Can someone explain this to me? (Opinel steel question)

O

Omega Leather Works

Joined
Jun 13, 2007
Messages
7,743
Hey guys. My friend and I have been modifying our Opinel's to make them suitable for wood carving. We both shorten the blades considerably. I use a Dremel to score a line then smack the blade to snap off the excess. My friend has had success using a set of bolt cutters.

Here's the part I'd like explained. When she went to cut the blade on an Inox blade (so far we had just been using the XC90 carbon blades) it wouldn't snap. You can tell from the pics that there was some effort put into this, yet it wouldn't break.

Is this a property of a softer heat treat maybe, or is there something in the inox that makes it less brittle than their carbon steel?

Take a look at the pics in post# 1194 -

http://www.bladeforums.com/forums/showthread.php?t=1046919

Thanks for any insight. :)
 
It's not that the stainless is softer. If anything, I'd bet it's several points higher in RC. Sandvik specs the 12c27Mod up to 59 RC, and I think Opinel took advantage of that. Their XC90 'carbone' blades are noticeably softer, which is much more apparent at very thin edge angles (I've seen this in the carbone and inox Opi's that I thinned; the carbone is much more vulnerable to edge dings and other impacts).

What you're seeing is the added ductility and toughness that almost always comes with the addition of large amounts of chromium to the steel. By 'ductility', I'm referring to the permanent plastic deformation seen when the steel gets bent. By 'toughness', this refers to the blade's resistance to breakage or fracture when subjected to hard impacts or other extreme force.

BTW, Sandvik is also known for the fineness of the grain in their stainless. That also contributes to toughness in a blade, as well as it's ability to take very fine edges.


David
 
I used to use the carbon Opinel's exclusively, but then I got a stainless Opinel that was a gift, and I was impressed at how much better edge holding it was. Noticeable. I now use the Inox Opinel's. They seem to be heat treated to a higher level, as well as being tougher. I once took two Opinel's, one carbon and one stainless, and pushed them to see how much abuse they would take. I'd always heard the stuff about how Opinel's are a good picnic knife, but not a hard work knife. I found out that was pure bunk. Both knives took a huge amount of deliberate abuse, but the carbon one snapped first. It took some real punishment to break the stainless knife.

Now, I'm not about to drop into the Amazon jungle with an Opinel as my survival tool, but it's nice to know how much one of your edc's will take. An Inox Opinel will take a lot. Since the Opinel is such a low cost knife, it was easy to justify testing it to destruction to find out if they were or were not a rugged little knife. They are. And the stainless is more so. Then I watched the bearded guy on youtube do the test on the Opinel number 13, and baton, pry, chop, and hack with no problem. It took it all in stride.

Carl.
 
Thanks guys. I know that XC90 has great impact strength, I didn't know that about their SS. That's a really low amount of carbon. Just north of what's necessary for hardening I believe, unless that only holds true for carbon steels. Alloying does funny stuff to steel... :)
 
Oh and jackknife, I had to cut the blade groove really thin before it would break! It was cool seeing the sparks from whacking it too. :D
 
Here's some pics so you don't have to dig through the carving thread.

IMAG1428_zpsdbe14614.jpg


IMAG1429_zps6f1ac23e.jpg


IMAG1433_zps24d29931.jpg


IMAG1434_zpsbea82331.jpg


IMAG1435_zpscd3b98c2.jpg
 
It's crazy that I'm complaining that my knife won't snap!
 
Strigamort said:
Thanks guys. I know that XC90 has great impact strength, I didn't know that about their SS. That's a really low amount of carbon. Just north of what's necessary for hardening I believe, unless that only holds true for carbon steels. Alloying does funny stuff to steel... :)
Click to expand...

Not exactly sure which you're referring to, as to the 'low amount of carbon'. I think the Sandvik is around 0.52%, which is decent for stainless, and made better by their very pure manufacturing process, as well as taking advantage of a good heat treat. Compare to Case's and Buck's 420HC, which both perform pretty admirably with only about 0.50% carbon. I think the XC90 is around 0.9% carbon, which is quite good for edge-holding, and compares in the same ballpark as 1095 and Case's CV. These numbers expressed as a fraction of a percent look small on paper, but big differences and improvements take place in rather large steps, as carbon increases in small steps above ~0.5% or so, which is the minimum threshold for what's known to be 'high carbon steel', useful for cutlery (stainless or otherwise).

And you're absolutely right; alloying definitely does some funny stuff to steel. Chemistry really changes things. :D


David
 
Last edited:
Obsessed with Edges said:
Not exactly sure which you're referring to, as to the 'low amount of carbon'. I think the Sandvik is around 0.52%, which is decent for stainless, and made better by their very pure manufacturing process, as well as taking advantage of a good heat treat. Compare to Case's and Buck's 420HC, which both perform pretty admirably with only about 0.50% carbon. I think the XC90 is around 0.9% carbon, which is quite good for edge-holding, and compares in the same ballpark as 1095 and Case's CV. These numbers expressed as a fraction of a percent look small on paper, but big differences and improvements take place in rather large steps, as carbon increases in small steps above ~0.5% or so, which is the minimum threshold for what's known to be 'high carbon steel', useful for cutlery (stainless or otherwise).

And you're absolutely right; alloying definitely does some funny stuff to steel. Chemistry really changes things. :D


David
Click to expand...

I'm no metallurgist, but I've read that .5% is the minimum for the ability to harden simple carbon steels (for cutlery, right). What that works out to for specific alloyed SS's I'm not sure, but if the same holds true for SS then .52% is right at the threshold. Usually when I'm referencing the knife steel app (you know the one) I'm looking for data on so called "super steel" alloy or carbon steels, both of which, typically, have a higher concentration of carbon. The number just looked lower than what I'm use to seeing, that's all. ;)
 
Wait, just re-read what you wrote. Okay, so .5% for HC stainless... Once it goes below that figure what contributes to hardening? I'm guessing it's the Chromium? That would be a really uneducated guess based on what I know about zdp189, which is just that it has a lot of Chromium, but it doesn't contribute to the stain resistant attribute... That's gotta be it.

I wish I had a class available for this topic specifically because it's kind of fascinating.

Just looked at the chart for zdp189 3% carbon(!) 20% chromium(!) okay, so most of the chromium is tied up in carbides. It's got more than enough carbon, although I don't know what's required for 67Rc(!)

Man, that's some extreme stuff. :D
 
Last edited:
Strigamort said:
Wait, just re-read what you wrote. Okay, so .5% for HC stainless... Once it goes below that figure what contributes to hardening? I'm guessing it's the Chromium? That would be a really uneducated guess based on what I know about zdp189, which is just that it has a lot of Chromium, but it doesn't contribute to the stain resistant attribute... That's gotta be it.

I wish I had a class available for this topic specifically because it's kind of fascinating.

Just looked at the chart for zdp189 3% carbon(!) 20% chromium(!) okay, so most of the chromium is tied up in carbides. It's got more than enough carbon, although I don't know what's required for 67Rc(!)

Man, that's some extreme stuff. :D
Click to expand...

Yep, it's extreme hard, and that's really about the carbon (which facilitates extreme heat treat to very high RC). I even have a ZDP-189 Leek with a broken tip to show for it's vaunted hardness. I actually broke the tip while stropping over a hard glass backing. When it's that crazy hard, it's also very brittle. That's the trade-off; extreme edge retention instead of toughness. I decided it wasn't for me, because I don't like worrying all the time about chipping, cracking or breaking the blade. With ZDP, a lot of that chromium has to be there, else the steel would rust like crazy (due to the extreme carbon). Some of that chromium is in carbides, but the steel doesn't seem as abrasion-resistant as the presumed carbide content might imply. It is hard, and a diamond hone is very handy for it, but still not like S30V or something similar, in wear-resistance. I've noticed the bevels polish up much more easily with AlOx or SiC compound on a strop, but that's a different story with S30V.


David
 
Generally speaking, enough Cr to make stainless will lower toughness, not increase it. However, the stainless used by Opinel has more going for it than just the Cr content. Lowering carbon is a very powerful tool to increase toughness. Cleanliness of the steel also has a large influence. I'd say these 2 combine to give the results you're seeing.

If everything goes right, 0.9 to 1.0 % carbon is all that's needed to get to 66-67. Oddly, the lower end of that scale the better, due to issues with excess retained austenite. Plotting hardness vs carbon content will show a peak right around 0.85, and above that you have to do everything just right to get higher hardness. This is for simple steels that don't get secondary hardening to 68-72 HRc.

You can harden steels with less than 0.5% carbon for cutlery, it just won't stay above 60 HRc after tempering. Hawks and choppers in the 55-58 range can be made, but there are almost no carbides for wear resistance, if you're into that sort of thing in a chopper/hawk. For the steels Opinel uses, carbon content is the sole element considered for hardness before tempering. Obviously, the other elements play a role in softening during/after tempering, but the as quenched hardness of 0.5ish simple stainless is governed by the carbon.
 
^^me2, thanks. :thumbsup:

What you mention about the lower carbon improving toughness does make sense. I had some of the simpler and lower carbon steels in mind, and was thinking about the combination of lower carbon and higher chromium found in a steel like 420HC, in making my earlier assumptions about toughness. The tenaciousness of the burrs in these steels is what always sticks with me (I haven't seen burrs like that in simpler non-stainless steels), and lends a lot to my impression of how 'tough' the steel seems to be, based on the character of the burrs. But, as you mention, that has a lot to do with the lower carbon content in it. Thanks for clarifying that for me. :)


David
 
Last edited:
Well, with burrs, I think it has a lot to do with the rather questionable heat treatment rather than the steel itself. My thinking is there will be a good bit of retained austenite, which is rather gummy and very ductile and tough. When I say toughness, I'm speaking in terms of either impact or fracture toughness. It keeps me from confusing myself. One day if you get a chance, try sharpening some 303 or 304 stainless. It gives stubborn burr a whole new level of meaning. These are austenitic at room temperature, for anyone who isn't familiar with them. As Roman Landes is fond of saying, to improve toughness, reducing carbon content is the proper lever/tool. Odd heat treatments and over tempering are fixes when you don't have anything else. This really hit me when I was doing impact testing of structural steels. We ooooh and aaah over an impact value of 60-80 foot pounds in a C notch or unnotched impact test. These structural steels regularly bring a 300 ft-lb hammer to a dead stop in a V notch test.
 
The Chrome is there to make the steel "stainless" and it's usually (and arbitrarily fixed at >10%) Some of the other alloying elements need to be considereing in conjunction with the carbon content because they add a "carbon equivalence" to the steel in terms of hardness. Tungsten, vanadium, Molybdenum all add a Carbon equivalence. Part of the reason US rebar is preferred (particularly the hi- tensile carbon version) is because it actually lists the carbon content, rather than have the " allowable" carbon equivalent. EU hi tensile rebar might not have any carbon in it at all (or not enough to harden to 59hrc)
 
@Strigamort: cool looking blade mods. Any trouble getting the blades out when damp?

@OWE (Dave), ME2 and the other steel people: I'm finding the discussion of the compositions and heat treatments to be fascinating.

At the risk of thread high-jacking (and in the hopes of shedding more light on the relationships), I was wondering if you could comment on the performance of 440A with Schrade's (USA) heat treat which supposed took their 440A (often labeled as Schrade+) up to the 56-58Rc range compared to Opinel's Sandvik 12C27.

Actually, the quote I've seen (will need to dig it up) attributed to Schrade employee was to the effect of that they used a 440 series steel made to their specifications. In other Schrade literature, they simply named it as 440A.

The mix for 440A as I've seen it is:
Carbon = 0.15-0.36%
Chromium = 12-14%
Molybdenum = 0%
Vanadium = 0%
Manganese = 1.00%
Silicon = 1.00%

The discussion in this thread leads me to think that something isn't adding up. How would Schrade be able to hit their claimed 56-58Rc with <.36% carbon? Could it be they used a higher carbon variant (e.g. to their specification) or could really good heat treat make up for the lack of carbon?

Had the chance to sharpen a new Opinel Inox this week as well as a (new to me) used Schrade 51OT with stainless. (The blade was stamped "Schrade", but the blade is definitely stainless, something that Schrade did occasionally >:( ). I assume the Schrade is 440A.

The Opinel Inox sharpened and honed up well without much burr drama. Very similar feeling to Buck's 420HC on the stones.

The Schrade 440A felt harder in that it took more pronounced pressure with the stones to make progress. Somewhat similar to working down their 1095. But could also be related to the thickness of the 51OT's blade near the edge too. Definitely more burr drama at the end. Somewhat similar to Case SS but nowhere near Victorinox Inox.
 
You have to dig for it, but Cr isnt set arbitrarily. There is a plateu in the graph of corrosion resistance that starts a little above 10% and ends around 13%. Thats where the Cr% for stainless comes from. I'm most familiar with carbon equivalence w/r to welding. In fact, I just gave a little talk about welding reinforcing steel. All the other alloying besides carbon has a great effect in the annealed, normalized, and tempered conditions. The maximum attainable quenched hardness is dictated basically solely by the carbon %. 12c27 and 5160 have the same range of as quenched hardness (62-64 HRc), depending on the exact treatment of each. The upper limit would require all the Cr carbides in 12c27 be dissolved.
 
I'll have to check, but I'm pretty sure that not the chemistry of 440A. Too little Cr and C. Its really a shame that 440A has such a bad rap. It has the potential to be a more wear resistant version of 12c27. The heat treatment has to be spot on though. I have not used any stainless Schrade blades, so can't comment on them. 440A will generally have higher wear resistance than 12c27 or 13c26. Its potential for taking and holding a very fine edge (<10°\side) is reduced though. Does Schrade say what they do to get that hardness range? Just guessing I'd say one would need a slightly higher than usual austenzing temperature, the equivalent of a slow oil quench, followed by cold treatment to dry ice or colder temperature. Then follow all that up with a rather low temper, <400 °F, possibly double temper and water quench after each. Thats how I'd do it, but I'm not making 10k blades a day. There are a few ways to get there, but control and selection of austenizing temperature and control/minimization of retained austenite are the keys.
 
Yes, something's amiss in the earlier chemistry description of 440A.

440A's carbon is around 0.65-0.75%, with chromium at 16-18%. In fact, 440A and 440C both carry the same high chromium content, but the difference is the carbon, which is much greater in 440C, at 0.95-1.2% carbon.

http://zknives.com/knives/steels/steelchart.php?snm=440a

And I agree, 440A is pretty darn good when it's done right. Doesn't deserve the blanket disrespect it sometimes gets. I have an older Camillus-made Buck 307 in 440A, and that's the knife that made me more favorably re-think my impression of that steel. Sharpens up to a fine edge, and very, very easy to maintain in that condition, on a strop or wet/dry sandpaper. Polishes beautifully too.


David
 
Last edited:
Strigamort,

Are you going to try and Dremel a nail knick into that wharnie Opinel?

Man that looks like a fantastic carver... I need to do that.
 
You must log in or register to reply here.
Back
Top