EDGE STABILITY IN BUTCHER’S AND KITCHEN KNIVES

[KnifeGrinders]

SEQUENCE #2
Using the same methods and equipment, we tested edge retention in two more brands of kitchen knives: one of less hardness, less Carbon contents and known to hold the edge worse than the Victorinox SWIBO knives, and the other of higher Carbon contents and known to hold the edge better.
The only difference with the previous tests was that to study relation between the initial sharpness and edge retention we used the initial sharpness of 100 BESS (i.e. next to razor sharp) versus 200 BESS (i.e. utility blade sharpness).
 
STEP DOWN
SCANPAN Classic Kitchen knife
Stainless steel X45CrMoV15, Hardness HRC 56-57, Carbon 0.45%



Link to SCANPAN Data >>  
We average data the same way we did for the SWIBO knives, mainly to boost trust in the data we’ve obtained, because the sets of data for each initial sharpness can be treated as a retention test for a given edge angle repeated on different knives to confirm reproducibility of the results.

10 dps (20° degrees included) edge
A shaving sharp 10 dps edge about 150 BESS by the Sharpness Chart, is dented by the test media line, scoring in the dent 700 BESS on the sharpness tester.

As it was in the case of Victorinox SWIBO knives, this indicates the angle at which the edge becomes unstable and easily deformable, rendering further tests both meaningless and impossible.



12 dps (24° degrees included) edge
Microscopy is unremarkable – no visible deformation after sharpness measurements.

***

The following charts show edge retention as a function of initial sharpness.

12 dps (24° included)

Initial Sharpness - Average Sharpness Score over 100 Cycles
100 BESS - 485
200 BESS - 632
The sharper is the edge the better is retention.

15 dps (30° included)

Initial Sharpness - Average Sharpness Score over 100 Cycles
100 BESS - 631
200 BESS - 611
No relation between the initial sharpness and edge retention (difference < 5%).

20 dps (40° included)

Initial Sharpness - Average Sharpness Score over 100 Cycles
100 BESS - 650
200 BESS - 624
No relation between the initial sharpness and edge retention (difference < 5%).

***

The following chart of averaged data shows edge retention as a function of edge angle.
EDGE ANGLE (dps)         12° - 15° - 20°
AVERAGE SHARPNESS   559 - 621 - 637

The thinner is the edge the better is retention.

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STEP UP
GLOBAL Classic Kitchen Knife
Stainless steel CROMOVA 18, Hardness HRC 56-58, Carbon 0.7% (other source 0.55%)


Link to GLOBAL Data >>

8 dps (16° degrees included) edge
An 8 dps edge under 100 BESS by the Sharpness Chart, is dented by the test media line, scoring in the dent 300-500 BESS on the sharpness tester.
I am aware that this contradicts common opinion that Global knives can be sharpened to 15 degrees included, yet at this angle the edge is too weak in the apex for testing by our methods.
Below are microscope images of several attempts to measure sharpness on the BESS tester.





10 dps (20° degrees included) edge
Microscopy is unremarkable – no visible deformation after sharpness measurements.

***

The following charts show edge retention as a function of initial sharpness.

10 dps (20° included)

Initial Sharpness - Average Sharpness Score over 100 Cycles
100 BESS - 429
200 BESS - 454
The sharper is the edge the better is retention.

12 dps (24° included)

Initial Sharpness - Average Sharpness Score over 100 Cycles
100 BESS - 475
200 BESS - 437
Reverse relation between the initial sharpness and edge retention.

15 dps (30° included)

Initial Sharpness - Average Sharpness Score over 100 Cycles
100 BESS - 473
200 BESS - 540
The sharper is the edge the better is retention.

20 dps (40° included)

Initial Sharpness - Average Sharpness Score over 100 Cycles
100 BESS - 509
200 BESS - 500
No relation between the initial sharpness and edge retention (difference < 5%).

***

The following chart of averaged data shows edge retention as a function of edge angle.

EDGE ANGLE (dps)        10° - 12° - 15° - 20°
AVERAGE SHARPNESS   441 - 456 - 507 - 504

The thinner is the edge the better is retention.

STEEL COMPARISON

The following charts and graphs compare the knives edge retention.

The following two charts show edge retention by steel, and include average sharpness and number of impact cycles it took to render the edge blunt i.e. over 500 BESS (a key indicator).



The following graph shows the best edge retention by averaged data for each of the steels tested; additionally shown is one more GLOBAL sharpened at 30 degrees included.



The lower ranking SCANPAN at 12 dps outperforming GLOBAL at 15 dps (though not for long) can be explained by the advantage of the lower edge angle, but what about the winning SWIBO?
The seemingly paradoxical SWIBO knives outperforming the higher ranked Global is discussed in the Conclusions below.

CONCLUSIONS

Softer steel (Step down)
The optimal edge angle for mainstream kitchen knives with HRC 56-57 and Carbon 0.45% is 12 dps (24° included), sharpened to 100 BESS; the 12 dps edges with the initial sharpness of 100 BESS stay sharp twice longer compared to the initial sharpness of 200 BESS.
Edges sharpened at 12 dps stay sharp twice as long as those sharpened at 15 and 20 dps, and the 15 dps edges hold sharpness somewhat better than 20 dps - the edge angle remains a strong determinant in the edge retention of softer steel.
Lower than 12 dps edges are too weak in the apex.

Compared to the SWIBO knives, the SET load causes early plastic deformation in the edge of the softer steel X45CrMoV15.

Absence of relation between the initial sharpness and edge retention that we see in easily deformed edges sharpened at 15 and 20 dps, suggests that the initial sharpness helps to hold edge only in the elastic deformation phase.

Harder steel (Step up)
The optimal edge angle for better kitchen knives like Global is 10 dps, sharpened to 100 BESS; the 10 dps edge stays sharp by 50% longer than the 12 dps edge, and over 2 times longer than the 15 dps edge.

Edge retention as a function of edge angle

Best edge angle for a knife is the lowest angle at which the edge can take a load without deforming.
Steels have a certain smallest angle at which the edge becomes unstable, the stronger the steel the smaller is this angle.

Edge retention as a function of initial sharpness

Generally, the sharper edge shows better retention.
Edge retention is improved by the initial sharpness under 100 BESS, however this relation becomes less consistent at and over 100 BESS, especially in lower ranking steels.

Comparison by steel

A knife made of weaker steel, but sharpened to its best at the acutest angle it can hold may outperform a knife of stronger steel sharpened at a more obtuse angle.

As seen by the test data, even a lower ranking SCANPAN knife sharpened to 100 BESS at 12 dps outperforms GLOBAL knife sharpened to 200 BESS at 15 dps.

The above is the common rule ensuing from our SET testing, but not without many exceptions, and when the exceptions are abundant, this usually tells us there is another hidden rule behind them we are yet to comprehend.

The main paradox we see is that SWIBO knives outperform the more wear-resistant Global in our tests.
Obviously, edge rolling is far not the same as wear resistance, and a mainstream steel like SWIBO can tolerate rolling better than a harder steel like GLOBAL; steel compressive strength seems to retain the edge within the elastic deformation range better than hardness.

SET testing method

Number of cycles had been guessed right because sharpness of all knives neared or exceeded 500 BESS (i.e was rendered blunt) by the 100th impact cycle – allowing us to watch the full life cycle of the edge within one test.

The testing procedure yields additional information about events happening in the edge as reflected by the test data. The testing regimen we’ve applied in these series of SET testing has 3 distinctive phases:

·        Phase I “Elastic deformation” from the 1st to the 5th impact cycle, when sharpness is measured after every cycle – considering that interval between subsequent impact cycles is about 30 sec, this break in impact allows the edge to partially recover from rolling. This phase takes about 2.5 min.

·        Phase II “Elasto-Plastic transition” from the 6th to 50th impact cycle, where the edge gets 5 impact cycles between sharpness measurements – edge is challenged for resistance to plastic deformation. The elastic deformation transits to plastic here, as seen by the lessening of variances in sharpness in the second half of this phase. Phase II is where the initial sharpness contributes the most to the edge longevity through the enhanced elasticity of the thinned edge. Weaker steel simply crashes in this phase. This phase takes 5 min.

·        Phase III “Plastic deformation” from the 51st to 100th impact cycle, where the edge is continuously rolled 10 times before each next sharpness measurement, testing the edge stability to permanent rolling. This phase takes about 3.5 min.

Key indicators:
- Overall average sharpness over 100 impact cycles;
- Average sharpness in the Phase I (elastic deformation) - calculated as an average sharpness scores in the first 5 impact cycles;
- Sharpness by the end of the Phase II (elasto-plastic transition) – calculated as an average of 3 sharpness scores: after 40, 45 and 50 impact cycles;
- Number of impact cycles to turn the edge blunt at 500 BESS (resistance to permanent rolling).

Overall, each SET test takes 11 minutes to estimate life cycle of the edge; for comparison, CATRA’s “knife sharpness and life tester” takes 15 minutes in the semi-automatic model and 10 minutes in the automatic model.

SET and CATRA testing should not be opposed, as they focus on different attributes of the edge:
of the two main constituents of the edge blunting, SET testing focuses on the edge rolling, while CATRA on the abrasive wear; of the three main steel properties playing role in the edge retention: strength, toughness and hardness - strength is more about resilience to rolling, while hardness to wear.
More tests are needed to estimate edge rolling in so called “super-steels”, and compare to the mainstream steels; the data we’ve got so far tell us that the “super-steels” are not necessarily superior in resilience to rolling unless compared to lower end steels.
Lately we and EOU have been getting more experimental facts that wear resistance and edge resilience to rolling may be unrelated.

The SET method has proved to distinguish different types of steels, even when this difference is subtle, just when interpreting SET data keep in mind that in SET compressive strength/elasticity wins over hardness, as it often does in the kitchen.

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