S30V vs M390 Toughness?

Jerry Hossom said:
In my opinion transverse toughness does matter, ... getting back to S30V, the target was to have a stainless with the wear resistance of D2 and the toughness of, get this, A2. A2? Yep, because A2 was long known for being a tough steel. In fact in the wood working world the very best chisels and planer blades were made of A2, and not that long ago - 10-15 years. Why A2? Because when they used it for the purposes those blades are designed they held their edge longer than other steels you might think would hold any edge longer - like maybe D2. Why? Toughness, not "wear resistance". Because what killed those edges wasn't wear, it was micro-chipping, one of the things that kills knife edges, probably more than what we think of as "wear". I did some work with people in the serious woodworker world some years ago, when they were searching for a better steel for those tools. What they went to was CPM-3V, not CPM-10V. Why? Because what really matters is what works. ... When you look back on the numbers however, you'll see that lateral toughness correlates reasonable well with actual performance. You can't say 8 is better than 6, but you can probably say 10 is better than 4.
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Due respect, you are talking about strength or "stability" as relates to fracture resistance - i.e. slow-loading of an edge that then chips-out - which is distinct from impact toughness (fast load, insufficient time for plastic deformation). The two do correlate, but again the relevance of transverse toughness vs. longitudinal toughness comes into play. In Landes work on "Edge Stability" he performs micro-loading of the edges to induce lateral stress and see which steels fracture out first (what you are talking about). His results show that above 15-dps it is difficult to distinguish between many steels because even large carbides are stabilized in the matrix.

When you load an edge, you are stressing it longitudinal until you induces sufficient fractures to allow propagation along the grain, whereupon it is transverse and propagates more easily, but getting there is the real challenge. If sufficient impact-stress is present to exceed the 28J limit of S30V, it is already sufficient to exceed the 10J limit, the latter is irrelevant. It could be 10 or 8 or 4 transverse, it doesn't matter because the stress was 28 and the effect is the same for all, i.e. failure. But A2 presents longitudinal toughness ~40J. 28J isn't enough to cause a fracture, the transverse toughness doesn't even come into play, again it's irrelevant.

What is the transverse toughness of A2? What about of any PM steel? Why would it matter?

As to wear-resistance being less important to wood-planer blades, Steve Elliot's work disagrees with that. In his testing, M2 steel (toughness ~20J like D2) performed above that of high-end A2 blades, and CPM-3V performed highest. The M2 and 3V blades showed the highest chip-resistance (again, this is slow-load strength/stability, not impact toughness) and the highest wear resistance which does indeed matter: http://bladetest.infillplane.com/html/summary_of_results.html
 
chiral.grolim said:
Due respect, you are talking about strength or "stability" as relates to fracture resistance - i.e. slow-loading of an edge that then chips-out - which is distinct from impact toughness (fast load, insufficient time for plastic deformation).
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From my own testing of a variety of knives in various steels I believe that it's more the slow loading that causes the edges to have issues more so than impact when cutting .... Can be micro chipping, rolling and flattening.....

In use the edge is going to flex back and forth kinda like bending a metal coat hanger back and forth until it breaks.

That's were proper cutting technique really comes into play as in keeping the spine behind the edge when cutting to reduce as much side loading as possible, especially with thin grinds and edge geometry.

Now with choppers impact does have more relevance.
 
Jerry Hossom said:
Just say it, James. I've been wrong before... :)
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It was nothing like that, I just... out-thunk myself for a minute there :o. I think I have my head wrapped around it now.

This is a little off-topic, but based on advice from Jerry and Brad Stallsmith among others, my current batch of medium-to-large Elmax and 3V blades will be run at 60Rc instead of 58Rc as I previously had them done. So I will be able to do some comparisons and see how much impact toughness I'm giving up and how much wear-resistance I gain...
 
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james terrio said:
It was nothing like that, I just... out-thunk myself for a minute there :o. I think I have my head wrapped around it now.

This is a little off-topic, but based on advice from Jerry and Brad Stallsmith among others, my current batch of medium-to-large Elmax and 3V blades will be run at 60Rc instead of 58Rc as I previously had them done. So I will be able to do some comparisons and see how much impact toughness I'm giving up and how much wear-resistance I gain...
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I doubt the impact toughness would be noticeable in use as I am sure the geometry would be right for that type of use.

The compression strength will go up with the hardness so there should be less edge deformation, flat spots, rolling etc...

Dunno about the edge retention, should be better, not sure about how noticeable it would be with the thicker grinds...
 
That's what I'm thinking, too; "wear-resistance" wasn't really the right term to use, but overall edge-retention should improve and I don't think I'll notice the lower toughness. There's only one way to find out of course... keep grinding the edges thinner until they start to fail...
 
james terrio said:
That's what I'm thinking, too; "wear-resistance" wasn't really the right term to use, but overall edge-retention should improve and I don't think I'll notice the lower toughness. There's only one way to find out of course... keep grinding the edges thinner until they start to fail...
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Yep, pretty much. :D

If you really thin it out for a cutter you could bump the hardness into the 62 range and the edge retention will skyrocket.... :)
 
I doubt you'll be able to see much difference, James. What you will gain is more edge strength, less likelihood of rolling or deforming on impact. Even your measurement of "impact toughness" may be hard to discern because the edge will bend, not chip. I've only seen a 3V edge chipped on one occasion and that was after it was hammered halfway through a thick walled steel pipe, where I'm sure it was work hardened beyond all reason. I think the photos of that were posted on the forums some years back.
 
Ankerson said:
If you really thin it out for a cutter you could bump the hardness into the 62 range and the edge retention will skyrocket.... :)
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Yeah... AKS has some 1" wide by .048" thick 3V that I plan to have run at 62Rc... should make nice little slicers :)

Jerry Hossom said:
I doubt you'll be able to see much difference, James. What you will gain is more edge strength, less likelihood of rolling or deforming on impact...
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Right on; I'm looking forward to it and I think my clients will be pleased. Waiting for blades to come back from HT reminds me of being a little kid waiting for Christmas :D
 
So just so i got this straight, A Crack occurs at the edge and as the knife is used more it stresses the crack into growing up the blade towards the spine of the blade... and Transverse impact toughness measures how the steel resists this crack spider webbing through the grain?

Also is not possible to micro crack or even macro crack with out exceeding the 28j of longitudinal toughness? Say cutting a material that is hard causes the cracks instead of an impact the transverse toughness would come in to play then right? Your response is appreciated...

I think i have this figured out..The longitudinal toughness is higher because its its taking the impact across the grain.... and the transverse toughness is the energy it can handle going with the grain.. similar to wood splitting its easier to spit down the grain than across the grain. In order for transverse toughness to come into play there has to be damage to allow the stress to run with the grain.... but how that damage occurs may not always exceed the longitudinal strength so it is important.. is that about right?
chiral.grolim said:
Due respect, you are talking about strength or "stability" as relates to fracture resistance - i.e. slow-loading of an edge that then chips-out - which is distinct from impact toughness (fast load, insufficient time for plastic deformation). The two do correlate, but again the relevance of transverse toughness vs. longitudinal toughness comes into play. In Landes work on "Edge Stability" he performs micro-loading of the edges to induce lateral stress and see which steels fracture out first (what you are talking about). His results show that above 15-dps it is difficult to distinguish between many steels because even large carbides are stabilized in the matrix.

When you load an edge, you are stressing it longitudinal until you induces sufficient fractures to allow propagation along the grain, whereupon it is transverse and propagates more easily, but getting there is the real challenge. If sufficient impact-stress is present to exceed the 28J limit of S30V, it is already sufficient to exceed the 10J limit, the latter is irrelevant. It could be 10 or 8 or 4 transverse, it doesn't matter because the stress was 28 and the effect is the same for all, i.e. failure. But A2 presents longitudinal toughness ~40J. 28J isn't enough to cause a fracture, the transverse toughness doesn't even come into play, again it's irrelevant.

What is the transverse toughness of A2? What about of any PM steel? Why would it matter?

As to wear-resistance being less important to wood-planer blades, Steve Elliot's work disagrees with that. In his testing, M2 steel (toughness ~20J like D2) performed above that of high-end A2 blades, and CPM-3V performed highest. The M2 and 3V blades showed the highest chip-resistance (again, this is slow-load strength/stability, not impact toughness) and the highest wear resistance which does indeed matter: http://bladetest.infillplane.com/html/summary_of_results.html
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Short answer to your question, Shinyedges, about can you crack below the rated transverse level? Absolutely, was what I was talking about re the little stress risers, and one of the reasons C-notch tests don't mean a lot on knife edges but to serve as rough guidelines. The macro and microstructures of knife edges is like nothing steel companies test, and that includes Rc hardness.
 
here's what chiral.grolim wrote:
When you load an edge, you are stressing it longitudinal until you induces sufficient fractures to allow propagation along the grain, whereupon it is transverse and propagates more easily, but getting there is the real challenge. If sufficient impact-stress is present to exceed the 28J limit of S30V, it is already sufficient to exceed the 10J limit, the latter is irrelevant. It could be 10 or 8 or 4 transverse, it doesn't matter because the stress was 28 and the effect is the same for all, i.e. failure. But A2 presents longitudinal toughness ~40J. 28J isn't enough to cause a fracture, the transverse toughness doesn't even come into play, again it's irrelevant

He says that if the Impact stress that causes the fracture that propagates through the grain exceeds the 28j of longitudinal toughness than it has already exceeded the 10j of transverse toughness... he is correct but what I'm wanting to know is does the initial fracture have to be caused by impact? what if the cause of the fracture isnt from an impact.. its from cutting hard material. In the case of the fracture resulting from a non impact event the longitudinal toughness doesn't come in to play and the fracture would now be a transverse toughness issue... and the higher the better correct?



Jerry Hossom said:
Short answer to your question, Shinyedges, about can you crack below the rated transverse level? Absolutely, was what I was talking about re the little stress risers, and one of the reasons C-notch tests don't mean a lot on knife edges but to serve as rough guidelines. The macro and microstructures of knife edges is like nothing steel companies test, and that includes Rc hardness.
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Well... Picture this on those lateral stresses in real life. What is your blade edge hitting that causes them, and at what angle, and what point on the blade, and what it is edge geometry at that point? And, as I was saying in particular, are there any stress risers at or near that point? And how about this, what is the grain structure of the steel that makes an issue of what is lateral or longitudinal? Higher is better, because you've got nothing else to go by, but at the end of the day the variables are too many and too interdependent to know without testing on a known sample with a known geometry and known finish under some known conditions that will define only how that blade performs under those conditions and no other. It is suggestive of how it will perform in general, but that's the best you're going to get. If the blade hits the face of a 2 x 4, it's one thing, the corner of a 2 x 4 is another, a steel pipe is yet another and another blade edge is probably worst of all. It depends, always, on a whole bunch of things that generalizations and number crunching won't tell you. Use the numbers to get you in the ball park, from there on you're on your own..

That's my story and I'm sticking to it. :)
 
Jerry Hossom said:
Well... Picture this on those lateral stresses in real life. What is your blade edge hitting that causes them, and at what angle, and what point on the blade, and what it is edge geometry at that point? And, as I was saying in particular, are there any stress risers at or near that point? And how about this, what is the grain structure of the steel that makes an issue of what is lateral or longitudinal? Higher is better, because you've got nothing else to go by, but at the end of the day the variables are too many and too interdependent to know without testing on a known sample with a known geometry and known finish under some known conditions that will define only how that blade performs under those conditions and no other. It is suggestive of how it will perform in general, but that's the best you're going to get. If the blade hits the face of a 2 x 4, it's one thing, the corner of a 2 x 4 is another, a steel pipe is yet another and another blade edge is probably worst of all. It depends, always, on a whole bunch of things that generalizations and number crunching won't tell you. Use the numbers to get you in the ball park, from there on you're on your own..

That's my story and I'm sticking to it. :)
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The only answer is that there really isn't a 100% correct answer. :)

That's why I always say it depends.... :D
 
very well said
Jerry Hossom said:
Well... Picture this on those lateral stresses in real life. What is your blade edge hitting that causes them, and at what angle, and what point on the blade, and what it is edge geometry at that point? And, as I was saying in particular, are there any stress risers at or near that point? And how about this, what is the grain structure of the steel that makes an issue of what is lateral or longitudinal? Higher is better, because you've got nothing else to go by, but at the end of the day the variables are too many and too interdependent to know without testing on a known sample with a known geometry and known finish under some known conditions that will define only how that blade performs under those conditions and no other. It is suggestive of how it will perform in general, but that's the best you're going to get. If the blade hits the face of a 2 x 4, it's one thing, the corner of a 2 x 4 is another, a steel pipe is yet another and another blade edge is probably worst of all. It depends, always, on a whole bunch of things that generalizations and number crunching won't tell you. Use the numbers to get you in the ball park, from there on you're on your own..

That's my story and I'm sticking to it. :)
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To shinyedges, Yes, it's like grains of wood ... except many MANY times smaller and also stronger, and again the PM process greatly reduces (but doesn't eliminate) the directionality of the grains.. is that even a word? Anyway, as Ankerson and Jerry have both noted, most edge damage isn't even caused by impacts, it is caused just as you describe, by cutting into hard materials or encountering slow-load lateral stress on the edge (google 'edge flex test'). When you split a piece of wood, for example, the grains of wood place lateral loads on the edge which can flex it out of alignment. If the edge is too soft, it will permanently deform (squash, roll/bend) which cold-works the edge to higher hardness and a thinner, more fragile geometry that can crack. If the edge is too thin and too brittle, it will resist bending but may fracture readily. If you cut into something hard, the edge will squash or try to bend around the object or may crack - same thing. This is all slow-loading, no impact-toughness value need be consulted.

Once a crack or other stress-riser (as Jerry described) is present, this focal point can lead to further crack-propogation... all in the absence of an impact. Now while some call this "toughness" it is important to qualify it as "fracture toughness" as a corollary of strength - i.e. the ability of a material to resist crack propagation, often discussed as "edge stability" in regard to knives) and not "impact toughness" (the ability of a material to resist crack formation, especially at a focal point, upon a sudden impact which can carry the crack to full failure).

As you've hit upon and Ankerson has been trying to put forward, impact toughness is not the most important property of knife steel because it isn't the most common method of failure. Even chopping wood, the wood is usually soft enough that it slows the impact force to a compression force relating to strength rather than toughness.

But if you are unlucky enough (as I have been) to encounter an embedded spike or lock-staple or nail/screw/etc. on a strike, THAT is impact force and even my GSO-10 in CPM-3V suffered a major chip from than encounter! But I may have you note that the chip was much wider than it was deep. Can you tell me why?
In case not, it's because the longitudinal toughness was sufficiently high as a thickness a little back from the edge that the crack couldn't propagate any deeper but it could propagate transverse along what little grain CPM-3V has to follow. Now if the transverse toughness had been higher, certainly the chip would have been smaller, but the chip would still be present because the threshold of longitudinal toughness was exceeded.

Anyway, you seem to have the right idea, I've learned somethings about S30V's origins that I didn't know before (and continue to be impressed about how responsive Crucible is to e-mails, this is the 3rd time I've read of quick replies from them to forumites :thumbup:), but the question I have is why oh why were both myself and Ankerson posting to this thread so late into the night? ;)

Sorry for another wall of text :o Listen to Jerry and Ankerson - impact toughness is important, but not that important for knives. Use the values as a guide, but make sure you are comparing the same values for each steel (i.e. transverse or longitudinal but don't mix-and-match). In the end, the best test is how the knife blade actually performs in use, not what value a generic test-specimen gives. SOOO many other variables for knife edges. :thumbup::thumbup:
 
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