an alternative way to classify stainless steels

[Cliff Stamp]

While tool and alloy steels have well known classes, the AISI D,A,S series for example, the stainless steels often get jumbled into one big group. They are often labeled by the carbon content, but that is flawed. Is L6 a low end blade steel because it only has 0.7% carbon? No, and thus saying the same thing about 440A is equally absurd. To rank the stainless steels requires consideration of the relationship between the elements. To start, consider the simplest stainless steels,C/Cr alloys, which mean the other elements are not in a large enough amount so as to induce a major influence on carbide fraction or hardness.

First, the solid lines are the carbon saturation lines which show the maximum solubility of carbon and chromium at 1100C and 1000C in the austenite. As would be expected, at the higher temperature the austenite can hold more carbon and chromium just like hot water can ohld more sugar. As the carbon content increases, the steel can dissolve less chromium as carbon dissolves and diffuses a bajillion times faster than chromium because chromium is much heaver. This is one of the reasons why increasing the carbon content and keeping everything else the same will decrease corrosion resistance.

Second , the dashed lines are called "tie lines". Any steel on a tie line has the same austenite C/Cr percentage at that given soak temperature which is the composition at the point where the tie line intersects the carbon saturation line. As the tie lines move to the left then their steels have the austenite percentage of carbon lowered (steel gets softer) and the chromium percentage gets higher (more corrosion resistance). As the tie lines move to the right then this reverses. As you move up along a tie line there is a higher carbide fraction in the hardened steel and thus a greater wear resistance and lower edge stability.

A few examples :

1) 8C13CrMoV vs AEB-L : They are close to the same tie line so have similar as quenched hardness and a corrosion resistance. 8C13CrMoV is to the right of AEB-L and thus has a higher carbide volume (this is basically a linear responce of the distance along the x-axis of the two points) and so more wear resistance and lower edge stability. 8C13CrMoV is a higher wear alternative to AEB-L and AEB-L is a higher edge stability alternative to 8C13CrMoV. Both of these differences are small because the distance between the steels is very small. To obtain a higher hardness or more corrosion resistance requires a steel on a different tie line. Note 8C13CrMoV is used on the Byrd line and AEB-L on high end customs.

2) 440C vs 6A : Same thing only this time the differences are large because these steels are quite far apart on that tie line. This is contrary to popular opinion which dismisses 6A but the above shows it would be expected to have a similar max hardness and corrosion resistance and a greater toughness, initial sharpness and edge stability. Hard to argue all of that has no benefit in any knives.

3) 440A vs 440C : These are on two different tie lines. Looking at where both tie lines intersect the carbon saturation line shows 440A has a lower carbon content in the austenite but a higher chromium content. The corrosponding distances along the x-axis from their positions to the saturation intersect points predicts that 440C has a much greater retained carbide fraction. Thus 440A would be expected to be weaker but tougher, more corrosion resistant but less wear resistant. This comparison of course isn't surprising.

4) AUS-6A vs 12C27 : Very close positions and thus would have very similar as hardened strength, corrosion resistance and wear resistance and edge stability. This is contrary to popular opinion which often dismisses AUS-6A but praises 12C27.

I'll follow this with a table which shows the austenite composition of these steels at a specific hardening temperature and what this means in terms of as-quenched hardness and carbide fraction. Just how much more carbide is in 440C vs AEB-L for example and how much harder is AEB-L than 12C27M. Note, and this is critical, optimal performance requires involved heat treatments which includes fairly high austenization temperatures for some of these steels, quenchants (not air cooling), and repeated cold treatments and tempers. Landes has given specific recipies for steels to optomize them for cutlery. These are not what is often given off a spec sheet because those methods are optomized for high volume production where the performance/cost ratio tolerance is very low. I'll eventually web page this and include the relevant references

-Cliff

 

[hardheart]

Awesome, how would you say steels compare in actual production now with the HT received? ie, Cold steel AUS8 in Voyagers and Kabar Doziers @ 56-58 Rc, or the Recon 1's with an advertised cryo, which has been reported to change the 'feel' of the steel and edge by some forumites (though possibly imagined), as well as Al Mar's at 57-59 & Benchmade Red Class at 58-60 advertised. I've been wondering if the couple points difference are very critical, as the specifics of the HT are not advertised, and those details would seem to be more important.

I would think the treatment of 8C13CrMoV would be of good quality, since I beleive that the hardness is reported to be around 61, and Spyderco is run by a self-proclaimed steel junkie like Sal, who appreciates such performance gains.

 

[kel_aa]

440C vs 6A : Same thing only this time the differences are large because these steels are quite far apart on that tie line. This is contrary to popular opinion which dismisses 6A but the above shows it would be expected to have a similar max hardness and corrosion resistance and a greater toughness, initial sharpens and edge stability. Hard to argue all of that has no benefit in any knives.

I get the feeling you are standing up for 6A based on its properties, but the reality is that no one bothers to treat to the same hardness as 440C so I see the comparison as a little lopsided. I don't see much of 440C (except S&W?) anymore now that Benchmade moved out of it for some of their folders which were spec'ed at 58-60. But I see Aus6 now mostly used as a softer and tougher alloy (SOG Seal Pup 56-57? for instance, where it would be hard to argue that 440C would be a subsitute with any similarity).

 

[db]

I find this very interesting and fairly sensible for ranking stainless. C vs. CR. Of course in the real world how they are heat-treated makes a great difference sometimes more so than the steel types, but that doesn’t change the potential of the steel.

 

[Dog of War]

Once again you've pushed me to better my understanding, Cliff. Thanks.

 

[Cliff Stamp]

I've been wondering if the couple points difference are very critical, as the specifics of the HT are not advertised, and those details would seem to be more important.

Yes, the small differences in hardness are not directly critical in terms of the actual effect of the hardness itself but there are other issues. For example if the austenization temper was lowered then the reduced carbide dissolution will degrade the corrosion resistance and the strength/carbide ratio will be lowered and thus the steel will be harder to hone because it will burr more readily. The edge stability will also suffer which reduces the optimal sharpness and high sharpness edge retention.

I would think the treatment of 8C13CrMoV would be of good quality, since I beleive that the hardness is reported to be around 61, and Spyderco is run by a self-proclaimed steel junkie like Sal, who appreciates such performance gains.

Yes, in the above when I commented about how these two steels are used in very different knives I was mainly noting how they are labeled very differently but are in fact very similar steels with 8C13CrMoV being simply higher up the tie-line. Looking at the above it should be obvious they are in the same class, but for example 440C and AEB-L are not.

I get the feeling you are standing up for 6A based on its properties, but the reality is that no one bothers to treat to the same hardness as 440C so I see the comparison as a little lopsided.

Indeed, the point of the above was exactly that, to illustrate how much of the performance is due to the steel and how much to simply to how it is heat treated. Often times steels are chosen simply because of the ability to sell and the above should make that obvious because given the intended properties there are usually much better steels.

Note you can use the above to see why steels can be severely effected by lower austenization tempers. For example imagine AUS-6A austenized well below 1100 C (most knifemakers can't actually use that temperature it damages the furnaces). You will see that it gets softer quickly and the carbides get more coarse.

Thus the edge starts to roll/indent easier, the corrosion resistance suffers and it gets harder to sharpen because the edge is weaker but contains larger carbides so it will burr far more readily. The edge stability is also lower and thus it loses this fine sharpness much faster. That sound familiar?

So when you see that in a knife, instead of calling it a junk steel critize the obvious problems with the heat treatment and demand that the manufacturers live up to the constant claims of superiority/optomization.

But I see Aus6 now mostly used as a softer and tougher alloy (SOG Seal Pup 56-57? for instance, where it would be hard to argue that 440C would be a subsitute with any similarity).

It would not be a substitute even with both optimally hardened for cutting performance in a knife because the carbide fractions are so dissimilar as they are on extreme ends of the same tie line. They are only substitutes in hardness/corrosion resistance, in regards to toughness, grindability, and edge stability and extended edge retention they are very different.

Of course in the real world how they are heat-treated makes a great difference sometimes more so than the steel types, but that doesn’t change the potential of the steel.

In general extreme viewpoints tend to be problematic. For example there are those that will argue heavily on steels and others who will take the approach of heat treatment. The reality is somewhere in the middle, you need to pick a suitable steel and use an optimal heat treatment. If either of these are incorrect then the final product will have significant problems. Consumers need to start being more demanding of heat treatments and not focus simply on steels.

Note you can do the same classification for steels with other elements you just have to generate the C/Cr diagrams taking into account the other alloys and do additional calculations. If you just put 154CM on the above diagram for example and ignored the 4% molybdenum it would be under the AEB-L tie line and thus have a much higher austenite carbon composition. However including the effect of the molybdenum predicts it is actually lower at 0.58/10.6% C/Cr composition of 1100 C austenite.

-Cliff

 

[Carl64]

I get the feeling you are standing up for 6A based on its properties, but the reality is that no one bothers to treat to the same hardness as 440C so I see the comparison as a little lopsided.

Going by the graph, I think the point is that the graph implies that someone SHOULD try making AUS6a as hard as they do 440c. It's a good comparrison because, if it turns out to be correct (I have no idea how hard 6a can handle), it shows how the graph can be used to actually learn something. Whether the graph ends up being right or not, this is a good example of how to test it's value.

 

[kel_aa]

To to fair to Boker I believe they still use 440C. So for something like the SOG Seal Pup with Aus6 spec'd at 56-57, would they get better properties if they instead of underhardening Aus6 drop to Aus4 instead for a similar hardness?

On a side note, I noticed on a kitchen cleaver the notation 3Cr13. If the notation is consistant, it would imply something like 0.3-0.4 carbon? Surprising, as I'd swear it works a lot better than that.

 

[Cliff Stamp]

Whether the graph ends up being right or not ...

The carbon saturation lines and the tie lines for C/Cr steels are well known, the only approximation made is that some of those steels are not pure C/Cr steels. Now in general you would want the exact data from the manufacturer of the steel if you were doing precise calculations, however this would not change the above classification system.

To to fair to Boker I believe they still use 440C.

A decent amount of custom makers use it, and there are similar steels being used on other knives, Bohler has 440B/C class steels which are being used by some.

So for something like the SOG Seal Pup with Aus6 spec'd at 56-57, would they get better properties if they instead of underhardening Aus6 drop to Aus4 instead for a similar hardness?

Yes. Consider 420HC and AUS-6A, note they are basically along the same diagonal. Harden 420HC at 1050C and AUS-6A at 1100C. Both are very close to the carbon saturation line so they have the same very low fraction of carbides which you would expect to both be very small and of very low volume, about one micron in size and 1-2% in volume fraction. Since AUS-6A would have more carbon and chromium in solution it would be harder and more corrosion resistant. It would thus be more resistant to edge roll, have higher edge stability and higher wear resistance plus the vanadium acts to keep the grain small in the austenizing. It sounds very much like a better cutlery steel.

Now however austenize them both at 1050 C. Now the AUS-6A will have the same amount of carbon and chromium in solution so offers no advantage in hardness or corrosion resistance. It has a larger carbide fraction so the toughness will be directly reduced and edge stability will be lower so it will have a lower optimal sharpness and be more difficult to sharpen because the strength/carbide ratio has been reduced. The wear resistance will be slightly increased but consider 420HC and how it blunts. Is the blunting usually deformation/roll or wear? If it is the former then the extra carbide in the AUS-6A at 1050C gives no advatnage and now all you have is all the drawbacks.

On a side note, I noticed on a kitchen cleaver the notation 3Cr13. If the notation is consistant, it would imply something like 0.3-0.4 carbon?

Generally you would assume 0.25-0.35. ZDP-189 for example is 30C20CrMoW, so 3% carbon, 20% chromium and <1% Mo/W. If the amounts were larger they are usually specified. like 1C14Cr4Mo for 154CM.

-Cliff

 

[kel_aa]

I see. So since SOG pressumably doesn't have the capacity to properly and economically heat-treat Aus6, they might as well use something like 420HC which requires a lower austentizing temperature. Ah well, I guess there are a lot more forces at play than the actual performance.

Their Seal Pup is a minority that uses Aus6. Most seems to use Aus8, and the choice doesn't appear to have a direct correlation to price, as the Field Knife or Wilderness Guide or something of that name is an Aus8 knife of similar size to the Seal Pup for similar price. So I guess the above conclusion would extend to their Aus8 as well, as they are only spec'd at 57-58. For someone who may at times need somewhat extended slicing aggression in a field knife, would they be better off altering the finish on a portion of their knife and prehaps have a small sharpening tool with the necessariy roughness or a file if it is really vital (as opposed to using a higher-carbide steel with many other undesirable qualities (Aus8 vs 40HC)?

 

[Cliff Stamp]

So since SOG pressumably doesn't have the capacity to properly and economically heat-treat Aus6, they might as well use something like 420HC which requires a lower austentizing temperature. Ah well, I guess there are a lot more forces at play than the actual performance.

Exactly, how much money does it cost to get optimal performance and how many people will realize the gains and in fact how many people will buy on perception rather than performance anyway. Once a steel becomes "hot" it tends to spam across every knife regardless of size or type. Now is it possible for a steel to be so uniformly optimal, of course not, but is it possible that it will sell the most and provide the highest profit ratio, yes.

It takes a lot of money to educate consumers that different steels are optimal, especially if you have to fight popular opinion. Plus if you decide to go that route you now you have to run different heat treatment cycles and deal with different steels responding differently to abrasives so you may have to use different belts and train the guys grinding/sharpening how to handle the varying responses. Simply consider :

http://www.bladeforums.com/forums/showpost.php?p=1342949&postcount=4

Especially in light of the above. Benchmade even noted publically they didn't use more M2 not because it wasn't a better steel, but simply because it would cost too much, in particular noting the cost to educate the consumer.

For someone who may at times need somewhat extended slicing aggression in a field knife, would they be better off altering the finish on a portion of their knife and prehaps have a small sharpening tool with the necessariy roughness or a file if it is really vital (as opposed to using a higher-carbide steel with many other undesirable qualities (Aus8 vs 40HC)?

Absolutely, the difference that grit makes is easily as large as the difference in cutlery steels. So consider what you have to give up, grindability, toughness, ease of sharpening, corrosion resistance, etc. . The real thing consumers have to do is get more educated on what steels can be expected to do and demanding proper steel selection and optimal heat treatment.

As an additional note, consider VG-1's position on the above graph. It is actually very close to the tie-line for AEB-L. So is there any way this could be described as a "low end" or inferior steel. Of course not. It got this label mainly because of the bias towards high carbide volume for stainless and of course Cold Steel's use doesn't help.

-Cliff

 

[kel_aa]

The real thing consumers have to do is get more educated on what steels can be expected to do and demanding proper steel selection and optimal heat treatment.

Okay, I'm educated now! What, still no M2 at 65? So how do we translate education into influence? Countervailing forces are so many, not to mention our small size and the fact that even for myself steel still remains to be a small factor as long as its workable?

 

[Larrin]

The real thing consumers have to do is get more educated on what steels can be expected to do and demanding proper steel selection and optimal heat treatment.

Okay, I'm educated now! What, still no M2 at 65? So how do we translate education into influence? Countervailing forces are so many, not to mention our small size and the fact that even for myself steel still remains to be a small factor as long as its workable?

CPM-M4 is picking up in popularity. There is a chance that you could convince a maker to heat treat it to 65 Rc for you, but maybe you're not interested in custom knives. As a side note, Carpenter has abrasive wear numbers on conventional M4 and the abrasive wear hardly changes between 60 and 65 Rc, so the main benefit would be strength; however, maybe these particular abrasive wear numbers don't correlate well to knife wear, and the numbers are rather low, so maybe it's just that there is so little wear that it doesn't change much.

 

[Ben Dover]

Larrin,

What advantages does M4 have over M2??

 

[Larrin]

Larrin,

What advantages does M4 have over M2??

Well, I was mostly talking about availability, but as for CPM-M4 vs. M2, CPM-M4 has greater toughness and wear resistance, and a finer carbide structure. CPM-M4 is extremely fine. It is being used by more and more in competition chopping knives because of its combination of toughness, edge retention, and sharpness, three key attributes in those knives. Those makers rarely go harder than 60 Rc, though Gayle Bradley says that his most recent knife is at 62 Rc. The latest Blade magazine (I got it today) has an article on CPM-M4.

 

[kel_aa]

Going back to the previous discussion, so 420HC at 1050 can get full carbon dissolution (assuming equilibrium conditions) for a marsenitic hardness past 60. So does any production company approach this target?

From my experience and reading on the board it seems most people put Aus8 better than Victorinox similar 420HC class steels. I understand the carbon/chrom ratio of Aus8 is higher than 420HC, so it can achieve a higher carbon dissolution. So assuming that they are both sub-optimally heat treated, do we see the "advantage" in Aus8 mainly because of the increase in resulting hardness?

but maybe you're not interested in custom knives

Call me frugal, democratic, a man of the people, whatever. Yes, I'm mainly interested in production knives.

 

[Larrin]

but maybe you're not interested in custom knives

Call me frugal, democratic, a man of the people, whatever. Yes, I'm mainly interested in production knives.

I hope you didn't take offense at my remark, I didn't mean to say you are cheap, poor, or any of the things that you mentioned. I simply tried to help while adding the disclaimer that the information is useless unless you want to purchase a custom knife.

 

[kel_aa]

Not at all, Larrin. I appreicate your entrance into the discussion. I just stated my temperment, that I would like to see something that would be available on a production level and can benefit multitudes of people. Now I know economics is a big part of things, but really the target cannot be that far beyond the horizon. For something like the Camillus Cuda EDC, it's a 20-25 dollar knife with a full grind and 420HC. Using that as a platform, how much more would it cost to produce with a thinner edge spec (like an Opinel) and push the hardness up? Diddo for other knives in the same budget range like Ka-bar Dozier (Aus8), Benchmade Fecas Gunstock (Aus8), Spyderco Byrd (8C13CrMoV)... I mean if Aus6 can reach 440C hardness and 440C can go past 60, then Aus8 can surely do better.

Correction: The Cuda EDC is Aus8.

 

[Cliff Stamp]

So how do we translate education into influence?

The reality is that the masses are not convinced by facts/logic but by "name". You have to be very popular or have your background give your arguement strength regardless of its content. There are obvious means by which to achieve these goals, however unless it is actually of central importance to your life this isn't practical unless indirectly achieved.

Countervailing forces are so many, not to mention our small size ..

Unless you want to make it a personal mission and devote a large part of your life to some kind of lobbying effort, the most you can achieve is to educate yourself so you can buy better knives and share this information with others who are of similar critical viewpoint. It will take a long time for popular opinion to shift simply due to the inertia of numbers and the fact that the people spreading hype have a monetary interest and you don't, so likely they will be far more aggressive. However you have one thing they don't which is the truth, and in the end the truth will always come out.

...and the fact that even for myself steel still remains to be a small factor as long as its workable?

It is simply one part of the system and it can be compensated for by adjustment to sharpening, method and adjusting class of use. It isn't like you are 100% focused on materials. The performance of a knife, the cutting efficiency, handling/security, ability to generate power, are all due to how the blade is ground. What the knife is made from just controls the scope of work. Given a particular scope of work, the materials can be irrelevant beyond a very low standard.

Carpenter has abrasive wear numbers on conventional M4 and the abrasive wear hardly changes between 60 and 65...

For such steels with a massive carbide content you would not expect it to, however the manner by which the wear takes place changes which effect the resulting conditions of the surface which would be of importance. You are also looking at effects like edge stability, ease of sharpening, and of course the influence on deformation resistance of the raw change in hardness. Consider for example that typical "low end" blades are about 52-54 HRC, such as the Cold Steel Bushman and high volume production "stainless" kitchen cutlery. If you accept that the jump from there to 58/60 HRC is significant then it would be obvious that the change from 58/60 to 65/67 would be as well.

What advantages does M4 have over M2?

It is a higher wear HSS, significantly more carbon and more alloy, in particular vanadium to improve wear resistance. Crucibles P/M version has a higher carbon content still to make it easier to harden, which means larger sections with less aggressive quenching. It is designed for very fast and light cutting, very fast means friction induced temperatures in the class of 1000F.

...as for CPM-M4 vs. M2, CPM-M4 has greater toughness and wear resistance, and a finer carbide structure.

This is M2 :

from Allen, Metallurgy Theory and Practice. The carbides in the hardened state, which is all you are interested in, are very small (several microns) and well distributed. Landes has a picture of a P/M of composition similar to M4 but not as heavy carbide, less vanadium/carbon. It isn't finer and the edge stability would be lower due to the higher carbide volume, especially at a much reduced hardness.

CPM-M4 is extremely fine. It is being used by more and more in competition chopping knives because of its combination of toughness, edge retention, and sharpness, three key attributes in those knives.

Those competitions, while extreme tests of maker skill do not place significant demands on wear, toughness or ultimate sharpness. Cashen noted awhile ago that the ABS smith tests could be passed by a mild steel blade and they included chopping a 2x4 and still shaving. Note for example the STIHL Lumbersports competitions, these are obviously highly skilled axemen, possibly the greatest speed, precision and raw power choppers in the world. The axe heads they use are cast stainless steel.

Going back to the previous discussion, so 420HC at 1050 can get full carbon dissolution (assuming equilibrium conditions) for a marsenitic hardness past 60. So does any production company approach this target?

Buck claims 58 HRC.

From my experience and reading on the board it seems most people put Aus8 better than Victorinox similar 420HC class steels. I understand the carbon/chrom ratio of Aus8 is higher than 420HC, so it can achieve a higher carbon dissolution. So assuming that they are both sub-optimally heat treated, do we see the "advantage" in Aus8 mainly because of the increase in resulting hardness?

What you see are both underhardened and AUS-8A being both harder and with a higher carbide fraction. Though the carbide fraction is still much lower than steels like ATS-34 which is why it commonly praised for ease of sharpening and high initial sharpness. You can estimate the carbide fraction from the length of the projection on the x-axis of the line segment which connects the position of the steel to its position on the carbon saturation line.

For something like the Camillus Cuda EDC, it's a 20-25 dollar knife with a full grind and 420HC. Using that as a platform, how much more would it cost to produce with a thinner edge spec (like an Opinel) and push the hardness up?

It isn't the raw cost as much as the cost vs benefits. You have to educate the consumers on the benefits which would include contradicting the mass opinion. If you even suggest something like AUS-6A is a better steel than 440C it is generally recieved with absurdity because of course the popular opinion says otherwise. Instead of going to the effort of actually optimally hardening 420HC it would likely have a better profit ratio to just use a steel with a better name which made it easier to reach 60 HRC.

-Cliff

 

[hardheart]

Would the volume of blades being batch heat treated also be a factor? I figured the steels were left a few points below optimal just for fudge factor.

But then it depends on who drives whom-custom makers or mass producers. You would expect more precision from one, and economies of scale to absorb mistakes of the other.