How sharp does your sebenza get compared to other knives?

[Cliff Stamp]

Professor :

I don't buy the "S30V at lower RC is a bad thing" concept as has been applied specifically to the Seb.

It doesn't apply specifically to the Sebenza, it is a general rule that deformation blunting is highly correlated to hardness. Physics doesn't make exceptions for Chris Reeve.

JoHnYKwSt :

[Reeve should use a lower edge angle]

I always thought they did, the blades I have used (older ones) were definatly not at 20-22 degrees. If this is a case of S30V influencing the geometry it certainly was a decline in performance.

CRK's S30V is not 57Rc, it is 58-59.

It was listed as 57-59 initially, I have seen three different ranges now.

As for the 420HC comparison, that sounds like quite a stretch.

High alloy steels at low hardness levels are the worst possible combination. You get the weakness of the lower HRC and the lower machinability of the higher alloy. If you are going to go soft there is little benefit to hard carbides because the edge can't cut well if it is bent regardless of the aggression of the carbides.

-Cliff

 

[FK]

"High alloy steels at low hardness levels are the worst possible combination. You get the weakness of the lower HRC and the lower machinability of the higher alloy. High alloy steels at low hardness levels are the worst possible combination. __________________________________________________

"Low Hardness levels" = Rc 58-59 ???? That is very interesting..??

"Weakness of the Lower HRC" --- this should be in the range of Rc 54-56 to equal relative weakness.

Cliff, this sounds like theoretical smoke and mirrors metallurgy.

"..it is a general rule that deformation blunting is highly correlated to hardness."

Yes, true however,,,,, higher hardness is directly correlated to increased edge chiping.

Regards,
FK

 

[JoHnYKwSt]

CRK's "Low Rc" has been discussed many times in the past when other makes were running their S30V at 59-60 or 60-61, but it seems that most other makers now harden their S30V to 58-59 (For example, Spyderco, Microtech, etc.) minus Paul Bos whom I believe still hardens to 59-60. When compared to BG42, this lower hardness is alarming but the fact that everyone else hardens in this range means that CRK was right all along, Chris did help develop the steel afterall. http://www.chrisreeve.com/pressawards.htm

I do wish that CRK would thin their edges but I also have faith that Chris knows what he's doing.

Cliff:

[Reeve should use a lower edge angle]

I always thought they did, the blades I have used (older ones) were definatly not at 20-22 degrees.

I think they were thinner in the past and have gotten a bit thicker at some point. This is based on what I've read a few sebenzas I have.. not much to go on so take it with a grain of salt. Anne has said they sharpen 18-20 degrees.

 

[Professor]

...people who draw performance conclusions based on numbers alone are the same people who get out of the shower to pee.



Just kidding (maybe). In all honesty, I do wish that the blade's edge was beveled thinner, and if a higher RC is what it takes, then I reckon I'm for a higher RC. Edge retention has not been a problem for me with the Seb, but the beveling has been less than ideal.

Professor.

 

[Cliff Stamp]

FK :

this should be in the range of Rc 54-56 to equal relative weakness.

Any drop in hardness is a loss in strength. You are willing to accept that 54-55 vs 59-61 is a weaker blade (most would argue the difference is very large, that scale is where you find cheap kitchen cutlery) but not that 57-59 is weaker than 59-61. Interesting math. If the performance loss was insignificant for the latter it would also be for the former. Unless you think the relationship between strength and hardness is highly nonlinear which isn't the case and is a known fact for about a hundred years.

Check out the edge retention work done by Goddard, Wilson and others who consistently showed noticable differences when a drop in one-two HRC points was made. I have also done blind comparisons on identical blades (made by Ray Kirk, no idea of the composition or hardness until after the test was done) and the difference that just a few HRC points makes can be dramatic.

higher hardness is directly correlated to increased edge chiping.

At 53 HRC S7 has an Impact Toughess of 105 ft.lbs, at 54 it is 115, at 55 it is 125. The dependence of toughness on HRC is much more complicated than resistance to deformation. Now it may be the case that S30V has a maximum toughness peak where Reeve runs it. This is impossible to know as Reeve won't even discuss the testing he has done, which to be frank inspires no confidence. Don't live in a vaccum, take a look outside and ask carious custom makers about similar issues and you can get very detailed answers about exactly what they have done and what happened at various tempers and soaks.

Anyway, even if this is the case, it clearly indicates that there will be a large performance drop over the BG-42 in many types of cutting because there is a significant HRC difference between the two. Wilson has studied this in particular with really high alloy blades and found that the edge retention is gutted by lower hardness levels, and for example you can get the edge retention of S90V to drop below ATS-34 by making the S90V significantly softer (just a few points), in spite of the massive advantage of high wear carbides for S90V. As noted, carbides do nothing once the edge is misaligned.

JoHnYKwSt :

... but the fact that everyone else hardens in this range means that CRK was right all along ...

Everyone else doesn't, there are still several customs at 60-62. Even if this was the case and everyone concluded that at a higher hardness it chipped out too readily (a problem which wasn't with BG-42) this tends to say something significant.

I do wish that CRK would thin their edges but I also have faith that Chris knows what he's doing.

There is no place for faith in metallurgy. This is supposed to be science, facts and logic not refusing to question a man on the presumption that he is infallable, that does nothing but encourage hype. Lots of makers do things in ways that are suboptimal for me, if I was buying one of their knives I would not get it made to suit them.

I think they were thinner in the past and have gotten a bit thicker at some point.

Maybe I just fluked out and got lower ones, a Project I used was just over 22 degree included, with a slight convex increase near the very edge, but not anywhere close to 20 per side. I would assume the Sebenza to be far more acute considering a large chopper to a small folder (the one I had certainly wasn't more obtuse than the Project). This was several years ago, and other users I talked to at the same time also had very thin edged blades, none close to 20. I don't even run large chopping blades at 20 degrees per side and I cut frozen wood on a regular basis. The only reason you need edges that thick is if you are cutting hardened metal or really energetic chopping into thick bone (like a Moose not a chicken). Is this the actual expected use of a Sebenza?

-Cliff

 

[stjames]

Originally posted by Cliff Stamp
I would assume the Sebenza to be far more acute considering a large chopper to a small folder (the one I had certainly wasn't more obtuse than the Project). This was several years ago, and other users I talked to at the same time also had very thin edged blades, none close to 20.

-Cliff

So all this number crunching is assumed on your part without the usual hard data from use I have seen you refer to in the past. I would be interested to hear any actual results from testing you could contribute without the speculation.

 

[Cliff Stamp]

There is lots of evidence based on users and makers using lots of steels showing the behavior noted, it goes back *years*, Mike Swaim and Alvin Johnson were two of the principles and showed that 1095 could offer better edge retention than many high alloy steels if it was significantly harder. This was back on rec.knives before Bladeforms existed, I have also seen in in print long before then (usually comparing simple Japanese steels at 65 HRC vs western high alloy steels at 58 HRC).

I have used S30V, however I would not have to do so to be confident that S30V significantly softer than BG-42 is going to see deformation problems because hardness is a direct measure of deformation. You are actually arguing that S30V while measuring significantly softer than BG-42 is not actually going to be significantly softer in use becuase it is a different steel. This is like saying that 30 g of S30V is not lighter than 40 g of Bg-42.

Not to mention that this exact issue (hardness / edge retention) has been studied extensively specifically for the CPM stainless steels in particular so you can't even argue that they are some exception to the rule (no arguement at all has been give as to how this is even possible) as when Spyderco came out with S60V much softer than VG-10 it proved readily that it didn't matter about carbides or the CPM process because the edge just bent too readily. Not to mention all the other makers who have noted the exact same thing even in more extreme cases such as S90V losing edge retention readily at low HRC levels which Wilson checked to see if the *huge* alloy content in S90V would allow it to overcome the lower hardness issues - it doesn't.

-Cliff

 

[stjames]

Cliff, I'm not arguing anything. You are dedicating thousands of words to an assumption. Until you have the genuine experience with the knives that others in this thread have you lack evidence to back up your continuing arguments.

 

[Geode]

Back to the original question:

My small sebbie and mnandi are not the sharpest folders I own, but are pretty darned close. My understanding is that the edge is V-ground then semi-convexed with a felt wheel and polishing compound. Therefore, I strop the blades to sharpen them.

They are very sharp indeed. Maybe something has to do with the blades being S30V.

 

[Cliff Stamp]

stjames :

Until you have the genuine experience with the knives ...

This is what the hardness measurement allows, it is a direct measure of resistance to deforming. It is actually measured by denting the steel with a ball and measuring the extent of the dent. It isn't some abstract quantity. Soft edges dent more because they are softer in the exact same way they dented more under the ball in the hardness test.

-Cliff

 

[FK]

http://www.mee-inc.com/rockhar.html

http://www.gordonengland.co.uk/hardness/rockwell.htm

The testing of resistance to deformation by the penetration of a diamond cone with a 0.2mm radius spherical tip into the body of a hardened steel sample is only one indicator of physical properties.


B. Levine wrote an excellent summary of hardness testing.

http://pweb.netcom.com/~brlevine/rockw.txt

"quote from Wilson Instruments, maker of Rockwell

hardness testers, "... the Rockwell test is a measure of the

resistance of a material to permanent indentation. Indentation

hardness is not a fundamental property of a material. However,

reliable relationships have been established between the various

tests and important properties of materials -- for example,

tensile strength and machinability. Furthermore, indentation

hardness has become one of the more reliable controls of the heat

treatment and quality of manufactured parts."

The knife community is reading too much into the Rc numbers without considering other physical properies. The Rc is mainly for consistent quality control when heat treating.

Regards,
FK

 

[Cliff Stamp]

FK :

the Rockwell test is a measure of the resistance of a material to permanent indentation.

Yes, and the major cause of blunting in many types of cutting is permanent indentation. This should be obvious because if it wasn't steeling a blade would do nothing. It can also be verified by simply looking at a used knife under magnification and noting the misaligned edge (and of course direct cutting comparisons as noted in the above).

-Cliff

 

[FK]

The major cause of knive edge blunting is actually permanent deformation accompanied with removal of the steel on the knife edge. The steel edge does not have the material backing up the area under the diamond point penetration used to measure permanent indentation with the Rc method.


Yes, the steeling or reforming the edge with cold working is a very old technique. I am using a small round piece of carbide with high polish for cold working and reforming of edge profiles. This eliminates going to the sharping stones and removing material to bring back the fine edge. This can be observed under high magnifaction and requires a very light touch with the carbide. I also believe that stropping on hard leather with CrO is contributing to cold working as well as material removal/polishing.

Regards,
FK

 

[Cliff Stamp]

FK :

The major cause of knive edge blunting is actually permanent deformation

Indentation is a type of deformation, steels deform in basically the same way regardless of the axis by adjustement of slip planes.

...accompanied with removal of the steel on the knife edge.

This is actually quite minimal, which should be obvious as if it wasn't then a smooth steel would do nothing.

The steel edge does not have the material backing up the area under the diamond point penetration used to measure permanent indentation with the Rc method.

Yes it does, just hit it from edge to spine. You can also get micro hardness testers which allow HRC readings on very thin metals, they basically apply smaller loads with smaller indentors so they get higher pressures with less force.

Rockwell doesn't have to use a Diamond point either, most HRC tests on steels are done using a round ball, you use Diamond on high alloy steels, carbide and so on. Though none of that makes any difference to what the test means, it means what most people would define hardness, how difficult it is to squash something by pressing on it.

Pine for example is a much softer wood than Birch. You can tell this by whacking both with a hammer. Now if you wanted to be really precise you would let the hammer fall under a specific energy, and maybe even pretreat the area with a suboptimal impact first (this is what Rockwell testors do with the minor load), but it is just a more precise version of the same thing, just are just giving it a whack and seeing how big of a dent is made.

This relationship (hardness and ability to stay sharp) goes back to primitive man fire hardening wood so his spears would stay pointier. It isn't revolutionary. It has also been specifically studied as applied to knives and even more so to the CPM alloys and even more so again to the CPM stainless alloys to see if they would violate it for some reason, they don't of course, but it is always interesting to check.

For some more reading check out the Crucible's (the guys who made S30V) orange book where they clearly note how wear resistance is weakly correlated to hardness and mainly depends on the carbide nature of the steel. Does this mean that you can ignore hardness, if so why did Reeve stop at 57-59, why not drop it down to 45, the steel would be much tougher and more ductile, and you could then readily sharpen it with a file, so it would be much more friendly for "field" sharpening. The performance would of course be gutted at that hardness as the edge would deform far too readily regardless of the carbide structure because the steel matrix was simply too soft.

On an ironic note, especially concerning James blatent focus arguement in the above, way back when Reeve was asked about Talonite in the Sebenza he commented that he would not use it because it was too soft regardless of its other abilities (also had jig problems as it wasn't magnetic). Sound familiar.

-Cliff

 

[FK]

Cliff,
My comments on too much emphasis on Rc was directed toward comparison between different steel chemistries not within one steel formula.

Comparing BG-42 at Rc59-60 with S30V at 57-59 and ATS-34 (154CM) at 59-60 and so on.

I understand and agree within one steel grade the correlation of hardness with wear resistance and edge holding is well established.

We (all the usere of high quality knives) are sometimes overly concerned that steel "X" is Rc57-59 and steel "Y" is 60-61 and therefore superior in properties. This is what my main discussion addresses,,,, hopefully.

Regards,
FK

 

[Cliff Stamp]

Yes, there is a lot more to a steel than its hardness, it can at times not even be an overly critical factor. For example in large knives I would want a decent hardness, but could easily ignore a HRC reading of 4 points in favor of a much higher impact toughness. I would take 5160 at 58 HRC over BG-42 at 62 HRC anyday. For a fillet knife I would look for more corrosion resistance and could infact almost ignore hardness because the edge is of little use if heavily rusted regardless of how great the edge retention would be if it was nice and dry.

-Cliff