Cutting ability and edge holding comparison between Dendritic D-440C, dendritic Cobalt and AUS-8A at a high polish and a medium finish.

Some background information

Two Boye drop point hunters, one in Dendritic 440C stainless steel (D-440C) and another in Dendritic Cobalt (D-Cobalt) were compared by cutting cardboard to a Deerhunter in AUS-8A from A. G. Russell . The method used to test sharpness has been described in detail in earlier work [ref].

First run

The blades were used to cut 2000 centimeters of cardboard with the sharpness checked for slicing and push cutting every 500 centimeters. A run was made with the blades sharpened to a high finish from an 800 grit ceramic rod (from Ben Dale and another with a much more coarse finish from a 600 grit DMT rod.

Before each sharpening waterstones were used to reset the edges to ensure all weakened metal was removed. A SiC coarse stone was first used and followed by an 800 grit AO hone. The cutting with the 600 grit DMT finish was performed on a different cardboard stock than the 800 grit ceramic finish as that was what was at hand. Only 1500 centimeters of cardboard were cut in that run as the blades were starting to tear the cardboard at that point.

The most obvious result is that with the more coarse finish from the Diamond DMT hone, all the knives performed lower on the initial push cut. Secondly it is obvious that the rate of blunting is not linear, and slowly down rapidly as more material is cut. Comparing the blades, with a fine finish the Boye blades have a slight advantage, but when all knives are sharpened with the rougher Diamond, the push cutting sharpness degrades at a very similar level.

Note that even though at the 1500 centimeter point the force required to cut the string is similar in both finishes, the DMT finished blades are much harder to use which is why the cutting had to be stopped at that point. The string measurements might be getting distorted due to point effects. Again to clarify, different cardboard stock was used for the runs so direct comparisons on the rate of blunting can not be drawn.

Looking at the loss of slicing sharpness :

When the blades have the high polish the Deerhunter loses aggression fast and is outclassed by the Boye Dendritic blades. However with the rougher DMT finish was used the performance is much closer with the blades and the Deerhunters performance is not significantly different from the Boye Dendritic D-Cobalt drop point hunter.

UPDATE : these types of comparisons should be repeated several times to insure that differences seen were not due to random effect such as differences in initial quality of finish, or variety in the cardboard. The results would also have been much more informative if the cardboard stock was the same in both runs. It should also have be clarified exactly how the cardboard was cut, from memory it was a push cut.

Extended comparison

A more extended run was also performed, again with the two finishes, this time the same cardboard stock was used for both runs. The sharpness check intervals were enlarged due to the greater amount of material being cut. A more difficult to cut fabric was also used to more clearly separate the behavior of the blades. Looking at the loss of push cutting sharpness :

Again, the more coarse DMT finish has a lower initial cutting ability on the strong. No significance difference was noted in regards to edge retention as measured by sharpness on the string. However the variance in the measurements is rather large. Also as noted before the edge holding in regards to slicing ability shows much greater changes :

Similar to the shorter run, for all blades the slicing ability is much better with the medium finish and the difference in edge retention at the fine finish is much larger than at a medium finish. Note the odd behavior of the Deerhunter with the fine finish at the 3000 centimeters mark, when it seems to actually improve.

UPDATE : as noted in the above these types of comparisons should be repeated several times to insure that differences seen were not due to random effect such as differences in initial quality of finish, or variety in the cardboard. Note for example while the D-440C blades is ahead of the D-Cobalt one with a high polish, the order is reversed at a medium finish. However neither difference is significant compared to the uncertainty in the cutting abilities so there is no contradiction.

Exploring some details - moving beyond the obvious

Examination of the edge of both the Deerhunter under magnification revealed a stress line that runs about 0.1 millimeters deep along most of its edge, which has broken away in some places. There are also many small chips about 0.1 to 0.2 millimeters deep. These fractures create a micro-serrated edge and allow the blade to saw the fabric enhancing the slicing ability which is why the performance increased after the the 2000 centimeter mark.

The Dendritic D-Cobalt blade did not have a similar stress line but had suffered more extensive damage in a few places. There were two areas two millimeters long missing pieces 0.2 millimeters deep, a couple of smaller sections had broken away producing gaps about one millimeter long and 0.1 millimeters deep. The Dendritic steel blade did not fracture and was only slightly reflecting light in a few places.

Thus due to the edge fracturing on the Deerhunter and D-Cobalt, the slicing performance can actually increase after extended work has been done as a pseudo-saw is created along the edge. Note that this behavior isn't seen with the rougher finish to the same extent as it already has a saw formed on the edge due to the initial micro-teeth left by the rougher finish. Note as well that even with this "self-sharpening" effect both blade are still behind the D-440C drop point hunter. As well the creation of this micro-saw due to the edge fractures, while helping slicing performance doesn't aid push cutting ability at all.

After the 3000 centimeter mark with all three blades on the 600 grit DMT finish run some cord was push cut with each blade, a six braided cord, much thicker than the string used in the above tables. The Dendritic steel hunter would cut it consistently at about 1800 g. The Dendritic D-Cobalt would cut about halfway through 2 of the braids at 2200 g (maximum reading), and the Deerhunter would only score the braids lightly at 2200 g. Thus the thicker cord sees the influence of the small fractures which are masked by the string cutting as it is too thin to catch them. This clearly shows the greater degradation due to fracture of the Deerhunter and Boye D-Cobalt blade.

Summary

The D-440C blade seems to have better edge retention in regards to slicing aggression, than the D-Cobalt and AUS-A Deerhunter. No significant differences were noted in regards to initial sharpness. The push cutting edge retention as measured on thread was also consistent for all the blades, but the thread was too slight to give an accurate indication of the edge condition and masked the extensive fracturing of the D-Cobalt and AUS-8A Deerhunter. Using heavier cord clearly showed the influence of the edge damage and the D-440C blade showed superior performance.

UPDATE : it would have been informative to compare the sharpening of the blades, as the D-440C one was honed back to 100% much faster than the others due to much less material needing to be removed from the edge to restore it to 100%. The D-Cobalt and AUS-8A knives needed to have all the fractures honed out before a crisp edge could be formed.

UPDATE : note the inherent aggression of the Dendritic steels was readily overshadowed by the choice of sharpening abrasive. The Deerhunter at the medium finish was much more aggressive and had much better edge retention than either of the Dendritic steels when they had a fine finish.

Comments and references

You can comment on this write-up by dropping me an email : cliffstamp[REMOVE]@cutleryscience.com. Feedback can also be seen in the the following thread on Bladeforums :

• Edge retention / cutting ability (Boye Dendritic 440C and Dendritic Cobalt, AUS-8A