all. Out of the quench I couldn't touch it with a file, let alone after an LN cycle. It was night and day difference between the two quench types. Just something you might consider.

Justin

blademan
07-20-2004, 07:32 PM
Here is what works for me, this formula comes up with a
58-59HRC, I'm sure there are other makers out their that have
there own formula that works just as good

CPM-S30V
Preheat thoroughly at 1400 /1500 F and hold for 30min
Then raise temperature to 1950 F and hold for 1 Hr
Air quench at room temperature

Tempering
1st Temper at 750 F for 2 hr, 1 hr for smaller blades.
Subzero quench for 8 hr?s
2rd Temper at 750 F for 2 hr, 1 hr for smaller blades (No subzero quench)

Spring Tempering
(For Slip Joint Springs)
1st Temper 1150 F for 1 hr
Subzero quench for 8 hr's
2rd Temper 1150 F for 1 hr, (No subzero quench)



James

Quenchcrack
07-20-2004, 07:59 PM
Steve, These tool steels operate on a slightly different principle. Below about .70%carbon, you can pretty much dissolve all of the iron carbides if you austenitize for a sufficiently long time. When you quench, most of the carbon will remain trapped in the martensite. The carbides will precipitate when you temper it.

With tool steels, a carbon content of 1.45% and more chromium, molybdenum and vanadium than can combine with the carbon, you get very stable alloy carbides. If the steel is annealed, you will have a lot of spheroidal carbides in a ferrite/pearlite matrix. When you austenitize it, you have to heat to very high temperatures, eg, 1900F to get some of the carbides to dissolve into the austenite. However, you will probably never dissolve all of the carbides. When you quench it, you will get martensite with a lot of carbides in it (the ones that did not dissolve). When you temper, you get even more carbides, but smaller. These carbides act like little ball bearings that contribute to the extreme abrasion resistance of the steel.

SteveS
07-20-2004, 10:07 PM
Wow Blademan, that seems high, but I'm tempted. 61 seems to high for a thin kitchen knife. The rest of your procedure is pretty much what I do.

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QC - Now you've really raised the question of brittleness. Seems to be it might hold a great edge, but won't hold together!

Steve

SteveS
07-21-2004, 11:37 AM
I put an edge on this blade. Chips way easier than the other S30V knives I've built. I've never failed to chop fir with even the finest edged kitchen knives. Never had chips. This blade is a whole different story.

It's back in the oven for 2 - 700 degree tempers.

However, I'm still worried that blades hardened in this temp range will always be brittle. Man I hope this works!

I'm going back to 1950 or 1975 if I can't reduce the brittleness.

QC: When this many carbides form (as you explained) is brittleness always going to follow? Will increased tempering temps solve it or make it worse?

Steve

RJ Martin
07-21-2004, 04:52 PM
Steve: Not knowing the geometry of the blade, edge thickness, etc, or your soak time, it's hard to comment on your question. I believe that you can re-temper this knife, although I'd probably start at 650, then go to 700 if the hardness didn't come down enough.
One of the benefits of the many small carbides is that they aren't supposed to reduce toughness as much as larger carbides do, so, I'm inclined to say that you can still end up woth a tough blade, if you just temper it back a bit.

Quenchcrack
07-21-2004, 06:25 PM
I am inclined to agree with RHJ Martin. Temper it again as he suggested, Big carbides usually pose less of a problem with brittleness unless you get them at the grain boundaries. The S30V is a powder metallurgy grade, if I am not mistaken, and problems associated with grain boundary carbides, as was typcial of D2 that did not get enough hot work, is much reduced.

SteveS
07-21-2004, 10:19 PM
Well, it's out of the oven - 700 degrees (my oven's setting and not double checked).

It still reads 61!

Personally I think the whole thing is strange. I've tempered 2 hours 2 times at 450. Then 2 hours 2 times at 600. Then 2 hours 2 times at 700.

Well at least it doesn't chip any more.

But I'm going back to 1975, that's for sure.

Steve

RJ Martin
07-22-2004, 08:12 AM
Steve: Have you checked your hardness tester using a test block of known hardness? What kind of tester do you have?What kind of oven are you tempering in?
Also, I am still curious to know what was your soak time during HT?I know, I'm asking a lot of questions, but, the answers may be part of your problem.

SteveS
07-22-2004, 10:38 AM
RJ,

Thanks for the help! It's no problem answering your questions. Actually now that it's stopped chipping it's not so much of a problem as a mystery - to me.

Next I have to make sure it doesn't chip on bones - it's a kitchen blade.

Blade is 8", 3/32" thick to a Convex edge

The treatment cycle:

Stress Relief:
1200 for 2 hours
Oven cool

Hardening:
Blade in foil
Into cold oven
Moderate speed Ramp to 1550
Soak 20 minutes
Fast ramp to 1985
Soak 20 minutes
quench in foil
1" thick aluminum quench blocks
Blow thru gap with compressed air

temper 1 hour 400
LN soak 8 hours

2 tempers 2 hours @ 450
62 HRC

2 tempers 2 hours @ 600
61 HRC
blade edge chips chopping wood

2 tempers 2 hours @ 700
61 HRC
blade edge doesn't chip chopping wood

The hardness tester is a portable "diamond service". It's like the Riehl tester. I do have a calibration block of 62.5HRC and another blade of known hardness. It tests fine. Of course it's not like a bench model, but it is consistent.

The mystery is probably the carbides. If I understand quenchcrack righly, those are going to stay there and stay hard. The iron matrix around then is probably getting softer. Maybe that's why it measures hard, but the performance changed?

I've never tempered at this temperature and never expected to see this. My fear of tempering even higher is running into the secondary hardening phase!

Steve

RJ Martin
07-22-2004, 02:07 PM
Steve: You could certainly go a bit hotter, say, 750F and see what happens without risking having the hardness go up. Sounds like you've solved the problem, though.

My only suggestion is to eliminate the forced air application during your quench. In combination with the foil, you are probably not cooling the blade uniformly, and, this steel simply doesn't need the extra speed that the air is adding. Those aluminum plates suck that heat out much faster than is necessary, even at the edge where you don't have contact with the plates.

Picture some parts of the edge contacting that foil, some not, and then how erratic the quench could be when the forced air is applied.........

One last point is that, I like to leave a bit of extra metal at the edge, so I can remove .010 or so without really eating into the blade. The unsaharpened edge of a knife blade often has little stress risers in it, and, taking off a bit more material during the first sharpening helps get rid of those.

rlinger
07-22-2004, 03:51 PM
Steve,

I moderately ramp to preheat temperature and hold only 7 min.
quickly ramp to 1950 minimum 1965 F maximum and soak 25 min.
rapid air quench
snap temper @ about 300 F / ! hr.
deep cryo minimum 12 hr.
temper @ 400 F / 2 hr (twice)

About 60 HRc

I am fairly sure there is a lose of toughness by using the higer austenitizing temperature. Tempering very high will result in lose of corrosion resistance. Except for BG-42 I try not to temper higher than 600 F. 400 F has worked well for me on S30V. I sometimes temper ATS-34 as high as 600 F and have gotten about 60 HRc.

RL