EDGE RETENTION/ROLLING PART II

[EOU]

This thread is a continuation of Edge Retention/Rolling Test Stand.

[Mark Reich]

Picking up where I left off...
(Thanks for fixing the thread, Mr. Grepper)

Mike Brubacher explained a few days ago;
(Quote) "This experiment is a big deal to us though because it represents the very clear opportunity to see if sharpening, in and of itself, somehow significantly affects the yield strength characteristics of the steel. If it doesn't, we move on. If it does we then can investigate if this is true of all sharpening methods or just some and to what degree." (Unquote)

I really didn't understand the true nature of the goal of heat treating sharpened edges until I read this. I've been pondering the variables in this equation for a few days, and I'm hitting a wall.

I certainly appreciate your effort to try to decode the theory of micro heating and edge softening due to sharpening, Mr. EOU. Maybe this has been one of the goals of the SET all along. At any rate, I think the SET should be our best opportunity to answer this question.

While we have been pondering the affect of hardening sharpened edges, we are overlooking another factor of greater significance to determining the affect of micro heating the EOTE of these blades.

If the hardness of the EOTE is so tenuous as to be affected by a completely un-observable, indeterminably small amount of heat, how much more would the infinitely greater affect of tempering have on the EOTE?  

Not only do we really not know if the EOTH is overly affected by hardening, we are trying to determine if virtually no heat has a minor influence to an amount of steel that has just been subjected to an enormous amount of heat.

IMHO, we are realizing why no one ever sharpens blades before HT. The EOTH is much too fragile to come away unscathed.

The current standards of not taking edges below .010- .015" prior to heat treat have been aided by advances in HT. Not only that, but standard procedure mandates removing several thousandths from the edge to get down to homogeneous steel.

If you hang out on sharpening forums much, I'm sure you've seen questions posed to the properties observed on factory edges. Most often it's from people appalled to see chippy edges on their brand new blades. It's very common to receive input guiding them not to worry about it. It's normal to assume that blades have not been ground back far enough to relieve the amount of steel that has been over-affected by the extreme heating and cooling during HT.

I'm sure we all had some concern that heat treating sharpened edges might not be appropriate. After all, nobody does that... ever. Brad claimed heat treating sharpened edges probably won't affect sharpness much. That's probably accurate. The amount of the increase in volume during phase change to martensite in steel with 1% carbon is about 3%. With inert gas shielding the affect of normal oxidation at 1500°F is magically diminished.

We could hope that the sharpness of pre-sharpened blades post-HT could remain mostly unaffected. That would be fine if that's what we wished for, but that is not what matters to us or anyone else.

What matters to everyone is the performance of the edge of a brand new blade. To that end, everyone takes certain measures to avoid over-affected steel to remain on the edge of a new blade.

I hope this exercise helps explain why certain procedures have developed into normalcy for the millions of new blades heat treated each year. I also hope this helps explain why certain procedures have developed into normalcy regarding the sharpening of hundreds of millions of edges per year.

I hope the people with questions pursue answers to their questions to the best of their ability. Perhaps EOU and BESS testing testing devices will shed new light on significant misconceptions. That would be very cool, and please do not think for one second that I wouldn't be the first to congratulate my esteemed colleagues.

I feel like an old dog. I've made mistakes like every dog my age, and (accidentally) been kicked and stepped on. I can handle it. I know I won't be in the right place every time. I'm just happy to still be in on the chase.

[EOU]

Very well thought through and expressed Mark and thank you as well for opening Edge Retention/Rolling part II with your thoughts. The thrust of these experiments is to increase understanding and to reduce speculation.  Conventional wisdom can be both the foundation of an industry as well as its adversary. Only thoughtful experimentation can advance a process. There could be a little blind luck involved, like accidently spilling your rubber mixture on a hot stove top, but lots of experimentation followed that famous accident. This is how scientists came to understand the vulcanizing process. 

Sharpening and then heat-treating wouldn't seem to be a viable solution to us. It does seem as if it might present us with a clue though. If pre-sharpened edges don't roll and post-sharpened edges do then we might surmise that something about the sharpening process changes the characteristics of heat treated steel. That's really all there is to this portion of the experiment.

But even a successful experiment doesn't leave us at the final chapter in this case. Seems to us that edges that don't roll might be brittle. Brittle edges chip. If brittle edges that chip were OK with us we'd all be making and using cutting edges composed of ceramic. It's all just about understanding the process and then designing to maximize for a particular process/application. One thing we've learned during our years is that "there is no free-lunch in the physical world". You want an edge that doesn't roll? Its likely to be prone to chip. You want an edge that doesn't chip? It's likely to roll. We're all on a bus trip here, we just don't know, yet, where it's going to let us off.

[EOU]

There's been a discussion going on over in KG's   EDGE STABILITY IN BUTCHER’S AND KITCHEN KNIVES concerning ceramic knives and edge rolling etc. We borrowed a couple of inexpensive Kitchen Aid from an unsuspecting wife's knife drawer and brought them to the office for test in the SET instrument. Here are the two ceramic paring knives:

                                   

The long one measured 335 and the short 382. There was a fair amount of edge sharpness disparity along both edges but that is what they measured at the two test points. We don't think that these edges had seen much, if any, previous use.

We performed our standard Phase I test procedure (5 up and back passes over the edge) and then measured again.
Our 335 now measured 326 and our 382 now measured 376. There is no obvious reason why the measurements would go down other than to chalk it up to the vagaries of this particular test. One thing appears obvious though and that is that ceramic edge don't roll. Here's a picture of one of the edges after the test: 

                                   

We tried sharpening one of the ceramic blades with a ceramic belt just to illustrate a point and we were successful in that regard because we were unsuccessful in doing so. It simply demonstrates how hard these blades really are relative to steel. We've got a couple of better quality ceramic knives headed our way and we're going to order some belts that will conquer ceramic. Then we'll test how sharper ceramic edges fair in the SET. We don't think that they'll roll but perhaps the standard SET piston design will damage (chip) the edge. 

In the meantime, KG is going to put some ceramic knives to test in a meat processing facility and that is going to be where the rubber meets the road.

[EOU]

Just a very short update and reminder here. The 12 sharpened A2 blades are in the hands of the heat treaters now. These samples represent a total of 24 edges in that both sides of each piece have been beveled and pre-sharpened. Eight pieces are double bevel at 15° and four have been chisel ground and sharpened at 30°. Three pieces each will be hardened at HRC50,54,58 and 62. When they arrive back here at our offices we will SET test the edges in their hardened state and then retest after re-sharpening has been conducted . 

The pre-heat treated edges were sharpened in an array that spanned the BESS range 250 - 450. Our previous SET results with sharpened edges indicate that this range of initial sharpness levels (250-450) all roll albeit at different rates. This sequence of SET tests will allow us to; (1) test for the edge structural parameters inherent to differing hardness levels and to (2) test for the structural effect, if any, of the sharpening process itself and finally (3) test for how initial sharpness levels correlate to hardness levels and the rate of degradation thereof. 

In other words, tests (1) and (2)  will involve re-measuring the sharpness level of the edges right out of the heat treaters box and then subjecting them to a SET (rolling) test in order to see how pre-sharpened, post heat treated, edges perform structurally. We will then re-sharpen those same edges and see how they fare comparatively speaking. This will all be evaluated against the HRC hardness levels 50 through 62. This experiment should answer the question of whether or not the sharpening process itself alters the physical structure of the steel somewhat, significantly or not at all. 

Test (3) will simply see how different initial sharpness levels fare at different hardness levels.

If you have a suggestion for a different or particular test that might be run with this group of samples we'd be happy to hear about it.

Edit: Oh yes, forgot to mention that we have recently witnessed our 3000th post come and go. That's a lot of quality content and a lot of keystrokes.  EOU and BESS thank you all wholeheartedly for your participation and support here.

[Jan]

There's been a discussion going on over in KG's   EDGE STABILITY IN BUTCHER’S AND KITCHEN KNIVES concerning ceramic knives and edge rolling etc. We borrowed a couple of inexpensive Kitchen Aid from an unsuspecting wife's knife drawer and brought them to the office for test in the SET instrument. Here are the two ceramic paring knives:                              

The long one measured 335 and the short 382. There was a fair amount of edge sharpness disparity along both edges but that is what they measured at the two test points. We don't think that these edges had seen much, if any, previous use.

We performed our standard Phase I test procedure (5 up and back passes over the edge) and then measured again.
Our 335 now measured 326 and our 382 now measured 376. There is no obvious reason why the measurements would go down other than to chalk it up to the vagaries of this particular test. One thing appears obvious though and that is that ceramic edge don't roll. Here's a picture of one of the edges after the test: 
                             
We tried sharpening one of the ceramic blades with a ceramic belt just to illustrate a point and we were successful in that regard because we were unsuccessful in doing so. It simply demonstrates how hard these blades really are relative to steel. We've got a couple of better quality ceramic knives headed our way and we're going to order some belts that will conquer ceramic. Then we'll test how sharper ceramic edges fair in the SET. We don't think that they'll roll but perhaps the standard SET piston design will damage (chip) the edge. 

In the meantime, KG is going to put some ceramic knives to test in a meat processing facility and that is going to be where the rubber meets the road.

Let me briefly recollect why ceramic blades are harder and brittle then steel blades.

When iron atoms are grouped together to form a piece of iron or steel, the outer electrons of iron atoms leave their parent atoms and become an electron cloud. The mobility of valence electrons makes it possible that iron atoms can slide past each other when the steel is deformed. Generally we can say that metallic bonds are delocalised.

Ceramic blade is usually made of zirconia which is an ionic compound of zirconium and oxygen atoms (ZrO2). The ionic bonding of atoms is much stronger than metallic bonding. On the Mohs hardness scale ceramic blade has some 8.5 while hardened steel has 7.5 to 8 and diamond has 10.

Ionic bonding in ceramics results in high hardness and high chemical inertness. On the other hand this strong bonding accounts for low ductility and low tensile strength. Most ceramics are poor conductors of electricity because they do not have free electrons like metals.

By adding small amount of yttrium oxide we get stable zirconia, material known as Transformation Toughened Zirconia (TTZ). TTZ is relatively tough ceramic material and should not undergo catastrophic blade failure.

Jan

[EOU]

Thanks Jan. While we're waiting on our test samples we're finding time here and there to examine ceramic blades. We just purchased two new ceramic knives and are testing them as well. Thanks for the heads up concerning TTZ. Don't know if all/most ceramic knives are composed of this material or not. It might explain the price differential between Kyocera ceramic knives and the run of the mill knives that can be purchased in the 10-15 dollar range.

By the way, just got notification from Peter's Heat Treating that our A2 samples are being tempered now so in a week or so, Jan, the mysteries of the world may be revealed.

[Mark Reich]

I bought a Kyocera Kyotop 5" slicer a few years ago when they came out. The major difference in this new line from Kyocera was that the blades are basically sintered twice. They are sintered like normal, then go through what Kyocera calls a "Hot Isostatic Press". This is supposed to make them harder and tougher than their regular ceramic blades.

They also look totally different than regular ceramic blades, so they made enough of a stir that I went ahead and got one, with the plan of seeing how sharp I could get it.

That plan was a bust. I never did get it sharper than new, and concluded that the new ceramic blades must indeed be harder and tougher than normal ceramic blades.

It seems Kyocera doesn't stand around waiting for others to catch up. They have more lines of ceramic knives now, but their "double sintered" ones are still top of the line.

I really do like the Kyotop knife. It looks great and doesn't seem to lose it's edge, but I still wish it was sharper.

[EOU]

Would certainly be interested in just how sharp a good sharpener like you can get a ceramic edge Mark. We just ordered a bunch of diamond belts and would like to know what is possible/feasible. Just received two more low cost knives and they measured around 300 out of the box. We're beginning to wonder, from a practical standpoint, if that isn't about where ceramic edges want to be. These knives actually seem to have a little tooth to them. These were sharpened with a primary bevel of, perhaps 12°, and then a very small micro bevel. 

                         

[KnifeGrinders]

Our SET-testing results of ceramic knives are in line with Mike's findings:
sharpened at the same dps and to the same initial sharpness, the ceramic edge lasts 6 times as long as a quality boning knife (0.5% carbon, 56-58 HRC), and twice as long as a Vanadis-10 knife (2.9% carbon, 9.8% Vanadium, 63 HRC).

The ceramic knife I've tested is Zirconium Dioxide of a cheap make, that you can buy under $10 retail, and for $1.00 bulk from China.

No rolling in the impact area as you would expect, but microscopic chipping as seen in the below image taken after 600 impact cycles when the edge started scoring blunt over 500 BESS. The chipping develops on the side of the blade opposite the impact roller, i.e. where on a steel blade you would feel the rolled/bent apex.



Now to the most interesting part - correlation between the ceramic edge retention and angle is reverse to what we've seen in steel knives.
The above results are for a ceramic edge sharpened at 12 dps.
The ceramic edge sharpened at 15 dps stays by 30% sharper and lasts by 3 times longer than the ceramic 12 dps - it took 600 impact cycles for the 12 dps ceramic edge sharpness score to go over 500 BESS, and more than 1500 cycles for the 15 dps.
The 15 dps (30 degrees included) ceramic edge stays sharp longer than a steel boning knife at least by 15 times, and longer than Vanadis-10 at least by 5 times.
I say "at least" because I didn't have patience to continue rolling the edge till its sharpness scores over 500 BESS - it didn't after 1500 impact rolls, and I stopped there.
Microscopically I didn't see any chipping in the impact area on the 15 dps edge.




Impact roller wear in the ceramic knife test: