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TOPIC: Question with a sebenza

Re: Question with a sebenza 1 year 11 months ago #4357

  • BassLakeDan
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CliffStamp wrote:
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!) :lol:



Note: low numbers= sharper, high numbers= duller
Last Edit: 1 year 11 months ago by BassLakeDan.
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Re: Question with a sebenza 1 year 11 months ago #4368

  • CliffStamp
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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.
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Re: Question with a sebenza 1 year 11 months ago #4387

  • Rlb
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CliffStamp wrote:
BassLakeDan wrote:
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).

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!)
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Re: Question with a sebenza 1 year 11 months ago #4393

  • CliffStamp
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Rlb wrote:
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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