12c27... why not more love?

[sabre cat]

last I heard sandvik had stopped producing 12c27 ? dosnt worry me as I brought a large sheet of it . I wont be running out any time soon.
its a great steel that was designed as a knife steel.

No more 12c? What is going to take it's place, do you know?

 

[marthinus]

One thing i would put to you is that the crack initiation can start at a very small inclusion and what it needs to propagate is grain-boundaries (weak joints) to follow. In those images, CPM154 has larger carbides with grain-boundaries around each one - a less-linear path for a crack to follow; and there is much greater distance between carbides making it much harder for a crack to jump across (actually proceed through) the tougher tempered-martensite/austenite to the next weak-joint around the next carbide. For the AEB-L, the path from carbide to carbide may be more linear with less matrix between. Using these slightly larger carbides in the midst of larger distances between may actually give the slightly higher-carbide volume CPM steel slightly greater fracture toughness than the lower carbide AEB-L though the difference might not be noticeable in actual use where threshold stresses are likely to be unreached or exceeded for both. But for ingot high-carbide steel, the larger carbide aggregates provide a path yet more linear with less matrix between than the lower-carbide AEB-L (or 420HC) hence the increased toughness over steels like 154CM and 440C. Using PM you get very high-carbide (high-wear) steels with toughness levels comparable to low-carbide (low-wear) ingot.

Here is a chart relating to fracture toughness in cemented carbide (<30% matrix WC-Co) that helps illustrate fracture toughness with regard to matrix volume and carbide size, specifically looking at the differences in toughness at 85% carbide between "submicron" vs "medium" and "coarse" carbide material. There are some excellent papers on the toughness of cemented carbide detailing how fracture-toughness relates to the ease with which a crack can follow a path through the material.

Again, Crucible suggests in its data sheets that the difference in longitudinal impact toughness between 440C (22J @58Rc) and S35VN (38J @58Rc?) may not be all that noticeable (or the values have a large range of deviation) so they push transverse impact toughness instead, and Sandvik won't even release notched impact toughness values for what they peddle. Based on the data from cemented carbides, I am suspicious that fracture-toughness values would be even closer, i.e. harder to differentiate steels that are between 4% and 24% carbide rather than only 4% to 24% matrix as is the case for cemented carbide - there is just so much more matrix in knife steel to absorb fracture energy, it is likely not a very good way differentiate between them for this purpose. *shrug*

Using cemented carbide as a form of showing fracture toughness with regard to matrix volume and carbide size is a double barrel IMO as cemented carbide is a completely different thing to compare to the steels we are using.

Yes the coarser carbide shows greater fracture toughness than the sub-micron, but the rest of the article shows that sub-micron cemented carbide has higher hardness and greater resistance to wear.

Reference:

http://www2.sandvik.com/sandvik/0130/HI/SE03411.nsf/7a5364adb7735b05412568c70034ea1b/651f6e334db04c46c125707600562c88/$FILE/Cemented+Carbide.pdf

By that standards we would want less carbides in our steels because it would give us greater hardness, resistance to compression and greater wear resistance.

This is opposite to what has been marketed in our cutlery steel and what we know with regards to abrasive wear resistance in our knife steels based on research done by Verhoeven and Totten etc. IMO to bring in cemented carbide really muddies the water far more than we need to. Totten states that one should not compare steels that fall in different categories such as HSS to HSLA etc as there are too many variables.

With regards to the linear path etc and the matrix I have not read anything to really contradict "Crack growth is governed mainly by the content, size and distribution of the primary carbides and the mechanical properties of the matrix. The content of primary carbides is determined by the amount of carbon and carbide forming elements like chromium, molybdenum, vanadium, tungsten and niobium. These elements improve wear resistance and hardness of the material but impair toughness,because of their strong tendency to segregate during solidification." (J. Blaha, C. Krempaszky, E.A. Werner and W. Liebfahrt (2006). CARBIDE DISTRIBUTION EFFECTS IN COLD WORK TOOL STEELS. 6TH INTERNATIONAL TOOLING CONFERENCE. Page 290)

Though PM process does improve the distribution etc of carbide and the matrix compared to ingot versions, IMO, the carbide content and larger size of the carbides will still more likely cause fracture growth in CPM-154 to occur than AEB-L based on the articles and books that I have read.

We are not even yet discussing grain size and matrix relation.

I was recently introduced to stress corrosion cracking (SCC) and corrosion fatigue on a project that I am working on and how carbides etc affect that. Really interesting but doubt knives will ever be exposed to those conditions.

 

[caseynz]

I think the 13c26 sabre cat.

 

[marthinus]

No more 12c? What is going to take it's place, do you know?

12C27M will replace 12C27

 

[chiral.grolim]

Using cemented carbide as a form of showing fracture toughness with regard to matrix volume and carbide size is a double barrel IMO as cemented carbide is a completely different thing to compare to the steels we are using.

Yes the coarser carbide shows greater fracture toughness than the sub-micron, but the rest of the article shows that sub-micron cemented carbide has higher hardness and greater resistance to wear.

By that standards we would want less carbides in our steels because it would give us greater hardness, resistance to compression and greater wear resistance.

You apparently misread those charts, we want MORE carbides of smaller size to increase hardness and wear - hardness and wear increase as carbides get finer and more prevalent (highest at 95% submicron carbides), it decreases as %Co the non-carbide matrix increases (lowest at 75% carbide, 25% matrix on those charts). Decreasing carbide size while maintaining the same carbide content improves wear-resistance, as does increasing carbide content.

This is opposite to what has been marketed in our cutlery steel and what we know with regards to abrasive wear resistance in our knife steels based on research done by Verhoeven and Totten etc. IMO to bring in cemented carbide really muddies the water far more than we need to. Totten states that one should not compare steels that fall in different categories such as HSS to HSLA etc as there are too many variables.

I agree, it muddies the waters, but it helps clarify how CPM-154 can be as tough or tougher than AEB-L while also being more wear-resistant, and it is specifically NOT the "opposite" of what is being marketed in regard to cutlery steel.

With regards to the linear path etc and the matrix I have not read anything to really contradict "Crack growth is governed mainly by the content, size and distribution of the primary carbides and the mechanical properties of the matrix. The content of primary carbides is determined by the amount of carbon and carbide forming elements like chromium, molybdenum, vanadium, tungsten and niobium. These elements improve wear resistance and hardness of the material but impair toughness,because of their strong tendency to segregate during solidification." (J. Blaha, C. Krempaszky, E.A. Werner and W. Liebfahrt (2006). CARBIDE DISTRIBUTION EFFECTS IN COLD WORK TOOL STEELS. 6TH INTERNATIONAL TOOLING CONFERENCE. Page 290)

Though PM process does improve the distribution etc of carbide and the matrix compared to ingot versions, IMO, the carbide content and larger size of the carbides will still more likely cause fracture growth in CPM-154 to occur than AEB-L based on the articles and books that I have read.

We are not even yet discussing grain size and matrix relation.

I was recently introduced to stress corrosion cracking (SCC) and corrosion fatigue on a project that I am working on and how carbides etc affect that. Really interesting but doubt knives will ever be exposed to those conditions.

The bolded part is absolutely fundamental to the principle. Again, crack propagation follows the path of least resistance usually at the grain-boundaries where carbides aggregate. The boundary around a carbides is especially weak, the path through the matrix grain-boundaries is much tougher, and toughest is the path through a carbide (i.e. cracking it apart). Crack initiation can begin at an inclusion of ANY size be it the boundary of a 2um carbide or a 10um carbide, but propagation of the crack requires a path to follow that is as close to linear as possible for dispersing the energy/stress inducing said crack. Thus propagating cracks around large carbides is more difficult than around small carbides which have have a greater surface area-to-volume ratio. What gives AEB-L an advantage over higher carbide ingot steels is that those small carbides are dispersed with a greater amount of matrix between carbides whereas steels like 154CM and 440C may have large carbide-aggregates with their associated weak boundaries spanning great distances through the steel so there is less need for the crack to proceed through tougher matrix to achieve the same amount of damage = lower fracture toughness. But when those aggregates are broken up to eliminate those clear paths and put more matrix between, the fracture toughness of the material increases (as shown by the PM marketing/publications including the CPM-3V patent and also by the work on cemented carbide previously illustrated in the marketing chart). The effect is SO great that a steel like CPM-3V can achieve even impact toughness some 50% higher than a tough ingot-steel like A2 with the same carbide load. HOW the carbides are distributed is essential to fracture-toughness behavior based on how that behavior (crack propagation) actually occurs, i.e. following paths of least resistance.

So which has a greater impact on fracture toughness - carbide size or carbide distribution or carbide load? In cemented carbide, impact toughness is measured at levels 10-fold below that of cutlery steel due to the much-higher carbide content (as previously described), it's not possible to differentiate between them on the scale used for steels as crack initiation and propagation proceeds too easily, they are that brittle. For fracture toughness (a slow-load stress-test), a larger carbide creates a less-linear boundary-path for a crack to follow thereby increasing fracture toughness, the sub-micron WC-Co cannot attain the level of toughness of the coarse WC-Co without a SUBSTANTIAL reduction in carbide-content (and associated wear-resistance) in the latter.

But what about in steel that has much lower carbide percentages? CPM-3V has 4-5% carbide and presents 85-ft.lbs of impact toughness at 58Rc, A2 steel has the same carbide load and presents only 33-ft.lbs at the same hardness. S-7 tool steel has HALF the carbide load of CPM-3V (~2%) and presents 85-ft.lbs at 59Rc, i.e. the same toughness achieved at half the carbide load of a PM-steel.

http://www.crucibleservice.com/eselector/prodbyapp/plastics/cpm3vp.html
http://www.crucibleservice.com/eselector/prodbyapp/plastics/airkoolp.html
http://www.crucible.com/eselector/prodbyapp/plastics/crus7p.html

But what about higher-carbide PM vs ingot steels? Again, look back to the 3V patent chart at D2 and D7 steels compared to PM 12Cr4V (essentially S30V) - the PM presents substantially higher impact toughness than ingot-steel with the same carbide load and the same toughness as ingot-steel with a substantially reduced carbide load. The evidence is there as to which matters more for toughness - carbide load vs size/distribution of that load. The difference is in how easy it is for a crack to propagate around carbides or across matrix.

With regard to slow-load fracture-toughness in steels, geometry must be taken very thin to differentiate between them if Landes work is taken as the standard (and this required careful application of forces and minute measurement to discern), establishing classes based on how thin an edge can taken and remain "stable" against side-loads, thin enough that one loses much in the way of practical application for generalized (as opposed to highly specialized) cutlery wherein most every steel requires a sufficiently stout geometry to endure the varied stresses it will be subjected to in use. A favorite reference of mine is Steve Elliot's work on planer blades where he found that going below 30-degrees inclusive in his use offers no performance advantage and significant durability issues on steels that Landes would classify as highly stable. In numerous writings on the subject, professionals discourage going below a similar angle for a wide variety of tasks and all using "edge stable" steels. And even Verhoeven's experiments showed that sharpening to angles sufficiently low for distinguishing high and low-carbide steels could prove challenging depending on the hardness of the steel and the skill/equipment of the sharpener due to burrs and plastic deformation of an edge with too little supporting material. I have seen the micrographs of "edge stable" razor-blades used to cut stock-paper, not pretty.

So what is the take away? 1) Geometry is king and 2) the PM process can make a steel as tough as one with half the carbide load because how those carbides are distributed is most relevant to the ease with which a crack propagates.

 

[mete]

Do you by any chance have info on toughness with eta carbides [from cryo] vs no eta ?
In my experience as a metallurgist I've found that all too often toughness is taken as an absolute ,while in the real world many variables can greatly change the picture. If we look at vanadium vs Niobium ,the V congregates in the grain boundaries mostly while Nb is found throughout the matrix.
If we look at the fracture surface of vey brittle steel we can see that fractures travel along grain boundaries AND there are also fractures perpendicular to the fracture path !! Don't drop it ,it's likely to shatter !!

 

[pinnah]

So what is the take away? 1) Geometry is king and 2) the PM process can make a steel as tough as one with half the carbide load because how those carbides are distributed is most relevant to the ease with which a crack propagates.

While the side bar on the relationship between carbides (abrasion resistance) and toughness has been illuminating (really), I am wondering if the collective expertise on display in this thread could be turned back to something more in line with the OPs original question.

Namely, can 12C27 (or 420HC) be considered as being roughly equivalent to 1095 (or 1095CV) in terms of overall performance assuming heat treatment to the same Rc level and similar edge geometry, where overall performance is understood be edge retention (abrasion), edge stability (rolling), toughness (chipping) and ease of sharpening (can be restored on crude or found stones)?

That is, several pages ago Ankerson (correctly) suggested that the OP's customers would be well served with a 1095 or 1095CV blade with a crude pocket stone on the sheath. So, the question becomes, does the OP's preferred 12C27 give a decent approximation to the overall performance of 1095/1095CV?

 

[chiral.grolim]

Do you by any chance have info on toughness with eta carbides [from cryo] vs no eta ?
In my experience as a metallurgist I've found that all too often toughness is taken as an absolute ,while in the real world many variables can greatly change the picture. If we look at vanadium vs Niobium ,the V congregates in the grain boundaries mostly while Nb is found throughout the matrix.
If we look at the fracture surface of vey brittle steel we can see that fractures travel along grain boundaries AND there are also fractures perpendicular to the fracture path !! Don't drop it ,it's likely to shatter !!

I would bow to your greater knowledge of such things, I am not a metallurgist. A cursory google-search netted this brief presentation: http://www.industrialheating.com/ext/resources/IH/Home/Files/PDFs/Role of Eta-Carbide-Meng.pdf

The wear resistance of an Fe-12.2wt%Cr-0.84wt%Mo-0.43wt%V-1.44wt%C alloy tool steel after cold treatment at 223K (-60f) and after cryogenic treatment at 93K (-292f) ...
From Fig. 5, the specimens austenitized at 1293K, subzero treated at 223K and ultra-subzero treated at 93K have almost the same volume fraction of retained austenite. However, the specimens after cryogenic treatment show wear resistance improvement considerably, as shown in Fig. 4. Although the specimens after cold treatment have a smaller volume fraction of the retained austenite than that of the as quenched ones, both have almost the same wear rate at sliding distances 400 and 600m. It is accepted that a major factor contributing to wear resistance improvement through subzero or ultra-subzero treatment is the removal of retained austenite and the formation of homogeneous martenisitic structure although the hardness is hardly changed. According to the scanning electron microscopy observation of the worm surface of hardened carbon tool steel tempered at temperatures lower than 573K by Huet al., the predominant wear mechanisms were ploughing fatigue, fracture and delamination. In this case, the wear rate may be controlled by crack nucleation and propagation beneath the surface, which is related to the strength and toughness of the materials. Retained austenite may prevent crack propagation either by changing the growth direction of an advancing g crack or by great energy absorption. It is suggested that cryogenic treatment makes a contribution to wear resistance due to fine n-carbides precipitation rather than the removal of retained austenite.

This fits with the cemented carbide data about finer carbides offering greater wear-resistance that coarse carbides at the same carbide:matrix loads. My understanding was that eta-phase carbides (carbon deficient) are smaller and much more brittle reducing hard-metal toughness in cemented carbide. One invention for rock-drilling puts as much eta-phase carbide in the cutting core with decreasing amounts toward the outside to present a tougher tool (similar to laminated steel). http://www.google.com/patents/US5279901

In steels, I'd imagine that eta-phase carbide plays the same roll as using vanadium in preference to chromium or even tungsten carbides - smaller/harder for more wear-resistance in a steel. In terms of increased toughness, that should come from the amount/nature of the matrix around the carbides and how the carbides/matrix are distributed in relation to one another per the principles discussed already about crack-propagation, no?

...can 12C27 (or 420HC) be considered as being roughly equivalent to 1095 (or 1095CV) in terms of overall performance...

Having no hard data on 12C27/420HC or even 13C26 toughness (fracture or impact) vs 1095, one must rely on the experience of users, right? In terms of edge-retention and "stability" comparing similar knives of either steel (e.g. opinels and Moras in each), Jim and others have found little difference between them (http://www.fotogaleria.sandor.sk/ine/ostrie/Vysledky_testov.pdf, http://www.bladeforums.com/forums/s...based-on-Edge-Retention-cutting-5-8-quot-rope). i think sharpening on a rock is not really to be entertained (e.g. http://zknives.com/knives/articles/hardvssoftedgesp4.shtml) so after that it is a matter of selecting a hone with proper abrasive to do the job and I think any hone capable of 1095 is capable of 12C27 and vice versa. Corrosion resistance - obvious. The only other difference I'd expect would be impact/fracture toughness which, again, we don't have much hard data comparing the two. 1095 spring-steel is widely use in applications requiring higher toughness like lawn-mower blades, it may be that's what'd take to see a difference. Condor uses 420HC in some large heavy machetes (e.g. Viking) and I've not heard of a failure *shrug* Buck is confident in their 420HC. Maybe ESEE should have chosen that over the 440C they won't warranty

 

[sodak]

[...] i think sharpening on a rock is not really to be entertained (e.g. http://zknives.com/knives/articles/hardvssoftedgesp4.shtml) [...]

Interesting article, thanks for finding that one.

 

[pinnah]

Having no hard data on 12C27/420HC or even 13C26 toughness (fracture or impact) vs 1095, one must rely on the experience of users, right?

Right. I still deal with use case development and documentation regularly in my work (different field) but your statement above is very true and leads us to an important insight often missed by engineering teams. If the user can't reliably distinguish between 2 alternatives, then any distinctions between them are "distinctions that make no difference". Taking this seriously is the basis for human factors engineering, ergonomics and good product management. Other than the obvious issue of tarnish and patina, can reasonably well educated and reasonably well trained knife users tell 1095, 12C27 and 420HC apart?

I've seen nothing in the technical discussions that would indicate that they should be able to and my read of discussions on BladeForums and other knife forums (assuming knife forum participants are, on average, better than average knife users) reasonably experienced users can't distinguish between them.

In terms of edge-retention and "stability" comparing similar knives of either steel (e.g. opinels and Moras in each), Jim and others have found little difference between them (http://www.fotogaleria.sandor.sk/ine/ostrie/Vysledky_testov.pdf, http://www.bladeforums.com/forums/s...based-on-Edge-Retention-cutting-5-8-quot-rope).

This is exactly as I would expect, right?

i think sharpening on a rock is not really to be entertained (e.g. http://zknives.com/knives/articles/hardvssoftedgesp4.shtml) so after that it is a matter of selecting a hone with proper abrasive to do the job and I think any hone capable of 1095 is capable of 12C27 and vice versa.

As long as we agree on the bolded part, then we agree on both the main technical point and the main user perception point. 1095, 12C27 and 420HC sharpen up similarly, all things being equal. Well, that's not entirely true. At lower Rc levels (under 56Rc), I think most experienced users will find these stainlesses a bit more prone to burr edges. Above 57Rc, they are more similar.

Regarding the underlined bit, we can agree to disagree. In my training, if the end user continually perceives a difference, then this must be accounted for. Note btw, the zknives article is about the difference between the same steels at higher Rc levels, not about comparing fine carbide steels like 1095, 12C27 and 420HC with medium carbide PM steels like S30V. But again, we don't have to agree on ease of sharpening across these steel categories.

It is enough that we agree that 1095, 12C27 and 420HC sharpen up similarly. That's exactly as we would expect.

Corrosion resistance - obvious. The only other difference I'd expect would be impact/fracture toughness which, again, we don't have much hard data comparing the two. 1095 spring-steel is widely use in applications requiring higher toughness like lawn-mower blades, it may be that's what'd take to see a difference. Condor uses 420HC in some large heavy machetes (e.g. Viking) and I've not heard of a failure *shrug* Buck is confident in their 420HC. Maybe ESEE should have chosen that over the 440C they won't warranty

I'd be interested to hear more about why you would expect a difference in impact/fracture toughness (again, at the level that a reasonably well educated user could distinguish).

FWIW, my understanding (could be very wrong on this) but I thought that high toughness applications (lawn mower blades, machetes, swords) tend to be answered with 1070 type steels, not 1095. But I digress.

 

[chiral.grolim]

At lower Rc levels (under 56Rc), I think most experienced users will find these stainlesses a bit more prone to burr edges. Above 57Rc, they are more similar.

More prone to burr than 1095 tempered just as soft? I was assuming same hardness for each...

I'd be interested to hear more about why you would expect a difference in impact/fracture toughness (again, at the level that a reasonably well educated user could distinguish).

FWIW, my understanding (could be very wrong on this) but I thought that high toughness applications (lawn mower blades, machetes, swords) tend to be answered with 1070 type steels, not 1095. But I digress.

It may be that i am overly suspicious of stainless cutlery steels, but I have never seen hard data that they quite reach the charpy impact values of basic carbon steels like O1 and 1095, even if it's only a 10 J difference, and again they aren't that popular in large choppers *shrug* And you may be right about the mower-blades not being spring-tempered 1095 ... at least not anymore, wouldn't be surprised if my old scythe blades are higher carbon, but I can't find a reliable reference to anything even close to 1080 being used.

 

[pinnah]

More prone to burr than 1095 tempered just as soft? I was assuming same hardness for each...

Yes, that has been both my experience and what I've heard in the comments of others, particularly in traditional forums (BladeForums and others).

1095 and 1086 at 55-56 Rc used to be quite common. Knarfang (Frank) has reported his testing of Case CV at around 55Rc. Opinel's Carbone (a variant close to 1086) is around 56Rc.

On the stainless side, Case's Tru-Sharp (420HC) is around 56Rc and current Victorinox Inox is in the 55-56Rc range, up from it's earlier 54Rc range.

I've found these fine carbide stainless steels to be noticeably more prone to forming a gummy wire edge during honing and this is in sharp contrast to soft carbon steel.

I find the differences between carbon and these fine carbide stainless steels to disappear at 58Rc. I have several old Schrade USA 1095 knives and their hardness was reportedly in the 58-59 range (and it feels like it). Mora's carbon steel is also in the 59Rc range. And of course, Buck and Opinel both take their stainless (420HC and 12C27) to 58Rc. The hone up much more cleanly than the same steels at a lower Rc. If there's any difference between them and carbon in this regard, it's very slight.

It may be that i am overly suspicious of stainless cutlery steels, but I have never seen hard data that they quite reach the charpy impact values of basic carbon steels like O1 and 1095, even if it's only a 10 J difference, and again they aren't that popular in large choppers *shrug* And you may be right about the mower-blades not being spring-tempered 1095 ... at least not anymore, wouldn't be surprised if my old scythe blades are higher carbon, but I can't find a reliable reference to anything even close to 1080 being used.

Given how much poorly heat treated 440 and 420 steel that there's been, one can understand the suspicions. My understanding is that one of the things that has worked in 1095's favor is the ease with which it can be adequately heat treated (exceptional heat treat is still exceptional). Buck certainly deserves credit for their work with Paul Bos and Schrade (USA) had a very good reputation for doing a good job with their 440A (usually marked Schrade+). The manufacturing success of the modern fine grained steel like 420HC and 12C27 is that is that it can fine blanked and now, thanks to good automation, can be reliably heat treated to the 58Rc range with good consistency. I suspect that despite these production improvements, these steels still suffer from that old "cheap stainless" reputation.

Regarding choppers, I'm not sure 1095 is the preferred steel for choppers. I think 5160 and 1075 are more common, no? 1095 is certainly used in a lot of big fixed blades and "hard use" work knives and, as you note, 420HC and 12C27 both commonly get used in work knives. Grab a Buck Vantage or an Opinel #9 or #10. Ignore the other aspects of the knives but beat on the blades and compare them to a good 1095 blade. If you want good fixed blades, try the Buck Bucklite Max or any of the Moras. Nobody is saying the stuff is a super steel or similar in anyway to PM steels. Just that it's close to comparably hardened 1095.

Been an interesting discussion. Thanks for the stuff you posted above.

 

[marthinus]

The bolded part is absolutely fundamental to the principle. Again, crack propagation follows the path of least resistance usually at the grain-boundaries where carbides aggregate. The boundary around a carbides is especially weak, the path through the matrix grain-boundaries is much tougher, and toughest is the path through a carbide (i.e. cracking it apart). Crack initiation can begin at an inclusion of ANY size be it the boundary of a 2um carbide or a 10um carbide, but propagation of the crack requires a path to follow that is as close to linear as possible for dispersing the energy/stress inducing said crack. Thus propagating cracks around large carbides is more difficult than around small carbides which have have a greater surface area-to-volume ratio.

So which has a greater impact on fracture toughness - carbide size or carbide distribution or carbide load? In cemented carbide, impact toughness is measured at levels 10-fold below that of cutlery steel due to the much-higher carbide content (as previously described), it's not possible to differentiate between them on the scale used for steels as crack initiation and propagation proceeds too easily, they are that brittle. For fracture toughness (a slow-load stress-test), a larger carbide creates a less-linear boundary-path for a crack to follow thereby increasing fracture toughness, the sub-micron WC-Co cannot attain the level of toughness of the coarse WC-Co without a SUBSTANTIAL reduction in carbide-content (and associated wear-resistance) in the latter.

Do you have any references where they discuss this path that is followed in Autenitic or Martensitic steel? Specifically related to small vs large carbide? Or is this based on cemented carbide? Would love to read it as most of the references I find is related to Silicone/Silicone Carbide and Stress Corrosion Cracking.

A favorite reference of mine is Steve Elliot's work on planer blades where he found that going below 30-degrees inclusive in his use offers no performance advantage and significant durability issues on steels that Landes would classify as highly stable. In numerous writings on the subject, professionals discourage going below a similar angle for a wide variety of tasks and all using "edge stable" steels. And even Verhoeven's experiments showed that sharpening to angles sufficiently low for distinguishing high and low-carbide steels could prove challenging depending on the hardness of the steel and the skill/equipment of the sharpener due to burrs and plastic deformation of an edge with too little supporting material. I have seen the micrographs of "edge stable" razor-blades used to cut stock-paper, not pretty.

This I find interesting as I have found issues related to the higher carbide content steels at low angles in my uses.

CPM M4 at low angles:

Yet I have used 12C27, 14C28N, Nitrobe-77 at similar angles without chipping issues. Increasing the edge apex angle to 10-12 dps resolved the issues I experienced with CPM-M4.

 

[HeavyHanded]

Not sure if it was already mentioned, but Bark River once used 12c27, which seems to have been predominantly replaced with S35VN. I've heard mostly great things about the knives they produced in 12c27.

I have a Bravo Necker 2 in 12c27, takes and holds a screaming edge. I also have an EKA H8 Hunter in that steel, pretty sure its run a bit higher RC than the Bark River - it takes and holds a fine edge well too. They hold up better then any of my 420hc blades, am surprised to hear they are so similar - if so the difference comes back to HT.

 

[chiral.grolim]

Do you have any references where they discuss this path that is followed in Autenitic or Martensitic steel? Specifically related to small vs large carbide? Or is this based on cemented carbide? Would love to read it as most of the references I find is related to Silicone/Silicone Carbide and Stress Corrosion Cracking.

There are many publications concerning crack propagation in structural steels, but most deal with the steel matrix itself as that makes up the majority (80-99%) of the material, and the effect of carbides as inclusions for crack-nucleation is much smaller than for ceramics.

With regard to cracks following the most linear path and fracture toughness being increased with "deflections" aorund that linear path, here is one illustrating the point, though it may be a bit heavy (i.e. off-topic) for this thread: http://www.sciencedirect.com/science/article/pii/S0013794414000757
Grains here refer to the steel matrix which is more easily cleaved (ductile transgranular cleavage) than carbides (generally fracturing via intergranular or boundary cleavage around the carbide).

...Similar effects of grain size on fatigue crack growth were found ... where specimens with a crack parallel to elongated grains revealed a shorter fatigue life than specimens with the crack orientation perpendicular to the elongation direction...

After HIP processing the threshold values for both orientations, i.e. crack growth normal or parallel to building direction, are increased and almost equal. The microstructural reason for this enhancement and similarity is shown in Fig. 6c.... Fig. 6c depicts schematically coarse grains accompanied by small sections of elongated grains. The absence of a preferred grain orientation leads to similar thresholds... The increase in threshold values can be explained based on the elevated grain size. Here, the crack with its tip positioned in front of a large grain is oriented towards a large boundary which acts as a barrier. For passing through as well as for growing around the large grain increased effort is required.

Here is one perhaps more relevant (and even applies to PM vs ingot steels: http://www.gkn.com/hoeganaes/media/...teels with a Heterogeneous Microstructure.pdf

FATIGUE CRACK GROWTH OF Fe-0.85Mo-2Ni-0.6C STEELS WITH A HETEROGENEOUS MICROSTRUCTURE
...
...Since the crack path appeared quite tortuous, due to the hetrogeneous microstructure of these steels, we have also quantified the degree of crack deflection on the measured &#8710;K, per the model of Suresh [28]. The crack appears to be highly dependent on the phase(s) at the crack tip, Fig. 12. For the Ni-rich regions, cracks tend to propagate in a linear fashion, suggesting that the Ni-rich regions offer little to no resistance to crack propagation ... Cracks propagating through the coarse pearlite, fine pearlite, and bainite all show large increases in the degree of fatigue resistance due the crack deflection, Fig. 14. Crack arrest is often present, and further induces crack deflection through branching...
...The crack deflections induced by the heterogeneous microstructure increased the fatigue resistance of the steel. The increase in the fatigue resistance of the steels can be quantified...

And another: http://iopscience.iop.org/1468-6996/14/1/014208/article

One of the most important characteristics of the microstructure is its influence on the ductile–brittle transition of the steel. In clean, lath martensitic steels the brittle fracture mode is transgranular cleavage, so we need to understand the role of the microstructure in inhibiting cleavage. As is well known, cleavage is best inhibited by refining the effective grain size to introduce barriers to crack propagation...

About trans vs intergranular cleavage and fracture toughness: http://www2.lbl.gov/ritchie/Library/PDF/brittle_grain_bridging.pdf

In every paper I've read, it is noted that the steel matrix will fracture via transgranular-cleavage (through the grains) unless there is significant deposition of inclusions/carbides/etc. at the grain boundaries providing a weaker path to follow that deflects from linear. In contrast, cracking through a carbide aggregate amidst the matrix is very difficult and requires intergranular deflection to induce tear-out.
In ceramics, finer carbides provide a more linear (less "tortuous" or deflected) intergranular path than larger carbides at the same load %, and the increased difficulty of trangranular crack propagation through the matrix is not that great so the carbide load must be significantly reduced, the matrix-binder significantly increased, to provide matching toughness.
In ingot steels, larger carbides presenting grain-boundaries roughly parallel to crack-stress provide an easier nucleation and propagation path than transgranular cleavage of the matrix. But dispersing those carbides heterogeneously eliminates that linear path such that, while nucleation begins at a carbide/inclusion, the crack must then propagate transgranularly through the matrix to the next inclusion however small or large that inclusion might be. As a result, P/M increases toughness of the material substantially. Toughness isn't as much about carbide load/distribution in steels as it is about MATRIX microstructure and distribution, i.e. grain-structure and spacing between potential inclusions. Again, CPM-3V has only ~5% carbide / 95% matrix but evinces the almost the same impact toughness (sudden nucleation and crack propagation) at ~58/59Rc as S7 shock-steel with only 2.5% carbide / 97.5% matrix. The same holds true for higher carbide loads where aggregation becomes an increasing concern. Again, in the images of CPM154 and AEB-L, look at the carbides - regardless of their size - as individual crack nucleation sites and the matrix between them as crack-propagation paths. It is certainly easier to nucleate a fracture at the larger carbides in CPM154, but the distance between them looks to be greater and may require significant deflection of the crack path for growth. As a result, the difference in fracture toughness between the high-carbide PM and the much lower carbide ingot may be undetectable in use if not present a slight advantage to the high-carbide PM. *shrug* When discussing steel toughness, it is backwards to look at carbide-content and distribution, the focus should be MATRIX structure, content, and distribution. MATRIX provides the fracture-resistance, not carbides.

 

[xanopos]

This is a good point, I often here people tout 12C27 as a "tough" steel but see no "choppers" made from the stuff other than maybe a few stainless Condor machetes from 420HC that I've never tried so cannot comment on the edge-durability. Anyone have a 420HC or 12C27 chopper they'd like to share about?

I have a FOX machete in 12c27. Even chopped down a tree with it. Has still the factory edge on it.

 

[mete]

I'm still at this point in life learning metallurgy.
It is my understanding that with large carbides fracture path is mostly carbide to carbide .Small carbides ,fracture usually is through the matrix. Lots of things to think about. Small carbides have more chance of having cohesion of the matrix. Eta carbides from cryo treatment have cohesion.Cohesion is an important strengthening mechanism.

1095 for lawn mower blades-the only comment I remember was from someone in the business and he mentioned a 15xx type . That might be better as a cost thing for mass production . Commercial
mowers different ?

One other point I now remember - I posted on this and maybe another forum that hypoeutectoid steel has both types of martensite ---I got NO responce !!!