Question with a sebenza

[Cliff Stamp]

I thought the effect was more logarithmic and therefore the factor becomes very significant across the range of say: 56RC to 61RC.

Hardness itself would have a linear change, however if the microstructure changes then the effect would not be linear with hardness. The problem is that too much is attributed to hardness when it only measures one thing which is the resistance of the steel to compression on a fairly large scale. For example 1095 and D2 at 60 HRC have the same hardness but they do not wear at all the same as the microstructure is completely different.

As manufacturers do not say exactly how they HT their steels everyone ends up guessing which is made complicated because the reported properties are from a user group which tends to be fairly biased and you can get large sheep like tendencies as there are no blind/controls being used so a lot of care has to be taken. I am not saying that the choices Reeve makes are optimal, but I would take care in heavy extrapolation from the user reports.

The question that needs to be asked is why exactly are Reeve’s knives slightly softer as there are many ways that they could be that way and all will produce different micro structures. For example he could simply reduce the soak temperature, put less carbon in solution. This will end up with a perfectly fine blade but it will suffer corrosion issues because the soak was not hot enough to get the chromium in solution. In general Reeve doesn’t have common reports of easy corrosion (relative) so I don’t think this is the case.

He could be doing a simple air quench (not oil or positive pressure) and no below room temperature quench and if he did this then the micro-structure would be adversely effected as instead of the ideal which is martensite with less than 5% retained austenite he would have other primary structures and a much higher fraction of retained austenite and possibly even embrittlement. If he is doing this then you could see effects beyond what you would expect from the small change in hardness.

I have seen the reports where people have had blades rehardened and they report excellent results, however I am always very leery of such reports as again that could just be expectation bias. As what you are doing is fairly quantitative and not overly influenced by subjectivity I am interested in your results. I would like to see, if you have the time/inclination if you would see a different relative performance if the edge was finished with a high and low polish.

If the Sebenza has a lower relative ability to hold the low grit finish it is a fairly strong indication of a micro-structure issue as the teeth are approaching the size of the micro-structure (micron scale).

[BassLake Dan]

…

As manufacturers do not say exactly how they HT their steels everyone ends up guessing ..

The question that needs to be asked is why exactly are Reeve’s knives slightly softer as there are many ways that they could be that way and all will produce different micro structures. ..
…
if you have the time/inclination if you would see a different relative performance if the edge was finished with a high and low polish. If the Sebenza has a lower relative ability to hold the low grit finish it is a fairly strong indication of a micro-structure issue as the teeth are approaching the size of the micro-structure (micron scale).

Thanks Cliff for the above explaination. I will have time today to continue my experimentationswith the ZT and may be able to re-polish the sebbie. I have equip here to take a blade only to 0.25u , and anyway I am not nearly as good at those efforts (or near as fast!) as Clay, who can polish a blade faster than you can say ready-set-go much finer than that! :ohmy:

I find it kind of interesting that you are sort saying, ‘well lets be careful about putting too much on the shoulders of the physics of the steel, because we can not really account for the RC (and other differences of the material) to give us these extreme user reports. I understand your cautions here, and cbwx34 has issued cautions in the same vein, they are duly noted and I understand what you are all saying there.

🙂 –> However, I note that you are a ‘knife steel junkie’. I see your posts about the perfomance of certain blades, I note that you recently began testing a DiamondBlade knife that uses a very sophisticated ultra high pressure forging and hardening technique to form the edge that takes the RC to +68. User reports claim 3x and 4x increase in blade life when doing field work with that knife. I pose the hypothechical: If you could have a sebbie with CBN pressure forged blade made by DiamondBlade would you not instantly grab it from my hand or would I hear you say, well “No, thanks anyway Dan keep your hybrid there, i am fully satisfied that I would only get at most a +5% increase in performance on that..” 😉

I need to clarify something about my use of the term 3x 4x. I am not using that term about the steels engineering specification: I am using it the same way DiamondBlades is using it in their advertising and product information. How we are using that term means (to use an example from DB..) “I can skin three wild boars with this blade vrs only one with Brand XYZ blade, so my DB is outperforming that blade by 3x.” I am *not* saying that the engineering metals specification has gone up by 3x. For myself, I do not skin wild boars, but do carry a pocket EDC thru a fairly steady daily routine. I notice that the sebbie makes it (sometimes) thru the end of the day before I find myself looking around for a handy hone ( I tend to use a Sypderco UF beach stone that I keep in my kitchen for that…), but when I ave the ZT it might take three or four days (or sometimes more!) before I even think about dressing it. I understand that it is just all very ‘seat of the pants’, but oh well…

[Cliff Stamp]

If you could have a sebbie with CBN pressure forged blade made by DiamondBlade would you not instantly grab it from my hand or would I hear you say, well “No, thanks anyway Dan keep your hybrid there, i am fully satisfied that I would only get at most a +5% increase in performance on that..”

I would tend to have a high attraction to anything which is new/different as I am at heart and always will be an experimentalist. With that blade I am really curious as to how they will behave in long term wear as there is no data I can find or even extrapolate to in these cases. What we have is D2 flash-soaked to such a high temperature that massive amounts of the primary carbide have dissolved in the matrix. This allows the hardness to get up to 68 HRC and the corrosion resistance to be in the stainless class. But here is the kicker – in order to get this the carbide volume was severely reduced so where did the edge retention end up, it is harder but has less carbides.

Questions aside, I tend to prefer extreme type blades, but I know extreme type makers and those blades suit my preferences. I have for example 1095 blades at 68 HRC (small paring knife) which is just wonderful to sharpen (and of course use). It is so hard, the grain so fine, the edge is almost buttery smooth on the stone, it takes an extremely high polish without any effort and will take an edge which shaves above the skin, push cuts a tomato, etc. almost by accident. Yet, I can take the same blade and cut up sods outside with minimal damage as the hardness is right at the torsional toughness peak (pictures of all of that in the reviews).

That is an example where there is both a large change in hardness (about 10 points), a large change in microstructure (reduction in retained austenite) and very large change in torsional toughness (double if you compare the peak at about 325 F to the valley at about 500 F). If you combine all of these and the fact that the maker ground the knife accordingly to take advantage of it then you end up with a blade which can be many to one over another 1095 blade.

I notice that the sebbie makes it (sometimes) thru the end of the day before I find myself looking around for a handy hone ( I tend to use a Sypderco UF beach stone that I keep in my kitchen for that…), but when I ave the ZT it might take three or four days (or sometimes more!) before I even think about dressing it.

That is what I understood, that is really dramatic, assuming of course it is consistent on average. The first few questions that would come to mind are :

1) It is really blunting or just perceived to be – you can confirm that with the force cutting.

But there is a much harder one because again we are irrational beings :

2) Are you forcing the results because you expect them to happen? That sounds quite idiotic but it is one of the hardest problems to eliminate in research – hence double blind runs. One way to check this is to just loan the knife to a friend and ask them to return it when it needs to be sharpened and see what happens.

If it seems implied that I am using you/we in the singular sense I am not, the same thing happens to me all the time and I know it is going on, but still it isn’t trivial to stop. As a recent example, I am in the middle of some edge retention work on kitchen knives. I started a group of very inexpensive blades, these are the ones marked simply “stainless”, you can get them for <$10. I have another group of actual named blades like VG-10 etc. which I have called the mid-range group. The idea is to measure the edge retention of a bunch of these and then when I get another knife I can say where it is relative to the groups (I have another extreme group, the S90V class).

Now when I started the inexpensive knives I was getting a decent initial sharpness, when I started with the "better" knives my initial sharpness more than doubled. I expected this for various reasons. However after a number of posts on Spyderco's forum about placebo and bias I asked myself truly if I was not forcing the results. I was finishing on a very coarse stone (the fine side of a hardware store hone so this would be about 30 micron. If the finish is of this order should I really see any effect of aus-grain, carbide distribution, etc. . Probably not. I then did something which takes advantage of the fact that you are easily manipulate your subconscious and just kept repeating to myself how one knife in particular (one of the cheap ones) was very easy to sharpen to an extreme finish.

I then took that knife and what do you know I matched the sharpness I was getting on the more expensive knives. I then started working through all the other blades and with this bias broken all of them now fell in line and took the same quality of edge. Bias is the constant bane of any experiment and one of the things you always need to be asking yourself – is what I a seeing really due to what I think it causing it or could it be anything else. If you come up with anything you then have to find a way to make sure that is not the causal effect. At times I have always found it helpful to discuss with other people even if it is just to act as a sounding board.

[BassLake Dan]

.. Bias is the constant bane of any experiment ..

ain’t it the truth!

Well here are some ‘un-biased numbers’… I have sent this data to Cliff, who will ‘crunch it’ and make his own comments later.. he knows what numbers really mean, I am just a hacker, but here is my following take on the situation so far:

The ZT steels performed almost identical to the sebbie.. The basis sharpness was a little worse (higher numbers mean less sharp) than the sebbie (I think just because my ‘dressing’ of the blade before the test was a little hurried..) . Also per cbwx34 I re-ran the ZT through the 10 slices of copy paper. It, again, performed much like (actually exactly like!) the sebbie: i.e. after 10-15 cuts it started to fail on the 90 pushes, then I put it back on the test jig and noted the following numbers from random points along the blade: 352; 362; 344; 342; 346; 370. This is just like the sebbies numbers after the paper cutting, in other words about a 100 gram jump from baseline numbers after sharpening:

you can get the xls raw data here or just look at the following chart:

for those of you that want to see how all this stuff was collected, here is a shot of the ‘test-bench’. You can build yourself your own test jig for under $50 bucks and settle all those arguments at the local beer-bar about whose blade is sharper than yours:

[cbwx34]

So, just out of curiosity…

Keep in mind: It is important to note that push cut apex testing reveals only very limited information about real world “cutting ability”.. No matter what the results of my testing tomorrow of the ZT0550 I stand by my statement that the ZT holds up 4x better than a sebbie in the real world.

… did you change your mind? 🙂

I’m curious, and I know your test method is new, so you may not have this info yet, but can you give a little better idea of what a “350g test” blade can still do? You said it started to fail cutting paper… can it, for example, still shave arm hair at all, or whatever description you want to add.

Also, do you have a parts list avail. for your testing setup? I’m getting tempted…

Thanks.

[BassLake Dan]

..

… did you change your mind? 🙂
.

Well, I am going to return them both to pocket carry over the next week then re-test them both. But it is probably clear that there have been some zealous overstatements that have been made prior to the testing 😆 Cliff had an interesting series of posts once about how knife ergonometrics can effect “apparent sharpness feedback” to the user. i think we need to find those comments and re-post them somewhere..

..

..can you give a little better idea of what a “350g test” blade can still do? You said it started to fail cutting paper… can it, for example, still shave arm hair at all, or whatever description you want to add.
Also, do you have a parts list avail. for your testing setup? I’m getting tempted…
.

that is still a rather sharp knife, it will easily shave arm hair, it will not direct push cut tomateo skin, but will cleanly slice a tomateo using an easy draw cut under the weight of the blade, it will still 90 push cut copy paper if you handle it carefully and keep the finger hold on the copy paper within 1/4 to 3/8″ of the blade contact. -Will work on getting a parts list together this weekend. the thing is so simple you can basically figure it out from the videos and the posted pictures. The part of it that necessary to understand more fully is the proper placement and use of the testing media, how to run a ‘test series’ properly, the nature of the whole concept of “testing sharpness”, and a few other details that we need to put in some sort of documentation sheet. The whole thing is still a work in progress. .. Cliff has been very kind to work out an Excel template that crunches the numbers. From a recent email to Cliff, here is how I feel about it so far:

... I am not yet confidant enough in the ‘testing jig’ to accept it as a tool to measure anything but gross ranges of data. In fact because of what it is, and how it is built, it probably has limits that must be “respected” (if that is the right turn of phrase -grin!-)..

I feel an acceptable level of comfort with it as a rough gauging tool, within 20 grams push cut increments starting from commercial razor blades thru a range to kitchen knives, in that range I feel it is testing well, and the data it is returning is meaningful for that use and can be relied upon. When we are taking about testing subtle variations on very sharp high end blades with subtle variations in sharpening technique, well then, … humm not so sure.

[Cliff Stamp]

A simple fit will turn up :

Sebenza has an initial sharpness of 237 (5) the ZT was 283 (6). This difference of about 15% is significant (meaning it isn’t random) both from the standard errors being much larger than the difference (it would pass a t-test for significant at 0.05%, and if you look at the data you can clearly see that while they are scattered, the Sebenza scatters around a lower value (and thus higher sharpness).

Now that just means *for this run* it was sharper, it does not mean that it always would be, Dan would need to sharpen a few more times and if the Sebenza was consistently sharper then an inference could be made about the steel. But I would bet that this small difference in sharpness is simply the expected variation, it isn’t like you would expect the initial sharpness to be exactly the same each time you sharpened a knife.

The slope for the Sebenza was 1.35 (12) and for the ZT 1.39 (14), both of them showing a strong non-null value. This can be further checked as the F-statistic for both (136 and 92) easily pass the test of significance which show that the data do not remain constant, there is a statistically significant decrease in sharpness during the test. This is also, as Dan noted, easily verifiable in the data itself because you can see that towards the end the data flattens to a value about 100 g higher.

Dan if you want I can generate a template for that type of work which will generate all of these values and the graphs and email it back to you.

If you have the time/inclination you could carry the Sebenza and ZT and then periodically just record the sharpness and see how well the numbers match your perceptions. The ideal way would be for you to make your notes on the physical handling, and ask a friend to measure the forces and only look at the forces when you have decided to stop the experiment. This way you have reduced one of the largest possible sources of bias.

[BassLake Dan]

..
(from an earlier post…–>) If the Sebenza has a lower relative ability to hold the low grit finish it is a fairly strong indication of a micro-structure issue as the teeth are approaching the size of the micro-structure (micron scale).

..Now that just means *for this run* it was sharper, it does not mean that it always would be, Dan would need to sharpen a few more times and if the Sebenza was consistently sharper then an inference could be made about the steel.

This is interesting, and I will put this on the ‘to-do’list. I was looking through my stack of testing that I have done with the jig and came across one early on that was done with the ZT and the baseline numbers started as low as 175g so I know that I can get at least that far with it. It would be interesting to see if that could be brought even lower, and if the sebbie could be brought along with it. It would be interesting to see if I could ‘find the bottom’ (so to speak..), but with S35vn I don’t think I have the equipment or the skill here to really find out the answer to that, but it would make an interesting afternoon of sharpening 🙂 I do know that I have found such “bottoms” with certain knife steels: for example I tried pushing a CRKT M16 every finer and found out that the jig told me I could never (no matter what I did!) get it sharper than 180 or thereabouts. CRKT says the alloy is 8Cr15MoV.

[Cliff Stamp]

It might be interesting to set a few benchmarks which should be extremely high (or low as it were) :

-standard utility razor blades as-new
-double sided razor blades
-“shave ready” straight razor

[BassLake Dan]

It might be interesting to set a few benchmarks which should be extremely high (or low as it were) :

-standard utility razor blades as-new
-double sided razor blades
-“shave ready” straight razor

I have tested utility knives like what you find sold as ‘utility knife replacement blades’ at Home Depot and True Value hardware under brand names like “Stanley” and “Irwin” but they are just very dull, usually baseline numbers in the 350-400g. The Gillette type double edge replacement blades in the pharmacy are much more like what you would want as a reference, as they are consistent and very sharp across their entire length. I do not have access to ‘straight razors’ so, sorry to say, can not test them

The following chart is an example of results from a generic double edge razor sold by the Rite-Aid pharmacy chain stores. This data is simply sorted low to high, not put thru a proper data cruncher, but I thought I would present it this way as an easy way for ‘laymen’ to get the gist of it. (throw out the lows and the highs!) 😆

Note: low numbers= sharper, high numbers= duller

[Cliff Stamp]

Dan,

That makes sense to me, on the light thread I normally cut a Super Platinum razor blade is 30 (4) grams, if a knife even gets to within half of that at 60 grams it is extremely sharp, i.e., it is long since push cutting a tomato, newsprint, etc. . Most knives that people consider sharp such as the production Spyderco’s will in general be about 2.5-3.5 times as high. It has been awhile since I hand polished to a very high grit, but getting past 60 is not easy so I would be curious if you could bring knives down to match the finish of the razor and if so how long and what grit was required.

[Robert Borowski]

I thought the effect was more logarithmic and therefore the factor becomes very significant across the range of say: 56RC to 61RC.

Hardness itself would have a linear change, however if the microstructure changes then the effect would not be linear with hardness. The problem is that too much is attributed to hardness when it only measures one thing which is the resistance of the steel to compression on a fairly large scale. For example 1095 and D2 at 60 HRC have the same hardness but they do not wear at all the same as the microstructure is completely different.

As manufacturers do not say exactly how they HT their steels everyone ends up guessing which is made complicated because the reported properties are from a user group which tends to be fairly biased and you can get large sheep like tendencies as there are no blind/controls being used so a lot of care has to be taken. I am not saying that the choices Reeve makes are optimal, but I would take care in heavy extrapolation from the user reports.

The question that needs to be asked is why exactly are Reeve’s knives slightly softer as there are many ways that they could be that way and all will produce different micro structures. For example he could simply reduce the soak temperature, put less carbon in solution. This will end up with a perfectly fine blade but it will suffer corrosion issues because the soak was not hot enough to get the chromium in solution. In general Reeve doesn’t have common reports of easy corrosion (relative) so I don’t think this is the case.

He could be doing a simple air quench (not oil or positive pressure) and no below room temperature quench and if he did this then the micro-structure would be adversely effected as instead of the ideal which is martensite with less than 5% retained austenite he would have other primary structures and a much higher fraction of retained austenite and possibly even embrittlement. If he is doing this then you could see effects beyond what you would expect from the small change in hardness.

I have seen the reports where people have had blades rehardened and they report excellent results, however I am always very leery of such reports as again that could just be expectation bias. As what you are doing is fairly quantitative and not overly influenced by subjectivity I am interested in your results. I would like to see, if you have the time/inclination if you would see a different relative performance if the edge was finished with a high and low polish.

If the Sebenza has a lower relative ability to hold the low grit finish it is a fairly strong indication of a micro-structure issue as the teeth are approaching the size of the micro-structure (micron scale).[/quote]

Sorry I had a interesting week to say the least so I have a bit of catching up to do on this thread.

Just curious do you have data for the linear changes of performance with hardness? I would assume some variation between different steels as well which would muddy the picture. If this was the case and microstructure was more indicative of performance or retention I would be willing to ascertain that hardness may have a congruent relationship with microstructure and retention. Although maybe not. So to say it is a good indicator of retention seems like it is to be determined still.

Even with a linear relationship there would be no question what knife I would choose if I was presented with two knives, one had 3x better retention!

I agree with the bias statement, but without properly reported methods and lack of generated data it is hard to avoid. Consider a phase II drug trail where x many patients are given a new drug and are observed for adverse reactions. Study participants will talk to each and generate a similar type of bias. It may even statistical significance. My point is that bias occur and are sometimes unavoidable but then again thats what statistically analysis is for right?! haha that was definitely a joke!

Back to the knife at hand. I agree that it’s a burr problem but I just don’t think it was made from sharpening. I know I formed an apex/burr and I felt. No, I did not feel a burr after the ceramic and I did not mean I felt a burr after it stop slicing, haha that would be ridiculous. What I meant by that was that I know that I apexed the edge and formed a burr on the lower grits.

Also for clarification, I have high standards and when I say it stopped slicing after 10 slices it still would but I actually had to move the blade as compared to just blowing right through it. I may have been a little histrionic! ha However when it was sharpened up the the 0.6micron ceramics and stropped down to 0.25 I expected a lot.

Anyway, interesting thread, Thanks guys! This is why I posted it here instead of the CRK forums on BF where I would have been eaten alive! (Disclaimer I am a CRK fan just not a fan boy!)

[Cliff Stamp]

Just curious do you have data for the linear changes of performance with hardness?

Yes, as one example see “The Effects of Carbon Content, Matrix Hardness and Microstructure on the Wear of Steel Grinding Balls During Wet Copper Ore Grinding”. Here 1090 carbon steel is examined for wear resistance when hardened from 64 to 31 HRC. The changes in wear (material lost) are linear incrementally. In regards to strength, hardness is a direct measure of compression strength, and the relation between hardness and tensile strength is linear and can be seen in any simple table where one is used to estimate the other.

I would assume some variation between different steels as well which would muddy the picture. If this was the case and microstructure was more indicative of performance or retention I would be willing to ascertain that hardness may have a congruent relationship with microstructure and retention.

This is the critical point, hardness is only a measurement of the materials resistance to being dented, if the hardness changes and the microstructure is the same class then you can make inferences, however blades at the same hardness can have vastly different properties if the microstructure is different (i.e. bainite vs martensite being an obvious striking example of difference in toughness at the same hardness).

As an example critical to edge holding, if a blade has a high level of retained austenite, even if it is hard as in 60 HRC, the edge will still be highly subject to localized rolling and chipping and denting in these patches of austenite as it is very soft. Similar the grain of the steel can have a large impact if it runs from spine to tip (blade and edge are very tough) vs spine to edge (blade and edge is now very fragile).

I agree with the bias statement, but without properly reported methods and lack of generated data it is hard to avoid. Consider a phase II drug trail where x many patients are given a new drug and are observed for adverse reactions.

Medical research is insanely difficult just for that reason, all of the bodies are different and all of the people will describe the exact same condition differently. It is why you have to just get so much of it and pool a big bunch of studies especially when you are dealing with people who can simply lie to you about what they did or ate or past history etc., and some who just do trials for the money. It makes looking a knives for sharpness, etc. trivial in comparison as you only have to deal with minor biases such as data forcing (i.e. we see what we want to see).

I have high standards and when I say it stopped slicing after 10 slices it still would but I actually had to move the blade as compared to just blowing right through it.

To be frank, if you are careful enough in how you measure sharpness, and you can do this by feel on paper, you should be able to detect a difference almost immediately as blunting is actually highly nonlinear so the change initially is extremely fast compared to later. As a rough rule of thumb, in order to see similar changes in blunting you have to keep doubling the material cut. This isn’t an exact rule but it gives you an idea.

So when you are cutting paper you would check at 1, 2, 4, 8, 16, 32, 64, 128, 256, etc. . You can quickly see how it will appear that any blade will plateau very fast, this behaviour alone is responsible for some of the worst misconceptions about steel performance. You can see this in Dan’s data, see how it changes sharply at first and then starts to slow down, this gets very dramatic later on.

It also produces curious effects such as if you are willing for example work with a blade at 5% of optimal sharpness vs 10% then AISI 420 will easily outperform S30V as it will take longer for AISI 420 to get to 5% vs S30V to get to 10% as they are in the very slow plateau region at that point.

This is one of the reasons why people who don’t sharpen knives often, but let them go extremely dull don’t tend to consider edge retention at all as they won’t even notice any difference because once you get in the plateau of very low sharpness it takes forever for a blade to change any more regardless of the steel.

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