Category: Edge Retention
4 thoughts on “Edge Retention”
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Does Acidic Food Affect Edge Retention?
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I was interviewed on the Knife Junkie Podcast, so make sure you check that out.
Acidic Food
Carbon steel knives are frequently used in kitchens, probably the majority of them made by Japanese bladesmiths and knife companies. Kitchen knives cut a variety of foods, and some of them are corrosive. There has been some debate about whether any of these potentially-corrosive foods can actually affect sharpness or edge retention of kitchen knives. Sharpness is controlled by the radius/width of the edge. You can read more in the article on sharpness vs cutting ability.
Cryogenic Processing of Steel Part 3 – Wear Resistance and Edge Retention
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Intro to Cryo and Wear Resistance
In Cryogenic Processing Part 1 I covered the effects of cryo on retained austenite and hardness. In Cryogenic Processing Part 2 I looked at the studies on cryo and toughness. Wear resistance is the most controversial aspect of cryogenic processing of steel. In particular there are claims that the use of cryogenic processing (liquid nitrogen) leads to an improvement in wear resistance that is not found with subzero processing (dry ice). Sometimes it is claimed that cryo can lead to massive increases in wear resistance [1]:
Which Steel Has the Best Edge Retention? Part 2
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Background Information and CATRA Curves
Make sure you read Part 1 first so that you understand all of the background information for this article.
Below I have another Youtube video of CATRA testing so that you can see how the curves are generated during testing.
You can get a feel for how differently these steels cut by plotting a few of them together with the same edge angle. The top curve is a high wear resistance steel which cut 835 mm of cardstock after 60 cuts, which is the TCC value (Total Cardstock Cut). The 244 mm is a medium-low wear resistance steel which shows much more sharpness loss than the higher wear resistance steels.
Edge Radius During the CATRA Test
I’m not sure where in the test the average person would decide that the knife needs to be resharpened, but I would guess that it is before cut 60 because the CATRA test wears the edge pretty significantly. In CATRA tests performed by Verhoeven [1] the edge radius was reduced from ~0.5 micron all the way to 3-5 microns after only the second cycle (note he calls the cycles strokes):
The tests performed by Verhoeven were with low wear resistance steels (AEB-L, 52100, 1086, and Wootz) but it still shows the relatively significant wear that occurs with the CATRA test. In the CATRA article on 154CM, it was found that the edge width (rather than radius measured by Verhoeven) was increased to 23 microns with a 20° edge angle and the 50° edge to 17 microns, which is quite dull. A ten micron edge width has been reported previously as a dull edge that needs sharpening [2].
Regression Factors Analysis
In part 1 I described the process by which we calculated the relative factors that affect edge retention which resulted in the equation below. CrC is a general term to refer to either Cr7C3 or Cr23C6 chromium carbides. CrVC is a general term to refer to M7C3 where M can be either Cr or V; when vanadium is added to a high chromium steel the chromium carbides are enriched with vanadium which increases the hardness of the Cr carbides. MC can refer to either vanadium carbides (VC) or niobium carbides (NbC). MN can refer to either vanadium nitrides (VN) or niobium nitrides (NbN). CrN refers to chromium nitrides. The formation of these particles is controlled primarily by the composition of the steel and secondarily by processing and heat treating. The equation below and the tables in Part 1 and here in Part 2 come from journal articles and books that have reported the carbide fractions after heat treatment.
TCC (mm) = -157 + 15.8*Hardness (Rc) – 17.8*EdgeAngle(°) + 11.2*CrC(%) + 14.6*CrVC(%) + 26.2*MC(%) + 9.5*M6C(%) + 20.9*MN(%) + 19.4*CrN(%)
Carbide Hardness
Taking an average value of the hardness of each carbide type we can compare between carbide hardness [3][4] and the calculated coefficient. Below I have plotted the carbide hardness in vickers (Hv) on the x-axis vs our calculated coefficient in the equation above for the relative contribution to edge retention for each carbide type. We get a very good correlation, demonstrating that carbide hardness strongly controls the effect of a carbide on slicing edge retention:
The coefficient for VC (listed as MC in the equation) is somewhat higher than the nitrogen version (VN or MN) despite their reported similar hardness [4]. Either the VN is actually somewhat softer or this is due to the VN coefficient being based on only two CATRA tests on one steel (Vanax 35). Either way it appears to be qualitatively accurate. Another possibility is that there may be some formation of V2N or chromium carbide/nitride which is lower in hardness than VN. The value for CrN also falls off the trend line of the others but that value comes from only one steel, Cronidur 30, and would likely change if further tests were performed. The M6C value is based on only two steels, CPM-M4 and M2, which both get wear resistance from VC so the accuracy of the M6C coefficient could definitely be improved with other high speed steels. No simple carbon steels with Fe3C cementite were tested which would be nice to add to the regression. Due to the low hardness of cementite it would be expected to have a relatively low value. This is confirmed by the Verhoeven study comparing 52100, 1086, and AEB-L where AEB-L with chromium carbides had superior edge retention to 52100 and 1086 with cementite [1]. If we extend the trendline in the plot above to the hardness of cementite we would estimate a coefficient of 5, or about half of chromium carbide. Experiments would be necessary to confirm that. Another interesting set of tests would be on the low-alloy tungsten steels such as the Blue series, V-Toku series, F2, O7, etc. The tungsten carbides in those steels are reported to be the very hard WC so it would be nice to know if that carbide improves edge retention to the same extent as VC. They are relatively niche steels so they have not received as much study as many tool steels and stainless steels. I wrote about these low-alloy tungsten steels
in an earlier article on this site
Which Steel Has the Best Edge Retention? Part 1
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Super Steels vs Regular Knife Steels
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Super Steel
I see frequent references to “super steel” online, and I was curious about how long that terminology has been around. I did searches on bladeforums as it is one of the oldest knife forums. The number of references to “super steel” has increased over time, but so have the number of posts on bladeforums. I saw how many references to “super steel” there were in each year, and then as a proxy to how many posts there were on bladeforums I did a search for “154” and saw how many references there were each year. Google tops out at 200 results but at that point the dataset was big enough to get an idea:
So referring to steels as “super steel” or the category of “super steels” is at least as old as still-existent knife forums on the internet. Reading through the descriptions of “super steel” now and for as long as bladeforums has existed, they are typically defined as one or more of the following [1][2][3][4]:
- New (relatively)
- Excellent edge retention
- Difficult to sharpen
- Stainless
Not all of those qualities are universally used. Sometimes non-stainless steels such as 3V or Infi have been called “super steels” [4]. I am not sure if a steel must have high edge retention to be called super but in general the “new” steels that come out have high wear resistance and edge retention. Super steels are often described as having high edge retention but greater difficulty in sharpening, however. Different steels slowly lose the title of “super” over time. In the early bladeforums era, VG-10 was sometimes called a “super steel” [5] but I don’t see it called super much anymore [6]. This confirms the “new” part of the definition. I’m not sure why edge retention or wear resistance became synonymous with super rather than other properties like toughness, but this is where we have ended up.
Pre-Internet History
Unfortunately, searching through magazines and books that predate the internet is not as easy as searching through bladeforums. However, I did find one reference from Outdoor Oklahoma 1978, where a very modern sounding description of “super steel” is found:
“Some hunters are a bit reluctant to opt for super steels because these have a reputation for being hard to sharpen. It’s true, good edge holding qualities go hand in glove with hard steels and hard sharpening. Some steels, especially stainless…”
And that’s where my free view through Google Books ends. Reading through the descriptions of super steels on bladeforums I find similar descriptions to this day. In 1978 basically none of the current “super steels” were even in existence, meaning that the article was likely referring to 154CM, 440C, or both as super steels. Those steels are not called “super” any more which again confirms that steels tend to lose their super title over time.
Edit 6/27/2022: I found an even earlier reference to the term “super steel” though the above quotes are still great because they reflect a similar mentality to today. This new reference I found is from the first issue of The American Blade Magazine (now Blade Magazine) from 1973 in an article by John Wootters called “Blades for Game”: “In this day of super-steels, there is no reason why a hunting knife shouldn’t have a hollow-ground blade. Such a grind offers less drag in meat-slicing and is easy to keep razor-sharp. the higher the hollow-grind bevel lies on the blade, however, the less “spine” or strength the blade will have, and the less abuse the knife can be expected to stand. If the steel is not absolutely top quality, however, a flat bevel offers more resistance to edge-chipping.”
The second issue of American Blade Magazine in an interview with Ted Dowell mentions that “He is still field testing the new ‘super stainless,’ 154-CM, and remains unconvinced although he offers it as an option to those who want it.”
Is “Super” a Positive or Negative?
Even in that 1978 article it was stated that some don’t want super steels because of difficulty in sharpening. The sometimes negative connotation of super steels as being nothing more than a “flavor of the month” or being too difficult to sharpen continues to this day. Therefore, it is not clear to me if the term super steel was originally coined as a negative or positive description. Many discussions on bladeforums about super steels continue to be about whether we need the so-called super steels or whether the old classics are good enough or even superior [7][8][9].
Current Views of Super Steels
While many decry the super steels as being unnecessary, the conflation of “high wear resistance” and super, or premium steel, continues. For example, in the Knife Informer article rating knife steels [10], the steels are categorized from “Super Premium” down to “Low End” with the differentiating property being edge retention. See this article for more information on articles that rate and rank steels. Because these steels are viewed as being superior, they are often perceived as also having high toughness despite their high wear resistance. In the linked article on steel ratings I pointed out that M390 is often given high scores for toughness despite Bohler not providing any toughness data on the steel. Toughness testing here at Knife Steel Nerds has also found unspectacular toughness values for M390, though it has only been tested at relatively high hardness:
The reason why almost any steel will have relatively low toughness that is designed for very high wear resistance is the large amount of carbide that is present in the microstructure. You can read more about the effect of carbides on toughness in the article I wrote on microchipping and in t
he summary of edge stability theory
Maximizing Edge Retention – What CATRA Reveals about the Optimum Edge
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CATRA
Update 1/6/2020: I have since written more articles about CATRA looking at the effect of steel type: Part 1 and Part 2
Cutlery and Allied Trades Research Association (CATRA) makes an edge retention tester that measures slicing of cardstock impregnated with 5% silica (sand). You can see a video of what the test looks like here:
The tester uses a fixed load, test speed, and stroke length. A typical test for a plain edge steel knife is 60 cuts with 50N load at 50 mm/s, which takes about 15 minutes [1]. The test is often used to compare different steels to determine the edge retention potential of each. However, the CATRA tester is not necessarily a device that compares steels; it compares knives. What I mean by that is that the edge angle, how the blade was sharpened, etc. all affect the measurement. Only if all of those variables are kept the same can steels be compared. Even then there is the possibility that with a different set of variables two steels may behave differently. In other words, a knife with a very thin finely polished edge may show Steel A is better than Steel B, but that may be reversed with a thick edge that has a coarse finish. We won’t know for sure until we do the test.
The Study Summarized in this Article
In 2012 a set of edge retention tests were performed by Wister Hill who commissioned the manufacture of a set of knives in 154CM and CPM-154. Though the tests were completed in 2012 the results have never been published. You can read an article about 154CM here: 154CM – Development, Properties, Use in Knives, and Legacy. These steels have the same composition but 154CM is produced by conventional ingot metallurgy and CPM-154 is produced by the powder metallurgy process. The use of powder metallurgy leads to a very different carbide structure, as the powder metallurgy version has much smaller and more evenly distributed carbides. The carbides are very hard particles that greatly contribute to wear resistance of steel. Based on their smaller size and better distribution we might expect the powder metallurgy version to do better in a thin edge. Here are micrographs taken from the steel used in the CATRA study:
Conventional 154CM
Powder Metallurgy CPM-154
Niagara Specialty Metals donated the steel, the blanks were then cut out and rough hollow ground, then heat treated by Peter’s Heat Treating. Knifemaker Butch Harner then did the final grinding of the blades. The blades were sharpened by Jason Bosman using DMT diamond plates of 325, 600, 1200, or 8000 grit with a DMT angle guide. The knives were given different edge angles of 56, 50, 34, 27, and 20° total (half of that per side). The final angle of the edges was then tested with a CATRA geniometer. The knives used a flat profile designed to work easily with the CATRA tester:
Sharpened Edges
Here are two edge-on micrographs of the CPM-154 and 154CM knives. I believe these were sharpened to 8000 grit as the scratches are all under 3 micron. If I get confirmation of what the edges were resharpened to prior to the micrographs I will update this article. The edges are approximately 1.5 microns at the apex. however, there are “holes” in the 154CM edge which are about 4 microns wide which may be evidence of “carbide pullout” where a large carbide or string of carbides is lost during sharpening.
Ingot 154CM
Powder Metallurgy CPM-154
Effect of Edge Angle
With a fixed edge thickness the height of the final edge increases with lower angles:
Micrographs taken of the profile of the edges show that the target angles were achieved:
50°
34°
20°
Edge angle had by far the strongest effect on edge retention, much stronger than other effects such as PM vs ingot and the finish it was sharpened to. Here are graphs showing the performance of 20, 34, and 50° vs cut length for an individual cut and also for total card cut in mm:
You can see that the initial cut length with a smaller angle is considerably higher and that the difference holds basically to the end of the test. This finding is significant because some have speculated that lower angle edges start out sharper but a more obtuse edge lasts longer [2]. And with the high wear that occurs in the CATRA test it isn’t likely that the situation would reverse with even further cutting. The initial blunting rate is relatively rapid regardless of angle and it then begins to level out. The highest TCC measured was over 1000 mm with an angle of 20°, and this decreased all the way to under 100 mm with 56°. In a CATRA study by Bohler-Uddeholm [3] with a range of steels, but unspecified edge geometry or sharpening, 154CM was measured at 547 mm, and M390 was measured at 959 mm. The 547 mm value would be with an edge angle around 30° in this study if other parameters are similar. So if the edge angle of a 154CM knife is reduced from 30° to 20° then it can match or exceed a steel with 75% greater wear resistance.
There was a difference in final thickness of the apex of the edge after the knives were run through the CATRA test. The 20° edge was about 23 microns after the test, 34 degrees led to about 19-20 microns, and the 50° were around 16-17 microns. All of these images are of the ingot 154CM steel. So it appears that with a lower angle edge it can wear down to a larger apex and still maintain better cutting ability than a higher angle edge.
20° worn edge (23 microns)
34° worn edge (19 microns)
50° worn edge (17 microns)
How the hell does CT’s xhp hold an edge so long?
Way slow in the reply here, but it has a large carbide volume and can hold high hardness. So, it’s not going to end up that tough, but it will be strong.
I did a plane iron test (for woodworking) two years ago and found it to plane double the linear footage that O1 will at same hardness.
Then larrin posted his results confirming that was in line with catra testing (which was a relief – I got a lot of grief from the woodworking community). What I’ve found since then is that you have to have something relatively easy on the edge (so planing rough lumber will reduce its advantage over more plain steels).
But I’ve made a couple of mules out of XHP for the kitchen and it’s great for low toughness work (and where it won’t lay in salt and rust).
I’m just curious looking at the large steel comparisons, I don’t quite understand, I’ve been told that high carbon steel holds an edge very well but all the high carbon steels on your list seem to be very poor at edge retention. I am just a very uneducated person on this topic who just happened upon this looking for interesting comparisons between knife steels so if I’m missing something or glossing over something on accident I apologize I am just very confused honestly.
Low alloy and simple carbon steels have a small volume of low hardness carbide called cementite. That means the west resistance is much lower than high alloy steels that have more carbide and the carbide is higher in hardness. This is discussed in the CATRA articles and Knife Engineering.